SYSTEM AND METHOD TO PROTECT THE PRIVACY OF ADS-B MESSAGES

Abstract

System and method to protect the privacy of ADS-B messages transmitted by aircraft. The system includes one or more ground stations with a ground station control unit and a ground ADS-B transponder for receiving an ADS-B message. The ground station control unit includes an aircraft position determination module for retrieving an aircraft position included in the ADS-B message; an operating conditions module for determining the fulfillment of operating conditions including determining if the aircraft position is an actual aircraft position; and a fake aircraft position generator for computing one or more fake aircraft positions. The ground station control unit broadcasts one or more fake ADS-B messages including the fake aircraft positions if the operating conditions are met. With this system only trusted receivers can obtain the real position of the aircraft.

Claims

1. A system for protecting the privacy of automatic dependent surveillance-broadcast (ADS-B) messages, the system comprising at least one ground station comprising: a ground ADS-B transponder configured for receiving an ADS-B message; a ground station control unit comprising: an aircraft position determination module for retrieving an aircraft position included in the ADS-B message; an operating conditions module for determining the fulfillment of one or more operating conditions, said operating conditions at least including determining if the aircraft position is an actual aircraft position; and a fake aircraft position generator for computing one or more fake aircraft positions; wherein the ground station control unit is configured for broadcasting one or more fake ADS-B messages including the fake aircraft positions if the operating conditions are met.

2. The system of claim 1, wherein the operating conditions further includes determining if the aircraft position is located inside a region of interest.

3. The system of claim 1, wherein the fake aircraft position generator is configured for computing the fake aircraft positions by using a transformation function.

4. The system of claim 3, wherein the operating conditions module is configured for determining if the aircraft position is an actual aircraft position by using the transformation function, the transformation function being such that: when applied to an actual aircraft position the transformation function generates the fake aircraft positions; when applied to a fake aircraft position the transformation function generates at least the same fake aircraft position.

5. The system of claim 3, wherein the transformation function is a geometric projective transformation over a surface.

6. The system of claim 5, wherein the operating conditions further includes determining if the aircraft position is located inside a region of interest, and wherein the region of interest is defined by the surface used in the geometric projective transformation.

7. The system of claim 5, wherein the geometric projective transformation is a projection over a conical surface.

8. The system of claim 5, wherein the geometric projective transformation is a projection over a convex surface.

9. A method of protecting the privacy of ADS-B messages, comprising: receiving an ADS-B message; retrieving an aircraft position included in the ADS-B message; determining the fulfillment of one or more operating conditions, said operating conditions at least including determining if the aircraft position is an actual aircraft position; if the operating conditions are met: generating one or more fake aircraft positions; and broadcasting one or more fake ADS-B messages including the fake aircraft positions.

10. The method of claim 9, wherein the operating conditions further include determining if the aircraft position is located inside a region of interest.

11. The method of claim 9, wherein the generation of fake aircraft positions is performed using a transformation function.

12. The method of claim 11, wherein the step of determining if the aircraft position is an actual aircraft position comprises using the transformation function such that: when applied to an actual aircraft position the transformation function generates the fake aircraft positions; when applied to a fake aircraft position the transformation function generates at least the same fake aircraft position.

13. The method of claim 11, wherein the transformation function is a geometric projective transformation over a surface.

14. The method of claim 13, wherein the geometric projective transformation is a projection over a conical surface.

15. The method of claim 13, wherein the geometric projective transformation is a projection over a convex surface.

16. A system for protecting the privacy of automatic dependent surveillance-broadcast (ADS-B) messages, the system comprising: an aircraft position determination module for retrieving an aircraft position included in an ADS-B message; an operating conditions module for determining the fulfillment of one or more operating conditions, said operating conditions at least including determining if the aircraft position is an actual aircraft position; a fake aircraft position generator for computing one or more fake aircraft positions; and a transponder for broadcasting one or more fake ADS-B messages including the fake aircraft positions if the operating conditions are met.

17. The system of claim 16, wherein the operating conditions further include determining if the aircraft position is located inside a region of interest.

18. The system of claim 16, wherein the fake aircraft position generator is configured for computing the fake aircraft positions by using a transformation function.

19. The system of claim 18, wherein the transformation function is a geometric projective transformation over a surface.

20. The system of claim 19, wherein the surface is a conical surface or a convex surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] A series of drawings which aid in better understanding the invention and which are expressly related with an embodiment of said invention, presented as a non-limiting example thereof, are very briefly described below.

[0026] FIG. 1 depicts a schematic diagram of a system for protecting the privacy of ADS-B messages transmitted by an aircraft.

