Detecting objects in space

Abstract

An apparatus (806) for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means (804) configured to attach to a space-borne artificial satellite; a first planar substrate (802) attached to the attaching means (804), the first planar substrate (802) comprising one or more homogenous layers of material; and a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the first planar substrate (802) such that each sensor is able to detect vibrations of the first planar substrate (802) and to generate an electrical signal indicative of those vibrations.

Description

DETECTING OBJECTS IN SPACE

[0001] The present application relates to an apparatus for detecting objects in space.Background

[0002] The number of artificial, or man-made, satellites that are orbiting in space is rapidly increasing. Modern day life relies on a sophisticated network of these satellites that orbit the Earth and support a number of major global systems. Examples of systems that are supported by artificial satellites include communications systems, weather systems, mapping systems and transportation systems. Maintaining these global systems requires significant organisation and planning.

[0003] All spacecraft, including artificial satellites, must be equipped with appropriate systems to protect them from space debris which can collide with and damage space equipment. The term "space debris" is used to define freely moving solid particles in space and covers the two sub-groups of interplanetary dust and orbital debris. The term "interplanetary dust" defines small natural objects that are created by collisions between extraterrestrial bodies such as comets and meteorites. The term "orbital debris" defines objects that are present in space because of human activities, such as parts of defunct artificial satellites.

[0004] As the number of artificial satellites in space increases, the amount of space debris (and, specifically, of orbital debris) also increases. Thus, the risk of damage to spacecraft as a result of collisions with this debris is constantly increasing. To combat this problem, space agencies desire to develop systems for detecting and characterising space debris so that their spacecraft can be designed to protect against collisions with this debris. The US Space Surveillance Network (SSN) currently monitors over 36,000 objects in orbit that are larger than 10cm in diameter using a network of ground-based radars and optical sensors. However, there are a large number of objects in space that are less than 10cm in diameter and that cannot be detected by this system. These sub-10cm objects, whilst small, have the ability to cause significant strategic and financial damage.

[0005] Solutions that are currently incorporated onto spacecraft to protect against space debris include the strategic placement of hardware on the craft and shielding, such as Whipple shielding, for shattering incoming objects. These solutions introduce engineering (e.g., weight) and financial disadvantages for companies wishing to incorporate them. A more detailed understanding of space debris would enable the construction of improved, customised, shields for spacecraft that would alleviate these disadvantages.Summary

[0006] The invention is defined by the claims. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter; variants and alternative features which facilitate the working of the invention and / or serve to achieve a substantially similar technical effect should be considered as falling into the scope of the invention disclosed herein.

[0007] In a first aspect there is provided an apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space- borne artificial satellite; a first planar substrate attached to the first attaching means, the first planar substrate comprising one or more homogenous layers of material; and a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the first planar substrate such that each sensor is able to detect vibrations of the first planar substrate and to generate an electrical signal indicative of those vibrations.

[0008] The apparatus may further comprise an electronic module attached to the first attaching means, the electronic module being configured to receive and process the electrical signals from the first array of piezoelectric sensors and to characterise an object that has collided with the first planar substrate in dependence on the processing of the electrical signals.

[0009] In a second aspect there is provided an apparatus for characterising sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means; a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the substrate such that each sensor is able to detect vibrations on the surface of the substrate and to generate an electrical signal indicative of those vibrations; and an electronic module attached to the first attaching means and configured to receive and process the electrical signals from the first array of piezoelectric sensors and to characterise an object that has collided with the first planar substrate in dependence on the processing of the electrical signals.

[0010] The apparatus may further comprise an electromagnetic sensor attached to the first attaching means, the electromagnetic sensor being configured to detect electromagnetic waves generated by a collision of an object with the first planar substrate.

[0011] In a third aspect there is provided an apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means; a first array of piezoelectric sensors adhered to a surface of the substrate such that each sensor is able to detect vibrations of the substrate and to generate electrical signals indicative of those vibrations; and an electromagnetic sensor attached to the first attaching means, the electromagnetic sensor being configured to detect electromagnetic waves generated by an impact of an object with the first planar substrate.

[0012] The electromagnetic sensor may be configured to detect visible light waves.

[0013] The apparatus may further comprise an impact detection mechanism configured to determine the time of impact for an object with the first planar substrate.

[0014] In a fourth aspect there is provided an apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space- borne artificial satellite; a first planar substrate attached to the first attaching means; an impact detection mechanism configured to determine the time of an impact for an object with the first planar substrate; and a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the substrate such that each sensor is able to detect vibrations of the surface of the substrate and to generate electrical signals indicative of those vibrations.

[0015] The first array of piezoelectric sensors may comprise four piezoelectric sensors and the impact detection mechanism is a piezoelectric sensor of the array.

[0016] The impact detection mechanism may be a layer of the first planar substrate.

[0017] The first attaching means may be a first frame and the first substrate is mounted to the first frame.

[0018] The first frame may surround the circumference of the surface of the substrate.

[0019] The first frame may be attached to a planar surface of the first planar substrate.

[0020] The first frame may be comprised of a lattice of hollow prismatic cells, and wherein a first surface of the lattice is adhered to the planar surface of the planar structure.

[0021] The first planar substrate may consist of a single base layer which is a homogenous layer of material.

[0022] the first planar substrate may comprise a base layer and a second layer that provides a protective coating over the base layer.

[0023] The base layer of the first planar substrate may be composed of a flexible film.

[0024] The flexible film may be made from Kapton.

[0025] The flexible film may be a piezoelectric film.

[0026] The base layer of the first planar substrate may be comprised of a solid metal plate.

[0027] The solid metal plate may be made from aluminium.

[0028] The piezoelectric sensors may be polyvinylidene fluoride "PVDF" sensors.

[0029] Each piezoelectric sensor may be adhered to the surface of the first planar substrate using an adhesive.

[0030] Each piezoelectric sensor may be printed onto the surface of the first planar substrate.

[0031] The surface may be square in shape.

[0032] The first frame may be configured to be fixed with respect to a space-borne artificial satellite when it is attached to the satellite.

[0033] The first frame may be configured to be moveable with respect to a space-borne artificial satellite when it is attached to the satellite.

[0034] The electronics module may be configured to calculate a time of impact of the object with the substrate by observing an arrival time of an electrical signal received from each sensor of the first array of sensors and selecting the earliest arrival time as the time of impact.

[0035] The electronics module may be configured to calculate the location of the impact on the substrate using the arrival time of the electrical signal received from each sensor of the first array of sensors.

[0036] The electronics module may be configured to calculate the location of the impact on the substrate by: calculating a time delay between the time of impact and the arrival time of each electrical signal received from a respective sensor of the first array of sensors; and comparing each time delay to a look-up table that plots time delays against coordinates of the substrate.

[0037] The electronic module may be configured to calculate the time delay between the time of impact and the arrival time of each electrical signal by subtracting the time of impact from the time of the peak voltage recorded by each of the piezoelectric sensors in the array.

[0038] The electronic module may be configured to characterise the object by determining one or more of the size, velocity and trajectory of the object.

[0039] The electronic module may be configured to calculate the size of the object using an amplitude measurement obtained from each sensor of the first array of piezoelectric sensors.

[0040] The electronic module may be configured to calculate the size of the object by comparing an average peak-trough amplitude measurement to a look-up table that plots peaktrough amplitude against diameter.

[0041] The electronics module may be configured to calculate the size of the object using the following steps: determining coordinates identifying a location of the impact on the substrate; calculating a distance between the location of the impact and the location of a sensor of the first array of piezoelectric sensors; calculating a raw peak-trough amplitude for the electrical signal received from the sensor of the first array of piezoelectric sensors; calculating a normalised peak-trough amplitude for the raw peak-trough amplitude using the calculated distance for the sensor of the first array of piezoelectric sensors; and calculating the diameter of the object from a plot of average peak-trough amplitude against diameter.

[0042] The apparatus may further comprise: a second frame configured to attach to a space- borne artificial satellite; a second planar substrate mounted to the second frame; and a second array of piezoelectric sensors comprising three or more piezoelectric sensors, the second array of piezoelectric sensors being adhered to a surface of the second planar substrate such that each sensor is able to detect one or more vibrations of the surface of the second substrate and to generate electrical signals indicative of those vibrations.

[0043] The second frame may be parallel to and spaced apart from the first frame.

[0044] The electronic module may be configured to calculate the velocity of the object by observing the difference between the time of impact of the object with the first substrate and the time of impact of the object with the second substrate.

[0045] The electronic module may be configured to calculate the velocity of the object using the following equation: d v = -^2 ~ ^1Where t1is the time of impact of the object with the first substrate, t2is the time of impact of the object with the second substrate, and d is the distance between the first and second planar substrates.

[0046] The electronics module may be configured to calculate the trajectory of the object using the difference between the coordinates of a location of impact of the object on the first substrate and the coordinates of a location of impact of the object on the second substrate.

[0047] According to a fifth aspect there is provided a method of manufacturing a satellite as described above.

