SYSTEM FOR DETECTING THE PATH OF MOVING OBJECTS

Description

[0001] The field of the invention relates to the detection of moving objects (hereinafter referred to as "moving objects") in space at low and high altitudes by optical methods employing an array of telescopes. More specifically, the field of the invention relates to the detection of satellites and space debris, as well as aircraft or any moving object, and the calculation of their orbit and trajectory in order to prevent them from falling to Earth, colliding in space, or entering a danger zone.

[0002] Document WO 2015 / 136102 A1 describes, in its abstract, a method for the optical detection of space mobiles comprising the generation of a plurality of fields of view by means of a first set of telescopes, the set of fields of each telescope having a spatial distribution inscribed in a conical ring of a defined plane of space, called the optical plane, said optical plane being not parallel to at least one of the optical axes of a telescope, said conical ring having a diameter defining a wide detection field.

[0003] The method also includes a step of detecting at least one track of a moving object in the field of view of at least one telescope by an electronic detector, the integration time of the electronic detector being defined to obtain a spread of the track over several pixels for a given maximum speed of the moving object and a minimum altitude of its orbit. The method further includes a step of deriving a trajectory of the moving object.

[0004] Detecting orbital debris is very problematic. Indeed, tracking an object whose size may be less than ten centimeters, at a distance of several thousand kilometers, is an arduous task.

[0005] On the other hand, since objects have very different trajectories from each other, it is very difficult for economic reasons to cover the entire celestial vault with adequate detection capabilities.

[0006] The ESA (European Space Agency) estimated the number of objects such as inactive satellites, remnants of space missions like rocket fairings, rocket bodies, and small fragments to be: 5,400 pieces of space debris larger than one meter; 34,000 pieces of space debris larger than 10 centimeters; 900,000 pieces of space debris larger than one centimeter; 130,000,000 pieces of space debris larger than one millimeter.

[0007] Currently, monitoring near-Earth space and in particular objects presenting a potential danger, whether natural or artificial, has become a major problem to ensure the safety and integrity of any target that may be involved in a collision.

[0008] This monitoring concerns both end-of-life or uncontrolled satellites and debris from previous collisions, or asteroids or comets potentially dangerous to Earth.

[0009] Space agencies and private operators have initiated various monitoring programs grouped under the name "Space Situational Awareness".

[0010] Even when a meteorite is relatively small, the risks posed by its fall remain significant. A danger persists for populations and infrastructure on Earth's surface from any falling object, as its fragmentation in the atmosphere can cause substantial damage. One of the challenges is cataloging small bodies in the solar system whose orbits intersect Earth's and tracking them to assess their potential hazard.

[0011] The number of artificial objects in low and high Earth orbits is steadily increasing. Space congestion appears to be growing each year due to the increasing number of satellites and the rise in catastrophic collisions, which in turn generate more debris. The poor or nonexistent tracking of their trajectories after their demise remains a growing problem.

[0012] Projections suggest a situation in which the increasing population of debris larger than 1 cm will make its monitoring and control increasingly difficult. The 1 cm threshold corresponds to the size of an object that could potentially render a satellite partially or completely inoperable, given the speeds involved: 3 km / s in geostationary orbit up to 8 km / s in low Earth orbit.

[0013] We will refer to these objects as "debris" in the rest of the text, knowing that this notion of debris encompasses debris proper, operational or non-operational satellites, or even meteorites.

[0014] The first problem concerns debris falling to Earth's surface, and the second concerns collisions between debris and active satellites. Monitoring debris in space, particularly in low Earth orbit, helps prevent both of these consequences.

[0015] Finally, the issue of monitoring the various mobile objects in space also concerns, by extension, those of discreet objects moving at very low altitude such as aircraft, for example ultralight aircraft or drones, which can define a danger, for example, when their overflight is carried out near a sensitive site, for example, a nuclear power plant.

[0016] One difficulty is to find a wide-field optical system capable of covering different altitudes, both near and far, and of tracking low-altitude objects with high speeds that are therefore difficult to detect.