[0027] FIG. 2 represents a flow diagram of the ADS-B obfuscation algorithm.

[0028] FIG. 3 illustrates the process of obtaining the geometric transformation data used by the ADS-B obfuscation algorithm.

[0029] FIGS. 4A-4C represent a conic mirroring process for obtaining the fake aircraft positions. FIG. 4D represents a convex mirroring process for obtaining the fake aircraft positions.

[0030] FIG. 5 depicts the application of a transformation function to actual aircraft positions.

[0031] FIG. 6 shows a diagram of the obfuscation method in which a geometric projective transformation is used to obtain fake aircraft positions.

[0032] FIG. 7 represents a system comprising two different ground stations near a runway.

[0033] FIG. 8 shows the application of the transformation function to differentiate the actual aircraft position from the fake positions.

DETAILED DESCRIPTION

[0034] FIG. 1 represents a schematic diagram of an embodiment of a system for protecting the privacy of ADS-B messages transmitted by an aircraft. The system includes one or more ground stations 100. Each ground station 100 is a ground-based infrastructure comprising a ground ADS-B transponder 102 (ADS-B receiver and ADS-B emitter) coupled to a ground station control unit 104. The ground station 100 is preferably located close to an airport.

[0035] An aircraft 140 using ADS-B services periodically emits ADS-B messages 120 including, among other data, the aircraft identifier, current aircraft position 112 and aircraft speed. When the aircraft 140 is flying near the ground station 100, the ADS-B message 120 is received by the ground ADS-B transponder 102. Thereafter, an aircraft position determination module 106 of the ground station control unit 104 analyzes the ADS-B message 120, retrieving the aircraft position 112 and determining whether this aircraft position 112 is located inside a region of interest or not. The region of interest is a first filter used by the ground station control unit 104 to determine if fake ADS-B messages are to be transmitted. As it will later be explained, additional filters or conditions may be used.

[0036] An operating conditions module 108 included in the ground station control unit 104 is responsible for determining if one or more operating conditions 114 are met. The operating conditions 114 at least include the condition of the aircraft position 112 being located inside the region of interest. If the operating conditions 114 are met, a fake aircraft position generator 110 computes at least one fake aircraft position 116 and the ground station control unit 104 broadcast, using the ground ADS-B transponder 102, at least one ADS-B message 130 including the fake aircraft positions 116.

[0037] FIG. 2 depicts a flow diagram showing the steps of a method of protecting the privacy of ADS-B messages according to a possible embodiment, implemented by the ground station control unit 104. An ADS-B message 120 is received and decoded 202 to obtain the aircraft position included in the message, taking into account the ADS-B position messages types 204 previously stored. In 206 a check to obtain a position of the aircraft (including the latitude, longitude and altitude of the aircraft) is performed. If no position can be retrieved, no further action is taken 220. If a position is obtained, said position is compared 208 with a region of interest, considering the geographic coordinates of the region of interest 210 previously stored.

[0038] If the aircraft position is inside the region of interest 212, the received ADS-B message is compared 214 with a plurality of fake messages stored in a fake messages database 216. Otherwise, no further action is taken 220.

[0039] If the received ADS-message is a fake message 218, a computation 221 to obtain one or more fake aircraft positions is performed. Otherwise, no further action is taken 220.

[0040] According to the embodiment shown in FIG. 2, the computation 221 includes a geometric projective transformation taking into account some geometric transformation data 222 previously obtained or stored in a memory or a database.

[0041] Once a fake aircraft position is computed 221, a fake ADS-B position message is generated 224 and broadcast 226. If several fake aircraft positions are obtained, the same number of ADS-B messages including said fake aircraft positions are generated and broadcast. When the last fake ADS-B message is broadcast, the system keeps waiting 228 for the next ADS-B message to be received.

[0042] As shown in FIG. 2, geometric transformation data 222 can be employed for the calculation of fake aircraft positions. FIG. 3 shows an obfuscation process of obtaining the geometric transformation data 222 according to an embodiment. First, a setup 300 of the obfuscation process is established. In 302 the type of geometric projective transformation to apply is selected, such as convex mirroring or conic mirroring. Several relevant data required for the selected geometric projective transformation is retrieved from a memory or repository 304. The relevant data may include, for instance, the geographical parameters of the region of interest, maps, charts, etc.

[0043] A set of geometric transformation parameters are then generated 306 and distributed 308 to one or more trusted receivers, including the ground station control unit 104. The geometric transformation parameters are stored in a repository 310. By accessing the repository 310, the geometric transformation data 222 is retrieved to calculate fake aircraft positions.