[0048] The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.Brief Description of the Drawings

[0049] Examples of the invention will be described, by way of example, with reference to the following drawings, in which:

[0050] Figure 1 illustrates a schematic arrangement of an apparatus for detecting subcentimetre objects in space according to a first aspect of the invention;

[0051] Figure 2 illustrates a schematic arrangement of an apparatus for detecting subcentimetre objects in space according to a second aspect of the invention;

[0052] Figure 3 illustrates a schematic arrangement of an apparatus for detecting subcentimetre objects in space according to a third aspect of the invention;

[0053] Figure 4 illustrates a schematic arrangement of an apparatus for detecting subcentimetre objects in space according to a fourth aspect of the invention;

[0054] Figure 5 illustrates a two-dimensional view of an exemplary apparatus for detecting sub-centimetre objects in space according to any of the first to fourth aspects of the invention;

[0055] Figure 6 illustrates a two-dimensional view of an alternative exemplary apparatus for detecting sub-centimetre objects in space according to any of the first to fourth aspects of the invention;

[0056] Figure 7 illustrates a three-dimensional view of a further exemplary apparatus for detecting sub-centimetre objects in space according to any of the first to fourth aspects of the invention;

[0057] Figure 8 illustrates an apparatus as described with respect of any of figures 1 to 7 attached to an artificial space-borne satellite;

[0058] Figure 9 illustrates a compilation of electronic signals received from the piezoelectric sensors of the apparatuses of any of figures 1 to 8.Detailed Description

[0059] Examples of the present invention are described below by way of example only. These examples represent the best mode of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved. The description sets forth the functions of the example and the arrangement of components for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0060] The invention described herein is an apparatus for detecting and / or characterising objects in space. The objects in space may otherwise be referred to as space debris, or cosmic dust. The objects may be either artificial (i.e., man-made) or natural. An example of an artificial object is a part of a defunct satellite. Examples of natural objects include micrometeoroids, ice and organic particles. Detecting and characterising objects in space allows organisations to characterise the risks posed to their spacecraft by these objects, which in turn allows them to develop customised systems for protecting the craft against the objects. Examples of organisations that may benefit from being able to detect and characterise objects in space include space agencies and commercial organisations such as commercial satellite operators, defence companies and insurance companies.

[0061] The invention described herein is configured to detect sub-centimetre objects in space. Sub-centimetre objects are objects that have a diameter of less than 1cm. Solutions for detecting objects of this size are not widely available as they cannot be identified using earthbased systems. Whilst the apparatuses disclosed herein are configured to detect subcentimetre objects, this does not mean that they are incapable of detecting objects that havediameters of greater than 1cm. The invention may be configured to detect objects with diameters up to 2cm, up to 5cm, up to 10cm, up to 50cm or more.

[0062] Figures 1 , 2, 3 and 4 illustrate schematic diagrams of examples of the invention. It should be understood that these schematic diagrams are, by their nature, simplified. They are provided to illustrate the general connections between parts of the apparatuses described herein but are not intended to impart any limitation on the specific design or configuration of components within the apparatuses.

[0063] A first example of the invention is illustrated in figure 1. The apparatus comprises an attaching means 102, a planar substrate 104 and an array of piezoelectric sensors 106. The attaching means 102 of the apparatus is configured to attach to a space-borne artificial satellite. The planar substrate 104 is attached to the attaching means 104. In figure 1 , the attaching means of the apparatus is a frame 102. The frame 102 may comprise one or more attaching features that enable its attachment to an artificial satellite. The attaching feature may be a hinge, a hole though which a fastener can be secured or a recess into which a corresponding part of an artificial satellite can be placed, for example. The attaching feature may be any alternative feature that enables connection of the apparatus to a satellite. The frame 102 may be connectable to a variety of different satellites, which enables it to be used by a number of different space agencies. The frame 102 may be referred to as a first frame for reasons that will be explained in further detail below.

[0064] The planar substrate 104 is mounted to the frame 102. That is, the planar substrate 104 may be installed, or fitted, onto the frame 102. The planar substrate 104 may be mounted to the frame 102 so that it is surrounded by the frame, or alternatively may be mounted in front of or behind the frame. The substrate 104 is defined as planar because it is substantially flat, or substantially longer in two dimensions (e.g., height and width) than it is in a third dimension (e.g., depth). The planar substrate 104 is the component of the apparatus that is configured to detect the objects in space. More specifically, the planar substrate 104 is configured to vibrate when objects collide with it. The vibrations of the substrate 104 may be recorded from a surface of the substrate. The surface of the substrate 104 with which the objects may collide is a surface of the substrate that has the greatest area. The surface of the substrate 104 with which the objects may collide may be referred to as a first surface of the substrate.

[0065] The frame 102 and the planar substrate 104 may together be referred to as an impact detection gate. The impact detection gate can generally be described as a component that allows objects to impact with it and to detect those impacts when they occur.

[0066] In the example of figure 1 , the planar substrate 104 comprises one or more homogenous layers of material. That is, the planar substrate 104 may be made up of one or more layers, and each layer is made of a single type of material. In some examples the planar substrate 104 may be constructed from, or consist of, a single homogenous layer of material.That is, the planar substrate may be constructed from a single layer of a single type of material. The single homogenous layer of material may be referred to as the base layer of the substrate. Manufacturing the planar substrate with a single layer of homogenous material provides costsaving advantages over manufacturing the substrate with a greater number of layers.

[0067] In other examples the planar substrate 104 may be made of multiple layers of material, but each layer of the substrate may be made of a single layer of material. In this example the planar substrate 104 may be referred to as a "laminar" substrate. For example, the planar substrate 104 may comprise a base layer and a second layer that provides a protective coating over the base layer. An advantage of providing a second coating layer to the substrate is that this may prevent the base layer of the substrate from eroding. The substrate 104 may comprise more than two layers of material, for example it may comprise a protective coating on either side of the base layer. The layers of material, in the planar substrate 104, are stacked along the dimension of the substrate that has the shortest length (e.g., the depth). That is, the layers of material in the substrate 104 may be stacked in a direction that is perpendicular to the first surface of the substrate. The planar substrate 104 may be referred to as a first planar substrate for reasons that will be explained in further detail below.

[0068] The apparatus further comprises an array of piezoelectric sensors 106. The array of piezoelectric sensors 106 is adhered to the planar substrate 104. The array of piezoelectric sensors 106 may be adhered to the first surface of the planar substrate 104. In other examples, the array of piezoelectric sensors 106 may be adhered to a second surface of the substrate that opposes the first surface. The first and second surfaces of the planar substrate 104 may be the surfaces of the substrate with the greatest surface area. The array of piezoelectric sensors 106 comprises three or more piezoelectric sensors. In figure 1 the array of sensors is illustrated as comprising three sensors 108, 110, 112. However, it is understood that this number of sensors is a minimum threshold value and that in alternative examples the array of sensors may comprise more than three sensors. An array of three or more piezoelectric sensors is advantageous as it allows a large surface-area sensitivity of the substrate. This number of sensors also provides an ability to measure impact location accurately (as explained in further detail below). This minimum number of sensors also provides redundancy in the case of a sensor failure. The array of piezoelectric sensors 106 may distributed around the first surface of the planar substrate.

[0069] Piezoelectric sensors are configured to convert applied parameters such as strain into electrical signals. Thus, as they are adhered to the planar substrate 104, the sensors of the array of piezoelectric sensors 106 are able to detect vibrations of the planar substrate and to generate an electrical signal indicative of those vibrations. More specifically, the piezoelectric sensors are configured to measure acoustic vibrations that are generated as objects impact the substrate by inducing an electric charge in response to strain in the material of thesubstrate. The strain is caused by acoustic waves in the material of the substrate when an object collides with the substrate. An example of a type of acoustic wave that may be generated when an object collides with the substrate is an acoustic shockwave. In other words, the piezoelectric sensors are configured to convert raw analogue acoustic signals into electrical signals. Analysis of the object that has collided with the substrate may be performed using the electrical signals received from the piezoelectric sensors.

[0070] An advantage of the substrate of figure 1 comprising one or more homogenous layers of material is that it means that the substrate does not have a resistive grid or any other type of grid running through any of its layers. Resistive grids negatively impact the vibrations that traverse the substrate, making the sensor less accurate. Thus, avoiding the use of resistive grids improves the accuracy of electrical signals generated by the array of piezoelectric sensors. Resistive grids are also prone to becoming detached from the substrate causing spurious signals and loss of sensitivity with time. Thus, avoiding the use of resistive grids also improves the reliability of the apparatus.

[0071] A second example of the invention is illustrated in figure 2. The second example of the invention comprises the same attaching means 202 and array of piezoelectric sensors 206 as the respective attaching means 102 and array 106 described above with respect to figure 1. The array of piezoelectric sensors 206 comprises three or more sensors 208, 210, 212 corresponding to the respective sensors 108, 110, 112 of figure 1. In some examples, the first planar substrate 204 of the second example may be same as the planar substrate 104. That is, in some examples the first planar substrate 204 may comprise one or more homogenous layers of material. In other examples, the planar substrate 204 may differ from the planar substrate 104 in that it may not comprise homogenous layers of material. The attaching means 202 of the apparatus of figure 2 is a frame.

[0072] The apparatus 200 of figure 2 further comprises an electronic module 214 attached to the attaching means 202. The electronic module 214 is configured to receive and process the electrical signals from the first array of piezoelectric sensors. That is, the electronic module 214 is configured to receive and process electrical signals from each sensor of the array of piezoelectric sensors. The electronic module 214 is further configured to characterise an object that has collided with the planar substrate 204 in dependence on the processing of the electrical signals. The electronic module 214 may otherwise be referred to as a processor, as it is configured to process the received electrical signals. The electronic module may be located at any suitable location on the apparatus. The processing performed by the electronic module will be described in further detail below.

[0073] An advantage of the apparatus 200 of figure 2 comprising an electronic module is that the apparatus is able to perform processing and output an object characterisation in-situ (i.e., in space). This differs from other known debris detecting systems which have earth-basedprocessing means. Performing processing on earth results in an increased power consumption and decreased reliability. The electronic module 214 also provides an in-unit processing means, allowing the apparatus to be connected to any artificial satellite without that satellite having to include its own electronic module for processing results from the apparatus. Thus, the apparatus of figure 2 is desirable for space agencies looking for a compact and simple means for characterising space debris.

