Antenna with spoof detection

Abstract

The present invention provides a directional sensor 100 for receiving and evaluating location signals from aerial vehicles 160 relative to the sensor, the sensor comprising: a plurality of aerials, me

Description

The present invention relates to an antenna with spoof detection, such as may be used for air traffic control of micro air traffic and to the air traffic control of micro aerial vehicles Background Technological miniaturization has caused the diversification of aerial vehicles in both size and purpose. Manned, remote controlled and autonomous vehicles simultaneously operate. Alongside traditional aeroplanes and helicopters, quadcopters and insectoid aerial vehicles fill different roles. Historical, manpower, fuel and carbon intensive processes, for example delivery by postman and van, or less capable systems including static surveillance camera are being substituted with unmanned aerial vehicles. Unmanned aerial vehicle (UAV) or drones finding increased use can be expected to lead to a more congested airspace, with more aerial vehicles. This leads to an increased potential for both unintentional failings, including collisions, and malicious behaviour. This is both a risk both between aerial vehicles and with the surroundings. A specific problem arises in terms of effective air traffic control with micro air traffic in that position determination by means such as radar is generally ineffective, particularly in built-up environments and with high concentrations, particularly as predicted, for such micro aerial traffic. As such Automatic Dependent Surveillance-Broadcast" (ADS-B) and variants thereof needs to be relied upon. ADS-B is a surveillance technology that uses GPS (Global Positioning System) to determine an aircraft's precise position and then broadcasts that information to ground stations and other aircraft equipped with ADS-B receivers. This allows air traffic controllers to have a more accurate and real-time view of aircraft positions, which enhances situational awareness and safety in the airspace. ADS-B is becoming a standard in modern air traffic management systems worldwide, as it provides improved efficiency, reduced separation standards, and more comprehensive traffic information for both pilots and controllers. However, this system relies upon accurate reporting from aircraft. Various occasions arise where inaccurate reports may be provided. There are a variety of circumstances where this occurs and most obviously where there is malicious intent or a military motivation. It is therefore important to have means to detect when such spoofing occurs. Spoofing is where erroneous information is provided with an intention to deceive. In particular, where the erroneous information appears to be valid but is intentionally invalid. Spoofing can be addressed by comparing the erroneous information received with validated information and noting a discrepancy. These challenges present particular problems with micro aerial vehicles and these problems require means to address them. Further, any system to mitigate such spoofing must itself be defended against exploitation lest false information inhibit operations. In addition to large number of elements, the quick and often erratic movement thereof must be comprehended. The present invention The present invention in its various aspects is as set out in the appended claims. The present invention provides: A directional sensor for receiving and evaluating location signals from aerial vehicles relative to the sensor, the sensor comprising: a plurality of aerials, means for receiving signals from the aerials, the signals being processed by a processor, the processor being configured to: determine for a given input signal a differential of a signal parameter between 2 or more of the aerials; this may be achieved when the signal received at any given time instance is comparable in magnitude in the time domain, differences in magnitude reflect signal attenuation. Whilst the present invention is primarily a direction sensor it very preferably provides a combination of direction and distance, i.e. position sensing. Therefore the present invention may be described as a position sensor for receiving and evaluating location signals, when the distance function is also utilised. For the purposes of the present invention both "aerial" and "antenna", as nouns, are synonymous and refer to the equipment used in radio communications to transmit and receive radio waves. The physical element for electromagnetic radiation reception and transmission is shown in the diagrams but in the present invention the supporting hard work for amplification decoding and processing should be taken as implicitly present. "aerial", as an adjective, as in the context of "aerial vehicle” refers to "in or of the air" as this is where such vehicles operate; aerial vehicles commonly have antenna, as described, however this is not eponymous and is not necessarily required. In the present invention the aerials of the directional sensor preferably comprise a Log-periodic antenna or a Yagi-Uda antenna element. Preferably all antenna elements are of the same type. A log periodic antenna is preferred as in the present application it is been found to give good signal reception for of both the 2.4 and 5 GHz bands combined with similar gain at the two frequencies coupled with lower noise than other antenna types. However, in some situations a Yagi-Uda antenna element may be used where one or other of the 2.4 or 5 GHz bands is to be measured as better frequency selectivity can be obtained. Such antennas are preferable with multiple antenna elements such as six or more in various