[0044] FIGS. 4A and 4B represent an example of a geometric projective transformation of an aircraft position using conic mirroring or conical projection on a conical/lateral surface 410 of a cone 406 aligned with a runway 404, the vertex 412 of the cone in the aiming point area of the runway 404, the cone 406 with a length such that it covers the region of interest.

[0045] The schematic view of an aircraft 140 approaching a ground station 100 is depicted in FIG. 4A, the ground station 100 being located close to a runway 404. For every actual aircraft position 400 inside the cone 406 (which can be considered the region of interest), a fake aircraft position 116 is computed as the intersection of a line 402 perpendicular to the cone axis 408 with the lateral surface 410 of the cone. The vertex 412 of the cone may coincide with the location of the ground station 100, although the ground station 100 may be positioned at a different location.

[0046] FIG. 4B depicts a projected view of FIG. 4A, showing the intersection E of the orthogonal line 402 with axis 408, the line segment D from the vertex 412 to the actual aircraft position 400, the line segment D′ from the vertex 412 to the intersection E and the angle α between line segment D and axis 408. As shown in FIG. 4B, the line 402 can also be extended on both sides to intersect the lateral surfaces 410 of the cone at two different points, obtaining two fake aircraft position 116.

[0047] FIG. 4C represents a flow diagram of the conic mirroring process of FIGS. 4A and 4B. Using the actual aircraft position 400 (latitude, longitude, altitude), the fake aircraft position generator 110: [0048] Calculates 420 the distance from the actual aircraft position 400 to the cone vertex 412 (length of line segment D), using the established cone vertex coordinates 422. [0049] Calculates 424 the angle α between segment D and cone axis 408, using the known cone axis direction 425. [0050] Calculates 426 the distance from vertex 412 to intersection E (length of line segment D′) and computes the coordinates of intersection E. [0051] Calculates 428 the intersection of orthogonal line 402 with conical surface 410 using the cone base 430, obtaining the fake aircraft position 116.

[0052] FIG. 4D represents another example of a geometric projective transformation, in particular a convex mirroring process for obtaining the fake aircraft positions 116. A convex surface 440 (hyperbolic surface, in the example of FIG. 4D) aligned with the runway 404 is defined. The focus of the hyperbola is in the aiming point area of the runway 404, and the length of the hyperbola is such the area of interest is covered. For every actual aircraft position 400 inside the hyperbolic volume 436, a fake aircraft position 116 is calculated as the intersection of a line 432 perpendicular to the hyperbola transverse axis 438 with the hyperbolic surface 440.

[0053] The fake aircraft position generator 110 calculates, for each actual aircraft position 400, one or more fake aircraft positions 116 using a transformation function, a projective transformation that maps lines to lines (but not necessarily preserving parallelism). FIG. 5 depicts an example of the application of a transformation function 500 to a plurality of actual aircraft positions 400 defining an actual flight path 502. The output of the transformation function 500 is a set of fake flight paths (504a, 504b, 504c), each fake flight path being composed of a plurality of fake aircraft positions. In this depicted case the transformation function 500 converts the input, actual aircraft positions 400, into three different outputs, fake aircraft positions (116a, 116b, 116c). The transformation function 500 may convert each actual aircraft positions 400 into any number of fake aircraft positions (one or more).

[0054] In an embodiment, the transformation function 500 employed is a geometric projective transformation. For example, geometric projective transformations employed may be projections over a conical surface 410 (FIG. 4A), a convex surface 440 (FIG. 4D) or a frustum surface. The position and orientation of these surfaces may vary, although they are preferably with a relative position to the runway 404 such that the nearer the original points (actual aircraft positions 400) are to the runway 404 the more similar are the transformed points (fake aircraft positions 116) to the original ones. The surfaces of the geometric projective transformations may also be used to define the region of interest; in this case, the fake aircraft positions obtained by the geometric projective transformations define flight trajectories inside the region of interest.

[0055] The geometric projective transformation is a projection over a surface (a conical surface, a convex surface such as a hyperbolic, a frustum surface, etc.). The selected geometric projective transformation has the following properties: when applied to an actual aircraft position 400, the output is one or more fake aircraft positions 116, and when applied to a fake aircraft position 116, one of the outputs is the fake aircraft position 116 itself.

[0056] In an embodiment shown in FIG. 6, the transformation function 500 applied by the fake aircraft position generator 110 is a geometric projective transformation, such as a conic mirroring function or a convex mirroring function, although other geometric projections over different surfaces aligned with the runway can be employed, as long as they comply with the same properties. The geometric projective transformation 500 employs the geometric parameters of the region of interest 520 and the selected type of geometric projective transformation 510 (e.g. conic mirroring 512, convex mirroring 514, Frustum mirroring 516) to generate the required geometric transformation parameters. With these parameters the geometric projective transformation 500 is completely defined.