[0074] A third example of the invention is illustrated in figure 3. The third example of the invention comprises the same attaching means 302 and array of piezoelectric sensors 306 as the respective attaching means and arrays of figures 1 and 2. The array of piezoelectric sensors 306 comprises three or more sensors 308, 310, 312 corresponding to the respective sensors of figures 1 and 2. In some examples, the first planar substrate 304 of the third example may be same (i.e. , comprised of homogenous layers) as the planar substrate 104. In other examples, the planar substrate 304 of the third example may differ from the planar substrate 104 in that it may not comprise homogenous layers of material. The attaching means 302 of figure 3 is a frame.

[0075] The apparatus 300 of figure 3 further comprises an electromagnetic sensor 314 attached to the attaching means 302. The electromagnetic sensor 314 is configured to detect electromagnetic waves generated by an impact of an object with the planar substrate. An electromagnetic sensor is an electronic device used to measure electromagnetic waves and convert the energy from those waves into electrical signals. The electromagnetic sensor 314 is configured to detect electromagnetic, or light, waves, emanating from the planar substrate as a result of a collision of an object with the substrate. The electromagnetic waves may be observable from ionised plasma that is generated from a collision.

[0076] The electromagnetic sensor 314 may otherwise be referred to as a verification means for the apparatus 300. That is, the electromagnetic sensor 314 may be used to verify, or validate, electrical signals generated by the array of piezoelectric sensors and confirm that those signals have been generated by a collision of an object with the planar substrate. The electromagnetic sensor 314 may be used to differentiate between impact vibrations caused by objects from and background vibrations from the satellite to which the apparatus is attached. Background vibrations from the satellite will not result in the generation of electromagnetic waves. Thus, if a first electrical signal from a sensor of the array of piezoelectric sensors is determined to be generated at substantially the same time as (e.g., within + / -0.5 seconds of) an electrical signal from the electromagnetic sensor, it can be determined that the signal from the piezoelectric sensor has been caused by an impact of an object with the substrate. If a first electrical signal from a sensor of the array of piezoelectric sensors is determined not to be generated at substantially the same time as (e.g., within + / - 0.5 seconds of) an electrical signal from the electromagnetic sensor, it can be determined thatthe signal from the piezoelectric sensor has not been caused by an impact of an object with the substrate. The electromagnetic sensor 314 may also be used to determine the exact time of impact of an object with the substrate, as this can be derived from the time at which the electromagnetic waves, or pulses, were detected by the sensor 314. The electromagnetic sensor 314 may further be used, in some examples, to further characterise an object that collides with the planar substrate. That is, the sensor data from the electromagnetic sensor 314 may be used to define one or more parameters of the object.

[0077] The electromagnetic sensor 314 may otherwise be referred to as an electromagnetic antenna. The electromagnetic sensor 314 may be configured to detect electromagnetic waves of any suitable frequency. In one example, the electromagnetic sensor 314 may be configured to detect visible light waves. This is advantageous as ionised plasma emits visible light, allowing the sensor 314 to detect emissions from object collisions with the substrate. Other examples of types of electromagnetic waves that may be detected by the sensor include radio frequency (RF) waves, ultraviolet waves, x-rays and gamma rays.

[0078] Whilst in figure 3 the verification means for the apparatus is an electromagnetic sensor, it should be appreciated that alternative or additional sensors may also be used as verification means. The verification means may be provided by a sensor that is configured to detect the presence of plasma. In some examples, such as the example of figure 3, the sensor may be a light sensor. In an alternative example, the sensor may be an electrical sensor configured to detect a change in voltage that is induced by the plasma. In other words, the sensor may be a voltage sensor. In alternative examples the sensor may be a charge sensor, or a current sensor.

[0079] Plasma is a physical manifestation of a hypervelocity impact that may be generated when an object collides with a satellite. Plasma is a cloud of ionised particles. If the impact is sufficiently energetic, electrons from the objects that have collided acquire sufficient energy to escape and form an electron gas. As electrons are very much lighter than their parent nuclei they move at very high speed. This leads to a spread of charge: the electrons carry the negative charge, and the parent nuclei carry the positive charge, and thus an electric field is generated in the impact area. If this plasma cloud passes over, or near, a conductor it will induce a charge in the conductor and lead to a movement of electrons within the conductor, i.e. a current. This current can be detected easily as a rapidly varying voltage.

[0080] In the apparatus described herein, the plasma detector may be an array of conductive lines of a predetermined length laid out with a determined spacing. Towards the rear end of the array is a feed. The individual lines are not connected to ground so when a charge, such as one generated by an electron cloud, passes close to these lines the electrons at one end of the line are forced to move to the other end of the line. This generates a charge distribution and an electric field across the line. This, in turn, induces a field in the adjacent line and soforth, giving a certain amount of amplification. At the feed-end, this induced movement of electrons is measured as a voltage. The layout of the lines for the verification means may be designed to be directional. Thus, the layout may be more sensitive in a first direction than in a second direction.

[0081] A fourth example of the invention is illustrated in figure 4. The fourth example of the invention is an apparatus 400 for characterising objects in space comprising the same attaching means 402 and array of piezoelectric sensors 406 as the respective attaching means and arrays of figures 1 and 2. The array of piezoelectric sensors 406 comprises three or more sensors 408, 410, 412 corresponding to the respective sensors of figures 1 and 2. In some examples, the first planar substrate 404 of the fourth example may be same (i.e., comprised of homogenous layers) as the planar substrate 104. In other examples, the planar substrate 404 of the fourth example may differ from the planar substrate 104 in that it may not comprise homogenous layers of material. In figure 4 the attaching means 402 is a frame.

[0082] The apparatus 400 of figure 4 further comprises an impact detection mechanism 414. The impact detection mechanism 414 is a different mechanism from the electromagnetic sensor 314 described above. The impact detection mechanism may be attached to the attaching means 402 of the apparatus, or alternatively may be attached to or comprised within the substrate 404. The impact detection mechanism 414 is configured to determine the time of an impact for an object with the first planar substrate. That is, the impact detection mechanism 414 is a component that is used to determine the time at which an object collides with the substrate 404. This is useful because the array of piezoelectric sensors comprises at least three electric sensors, and the time at which a given sensor in the array will generate an electrical signal indicative of the collision of the object with the substrate will depend on the distance between that sensor and the location of the collision on the object. By incorporating an impact detection mechanism, the exact time of a collision can be determined despite the different times of signal generation by the sensors in the array.

[0083] The impact detection mechanism 414 may take several different forms. In one example, the first array of piezoelectric sensors 406 may comprise four piezoelectric sensors and the impact detection mechanism may be a piezoelectric sensor of the array. That is, one of the four piezoelectric sensors can be used to determine the time at which an object collides with the substrate 404. The use of this piezoelectric sensor provides a simple and compact impact detection mechanism. The method used to implement a piezoelectric sensor as the impact detection mechanism is described in further detail below with respect to figures 8 to 12. In an alternative example the impact detection mechanism may be incorporated within the planar substrate 404. In a more specific version of this example, the impact detection mechanism may be a layer of the planar substrate 404. In this example, the impact detection mechanism may be a piezoelectric layer of the planar substrate 404. The piezoelectric layerof the planar substrate 404 may be a homogenous layer of the planar substrate. The incorporation of an impact detection mechanism within the planar substrate provides a reliable mechanism that, as with the piezoelectric sensor example, is also compact.

[0084] It should be understood that the features in the examples illustrated in figures 1 to 4 can be combined together in further examples of the apparatus. For example, the apparatus may comprise a substrate with homogeneous layers as described with respect to figure 1 and may also comprise an electronic module as described with respect to figure 2 and / or an electromagnetic sensor as described in figure 3 and / or an impact detection mechanism as described in figure 4. In alternative examples an apparatus comprising an electronic module as described with respect to figure 2 may also comprise an electromagnetic sensor as described in figure 3 and / or an impact detection mechanism as described in figure 4. In further examples, an apparatus comprising an electromagnetic sensor as described in figure 3 may also comprise an impact detection mechanism as described in figure 4. In examples where the apparatus does not comprise an electronic module, the apparatus may have means to transmit the signals detected by its sensors to an electronic module that is external to the apparatus. The external electronic module may be located elsewhere on the satellite to which the apparatus is attached, or alternatively may be located on earth.

[0085] A two-dimensional view of an exemplary apparatus for detecting sub-centimetre objects in space according to any of figures 1 to 4 (or a combination of the features of any of these figures) is illustrated in figure 5. Figure 5 illustrates a front facing view of the apparatus 500 according to the invention. The apparatus comprises an attaching means 502 and a substrate 504 corresponding to the attaching means and substrate of figures 1 to 4 (or any combination of the features of these figures). In figure 5 the attaching means 502 is a frame. The frame 502 surrounds the circumference of the first surface of the substrate 504. That is, the frame 502 is positioned around the edges, or the outer periphery, of the first surface 518 of the substrate 504. This means that the frame 502 encapsulates the substrate. An advantage of this configuration is that the substrate, which is flexible, is supported around its border by the frame. This imparts structural integrity to the substrate 504.