directions as these antenna elements can be highly directional with narrow beam width. The overfull a suitable for the directional sensor of the present invention were differences in signal reception between aerials is the primary determinant rather than antenna element direction. For clarity the term antenna element is used here to connote the actual physical reception element. The present invention operates on the differential signal parameter to a determined position relative to the aerials by means of a correlation or covariance; The present invention compares the determined position relative to the aerials with a reported position; position may be in terms of direction and / or distance, preferably direction and distance from the directional sensor. Direction may be provided using any convenient metric such as angle compared to true North. The present invention determines a difference between the reported and determined positions and compare this to equivalence criterion; this equivalence criteria preferably comprises protocol and packet information. Further, the present invention upon carrying out the determination where the difference does not match the equivalence criteria to report a spoofing attempt. This can flag the discrepancy of the difference between location. As will be appreciated the above method steps are steps for which the directional sensor of the present invention is also configured to carry out, the present invention encompassing a method and apparatus for carrying out that method. The step, to determine, for a given input signal a differential of a signal parameter between 2 or more of the aerials; the signal received at any given time instance may be compared in magnitude in the time domain, differences in magnitude reflect signal attenuation. In the present invention the differential of the signal parameter is selected from one or more of: a) reception signal strength; this is particularly useful when monitored over time such that were a change of signal strength does not correspond to a changing distance of a micro aerial vehicle ADS-B signal reported position then this can be used as an effective determinant of a spoofing attempt. This is particularly useful when a change in position, such as a combination of distance and angle of a reported position determined by an ADS-B closely corresponds with a signal direction determined over time but for which the spoofing places the micro aerial vehicle either nearer to or further away from the directional sensor. b) time of arrival of the signal at the sensor; preferably by means cross-correlation as used to determine a time offset; this is particularly useful when the time of arrival, compared to a departure time, i.e. a sending time of the ADS-B signal does not correspond to an expected arrival time, this can be used as an effective determinant of a spoofing attempt. This is particularly helpful in determining spoofing as it enables differences in distance, even for an equivalent direction, of the source of a spoofing signal do not obscure a spoofing attempt. c) phase angle of the signal; this is particularly useful in enhancing direction finding. Phase differences of signals received at multiple, aerials of the direction sensor is a basis to determine the direction from which a signal is originating. This is used in applications like locating the source of a radio transmission. In the present invention the correlation of the differential signal parameter determines one or more of: a) a distance of the signal source from the aerial; as previously mentioned reception signal strength and time of arrival particular suitable for this determination. b) a geographic location of the signal source; this is particularly suitable for determination based upon changes in signal reception over time and the accuracy of determination. The use of a GPS as part of the directional sensor is also advantageous in this respect as it provides a clear and unambiguous reference point and more importantly, provides an indication, at any given time, of the GPS error. Specifically, whilst an incoming signal may appear to be spoofed in that there is a mismatch between the ADS-B reported location and the location determined by signal reception this in itself may be erroneous if there is significant disturbance in the GPS signal, therefore the signal parameter of location (i.e. a combination of direction and distance) may be more accurate than a GPS signal at any given time. On this basis it is advantageous in the present invention if the GPS location determined by GPS of the location sensor is repeatedly measured over time so as to progressively give a more accurate geographic location (provided that the sensor itself and with it the aerials are in a fixed position). Hence, the aggregated, more accurate geographic position (such as aggregated by an averaging process) can be compared at any given time with natural GPS signal and this error mapped onto a narrow boundary for the ADS-B provided location. Hence provided that the ADS-B location is within an error commensurate with the error determined from the directional sensor itself then erroneous spoofing alerts can be avoided. c) a height of the signal source from the aerial; this is particularly useful in determining spoofing attempts as elevation is a primary determinant of the type of traffic involved. For example, outside an airfield landing strip conventional aerial vehicles, as opposed to micro aerial vehicles d) location in 3 dimensions of the signal source from the aerial. In the present invention the compare provides one or more of: a) a difference in magnitude of one or more parameters, such as distance and angle; b) a