[0017] Indeed, one problem with the detection and monitoring of space debris, whose orbit and / or trajectory is not known a priori, is taking into account the light intensity of third-party sources which disrupts detections.

[0018] These sources can originate from the celestial vault, the sun, the moon, and local weather conditions that can alter the stability of the image capture. The surveillance system must be able to account for a multitude of lighting conditions to maximize detection under all circumstances. Since detection is performed by considering a point or area on the Earth's surface, the observer's field of view is an extremely important factor in calculating the probability of detecting a moving object and determining its trajectory.

[0019] The challenge of debris monitoring involves considering different orbits in space object detection methods. For natural objects such as meteorites, their orbit is generally heliocentric, meaning they can potentially approach Earth at any altitude and from any direction. For artificial objects, their Earth orbit can fall into different orbital categories.

[0020] The first family of orbits is known by the acronym LEO, which stands for "Low Earth Orbit" in English. It comprises low Earth orbits extending up to 2000 km. This family of orbits is heavily used by communications, military, detection, meteorological, and other satellites.

[0021] A second type of orbit is known by the acronym GEO, meaning "Geostationary Orbit," which includes a geostationary orbit defined at 35,784 km above the equator. A revolution of a spacecraft at this altitude takes 24 hours. A spacecraft in a geostationary orbit is fixed relative to a position on Earth. However, debris can leave its orbit and follow a swashplate path. This orbit is heavily used by communication satellites (military and civilian), remote sensing satellites, meteorological satellites, and so on.

[0022] A third family of orbits is better known by the acronym MEO, which stands for "Medium Earth Orbit." This family of medium-sized orbits is generally elliptical. This is the case for GNSS satellites.

[0023] A fourth family of orbits is designated by the acronym HEO, including highly elliptical orbits such as Molniya or Tundra orbits which allow communication or monitoring of high latitude regions.

[0024] A fifth family of orbits is designated by the acronym GTO, meaning "Geostationary Transfer Orbit." This family comprises elliptical orbits. Their apogee is approximately 42,000 km and their perigee is approximately 650 km. This family of orbits is very practical for injecting satellites into a geostationary orbit; it is therefore used during satellite launches as a transition orbit for geostationary placement.

[0025] Today, various methods exist for detecting space debris and its trajectory.

[0026] In particular, there is a family of methods known as "active methods," notably for detecting debris in LEO orbits. Active methods rely on a "radar-like" operation in which a moving object is illuminated by a signal-emitting source. The signal is then reflected, and it is this reflection that informs a receiver of the object's position data.

[0027] A primary drawback of this method is that the received power varies as 1 / d ≤ 4, where "d" is the distance from the mobile device to the transmitter / receiver. Consequently, the received flux will remain low during detection, even if a high transmission power is considered.

[0028] A second drawback is the relatively large-scale installation of radar-type systems that this method requires. These installations are expensive, require significant maintenance, and are easily detectable. Furthermore, these systems consume a great deal of energy and therefore must be installed near a power grid.

[0029] Among active methods, there are also LiDAR systems, which rely on illuminating a moving object with a laser. This method achieves better results than radar in terms of detected power because the laser light is more focused. However, the detection cones are much smaller and poorly suited to "blind" detection of objects in low and elliptical orbits. Another family of methods exists: passive methods, in which targets are not illuminated by a terrestrial source. With passive methods, the light flux captured by a detector varies with the distance "d" from the object as a function of 1 / d, which offers better results than active methods in terms of the light flux captured from the object. However, the major drawback is the strong dependence on illumination from external sources such as the sun, stars, or the moon.The advantage of these solutions lies in their low cost and the relative simplicity of their implementation using detectors based on optical instruments capable of visualizing small objects at all altitudes.

[0030] Regarding objects in geostationary orbit, a telescope, radar, or other electromagnetic instrument can detect a stationary point against a background of moving stars during the exposure time. With a wide-field telescope, it is then possible to detect spacecraft in a geostationary orbit and track their trajectories.