[0086] The apparatus further comprises an array of piezoelectric sensors comprising three or more piezoelectric sensors. In figure 5 the array of piezoelectric sensors comprises four piezoelectric sensors 506, 508, 510, 512. It should be appreciated that the apparatus may alternatively comprise more or fewer sensors. The piezoelectric sensors 506, 508, 510, 512 of figure 5 may be spaced apart on the substrate. Each piezoelectric sensor 506, 508, 510, 512 may be located at a respective corner of the first surface 518 of the substrate. For example, in figure 5 a first sensor 506 is located at a first corner 534 of the first surface 518, a second sensor 508 is located at a second corner 536 of the first surface, a third sensor 510 is located at a third corner 540 of the first surface and a fourth sensor 512 is located at a fourthcorner 538 of the first surface. The piezoelectric sensors 506, 508, 510, 512 may be located orthogonally to each other. That is, the piezoelectric sensors 506, 508, 510, 512 may be located at right-angles to each other. The piezoelectric sensors 506, 508, 510, 512 may be angled relative to each other. Each piezoelectric sensor may extend diagonally from a respective corner of the substrate towards the centre of the substrate. Each piezoelectric sensor may be orientated at a 45-degree angle to the edges of the substrate. For example, first sensor 506 may be orientated at a 45-degree angle to a first edge 520 of the substrate, and at a 45-degree angle to a second edge 522 of the substrate. Second sensor 508 may be orientated at a 45-degree angle to the second edge 522 of the substrate, and at a 45-degree angle to the third edge 524 of the substrate. This means that the second piezoelectric sensor is orientated at 90 degrees to the first piezoelectric sensor. Note that, for the sake of simplicity, the fourth edge of the substrate has not been identified in figure 500 as it is not referenced herein.

[0087] In an alternative example that is not illustrated in figure 5, the array of piezoelectric sensors may be positioned linearly along a single edge of the substrate. That is, the array of piezoelectric sensors may comprise a first sensor located at a first corner 534 of the substrate, a second sensor located at a second corner 536 of the substrate, and one or more additional sensors located at respective positions along the edge 522 of the substrate that extends between the first and second corners. The sensors of the array of piezoelectric sensors may be positioned linearly along two or more edges of the substrate (e.g., along edge 520 and opposing edge 524 of the substrate). The sensors of the array of piezoelectric sensors may be positioned linearly along all of the edges of the substrate. In a further example, every sensor of the array of piezoelectric sensors may be positioned along a respective edge of the substrate at a location that is between the first and second corners that define that edge. In this example, none of the sensors of the array of piezoelectric sensors are located in the corners of the substrate.

[0088] Each piezoelectric sensor of the array of piezoelectric sensors may have a respective connection 526, 528, 530, 532 to an electronic module for processing the electrical signals generated by the sensors. In a first example, the connections 526, 528, 530, 532 may be printed tracks. In a second example, the connections 526, 528, 530, 532 may be cables. In a third example, the connections may be 526, 528, 530, 532 partially printed tracks (e.g., printed on the surface of the substrate) and partially cabled (e.g., connecting the printed track on the substrate to the electronic module). The connections 526, 528, 530, 532 may be any other suitable electrical means. Alternatively, the piezoelectric sensors may be wirelessly connected to an electric module. As described above, the electric module may either be comprised within the apparatus or located separately to the apparatus.

[0089] Each piezoelectric sensor 506, 508, 510, 512 may be located close to a respective corner 534, 536, 538, 540 of the substrate 504. For example, each piezoelectric sensor may be located less than 70mm from a respective corner of the substrate. Minimising the distance between the piezoelectric sensors and their respective substrate corners maximises the distance between each piezoelectric sensor and its adjacent sensors. Maximising the distance between the piezoelectric sensors, in turn maximises the sensitive area 514 of the apparatus. The sensitive area 514 of the substrate is an area on the substrate within which vibrations from objects colliding with the substrate can be recorded accurately by the sensors. In other words, the sensitive area 514 is the acoustically sensitive region of the substrate. The sensitive area 514 is illustrated as the area within the dotted lines of figure 5. The circumference of the area 514 is defined by the location of the sensors 506, 508, 510, 512 on the substrate. More specifically, the location of each sensor 506, 508, 510, 512 on the substrate defines a respective corner of the circumference of the sensitive area 514. In other words, the distance between the sensors 506, 508, 510, 512 defines the size of the sensitive area. In one example, the distance between each piezoelectric sensor and its adjacent sensors may be 10cm. Impacts that occur outside of the sensitive area may still be recorded by the sensors 506, 508, 510, 512, but extra processing may be required to process such impacts.

[0090] The apparatus 500 also comprises an attaching feature 516. The attaching feature 516 enables attachment of the apparatus to a satellite. As described above, in some examples the attaching feature may be a hinge, a hole through which a fastener can be secured or a recess into which a corresponding part of an artificial satellite can be placed. In other examples, such as the example illustrated in figure 5, the attaching feature may be a protrusion such as an arm that allows the frame to be attached to a satellite. The attaching feature may be any alternative feature that enables connection of the apparatus to a satellite.

[0091] A three-dimensional view of an alternative exemplary apparatus for detecting subcentimetre objects in space according to any of figures 1 to 4 (or a combination of the features of any of these figures) is illustrated in figure 6. The apparatus 600 of figure 6 comprises an attaching means (i.e., a frame 602) and a planar substrate 604 attached to the attaching means. The frame 602 and substrate 604 may correspond to the frame and substrate of figures 1 to 4 (or any combination of the features of these figures). The substrate 604 has attached to it an array of piezoelectric sensors (not illustrated) comprising three or more sensors as described with figures 1 to 5.

[0092] In figure 6, the frame 602 is attached to a second surface 608 of the first planar substrate that opposes the first surface 610 of the first planar substrate. In the example of figure 6, the second surface 608 of the planar substrate may be the surface that opposes the surface with which objects may collide. In other words, the second surface faces 608 in an opposite direction to the first surface 610. If the first surface 610 is described as forward facing,then the second surface 608 may be described as backward facing. Correspondingly, if the first surface 610 is described as backward facing then the second surface 608 may be described as forward facing. Thus, instead of supporting the substrate around the outer periphery of the substrate as illustrated in figure 5, the frame 602 of figure 6 provides support to the substrate 604 across a planar surface of the substrate. The array of piezoelectric sensors of the arrangement of figure 6 may be adhered to either the first surface 610 or the second surface 608 of the substrate.

[0093] The frame 602 of figure 6 may be comprised of a lattice of hollow prismatic cells 612. That is, the frame 602 may be comprised of a mesh of adjoining prismatic structures. In other words, the frame 602 may be comprised of an array of hollow cells. Each cell may be separated from its adjoining cell by a thin wall. This type of arrangement is referred to as a honeycomb structure. A first surface 614 of the lattice may be adhered to the second surface 610 of the planar structure. More specifically, the lattice may be adhered to the second surface 610 of the planar structure by a surface 614 that is perpendicular to the walls of the lattice.

[0094] Honeycomb structures are advantageous for use as frames as they provide exceptional strength to weight ratio, high toughness and cost efficiencies. For this reason, many spacecrafts are manufactured with exterior panels that are made from a honeycomb structure. Thus, in addition to the general advantages of the use of a honeycomb structure, a further advantage of constructing a frame 602 as illustrated in figure 6 that it means that the apparatus can be integrated into a satellite as an exterior panel on the body of the satellite. In other words, the apparatus of figure 6 can form part of the outer body of a satellite to which it is attached. In this configuration, the frame 602 would face the interior of the satellite and the substrate 604 would be positioned as an externally facing surface of the satellite. This configuration provides a very compact apparatus for detecting objects, as it can be built into the body of the satellite instead of extending outwardly from it. The apparatus 600 may further comprise panel 606 that is attached to a second surface of the frame 602. The second surface of the frame 602 is a surface that opposes the first surface 614 of the frame. As with the first surface 614 of the frame, the second surface of the frame is perpendicular to the walls of the lattice of the frame. The panel 606 can act to separate the internal components of a satellite from the detecting mechanism of the apparatus 600. The combination of the frame 602 and the two panels 604, 606 defines a sandwich structure which may be referred to as a honeycomb sandwich structure.

[0095] The hollow prismatic cells in figure 6 are illustrated as hexagonal cells. That is, the cross-sectional area of each cell of the frame 602 is hexagonal. Hexagonal cells have a particularly high strength-to-weight ratio. In other examples, the prismatic cells of the frame 602 may have a cross-sectional area of any other suitable shape. Examples of shapes of thecross-sectional area of a prismatic cell in the frame 602 of structure 600 are triangular, square or pentagonal. The frame may be comprised of any number of adjoining prismatic cells.

[0096] In further examples of the invention, which are not illustrated, the attaching means of the apparatus may not be a frame. In a first of these examples, the substrate may be attached to the attaching means because the attaching means may be an attaching feature, such as a hole or a protrusion, that is added to the substrate. The attaching feature allows the substrate to be connected directly to a satellite. In this example, the substrate of the apparatus may act as the surface of a satellite body as a single panel (e.g., similarly to the arrangement illustrated in figure 6 but without the frame 602 and second panel 606 of this arrangement). In a second of the examples the attaching means may be an adhesive layer that is attached to the first or second surface of the substrate. For example, the adhesive layer may be an adhesive film. The adhesive layer may otherwise be referred to as an adhesive skin. The adhesive skin may be stuck to the first or second surface of the substrate. The adhesive skin may be used to attach, or stick, the substrate onto the body of a satellite. An advantage of the two examples described herein is that the detecting apparatus can be simplified and can be attached to a satellite in a minimally invasive manner, both with respect to weight and to surface area.

[0097] The substrate of the planar substrate of the apparatuses illustrated in figures 1 and 6 may have first and second surfaces of any suitable shape. The first and second surfaces of the planar substrate may be of the same shape and size. In the example of figure 5, the first surface of the substrate is square in shape. Consequently, the second surface of the substrate is also square in shape. Designing the first surface of the substrate as square in shape is advantageous as it allows adjacent piezoelectric sensors to be evenly spaced apart on the substrate if they are attached to the corners of the first surface of the substrate. In alternative examples, the first (and second) surface of the substrate may be in an alternative shape that has sides of equal sides, such as a triangle, a pentagon or a hexagon. Alternatively, the planar substrate may have a first (and second) surface in the shape of an alternative quadrilateral, or in a shape without edges such as a circle.