rate of change, first differential, of one or more parameters, such as distance and angle. This is particularly useful when compared to predetermined information for normal parameters for a micro aerial vehicle. For example acceleration and speed can be compared to acceptable limits and if outside those limits a spoofing detection alert can be raised. c) a second differential, of one or more parameters, such as distance and angle. In the present invention the compare may provide one or more of: a) a difference in magnitude of one or more parameters, such as distance and angle; b) a rate of change, first differential, of one or more parameters, such as distance and angle; c) a second differential, of one or more parameters, such as distance and angle. In the present invention a signal attenuation means, in addition to air space, may be present between two or more of the one or more aerials. In the present invention the signal attenuation means, may be a metal ground plane. This is a most effective signal attenuation means may be preferably provided in the form of a metal mesh having apertures in the order of 3 cm, such as apertures in the range 2 to 4 cm. This is on the boundary of effective microwave screening in the signal frequency relevant to ADS-B most relevant to the present invention namely 2.4 and 5 GHz. The present invention preferably being configured to selectively receive, and process, signals at one or both of these frequencies. Further, such a metal ground plane may be configured for selective activation by means of grounding or being left floating (i.e. unattached to any electrical reference). This enables the signal attenuation effect to be rapidly varied such as to assist in confirmation of a signal direction determination. In the present invention the signal attenuation means, may be a ferrite sheet or a conductive plastics sheet. Such signal attenuation means are advantageous in that they can be configured to provide partial signal attenuation. This enables the direction of reception of a signal to be readily determined by checking for a given signal which aerial receives the strongest reception, the intervening signal attenuation means attenuating the signal to provide a clear differential in signal strength stood a high degree of confidence can be obtained. This is particularly useful with micro aerial vehicles were a signal source may be very close and hence signal strength high and therefore the linear portion of an amplified signal as in linear correlation between distance and strength is distorted. This situation is overcome by use of the partial signal attenuation were a clear differential in signal strength may be created between two closely spaced aerials. In the present invention the signal attenuation means may provide no line of sight between adjacent aerials. This is useful as it enables a clear receive / no receive determination to enable direction of the signal to be rapidly determined. In the present invention the plurality of aerials is preferably 4 aerials mounted in a rectangular array. Whilst in principle the present invention simply requires a plurality of aerials receiving a signal, particularly in the required frequency bands of 2.4 and 5 GHz is been found in practice that for aerials is optimal in providing directional resolution combined with economy of construction. In particular, having more than four aerials is not necessarily advantageous since for a given signal, whilst a signal duration for an ADS-B may be in the order of In the present invention a cruciform signal attenuation means may be present between the aerials. By this is mean an extruded X shape having 5 axes of symmetry. In the present invention the aerials are preferably in a known and fixed proximity to a GPS sensor which is in communication with the processor for providing a location of the aerials to the processor. Preferably the GPS sensor is located between two or more of the aerials. This is advantageous in that it better approximates the overall geographic position of the aerials are more importantly because the aerials are positioned, preferably, around a horizontal periphery of the GPS sensor than the aerials act to partially shield near horizon GPS signals, which are generally the least accurate signals. Preferably the signal attenuation means comprises an aperture for reception of radio signals by the GPS sensor. This further improves the shielding from low angle, horizon, GPS signals. Since the directional sensor of the present invention is preferably configured in a stationary position then the lack of GPS signals, i.e. there being less occasions when necessary four satellites are visible. However, for us stationary directional sensor accurate determination is more important than frequency of determination. Since the present invention, is particularly useful in high-density urban situations (where larger scale antennas are not practical) in these some situations, like urban canyons or areas with obstructions, the GPS receiver might have difficulty receiving signals from four or more satellites. In such cases, the accuracy of the GPS reading could be compromised. Advanced GPS receivers and systems might incorporate techniques like Differential GPS (DGPS) or Real-Time Kinematic (RTK) corrections to enhance accuracy even further. Very preferably the GPS sensor is orientated in relation to the aperture such that a 45° angle, more generally and angle in the range of 50° to 25°, more to the vertical (i.e. defining a notional: of reception, is available to the GPS sensor and is not shielded by the attenuation means. Preferably a range of 40 to 50°, most preferably 45°. This enables the directional sensor