[0031] In the case of other orbits, called "sliding" and therefore non-geostationary, that is to say not fixed with respect to an observation point on Earth, it is difficult to detect space mobiles, their height, their inclination, the different nodes of their orbit, including the ascending and descending nodes.

[0032] One difficulty arises from the speed at which spacecraft move, which can exceed 1° / s at the zenith for a low Earth orbit. Detection is achieved by capturing a trace (in a sequence of images) relative to point traces or trails, depending on the sidereal motion and therefore the observation window in the sky.

[0033] The method then consists of discriminating between the tracks to detect the presence of space debris. While the inclination can potentially be detected by analyzing the track left by the object, obtaining the object's actual speed remains very difficult due to the unknown altitude. Consequently, it is difficult to deduce elements of its trajectory by extrapolating from the track analyses. In general, three angular position measurements of the object are necessary to derive its orbit. Two measurements are sufficient if the object is in a circular orbit.

[0034] The problem can be solved by increasing the field of view of a telescope to increase the traces and their number, but the images detected, as explained previously, can become difficult to analyze due to the complexity of the telescopes to be implemented, the surrounding light pollution, the strong confusion caused by all the objects in the field, and the very large size of the sensors required.

[0035] Furthermore, the construction of a wide-field telescope is hardly feasible beyond a few degrees, unlike photographic lenses.

[0036] Indeed, a wide-field optic allows us to deduce information about the trajectory of the moving object; however, a wide field is more susceptible to being affected by stray light sources. Furthermore, designing wide-field telescopes remains very difficult without encountering design problems and complex specific optical schemes. The presence of a large focal plane also leads to numerous aberrations. When an electronic detector is coupled to a wide-field optic, it must be very large; the size and number of pixels can be very high, resulting in significant design costs and challenging operation. State of the art

[0037] There are telescope systems that allow for very large fields of view by coupling a matrix of large-field telescopes forming a single field, as proposed by the company ASTRIUM, notably with the solution described in patent document WO2012 / 007361.This patent describes an optical system for a space surveillance system characterized in that it comprises an array of NxP telescopes each with a field of view greater than or equal to 5° and preferably greater than or equal to 10°, said telescopes being coupled to NxP image sensors of sensitivity adapted to an integration time on the order of 10 to 100 milliseconds, the telescopes being mounted on one or more motorized mounts, the telescopes being servo-controlled together and grouped so as to operate simultaneously to provide a wide field and in that the speed of movement of the mounts of the telescopes is such that each object crossing the scanned area is detected at least three times so as to obtain at least 3 dated position measurements distributed over the arc of transit of the object in the sky area, the exposure time or integration time being defined to obtain a spreading of the signal over several pixels.

[0038] One drawback of this solution is the cost of such a system, which requires numerous very wide-field telescopes. One solution is to reduce the number of telescopes and combine a motorized tracking system with wider-field telescopes having a field of view of at least 5°, and in practice 14° in the example cited in the patent document (10° x 10° on the square detector).

[0039] French patent FR2962556 describes an optical system for a space surveillance system characterized in that it comprises at least one telescope with a field of view greater than or equal to 5° and preferably greater than or equal to 10° mounted on a movable mount along two axes, said telescope being coupled to an image sensor with sensitivity adapted to an integration time on the order of milliseconds.

[0040] This solution requires the use of wide-field telescopes with modest magnification, insufficient to take into account very small moving objects.

[0041] French patent FR2962411 proposes an alternative solution for a space surveillance system in LEO zones of Earth orbit, comprising a step of arranging and networking optical surveillance system stations on the Earth's surface according to a grid adapted to provide an effective daily 24-hour system cycle and a chosen revisit time for the observed LEO zone. For said grid, a latitude positioning of the optical systems is defined according to preferred latitudes based on the types of orbits to be monitored, and the following is provided: sites at high latitudes >50° North or South, sites at mid-latitudes between 30° and 50° North or South, sites at latitudes below 30° near the tropics and subtropics, a selection of sites offering a longitude spacing of 20° or less. The deployment and networking of optical surveillance systems across the globe is carried out according to the following steps: We define a longitude positioning of the optical systems from a computer calculation of the revisit times of each point of the globe, we optimize the arrangement of the optical systems according to a computer analysis of the statistical weather conditions at the intersections of said latitudes and longitudes to obtain a maximum revisit time of the system of no more than one week, we place the optical systems at the nodes of the mesh constituted according to said latitude and longitude positionings according to a step adapted to respect said maximum revisit time.