[0098] As mentioned above, the planar substrate of any of the apparatuses illustrated in figures 1 to 6 may be comprised of a single homogeneous layer of material, or alternatively of two or more layers of homogenous materials. Where the planar substrate is made from a polymer or a similar material, it may have an overall thickness of between 2pm and 100pm. Where the substrate is made from a metal, it may have an overall thickness of between 1mm and 5mm. In a specific example, the planar substrate may have a thickness of 25pm. In an example where the planar substrate is comprised of two or more layers of material, it may comprise a first base layer and a second layer that provides a protective coating over the base layer. In the example where the substrate is comprised of a single homogeneous layer of material, that single layer of material may be defined as the base layer of the substrate.

[0099] The base layer of the planar substrate may be made from any suitable material. In a first example, the planar substrate may be made of a flexible film. That is, the base layer of the substrate may be made of a fine membrane. An advantage of constructing the base layer of the substrate out of a flexible film is that the film may react in a favourable acoustic manner when an object collides with it. That is, the film may vibrate robustly in response to the collision of an object with its first surface. The film may be thin enough that, when an object collides with the first surface of the substrate, the object passes through and causes a hole to form in the substrate. In other words, the film may be permeable to objects that collide with the substrate. At the same time, the film may not disrupt the trajectory of the object through space. Thus, after multiple collisions of an object with the substrate the film may become porous.

[0100] In a specific example, the flexible film of the base layer of the substrate is made from Kapton. Kapton has the chemical composition C22H10O5N2 and is a space-qualified polymer comprised of imide monomers known as polyimide. The use of Kapton as base layer for the substrate of the invention described herein is advantageous as it is highly suited to space applications. For example, Kapton is resistant to degradation from atomic oxygen and resistant to extreme temperatures, such as those found in space. Kapton also has high strength, low density, remains stable between a temperature range of 4K to 673K and has a very low outgassing variation.

[0101] In examples where the planar substrate comprises a first base layer and a second layer that provides a protective coating over the base layer, the protective coating may prevent the base layer of the substrate from erosion. This advantage is beneficial for apparatuses that are designed to have a long exposure in space. Examples of materials that may be used to provide the protective coating layer of the substrate are Aluminium and Gold. In the example where the base layer of the substrate is a Kapton layer, the substrate may be made from Al- coated Kapton, Au-coated Kapton or Black Kapton. It should be appreciated that other materials may alternatively be used to provide the protective coating.

[0102] The planar substrate may be either a passive substrate or an active substrate. A passive substrate is a substrate that serves primarily as a structural support or mounting surface for functional circuit elements. The Kapton described above is an example of a passive substrate as it allows vibrations to pass through it, to the array of piezoelectric sensors, but does not generate any electrical signals itself in response to the acoustic vibrations. In other words, a passive substate takes a passive role in sensing a collision of an object with the substrate. The passive substrate provides a medium for acoustic vibrations to travel through, allowing the piezoelectric sensors adhered to the substrate to measure impacts of objects with the substrate. An active substrate is a substrate that, in addition to allowing vibrations to pass through it, is also able to convert those vibrations into electrical signals. That is, an active substrate takes an active role in sensing a collision of an object with the substrate. In otherwords, the active substrate is sensitive to vibrations and can directly measure impacts. In one example, the flexible film of the planar substrate may be a piezoelectric film. A piezoelectric film is constructed of microscopic interlocking crystal domains with positive and negative charges. The piezoelectric film is able to transform acoustic vibrations into electrical signals. Electrical signals generated by the piezoelectric film may be sent to an electronic module for processing. As described above, the electronic module may either be incorporated in or separate to the apparatus. Advantages of the use of a substrate comprising a piezoelectric film include that it enables better impact timing and, in the case where the apparatus comprises two or more impact detection gates, it allows the space between the gates to be reduced.

[0103] In some examples, the base layer of the first planar substrate may be comprised of a solid plate. That is, instead of comprising a permeable membrane, the planar substrate may be comprised of an impermeable layer of material. In this example an object that collides with the first surface of the substrate may not pass through the substrate. Instead, the object may be repelled from the surface of the substrate after it collides with the substrate. The solid plate is still able to vibrate in response to a collision of an object with its first surface. The solid plate may either active (i.e., piezoelectric) or passive. In a more specific example, the planar substrate may be comprised of a solid metal plate. The solid metal plate may be constructed of aluminium, or of any other suitable metal. The construction of the planar substrate out of a solid metal plate may be particularly valuable in the configuration illustrated in figure 6, where the apparatus 600 may function as an exterior panel on the body of a satellite. In this configuration the solid metal plate may act, in addition to enabling the detection of an object, to protect the interior components of a satellite from objects colliding with the exterior of the satellite. Advantages of the plate being formed specifically of aluminium include that this material is light and corrosion resistant, providing weight saving and durability benefits.

[0104] In one example, the frame of the apparatus illustrated in figures 1 to 6 is configured to be fixed with respect to a space-borne artificial satellite when it is attached to the satellite. That is, the frame is not configured to move with respect to the satellite after it has been assembled onto the satellite. In other words, the frame is configured to be static with respect to the satellite when the satellite is in use (i.e., in space). In this configuration, the attaching feature that enables attachment of the frame to the satellite may be a static arm, panel, or one or more fasteners such as a bolt, for example. An advantage of the frame being fixed with respect to the satellite is that it reduces the number of parts in the apparatus, and therefore the susceptibility of the apparatus to breaking. This configuration also decreases the complexity and reduces the cost of the apparatus.

[0105] In an alternative example, the frame is configured to be moveable with respect to a space-borne artificial satellite when it is attached to the satellite. In other words, the frame maybe configured to be deployed with respect to the satellite after it has been assembled onto the satellite. The frame may be deployed between a first, retracted, configuration and a second, extended, configuration relative to the satellite. For example, the apparatus may be retracted when its satellite is being transported into space and may fold out of the exterior of the satellite when it is in space. In this configuration, the attaching feature that enables attachment of the frame to the satellite may be a hinge or an alternative moveable joint that allows movement of the apparatus relative to the satellite. An advantage of the frame being moveable with respect to the satellite is that it allows a satellite incorporating the apparatus to have a lower volume during launch. Another advantage is that the apparatus can be designed with a larger deployed surface area, a lower mass and an increase in the number of locations on a satellite to which it can be mounted.

[0106] In some examples, where the apparatus is deployable, it may fold out a single time from the body of the satellite. That is, the apparatus may comprise a single impact detection gate with a single attaching feature that allows it to fold out from the body of the satellite. In alternative examples the apparatus may fold out multiple times from the body of the satellite. In these examples, the apparatus may comprise two or more impact detection gates that are, in a deployed configuration, arranged parallel to each other and attached along a respective edge. An advantage of these latter examples is that they enable the surface area of the apparatus (and, more specifically, the overall substrate surface area) to be maximised.

[0107] The frame of the apparatus of figures 1 to 6 may be constructed of any suitable material. In a specific example, the frame may be made from aluminium. As described above, the use of aluminium is advantageous as this material is light and corrosion resistant. The aluminium frame may be rigid. This provides structural integrity to the apparatus. In an alternative example the frame may be made of a plastic material, such as a polymer or carbon fibre.

[0108] The piezoelectric sensors may be adhered to the first surface of the planar substrate in a number of different ways. In a first example, the piezoelectric sensors may be adhered to the first surface using an adhesive. That is, the sensors may be stuck to the first surface using a glue. An advantage of this adhering method is that it simple and cost-efficient, and that it allows the piezoelectric sensors to be obtained off the shelf instead of being custom made. In a second example, each piezoelectric sensor may be printed onto the first surface of the substrate. That is, the sensors may be made with polymers and / or speciality inks that are placed directly onto the substrate. The sensors may be in the form of a plastic, a foil, a paper or a textile. Advantages of the sensors being printed onto the substrate include that the size of the sensors can be decreased, and that their flexibility can be improved. It should be appreciated that the piezoelectric sensors may be adhered to the first surface of the substrate in any other suitable manner.

[0109] Where the piezoelectric sensors of the apparatuses of figures 1 to 6 are adhered to the first surface of the substrate using an adhesive, these sensors may be any suitable type of piezoelectric sensor. For example the sensors may be comprised of metal tabs that are attached to a surface of the substrate. In a specific example, the piezoelectric sensors may be polyvinylidene fluoride "PVDF" sensors. PVDF sensors are acoustic impact sensors that are constructed by placing a thin strip of polarised piezoelectric polymer between two electrodes. The polymer from which these sensors are constructed induces an electric charge in response to material strain. This property is advantageous as it means that the sensors are not reliant on a power source and can therefore remain continuously active. This means they provide real-time operation, long-life capability and low-cost advantages.

[0110] It is worth noting that, in the examples illustrated in figures 1 to 5, objects may collide with a second surface of the substrate in addition to the first surface of the substrate. The second surface of the substrate, as described above with respect to figure 6, is a surface that opposes the first surface of the substrate. In the example where the array of piezoelectric sensors is adhered to the first surface of the substrate, the piezoelectric sensors may be able to detect vibrations caused by objects hitting the substrate irrespective of whether the object collides with the first or second surface of the substrate. Similarly, in the example where the array of piezoelectric sensors is adhered to the second surface of the substrate, the piezoelectric sensors may be able to detect vibrations caused by objects hitting the substrate irrespective of whether the object collides with the first or second surface of the substrate.