[0057] The system may comprise a plurality of ground stations 100 for protecting the privacy of ADS-B messages transmitted by an aircraft 140. FIG. 7 shows an overall view of the system comprising two different ground stations (100a, 100b) located near a runway 404. A region of interest 700 is defined by a close curve 702 surrounding the runway 404 and an altitude interval (e.g. 0-400 m).

[0058] An aircraft 140 equipped with “ADS-B Out” technology periodically broadcasts its position using ADS-B messages 120. In FIG. 7 a first ground station 100a is close enough to the aircraft 140 to receive the aircraft position 112 included in ADS-B messages 120. The first ground station 100a verifies that the aircraft 140 is located inside the region of interest 700 (in the example shown in FIG. 7, latitude and longitude defining a position within the area defined by the close curve 702, and altitude between 0 and 400 m). When the received aircraft position 112 included in an ADS-B message is outside the region of interest 700, the ground stations 100 do not further process the ADS-B message.

[0059] If the received aircraft position 112 is located inside the region of interest 700, the first ground station 100a additionally verifies that said aircraft position 112 is an actual aircraft position 400, and not a fake aircraft position 116 previously generated by another ground station. In that case, the first ground station 100a employs a transformation function 500 to derive from that aircraft position a number of alternative fake positions 116.

[0060] For each actual aircraft position 400 received, the first ground station 100a generates and broadcasts an array of fake ADS-B messages 130, each fake ADS-B message 130 incorporating a different fake aircraft position 116. All ground stations receiving ADS-messages (120,130) process them to broadcast fake aircraft positions if the operating conditions 114 are met. This way, when the aircraft 140 is close enough to a second ground station 100b, the latter receives the actual aircraft position 400 and broadcasts fake aircraft positions 116. If the second ground station 100b receives fake ADS-B messages 130 broadcast by the first ground station 100a, the second ground station 100b is aware that they contain fake aircraft positions 116 and do nothing.

[0061] FIG. 7 also shows several trusted receivers 704 in the vicinity of the runway 404. Trusted receivers 704 receive both the actual aircraft position 400 transmitted from the aircraft 140 and the aircraft fake positions 116 broadcast by the first ground station 100a. As shown in FIG. 8, the trusted receivers 704 use the same transformation function 500 (i.e. the transformation function previously used to calculate the fake aircraft positions 116) to differentiate the actual aircraft position 400 from the fake aircraft positions 116.

[0062] The key parameters to apply the transformation function 500 is shared among trusted receivers 704 and ground stations (100a, 100b), for instance using a secure wired network 706. This way the trusted receivers 704 can differentiate between fake ADS-B messages 130 and actual ADS-B messages 120. Similarly, the ground stations (100a, 100b) also use the transformation function 500.

[0063] On the contrary, untrusted receivers 708 receive both actual and fake aircraft positions, but cannot differentiate them since they are not aware of the transformation function 500 to apply. The untrusted receiver 708 receive ADS-B messages including the position of the aircraft 140, but they are not consistent since the aircraft 140 seems to follow several different paths at the same time. Therefore, untrusted receivers 708 cannot distinguish the actual position of the aircraft 140.

[0064] FIG. 8 depicts the application of a transformation function 500 to an array of aircraft positions 802 received during a short time interval by a trusted receiver 704 or a ground station (100a, 100b). In particular, the array of aircraft positions contains three different positions: position A, which corresponds to the actual aircraft position, and positions B and C, which are fake aircraft positions. To determine which position is the actual aircraft position 400, the transformation function 500 previously used to calculate the fake aircraft positions 116 is now applied.

[0065] When applying the transformation function 500 to position A, the other two positions B and C are obtained, which determines that position A is an actual aircraft position 400. However, when applying the transformation function 500 to position B, the position B itself is obtained along with another position (position X), which determines that position B is a fake aircraft position 116. Similarly, when applying the transformation function 500 to position C, the position C itself is obtained along with another position (position Y), which determines that position C is also a fake aircraft position 116. Therefore, the transformation function 500 applied to a fake aircraft position does not generate the other aircraft positions; however, the transformation function 500 applied to an actual aircraft position 400 generates all the other fake aircraft positions 116. This way the receiver can decide which ADS-B message is real and which is fake. In other words:

[0066] When applying the transformation function 500 to an actual aircraft position 400, the other fake aircraft positions 116 are obtained.

[0067] When a transformation function 500 is applied to a fake aircraft position 116, at least the same fake aircraft position 116 is obtained.