[0042] French patent FR3018612 describes another known solution for detecting a moving object in space, characterized in that it comprises: a generation of a plurality of fields of view (Zkp) by means of a first set of telescopes (T), each telescope defining a detection telescope, the set of fields (FOV) of each telescope (Ty) having a spatial distribution in at least one plane of space inscribed in an open geometric shape (CC), said open geometric shape (CC) defining a wide detection field; a detection of at least one trace of a mobile (Mi) in the field (FOVy) of at least one telescope (Ty) by an electronic detector coupled to each telescope (Ty), the integration time of the electronic detector being defined to obtain a spreading of the trace over several pixels of the electronic detector for a given maximum speed (VM) of a mobile and a minimum altitude of its orbit; a deduction of a trajectory (TJSAT) of the mobile (Mi) in the image plane of said telescope (Ty). Solution provided by the invention

[0043] To manage these risks, it is essential to catalog all potentially hazardous debris and associate it with valid orbital parameters that describe its trajectories. Observed from a fixed point on Earth, objects in low Earth orbit are characterized by their rapid movement across the sky. Furthermore, at any given moment, several objects traverse the sky at various points. Depending on its orbital parameters, each object crosses the local sky at more or less regular intervals, ranging from a few tens of minutes to several hours.

[0044] For this purpose, the invention relates, in its most general sense, to a system for detecting the trajectory of moving objects having the characteristics stated in claim 1.

[0045] The system for detecting the trajectory of moving objects includes: A first array of N1 telescopes, each with a field angle of M1 degrees, oriented to cover an arc of C° in a plane P1, M1 being between 1 and 4°, N1 being greater than C / M1, the ALPHAN1 sighting angles of each of said telescopes being in a plane P1. At least a second array of N2 telescopes, each with a field angle of M2 degrees, oriented to cover an arc of C° in a plane P2, M2 being between 1 and 4°, N2 being greater than C / M1, the ALPHAN1 sighting angles of each of said telescopes being in a plane P2 perpendicular to P1 comprising the sighting axis of a reference stellar object. The orientation of said telescopes being controlled to maintain the intersection between planes P1 and P2 being in the axis of said reference object. At least a third array of N3 telescopes, each with a field angle of at least M3 degrees, oriented to cover a conic segment of 360° with an E3 elevation between (180 - C) / 2°,M3 being between 1 and 4°, N3 being greater than 360 / M3. Each of the telescopes being equipped with a photosensitive sensor periodically delivering an image I. The system further includes a computer performing: ∘ for each of said images I, a recognition process for the coordinates of a segment S i (T, H, CS) corresponding to a displacement of an object in the field of the corresponding telescope T, where the variable H designates the timestamp of the image and C designates the coordinates of said segment in said image; ∘ a process applied to all of said recorded segments to estimate the trajectories of said objects. Detailed description of a non-limiting example of implementation

[0046] The present invention will be better understood upon reading the following description, concerning a non-limiting embodiment illustrated by the accompanying drawings where: [ FIGURE 1 ] there figure 1 represents a schematic view of an installation according to the invention Telescope placement

[0047] There figure 1 represents a schematic view of the telescope layout.

[0048] The invention employs a plurality of telescopes at a suitable geographical site, meeting criteria favorable to stellar observation: Arid climate guaranteeing low cloud cover; altitude allows for reduced atmospheric agitation; proximity to the sea ensuring minimal thermal difference between the ground and the atmosphere; geographical isolation allowing one not to be disturbed by human activities, particularly with regard to lights and pollution clouds.