[0111] An alternative example of the invention disclosed herein is illustrated in figure 7. The apparatus of figure 7 is the same as the apparatus of figure 5 except for that it comprises two of each of the components illustrated in figure 5. That is, the apparatus of figure 7 comprises a first arrangement 702 corresponding to the apparatus of figure 5 and a second arrangement 704 which comprises the same components of the first arrangement. The attaching feature of the apparatus 700 has not been illustrated in figure 7 for the sake of simplicity. However, it should be appreciated that the apparatus of figure 7 comprises at least one attaching feature for attaching it to a satellite. In a first example the apparatus may comprise a single attaching feature that attaches to both of the arrangements 702, 704 and connects those arrangements to a satellite. In a second example the apparatus may comprise two or more attaching features, with at least one attaching feature connecting each of the arrangements 702, 704 to a satellite.

[0112] For the purpose of distinguishing between the first and second arrangements, the frame of the first arrangement 702 of apparatus 700 may be referred to from this point as a first frame 714. Similarly, the substrate of the first arrangement 702 may be referred to as a first substrate 716. The array of piezoelectric sensors of the first arrangement 702 may be referred to as a first array of piezoelectric sensors. A representative sensor of the first array ofpiezoelectric sensors of the first arrangement 702 is identified by reference 718 in figure 7. The first frame 714 and first substrate 716 may, in combination, be referred to as a first impact detection gate.

[0113] The apparatus of figure 7 differs from that of figure 5 in that it further comprises a second frame 720 configured to attach to a space-borne artificial satellite. The apparatus also comprises a second planar substrate 722 mounted to the second frame 720. That is, the apparatus comprises, in addition to its first impact detection gate, a second impact detection gate. The first and second impact detection gates form a pair of impact detection gates. The first and second impact detection gates may be parallel to (i.e. , aligned with) and spaced apart from each other. That is, the second frame 720 may be orientated parallel to and spaced apart from the first frame 714. The distance between the first and second gates may be any suitable distance. In a specific example, the distance between the first and second gates may be 10cm. That is, the distance between the first frame 714 and the second frame 720 may be 10cm.

[0114] The apparatus of figure 7 also comprises a second array of piezoelectric sensors comprising three or more piezoelectric sensors. A representative sensor of the first array of piezoelectric sensors of the second arrangement 704 is identified by reference 724 in figure 7. The second array of piezoelectric sensors is adhered to a first surface of the second planar substrate 722 such that each sensor is able to detect one or more vibrations of the first surface of the second substrate and to generate electrical signals indicative of those vibrations.

[0115] The first and second arrangements 702, 704 may be identical in their layout. That is, the first frame 714 may be the same size and shape as the second frame 720 and the first substrate 716 may be the same size and shape as the second substrate 722. The sensors of the first array of piezoelectric sensors may also be arranged in the same location on the first substrate as the sensors of the second array of piezoelectric sensors are arranged on the second substrate. This means that the sensitive area of the first arrangement 702 is the same as the sensitive area of the second arrangement 704.

[0116] The first and second substrates 716, 722 may comprise a flexible, permeable film as described above. Thus, an object colliding with the first substrate 716 may pass through the first substrate on impact and will move on to collide with the second substrate 722. The thin nature of the substrates means that the path of the object through space is not substantially affected by its collision with the first substrate 716. An example of an object that may collide with the apparatus 700 is identified by reference 706 in figure 7. The trajectory of the object through space is illustrated by reference 708. The object may collide with the first substrate 716 at a first location 710 on the substrate. The object may then pass through the first substrate and continue on the same trajectory until it collides with the second substrate 722 at a second location 712 on the substrate. The coordinates of the first and second locations 710, 712 may be calculated using the electrical signals generated by the first and secondarrays of piezoelectric sensors respectively. The coordinates may be used, in combination with the known separation distance between the first and second arrangements 702, 704, to calculate the trajectory of the object through the apparatus. The trajectory of the object can be used to calculate further parameters of the object. The object 706 in figure 7 is illustrated as passing through the first arrangement before the second arrangement. However, it should be appreciated that, in alternative arrangements, the object 706 may travel in the opposite direction, and so may pass through the second arrangement before the first arrangement. This reversed trajectory may also be detected and analysed by the apparatus.

[0117] Figure 8 illustrates first and second apparatuses 802, 806 as described in figure 7 attached to a satellite 804. In the example illustrated in figure 8, each apparatus 802, 806 comprises first and second frames, substrates and arrays of piezoelectric sensors. In alternative examples it should be appreciated that the first and second apparatuses 802, 806 could each contain only a single instance of each of a frame, substrate and array of piezoelectric sensors as illustrated in figure 5. In the arrangement of figure 8 two detecting apparatuses 802, 806 are connected to the satellite 804. This increases the number of collisions that can be detected on the satellite. However, it should be appreciated that in alternative arrangements the satellite may only be attached to a single apparatus, or alternatively may be attached to three of more apparatuses. The apparatuses 802, 806 of figure 8 extend outwardly of the satellite 804 of figure 8. However, it should be appreciated that in alternative arrangements one or more apparatuses may be incorporated into the body of the satellite, as described with respect to figure 6.

[0118] The function of the electronic module that may be incorporated into any of the apparatuses illustrated in figures 1 to 8 will now be described. The electronic module comprises a memory store and a processor. The electronic module may comprise further hardware components such as one or more filters and one or more gain modules. The processor may comprise processing logic for processing received electrical signals. The electronic module is configured to receive electrical signals indicative of vibrations of the planar substrate from the sensors of the array of piezoelectric sensors. The electronic module configured to store the received signals received by the sensors in its memory store. The electronic module is further configured to process the electrical signals. The electronic module may also be configured to remove background noise (i.e., signals received between time periods between impacts) from its received electrical signals.

[0119] The electronic module is configured to process electrical signals to determine the number of impacts of objects with the apparatus, which in turn identifies the population of objects in the orbit of a satellite. The electronic module may be configured to determine the number of impacts of objects with the apparatus by implementing a counter, in its processing logic, that increases its count each time a set of electronic signals are received from the arrayof piezoelectric sensors. A set of electronic signals includes an electronic signal generated by each sensor of the array of sensors. Where the apparatus comprises an electromagnetic sensor, the processing logic of the electronic module may only increase its counter if, in addition to receiving a set of electronic signals from the array of piezoelectric sensors, it receives an electrical signal from the electromagnetic sensor at substantially the same time. As described above, the electromagnetic sensor is used as a verification means for a collision of an object with the substrate by detecting ionic plasma generated by a collision.

[0120] The electronics module is also configured to calculate a time of impact (t of an object with the substrate. The electronics module is configured to calculate the time of impact by observing an arrival time of an electrical signal received from each sensor of an array of sensors on a common substrate (e.g., of a first array of sensors). The electronics module may select the earliest determined arrival time of the received arrival times as the time of impact. As an example, a graph illustrating the individual electrical signals received from each sensor of an array of piezoelectric sensors on a substrate is illustrated in figure 9. In figure 9 the signal of each piezoelectric sensor is translated across the y-axis, for ease of view. The signal from a first sensor of the array is identified by line A. The signal from a second sensor of the array is identified by line B. The signal from a third sensor of the array is identified by line C. A signal from a fourth sensor of the array is identified by line D. The voltage, or amplitude, of the electrical signal received by each signal recorded by the variation of each of lines A, B, C, D along the y-axis. Figure 9 has been provided to illustrate the analysis of electrical signals by the electrical module. It should be noted that the graph illustrated in this figure may not be generated by the electrical module.

[0121] The electronics module is configured to receive and determine, from the electrical signals received from each sensor in the first array of sensors, the signal that displayed a voltage peak at the earliest time. In the example of figure 9 the electronics module is configured to receive signals A, B, C and D and to determine, out of the four signals, the signal that displayed a voltage peak at the earliest time. That is, the electrical module is configured to compare the times U, te, tc and to, and to determine which of these times is the earliest. From figure 9, it can be seen that the earliest time was tc (i.e., the time of the peak voltage recorded by signal C). Thus, the third sensor is determined to have detected the time of impact (ti) of the object with the apparatus.

[0122] In the example illustrated in figure 9 the third sensor, in identifyinghas acted as the impact detection mechanism of the apparatus. That is, the third sensor has been used to determine the time of an impact for an object with the apparatus. The remaining sensors, generating signals A, B and D, are used to characterise the object that has collided with the substrate as explained below. In the example where a piezoelectric sensor acts as the impact detection mechanism for the apparatus, the specific sensor that acts as the mechanism mayvary depending on the location of the impact of the object on the substrate. In alternative examples, instead of a sensor of the array of sensors acting as the impact detection mechanism, the substrate may comprise a piezoelectric layer that generates an electrical signal similar to signal C in figure 9. In these other examples, the signal from the piezoelectric layer of the substrate is used to determine the time of impact of the object with the apparatus.

[0123] After the time of impact of the object with the apparatus has been determined, the electronic module may then calculate a time delay between the time of impact and the arrival time of each electrical signal received from a respective sensor of the first array of sensors. These calculations are performed by subtracting the time of impactfrom the time of the peak voltage (or amplitude) recorded by each of the piezoelectric sensors in the array. For example, for the four sensors described with respect to figure 9, the calculations are performed as follows:AtA=tA ~ tl tB=tB ~ tl tc=tc ~ ^1Where, as illustrated in figure 9, tAis the time of the peak voltage recorded by the first sensor, tBis the time of the peak voltage recorded by the second sensor and tDis the time of the peak voltage recorded by the fourth sensor. At4, AtB, Atcand AtDare the time delays between the time of impact and the signal received by the first, second, third and fourth sensors in the array respectively. tcis the time of the peak voltage recorded by the third sensor, however this time has been determined in the example above as the time of impact. Thus, tc= t^ and Atcis 0.