of the present invention at any given position on Earth's surface, to simultaneously have access to four satellites within a 45° range of the vertical multiple times a day. As mentioned, for a static directional sensor this is adequate and these accurate measurements can be cremated over time to give an even more precise position determination. This therefore provides an ideal reference for transitory GPS signals determined at any given time to, as mentioned, provide an indication of GPS error over ideal. In a preferred embodiment the directional sensor of the present invention has access to 2 GPS signal determinations, are shielded and unshielded determination. Shielded determination, as mentioned being more accurate though available less frequently in the unshielded been representative of a position determination from a micro aerial vehicle such as sending in an ABS-B signal to the directional sensor. This access may be provided by using a switchable signal attenuation which means, which can be switched on and off by the provision of two GPS devices, one within the aperture and another outwith. Preferably the aperture is perpendicular to vertical faces of the signal attenuation means. This in the context of an directional sensor of the present invention when installed, the notional angles to the vertical mentioned previously being in reference to this perpendicular. In the present invention, to mitigate the issue of reflections causing false positives / false spoofing scenarios, this sensor can be linked as part of an array of sensors that can be placed strategically in the environment to mitigate this issue. The array of sensors can compare and combine RSSI values to determine the possibility of spoofing and or reflections of signals. In the present invention a ground / shielding plane can also be modified through the application of RF absorption sheets / materials. This has the further benefit to reduce the effect of reflections from the shielding planes themselves. By using an array of directional antennas and by recording the RSSI (Relative signal strength indicator) of detections, the directional sensor of the present invention is able to detect and prevent ‘spoofing’ of drone locations on a map. ‘Spoofing devices’ send packet information of a drone’s location to place drones in false locations. For instance, if our sensor receives a packet of data placing a drone to the south of the sensor, we can check the RSSI and if the directional antenna pointing south has the greatest value amongst the rest of the directional antennas, it can be assumed that the information is correct and not ‘spoofed’. However, if the directional antenna pointing south does not have the greatest RSSI value but rather another directional antenna does (for instance one pointing North) it can be assumed that the signal of the drone is not coming from where the packet information is indicating the drone is and thus is spoofed. Drawings The present invention is illustrated by means of the following drawings in which like features are designated with like numerals. The figures provide figure 1 shows an aerial arrangement suitable for use in the directional sensor the present invention; figure 2 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cross; figure 3 shows a schematic representation of the aerial arrangement of figures 1 and 2; figure 4 shows a schematic of a spoofing attempt by a drone upon a directional sensor; figure 5 shows a schematic of the effect of planar shielding from figure 2 in the context of the schematic of figure 4; figure 6 shows a schematic of the effect of a selective planar shielding from figure 2 in the context of the schematic of figure 4; figure 7 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cross with a central cylindrical aperture; figure 8 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cross with a central rectangular channel; figure 9 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of rectangular channel with faces planar to the aerials; figure 10 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cylindrical member; figurel 1 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a rectangular channel with faces at 45° to the basis of the aerials. The features of the drawings are listed as follows: 100 Directional sensor assembly 110 Base 140 Arial 142 Shielded Arial 150 Arial Element 160 Drone 170 Spoofed Drone Position 180 Schematic 200 Cross Cylinder Shield 210 Cross Cylinder wing 215 Cross Cylinder core 220 Cross Cylinder aperture 230 Cross Cylinder join 300 Cross 310 Cross wing 310G Activated, Grounded, Cross wing 320 Cross join 330 Further Cross wing 330F Dea ctivated, Ungrounded, Floating Cross wing 400 Cross Tube Shield 410 Cross Tube wing 415 Cross Tube core 420 Cross Tube aperture 430 Cross Tube join 500 Cuboid Shield 515 Cuboid core 520 Cuboid aperture 530 Cuboid join 600 Cylindrical Shield 615 Cylindrical core 620 Cylindrical aperture 700 Offset Cuboid Shield 715 Offset Cuboid core 720 Offset Cuboid aperture 730 Offset Cuboid core join 240, 340, 440, 540, 640, 740 Aerial 250, 350, 450, 550, 650, 750 Aerial Elements The same numbers identify the same features even if depicted differently. Detailed description Figure 1 shows an aerial arrangement suitable for use in the directional sensor 100 of the present invention. The sensor comprises a base 110 which is a printed circuit board upon which circuitry for providing control, analysis and communication of the device is mounted. In the present figure for aerials are shown, with the angles to the vertical mentioned below. Each aerial is a multi-element log periodic dipole array and is electrically connected to the base 110 were