[0049] Telescopes offer a wide field of view, approximately 3°, allowing observation over a solid angle of around 10 square degrees. A Schmidt telescope is an example. Each telescope is equipped with a photosensitive sensor, such as an accumulation sensor with a pixel array that measures the number of pixels accumulated on each pixel during an acquisition period.

[0050] The telescopes are distributed among three arrays, two arrays in two perpendicular planes (10, 20) whose intersection defines a reference direction (40) forming a zenith angle ALPHA. The telescopes of the first array (11 to 13) and the second array (21 to 23) are distributed to cover the entirety of two arcs (15, 25) extending over approximately 120°, the lowest parts on either side of the reference direction (40) being made up of a mask rising over about thirty degrees.

[0051] The third network consists of a series of telescopes (31 to 33) observing an azimuthal corona (30).

[0052] The reference direction (40) ensures the immobility of celestial objects. This reference direction (40) can be periodically changed, with the telescopes being realigned according to the new reference direction (40). This telescope architecture makes it possible to determine the trajectories of objects without covering the entire celestial sphere.

[0053] The trajectories are established by measuring the image trails created by the intersection of the object with one of the three telescope arrays.

Claims

1. A system for detecting the path (100) of moving objects having: • A first network of N1 telescopes (11 to 13) each having an orientation, a field angle of M1 degrees and an observation angle ALPHAN1 and an observation axis of a reference object, oriented so as to cover an arc of C° in a plane P1 (10), M1 being between 1 and 4°, N1 being greater than C / M1, said observation angles ALPHAN1 of each of said telescopes (11 to 13) being in said plane P1 (10) • At least a second network of N2 telescopes (21 to 23) each having a field angle of M2 degrees, oriented to cover an arc of C° in a plane P2 (20), M2 being between 1 and 4°, N2 being greater than C / M1, the observation angles ALPHAN1 of each of said telescopes being in said plane P2 (20) perpendicular to the plane P1 (10) comprising said observation axis of a reference stellar object • Said orientation of said telescopes (11 to 13, 21 to 23) being controlled in order to maintain the intersection between said planes P1 and P2 being in the axis of a reference object • At least one third network of N3 telescopes (31 to 33) observing an azimuth ring (30), each having a field angle of M3 degrees, oriented so as to cover a conical segment of 360°; • Each of the telescopes (11 to 13, 21 to 23, 31 to 33) being provided with a photosensitive sensor which periodically supplies an image I, • the system further comprising a computer executing ∘ for each of said images I, a processing operation for recognising the coordinates of a segment Si (T, H, CS) corresponding to a movement of an object in the field of the corresponding telescope T, where the variable H denotes the timestamp of the image and CS denotes the coordinates of said segment in said image ∘ a processing operation applied to all of said recorded segments for estimating the paths (100) of said objects.

2. The system for detecting the path (100) of moving objects according to claim 1, characterised in that said third network consists of N3 telescopes (31 to 33), which each have a field angle of M3 degrees, oriented to cover a conical segment of 360° with an elevation E3 of ((180 - C) / 2)°, M3 being between 1 and 4°, N3 being greater than 360 / M3.

3. The system for detecting the path (100) of moving objects according to claim 1 or 2, characterised in that said paths are established by measuring the image trails established by the intersection of the object with one of the three telescope networks.

4. The system for detecting the path (100) of moving objects according to claim 1 or 2, characterised in that said telescopes are Schmidt telescopes.

5. The system for detecting the path (100) of moving objects according to claim 2, characterised in that telescopes are distributed between three networks, two networks in two perpendicular planes (10, 20), the intersection of which defines a reference direction (40) forming a zenith angle ALPHA and in that the telescopes of the first network (11 to 13) and the second network (21 to 23) are distributed to cover the entirety of two arcs (15, 25) extending over about 120°, the lowest parts on either side of the reference direction (40) consisting of a mask rising over about thirty degrees.