[0124] The time delays calculated above can be used to calculate the location of the impact of an object on the substrate. That is, the electronics module may be configured to calculate the location of the impact on the substrate using the arrival time of the electrical signal received from each sensor of the first array of sensors. The location of the impact of an object on the substrate may be calculated mathematically (e.g., using Pythagoras' theorem) by the electronic module. Alternatively, the location of the impact of an object on the substrate may be calculated by comparing the time delay calculated for each sensor in the array to a lookup table of time delays against coordinates of the substrate. That is, each time delay may be compared to a plot of time delay against impact coordinates. The look-up-table, or plot, is a simple indexing array that is used to minimise computational runtime. The look-up table may have a resolution of 1 mm per coordinate, within the sensitive area on the substrate. The lookup table is stored in a memory of the electronic module. In some examples, the memory of the electronic module may store four look-up tables. Each of the four look-up tables may storecoordinate data for a respective quadrant of the sensitive area of the substrate. As each of the sensors of figure 9 is located in a different quadrant of the substrate, the look-up table that is consulted by the electronic module is the look-up table for the quadrant of the sensor that detected tr.

[0125] The electronics module is configured to determine, from the electrical signals received from the array of piezoelectric sensors, coordinates identifying a location of the impact on the substrate. That is, the electronics module is configured to calculate the location of the impact on the substrate using the arrival time of the electrical signal received from each sensor of the first array of sensors. The processing logic of the processor of the electronics module that is used to identify the location of the impact on the substrate may be referred to as an impact cartesian coordinate (ICC) subsystem. The ICC subsystem may use an algorithm in conjunction with electrical signals received from the sensors to calculate the impact coordinates of the collision of the object with the substrate.

[0126] The electronic module may be further configured to characterise an object that collides with the apparatus. More specifically, the electronic module may be configured to characterise an object by determining one or more of the size, velocity, and trajectory of the object. In some examples, the electronic module may also be configured to calculate the mass and / or kinetic energy of the object using signals from the apparatus described herein. Determining these parameters is advantageous as it allows a profile of the objects in the orbit of a satellite to be detected, which allows for further customisation of protection mechanisms for spacecraft in the orbit from the objects.

[0127] The electronic module of the apparatus is configured to calculate the size, and in some examples the mass and kinetic energy, of an object that collides with the substrate by using an amplitude measurement obtained from each sensor of the first array of piezoelectric sensors. In specific examples, the amplitude measurement is a peak-trough measurement. The peak-trough amplitude measurement may be used to determine the diameter of the object. A peak-trough amplitude is the difference between the midpoint of a maximum peak of a sensor signal and the midpoint of the minimum trough of an electric wave within a specific time period. More specifically, the electronic module may be configured to calculate the size of the object by comparing an average peak-trough amplitude measurement to a plot of peaktrough amplitude against diameter.

[0128] More specifically, the electronic module is configured to calculate the size of an object using the following steps:1. Determine coordinates identifying a location of the impact on the substrate;2. Calculate a distance between the location of the impact and the location of a sensor of the first array of piezoelectric sensors;3. Calculate a raw peak-trough amplitude for the electrical signal received from the sensor;4. Calculate a normalised peak-trough amplitude for the raw peak-trough amplitude using the calculated distance for the sensor;5. Calculate the diameter of the object from a plot of normalised peak-trough amplitude against diameter.

[0129] The method of step 1 is described above with respect to figure 9. From the determination of the impact coordinates, at step 2 the distance of the location of the impact from the location of each sensor of the array can straightforwardly be calculated using Pythagoras' theorem, the impact coordinates, and the known distances between piezoelectric sensors in the array of piezoelectric sensors. That is, the distances can be calculated as follows:dD= x2+ y2Where dA, dB, dcand dDare the distances of the location of the impact from the location of the first, second, third and fourth sensors respectively, Lyis the height of the sensitive area of the substrate (e.g., the distance between sensors 506 and 510 in figure 5), Lxis the width of the sensitive area of the substrate (e.g., the distance between sensors 506 and 508 in figure 5), x is the x coordinate of the impact location and y is the y coordinate of the impact location. In some examples, step 2 comprises calculating the distance of the location of the impact from the location of a single sensor in the array. In other examples, step 2 comprises distance of the location of the impact from the location of each sensor of the array.

[0130] Step 3 comprises calculating the raw peak-trough amplitude of the acoustic signals recorded by the sensor(s) for which the distance of step 1 has been calculated. As an example, with respect to figure 9, the raw peak-trough amplitude of signal C is the difference between maximum voltage value 902 and minimum voltage value 904 (i.e., value 902 - value 904). The electronic module is configured to analyse the electrical signals received from the piezoelectric sensor(s) of the array of piezoelectric sensors and to calculate the difference between the maximum and minimum voltage values for the signals.

[0131] Step 4 comprises calculating a normalised peak-trough value for the peak-trough value calculated for the sensor(s) for which steps 2 and 3 have been performed. Normalising the calculated peak-trough values adapts the values to account for the distance of each sensor of the array from the impact location. A normalised peak-trough value is calculated using anominal impact distance that is applied to the value. The nominal impact distance is a predetermined value stored in the memory of the electronic module. The nominal impact distance may, in some examples, be selected as a value that is half the value of the distance between adjacent sensors of the array of piezoelectric sensors. The equations for calculating a normalised peak-trough amplitude for each of the four sensors of figure 9 may be laid out as follows:PTAN, PTBN, PTCjVand PTDNare the normalised peak-trough values for signals A, B, C and D respectively. PTA, PTB, PTCand PTDare the raw (i.e. , non-normalised) peak-trough values for signals A, B, C and D respectively (e.g., value 902 - value 904 for signal C). As described above, dA, dB, dcand dDare the distances of the location of the impact from the location of the first, second, third and fourth sensors respectively. dNis the nominal impact distance.

[0132] Finally, at step 5, the electronic module consults a look-up table, or plot, of normalised peak-trough amplitude against diameter to determine the diameter of the object. The plot may be stored in the electronic module. The plot may be generated from predetermined calibration experiments that are performed before the apparatus is manufactured. The relationship between normalised peak-trough amplitude and diameter, as stored in the plot, may be linear. That is, as normalised peak-trough amplitude increases, diameter may increase at the same rate.

[0133] In some examples, where normalised peak trough the method for calculating the size of an object may further comprise calculating an average normalised peak-trough amplitude using the normalised peak-trough amplitudes of step 4. That is in some examples, after step 4, an average normalised peak-trough value is calculated from the normalised peak-trough values obtained from each sensor in the array of sensors. The average value may be the mean value of normalised values. For the example illustrated in figure 9, the average normalised peak-trough value may be calculated as follows:PT_PTAN+PTBN+PTCN+ PTDNWhere PTNis the average normalised peak-trough amplitude. In this example, the look-up table of step 5 may comprise a plot of average normalised peak-trough amplitude against diameter. Using an average normalised peak-trough amplitude, instead the peak-trough amplitude of a single sensor, is more reliable as it accounts for the differences in the sensor measurements due to the distances between the sensor locations and the location of impact.

[0134] For apparatuses comprising two impact detection gates as illustrated in figures 7 and 8, the electronics module may be further configured to calculate the velocity of an object. Morespecifically, the velocity of the object may be calculated by observing the difference between the time of impact of the object with the first substrate and the time of impact of the object with the second substrate. The velocity of the object is also determined using the distance between the first substrate and the second substrate.

[0135] The time of impact of the object with the first and second substrates is calculated as described above with respect to figure 9. That is, the time of impact of the object with the first substrate, is calculated by determining, from the electrical signals received from each sensor in the first array of sensors, the signal that displayed a voltage peak at the earliest time. The time of impact of the object with the second substrate, t2, is calculated in the same way. That is, t2is calculated by determining, from the electrical signals received from each sensor in the second array of sensors, the signal that displayed a voltage peak at the earliest time. The distance between the two arrangements of the apparatus (i.e., between the first and second arrays of sensors) may be known at the time of manufacturing, and is identified by reference d. Alternatively, the distance between two arrangements may be determined by an integration operation. Thus, the velocity of the object, v, can be calculated by the electronics module using the following equation: d v = -^2—l

[0136] The electronics module may also be configured to calculate the trajectory of the object. The electronics module may calculate the trajectory using a difference between the coordinates of the location of impact of the object on the first substrate and the coordinates of the location of impact of the object on the second substrate. The electronics module may be configured to calculate the trajectory of the object as a three-dimensional unit vector. The method may comprise the following steps:1. Determine the impact coordinates on the first and second substrates;2. Calculate a vector between the impact coordinates on the first and second substrates;3. Calculate a magnitude of the vector;4. Calculate a unit vector.

[0137] In step 1 , the coordinates of the location of impact of the object on the first and second substrates may be calculated as laid out above. That is, the coordinates of the location of impact of the object on the first substrate may be calculated using the calculated time delays of the electrical signals of the piezoelectric sensors and a look-up table of time delays against impact coordinates of the first substrate. Similarly, the coordinates of the location of impact of the object on the second substrate may be calculated using the time delays of the electrical signals of the piezoelectric sensors and a look-up table of time delays against impact coordinates of the second substrate.

[0138] In step 2 a vector between the impact coordinates on the first and second substrates is calculated. The vector between the impact coordinates on the first and second substrates may otherwise be referred to as a trajectory vector. It may be identified by the following equation:Where v is the vector with components i, and k, and Ax, Ay and Az represent the magnitudes in the i, j and k directions respectively. To find the values of Ax and Ay , the impact coordinates (Xi, y of the object on the first substrate are subtracted from the impact coordinates of the impact coordinates (x2, y2) of the object on the second substrate. The equations used to calculate these values are as follows:Ax = X2— X-LAy = y2-yiThe value of Az is known by the module (i.e., stored in the module) as it is the distance between the first substrate and the second substrate.