amplification and signal conditioning is located. The aerials are located such that the planes do not overlap, thus most efficiently harvesting electromagnetic signals in the space available. The array for aerials as particular beneficial as whilst high resolution can be obtained using more aerials it has been found that in practice, for aerials provide a good compromise between the angular resolution of signal direction and space utilisation. Centrally on the circuit board there is mounted a GPS sensor for configured to determine the location of the directional sensor. Whilst in most applications the directional sensor will be fixed in a given position the ongoing obtaining of GPS signals and the evaluation of those signals to determine a geographic location is beneficial. In particular, by obtaining, such as daily, a GP office position then, punitively, over time, an extremely accurate geographic position can be established by interpolating, such as by averaging, the various position readings. Hence, repeated GPS location acquisition for a fixed device is beneficial. In the first example in addition, what in the context of the overall invention, optionally separately the GPS sensor is configured to determine the location of the directional sensor on a frequent basis, such as hourly, and upon each determination to compare this with a reference position (such as the accurate position mentioned above) so is to determine the level of noise in the GPS environment. Specifically, the present invention is concerned with the question of spoofing, that is whether an aerial vehicle, or other source, is providing inaccurate information regarding position, as compared to an actual position of the source of the signal. By determining the level of noise in the GPS environment, such as determined and error between actual and determined GPS location of the directional sensor 100 this can be used to enhance the evaluation of spoofing. The example during electromagnetic disturbance in the atmosphere electrical storms, they can potentially affect GPS signals due to increased ionization in the ionosphere caused by lightning activity. This ionization can lead to signal delays and disruptions. Thus, to prevent erroneous determinations of spoofing the directional sensor changes a threat foothold for spoofing determination dependent upon system error, but error may be consolidated such as in a measure as indicated above, for example by means of a horizontal dilution of precision" (HDOP) value. Horizontal Dilution of Precision (HDOP) is a measure of the geometric quality of satellite positioning signals. It provides an indication of the accuracy of horizontal (latitude and longitude) positioning based on the distribution and arrangement of the satellites in view. HDOP is a dimensionless value that reflects the potential error introduced into a position calculation due to poor satellite geometry. HDOP in the present invention is calculated using the positions of visible satellites relative to the user's position. The formula for HDOP involves the positions of the satellites and their corresponding measurement uncertainties. It is preferably calculated as follows: HDOP = sqrt((o_xA2 + o_yA2) / o_zA2) Where: o_x: Standard deviation of satellite positions in the East-West direction. o_y: Standard deviation of satellite positions in the North-South direction. o_z: Standard deviation of satellite positions in the vertical direction. HDOP takes into account the spread of satellites in the sky relative to your position. A lower HDOP value indicates better satellite geometry and thus higher accuracy. In the present invention the threshold for alerting spoofing is adapted (such as in real time) based upon the HDOP of the directional sensor preferably according to the following classification HDOP <1: Excellent accuracy: All mismatches between purported and determined position of an aerial object being tracked differing by more than 1m are signalled as spoofing. HDOP between 1 and 2: Good accuracy : mismatches between purported and determined position of an aerial object being tracked differing by more than 4m are signalled as spoofing. HDOP between 2 and 5: Fair accuracy, mismatches between purported and determined position of an aerial object being tracked differing by more than 16m are signalled as spoofing. HDOP >5: Poor accuracy, mismatches between purported and determined position of an aerial object being tracked differing by more than 50m are signalled as spoofing. However, in practice with the present invention it is been found more reliable to compare the HDOP with a difference in reception angle (angle as in general and 60° points of the compass). Therefore the following scheme may be used HDOP <1: Excellent accuracy: All mismatches between purported and determined angle of signal arrival of an aerial object being tracked differing by more than 1m are signalled as spoofing. HDOP between 1 and 2: Good accuracy : mismatches between purported and determined angle of signal arrival of an aerial object being tracked differing by more than 4m are signalled as spoofing. HDOP between 2 and 5: Fair accuracy, mismatches between purported and determined angle of signal arrival of an aerial object being tracked differing by more than 16m are signalled as spoofing. HDOP >5: Poor accuracy, mismatches between purported and determined angle of signal arrival of an aerial object being tracked differing by more than 50m are signalled as spoofing. Whilst this means that resolution at distance may in principle be poorer resolution at close range, ranges which are often more pertinent for micro aerial vehicles are improved. Thus providing more reliable spoof detection. Whilst