[0139] In step 3 a magnitude |v| of the vector of step 2 is calculated. The magnitude of the vector may otherwise be referred to as the trajectory vector magnitude. The magnitude of the vector is calculated using the following equation:

[0140] Finally, in step 4, unit vector v is calculated by dividing the x, y and z components of the vector v by the magnitude | v| of the vector. This is calculated using the following equation: v xi + Ayj + Az / c1121 ^ / Ax2+ Ay2+ Az2The unit vector can be used to find the flight trajectory angle (°) of the object along the x and y axes using the equations below. 6 and p represent angles in the x and y axes respectively.6 = tan p = t .an

[0141] The invention described herein provides the general advantage of enabling accurate measurement of the number of collisions of its substrate with objects in space. This means that the number of potentially damaging objects in the vicinity of a satellite incorporating the invention can be reliably determined. Furthermore, signals generated by the piezoelectric sensors of the invention can be used to calculate the size, speed and trajectory of objects. This allows for a detailed analysis of space debris surrounding a satellite to be performed. The analysis can be used to develop a customised protection mechanism for spacecraft operating in the same orbit as the apparatus. The customised mechanism could be designed to account for the determined characteristics of the objects in the orbit of the satellite.

[0142] Whilst the invention has been described herein with an application to space technologies, it should be appreciated that some examples may also have applications within the earth's atmosphere. For example, one or more examples of the invention described herein may be attached to the body of an earth-based aircraft. The invention may be used to measure the damaging objects surrounding the aircraft whilst it flies through hazardous environments such as volcanic ash clouds.

[0143] It will be understood that the above description of a preferred example is given by way of example only and that various modifications may be made by those skilled in the art. It is, of course, not possible to describe every conceivable modification and alteration of the above devices or methods for purposes of describing the aforementioned aspects, but one of ordinary skill in the art can recognize that many further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

CLAIMS1. An apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means, the first planar substrate comprising one or more homogenous layers of material; and a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the first planar substrate such that each sensor is able to detect vibrations of the first planar substrate and to generate an electrical signal indicative of those vibrations.

2. The apparatus of claim 1 further comprising an electronic module attached to the first attaching means, the electronic module being configured to receive and process the electrical signals from the first array of piezoelectric sensors and to characterise an object that has collided with the first planar substrate in dependence on the processing of the electrical signals.

3. An apparatus for characterising sub- centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means; a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the substrate such that each sensor is able to detect vibrations on the surface of the substrate and to generate an electrical signal indicative of those vibrations; and an electronic module attached to the first attaching means and configured to receive and process the electrical signals from the first array of piezoelectric sensors and to characterise an object that has collided with the first planar substrate in dependence on the processing of the electrical signals.

4. The apparatus of any preceding claim, further comprising an electromagnetic sensor attached to the first attaching means, the electromagnetic sensor being configured to detect electromagnetic waves generated by a collision of an object with the first planar substrate.

5. An apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means;a first array of piezoelectric sensors adhered to a surface of the substrate such that each sensor is able to detect vibrations of the substrate and to generate electrical signals indicative of those vibrations; and an electromagnetic sensor attached to the first attaching means, the electromagnetic sensor being configured to detect electromagnetic waves generated by an impact of an object with the first planar substrate.

6. The apparatus of claim 4 or claim 5, wherein the electromagnetic sensor is configured to detect visible light waves.

7. The apparatus of any of claims 4 to 6, wherein the electromagnetic sensor is configured to detect the presence of plasma.

8. The apparatus of any preceding claim, further comprising an impact detection mechanism configured to determine the time of impact for an object with the first planar substrate.

9. An apparatus for detecting sub-centimetre objects in space, the apparatus comprising: a first attaching means configured to attach to a space-borne artificial satellite; a first planar substrate attached to the first attaching means; an impact detection mechanism configured to determine the time of an impact for an object with the first planar substrate; and a first array of piezoelectric sensors comprising three or more piezoelectric sensors, the first array of piezoelectric sensors being adhered to a surface of the substrate such that each sensor is able to detect vibrations of the surface of the substrate and to generate electrical signals indicative of those vibrations.

10. The apparatus of claim 8 or claim 9, wherein the first array of piezoelectric sensors comprises four piezoelectric sensors and the impact detection mechanism is a piezoelectric sensor of the array.

11. The apparatus of claim 8 or claim 9, wherein the impact detection mechanism is a layer of the first planar substrate.

12. The apparatus of any preceding claim, wherein the first attaching means is a first frame and the first substrate is mounted to the first frame.

13. The apparatus of claim 12, wherein the first frame surrounds the circumference of the surface of the substrate.

14. The apparatus of claim 12, wherein the first frame is attached to a planar surface of the first planar substrate.

15. The apparatus of claim 14, wherein the first frame is comprised of a lattice of hollow prismatic cells, and wherein a first surface of the lattice is adhered to the planar surface of the planar structure.

16. The apparatus of any preceding claim wherein the first planar substrate consists of a single base layer which is a homogenous layer of material.

17. The apparatus of any of claims 1 to 15 wherein the first planar substrate comprises a base layer and a second layer that provides a protective coating over the base layer.

18. The apparatus of claim 16 or claim 17, wherein the base layer of the first planar substrate is composed of a flexible film.

19. The apparatus of claim 18, wherein the flexible film is made from Kapton.

20. The apparatus of claim 18, wherein the flexible film is a piezoelectric film.

21. The apparatus of claim 16 or claim 17, wherein the base layer of the first planar substrate is comprised of a solid metal plate.

22. The apparatus of claim 21 , wherein the solid metal plate is made from aluminium.

23. The apparatus of any preceding claim, wherein the piezoelectric sensors are polyvinylidene fluoride "PVDF" sensors.

24. The apparatus of any preceding claim, wherein each piezoelectric sensor is adhered to the surface of the first planar substrate using an adhesive.

25. The apparatus of any of claims 1 to 22, wherein each piezoelectric sensor is printed onto the surface of the first planar substrate.

26. The apparatus of any preceding claim, wherein the surface is square in shape.

27. The apparatus of any preceding claim when dependent on claim 12, wherein the first frame is configured to be fixed with respect to a space-borne artificial satellite when it is attached to the satellite.

28. The apparatus of any of claims 13 to 26 when dependent on claim 12, wherein the first frame is configured to be moveable with respect to a space-borne artificial satellite when it is attached to the satellite.

29. The apparatus of any preceding claim when dependent on claim 2 or claim 3, wherein the electronics module is configured to calculate a time of impact of the object with the substrate by observing an arrival time of an electrical signal received from each sensor of the first array of sensors and selecting the earliest arrival time as the time of impact.

30. The apparatus of claim 29, wherein the electronics module is configured to calculate the location of the impact on the substrate using the arrival time of the electrical signal received from each sensor of the first array of sensors.

31. The apparatus of claim 30 wherein the electronics module is configured to calculate the location of the impact on the substrate by: calculating a time delay between the time of impact and the arrival time of each electrical signal received from a respective sensor of the first array of sensors; and comparing each time delay to a look-up table that plots time delays against coordinates of the substrate.

32. The apparatus of claim 31 wherein the electronic module is further configured to calculate the time delay between the time of impact and the arrival time of each electrical signal by subtracting the time of impact from the time of the peak voltage recorded by each of the piezoelectric sensors in the array.

33. The apparatus of any preceding claim when dependent on claim 2 or claim 3, wherein the electronic module is configured to characterise the object by determining one or more of the size, velocity and trajectory of the object.

34. The apparatus of claim 33, wherein the electronic module is configured to calculate the size of the object using an amplitude measurement obtained from each sensor of the first array of piezoelectric sensors.

35. The apparatus of claim 34, wherein the electronic module is configured to calculate the size of the object by comparing an average peak-trough amplitude measurement to a lookup table that plots peak-trough amplitude against diameter.

36. The apparatus of claim 33 when dependent on claim 31 , wherein the electronics module is configured to calculate the size of the object using the following steps: determining coordinates identifying a location of the impact on the substrate; calculating a distance between the location of the impact and the location of a sensor of the first array of piezoelectric sensors; calculating a raw peak-trough amplitude for the electrical signal received from the sensor of the first array of piezoelectric sensors; calculating a normalised peak-trough amplitude for the raw peak-trough amplitude using the calculated distance for the sensor of the first array of piezoelectric sensors; and calculating the diameter of the object from a plot of average peak-trough amplitude against diameter.

37. The apparatus of any preceding claim when dependent on claim 12, further comprising: a second frame configured to attach to a space-borne artificial satellite; a second planar substrate mounted to the second frame; and a second array of piezoelectric sensors comprising three or more piezoelectric sensors, the second array of piezoelectric sensors being adhered to a surface of the second planar substrate such that each sensor is able to detect one or more vibrations of the surface of the second substrate and to generate electrical signals indicative of those vibrations.

38. The apparatus of claim 37, wherein the second frame is parallel to and spaced apart from the first frame.

39. The apparatus of claim 37 or claim 38 when dependent on claim 29, wherein the electronic module is configured to calculate the velocity of the object by observing the difference between the time of impact of the object with the first substrate and the time of impact of the object with the second substrate.

40. The apparatus of any of claims 37 to 39 when dependent on claim 29, wherein the electronic module is configured to calculate the velocity of the object using the following equation: d v = -^2—l is the time of impact of the object with the first substrate, t2is the time of impact of the object with the second substrate, and d is the distance between the first and second planar substrates.41 . The apparatus of any of claims 37 to 40, wherein the electronics module is configured to calculate the trajectory of the object using the difference between the coordinates of a location of impact of the object on the first substrate and the coordinates of a location of impact of the object on the second substrate.

42. A method of manufacturing a satellite as claimed in any preceding claim.

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