not shown in the diagrams, the base 110, or if not the based on the equipment as a whole, will comprise processing equipment, such as processor’s memory and communications equipment configured to carry out the tasks identified herein. The equipment is externally powered. A further advantage of the four antenna array design shown is that the 4 antenna design can be considered either ‘off centre’ or facing perpendicular to the side of the sensor they are placed upon. This provides a space efficient directional sensor and one which will also conveniently fit within the space limitations, such as the footprint allowed of a standardised mobile casing. Figure 2 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cross. In this figure, and in subsequent figures the base 110 is not depicted so as to aid clarity. The directional sensor here comprises the previously identified aerials 340 with antenna elements 350, in this instance four aerials, and placed between the aerials is electromagnetic shielding 300. The shielding 300 is in the form of an extruded cross, typically oriented in the vertical direction, here extruded in what is termed a principal elongate axis (axis for short). The axis being at an angle to the principal axis of the aerial elements are between 20 and 60°, preferably between 30 and 50° more preferably between 44 and 46°. As mentioned, the axis is typically vertically orientated and this provides an optimal angle for a small number of aerials to detect micro aerial vehicles. This is been found in practice since, unlike, conventional aerial vehicles flight at low altitudes, often at altitudes commensurate with the height positioning of the directional sensor, to be optimal. Whilst large number of aerials is certainly possible this is less efficient. Electromagnetic shielding 300 comprises planar sheets of material in the form of crosswings 310 converging at cross joint 320. In the present figure the electrolytic shielding is electrically connected at cross joint 320. This enables efficient space utilisation together with a shape of strong physical integrity. Electrolytic shielding is preferably supported at a lower edge of the electromagnetic shielding attached to the base 110, preferably by means of attachment to a printed circuit board forming the base 110. In general, shielding of antennas with ground planes in the present invention has been found to improve the RSSI reliability. figure 3 shows a schematic representation of the aerial arrangement of figures 1 and 2, this schematic is to be understood in conjunction with figures 1 (left-hand schematic) and figure 2 (right-hand schematic). These schematics are then used in the following figure. figure 4 shows a schematic of a spoofing attempt by a drone upon a directional sensor 100. Directional sensor 100 detects a radio signal from a drone 160, this for example being a micro aerial vehicle, north-west of the sensor. The drone provides an Automatic Dependent Surveillance-Broadcast (ADS-B) the information from which would normally correspond to information corresponding to its best-known position. However, in a spoofing attempt the ADS-B position corresponds to that of spoofed drone position 170 south-east of the sensor. In a first mode, by correlating the received information, the direction of the received information and extracting from these the relevant position the disparity between the two positions is used by the directional sensor to detect a spoofing attempt and for this then to be reported. The directional sensor of the present invention is also configurable in a further mode in which a purported position ADS-B does correspond to received signal direction but were the purported position is either further away or nearer than the sender location. This is detected by the sensor being configured to compare the movement in the purported position over time with either or both of: a) the signal strength. Whilst the absolute value of signal strength is unknown and therefore is not itself a marker of distance the diminution of signal strength with changing distance is known and in the present invention the correlation of expecting signal strength change with distance to the actual signal / change with distances can paired and where a mismatch is determined then a spoofing attempt is reported. b) the correspondence of the angle change with purported change in distance as compared to typical speed parameters for the type of vehicle. For example, the present invention may be configured that detecting a micro aerial vehicle that a speed between one and 20 m / s is to be expected. However, a micro aerial vehicle changing emitted signal angle of 3° over a period of one minute can be no further than 1200 m away or the present invention will report a spoofing attempt. In a further third configuration of this mode both methods a) and b) are combined in the determination of a spoofing attempt, that is expected changes signal compared to expected movement velocity are compared to preconfigured values for a micro aerial vehicle and a change outside an expected range is reported as a spoofing attempt. This is particularly useful in that a purported location which changes by 3° over the period of one minute but for which there is a signal increase of 2% in that time indicates that even faster movement must be present as the micro aerial vehicle is moving towards the observer and as such the directional sense of the present invention reports a spoofing attempt with a higher confidence level. figure 5 shows a schematic of the effect of planar shielding from figure 2 in the context of the schematic of figure 4. The manner in which directional signal information may be obtained by a directional sensor is known in the art. Signals are determined by a plurality of aerials and the signal strength determined by an aerial can be correlated to an angle of incidence. However, system sensitivity may be increased by shielding signals from other directions and in this respect in the present invention the directional sensor 100 may further comprise, in addition to aerials 140 signal attenuation means 310. This acts by shielding some of the plurality of aerials 140 S from an incoming signal so as to unambiguously determine signal direction. The signal attenuation means preferably provides no line of sight between adjacent aerials and therefore provides improved selectivity to an incoming signal as a function of direction. However, a signal difference as shown in figure 4 is still possible to be spoofed within the error margins of a small antenna array as the spoofed position could be determined to have arisen from its purported direction provided at the alternative direction is on a plane of symmetry of the incoming signal and 180° away from the signal source. This can be overcome by the use of signal attenuation means of the present invention were two of the four aerials 140 can receive the signal and to the aerials 140 S are shielded from the signal. This provides a more accurate and efficient directional sensor having a limited number of aerials, such as a number of aerials from 3 to 6, preferably 4 aerials. figure 6 shows a schematic of the effect of a selective planar shielding from figure 2 in the context of the schematic of figure 4. The directional signal sensor in the present invention may be further improved by the use of a selectively activated signal attenuation means. In this figure a configuration of the present invention is provided in which the extruded across 300 attenuation means has two of the wings 310 G selectively activated, for example by grounding and two of the wings 330 after the activated, for example I have been floating relative to ground. This enables selective combination of aerials to be presented to an incoming signal for the optimisation and checking of an incoming signal and angle. The present invention is therefore configured to attenuation means so as to present different combinations of aerials to an incoming signal and thereby selectively detect that signal and specifically excluding potential incoming directions of the signal from the required determination and thereby increasing the accuracy as limits to the range of incoming signal detected are therefore achieved. figure 7 shows the aerial arrangement of figure 1 further comprising a planar shielding member 200 in the form of a cross with a central cylindrical aperture. In this feature of the present invention a central aperture is presented such that the signal attention means, being selective or otherwise, shields the antenna such as to 40 so that the antenna elements 250 are shielded from incoming signal. The signal attenuation means 200 comprises wings to turn akin to those in the extruded cross 300 with the further addition of cylinder core to 15 enclosing aperture 220. This configuration has the advantage that a GPS tracking device can be located within aperture 220 yet shielded from lateral incoming signals, such as signals which may be designed to disrupt the incoming signal. The height of the signal attenuation means may be varied along with the depth of the GPS receiver within the aperture such that a suitable portion of the sky maybe available for signal reception. This has the advantage that without any modification, such as in processing or programming only GPS signals with low delusional precision , in particular such as preferably HDOP (Horizontal Dilution of Precision), and VDOP (Vertical Dilution of Precision), which can affect positioning accuracy can be minimised. Whilst restricting the angle of view of GPS signals is usually considered disadvantageous, as mentioned, directional centres of the present invention are typically secured, or are secured, in a fixed location and therefore time determination of GPS position is not a factor where hours accuracy is. Hence, this modification of the present invention enables the most accurate GPS signals to be achieved by restricting the incoming GPS satellite signals which may be used in the GPS determination. Hence, what is normally a negative or undesirable feature for a GPS signal is here accentuated for benefit. While GPS receivers are capable of receiving signals at low angles close to the horizon, this might result in decreased accuracy and potential signal blockage due to obstacles like buildings, trees, and terrain. This is avoided by the present invention. figure 8 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cross with a central rectangular channel. This arrangement of the present invention is further answered by dangerous over that shown in figure 7, which it otherwise shows the aforementioned advantages in that shielding of the aerials one from another, achieved by the signal attenuation means is maintained but a larger court for 154 the shield 400 with crosswings for 10 is obtained because the core 415 for a given distance away from an aerial encompasses a larger area and therefore permits a greater GPS signal providing a better compromise, for a four aerial array with GPS to both encompass shielding and central GPS access. For both the arrangements of figure 7 and 8 century placing the GPS gives the highest potential for accuracy relative to the position of the aerials. The GPS detector may for example be located on a printed circuit board forming the base 110 of the detector 100. In summary, the use of the shielded signal attenuation means of the present invention provides for GPS that satellite signals received at higher angles (closer to overhead) which pass through less of the Earth's atmosphere, reducing the potential for signal degradation due to atmospheric effects such as ionospheric delay and multipath interference are preferentially received. As the angle decreases and the signals approach the horizon, they pass through a thicker portion of the atmosphere, potentially encountering more atmospheric disturbances that could affect accuracy and these are shielded from reception. Whilst reducing GPS satellite signals may be counterintuitive it enables pre-existing pre-configured sensors which are normally configured to solely establish a reading when sufficient satellites are detected to be used more effectively by shielding it from the more ineffective readings. Figure 9 shows the aerial arrangement of figure 1 further comprising a planar shielding member 500 in the form of rectangular channel with faces 515 planar to the aerials. This arrangement of the shielding means of the present invention is advantageous in situations where the area accessible to receive signals by the GPS antenna must be maximised due to the large area of aperture 520 in comparison to the aerial encompassed by the aerials 540 with elements 550. This arrangement Which is advantageous particularly in built-up areas, such as with high-rise buildings surrounding the directional sensor whilst the shielding of the aerials one from another so as to improve directional accuracy is still maintained. Figure 10 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a cylindrical member. This arrangement of the present invention in which the shielding member 600 comprises a cylindrical core 615 surrounding a aperture 620 gives less efficient aerial to aerial shielding but is direction agnostic and therefore enables more efficient mounting of the directional sensor and is in particular useful where the directional sensor is mobile. Figure11 shows the aerial arrangement of figure 1 further comprising a planar shielding member in the form of a rectangular channel with faces at 45° to the basis of the aerials. This arrangement of the present invention is particularly useful in areas with low signal intensity, such as in more rural areas where micro-aerial vehicles may need to be detected over a wider area and the faces of the attenuation means 700 in the form of a tubular cuboid core 715 enclosing aperture 720. Shielding between aerials is less effective but this provides maximal overall signal reception area for a given real estate area of signal reception whilst enabling potential reflection from faces 715 back to the aerials. A further function of the attenuation mean is to help isolate noise generated from the internal components of the sensor (PSU, buck converters, signal generators).

Claims

1. A directional sensor configured to receive and evaluate location signals from aerial vehicles relative to the sensor, the sensor comprising:a plurality of aerials, means for receiving the location signals received by the aerials, the signals being processed by a processor, the processor being configured to:a) determine for a given input signal from an aerial vehicle, a differential of a signal parameter of the input signal between 2 or more of the aerials;b) operate on the differential signal parameter to determine a position of the aerial vehicle relative to the aerials by means of a correlation;c) compare the determined position relative to the aerials with a reported position of the aerial vehicle;d) determine a difference between the reported and determined position and compare this to an equivalence criteria;e) upon carrying out the determination where the difference does not match the equivalence criteria, to report a spoofing attempt;wherein a signal attenuation means, in addition to air space, is present between two or more of the one or more aerials;wherein the plurality of aerials is 4 aerials mounted in a rectangular array; andwherein a cruciform signal attenuation means is present between the aerials.

2. The directional sensor of claim 1 wherein the differential of the signal parameter is selected from one or more of:a) reception signal strength;b) time of arrival of the signal at the sensor;c) phase angle of the signal;3. The directional sip sensor of claim 1 or claim 2 wherein correlation of the differential signal parameter determines one or more of:a) a distance of the signal source from the aerial;b) a geographic location of the signal source;c) a height of the signal source from the aeriald) location in 3 dimensions of the signal source from the aerial.out the4. The directional sensor of claim 1 wherein a signal attenuation means, is a metal ground plane.

5. The directional sensor of claim 1 wherein a signal attenuation means, is a ferrite sheet or a conductive plastics sheet.

6. The directional sensor of claims 1, 4 and 5 were in the signal attenuation means provides no line of sight between adjacent aerials.

7. The directional sensor of any preceding claim wherein the aerials are in a known and fixed proximity to a GPS sensor which is in communication with the processor for providing a location of the aerials to the processor.

8. The directional sensor of claim / wherein the GPS sensor is located between two or more of the aerials.

9. The directional sensor of claim 8 wherein the signal attenuation means comprises an aperture for reception of radio signals by the GPS sensor.

10. The directional sensor of claim 8 wherein the aperture is perpendicular to vertical faces of the signal attenuation means.04 02 25

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