A method for detecting satellite signal spoofing and a global navigation satellite system (GNSS)
Patent Information
- Application Number
- PCT/EP2026/060357
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-04-14
- Filing Date
- 2026-04-09
- Publication Date
- 2026-10-22
Smart Images
Figure EP2026060357_22102026_PF_FP_ABST
Abstract
Description
[0001] A method for detecting satellite signal spoofing and a Global Navigation Satellite System (GNSS)
[0002] FIELD OF THE DISCLOSURE
[0003] The present disclosure relates to a method and system for detecting satellite signal spoofing within a Global Navigation Satellite System (GNSS). Specifically, the disclosure involves utilizing Low Earth Orbit (LEO) satellites equipped with an antenna and GNSS receivers to monitor, track, and analyze satellite signals for enhancing the accuracy of signal source location determination and spoofing detection.
[0004] BACKGROUND OF THE INVENTION
[0005] A Low Earth Orbit (LEO) requires the lowest amount of energy for a satellite placement. Further, it provides high bandwidth and low communication latency, satellites and space stations in LEO are more accessible for crew and servicing and need less powerful amplifiers for successful signal transmissions to earth. Therefore, a majority of artificial satellites are placed in LEO. LEO is used for example for many communication applications, such as the Iridium telecom satellites (orbiting at about 780 km) and Earth observation satellites or Earth imaging satellites. Also satellite internet constellations such as Starlink are using satellites in LEO. Therefore, a high number of LEO satellites is available for signal transmission from and to the Earth.
[0006] Normally, each (LEO) satellite is equipped with at least one antenna for receiving and sending signals. Further, each (LEO) satellite normally has a signal receiver like a GNSS signal receiver, onboard.
[0007] In the field of satellite navigation and positioning, it is common to rely on signals from Global Navigation Satellite Systems (GNSS) to determine precise locations on Earth or in LEO. These systems, which include GPS, GLONASS, Galileo, and BeiDou, utilize a constellation of satellites that transmit signals to receivers on earth. The satellites of a GNSS normally are orbiting in Mid Earth Orbit (MEO), which is between 2.000 and 35.786 km altitude. GNSS satellites of the Galileo system for example are orbiting at 23.222 km altitude. The receivers process these signals to calculate their position, velocity, and time (PVT). However, the integrity and reliability of GNSSsignals are critical for applications ranging from personal navigation to military operations and aviation safety. Known systems typically involve ground-based receivers that track signals from multiple GNSS satellites to ensure accurate positioning. Despite the robustness of these systems, they remain vulnerable to various threats, including signal interference and spoofing.
[0008] Spoofing refers to the deliberate transmission of false GNSS signals by transmitting antennas (spoofers) located on earth with the intent that a (victim) receiver is accepting the received signal as an authentic signal and to deceive the receiver into calculating incorrect positions and / or a clock offset. This can lead to significant disruptions in navigation and timing applications, posing risks to safety and security. Spoofing attacks can be sophisticated, mimicking legitimate satellite signals to mislead receivers. According to known methods, detecting spoofing often involves monitoring signal characteristics such as power levels, carrier-to-noise ratios, Doppler shifts and signal timing. However, these approaches can be limited in their effectiveness, especially when spoofing signals closely resemble genuine ones. Additionally, ground-based systems may have limited coverage and may struggle to detect spoofing across large geographical areas.
[0009] Despite the substantial advances in the field of GNSS signal monitoring and spoofing detection, there remains a need for improved methods that are reliable and cheap and do not need additional hardware components to be implemented in existing GNSS. It is therefore a technical problem underlying the present invention to provide a method for detecting satellite signal spoofing that is cheap and reliable and is providing actual locations of spoofers on earth.
[0010] SUMMARY OF THE DISCLOSURE
[0011] It is an object of this invention to provide methods that reliably and cheaply can locate spoofers on earth.
[0012] The following key terms are used in the disclosure of the invention:
[0013] - GNSS (Global Navigation Satellite System): A system that provides geolocation and time information to a GNSS receiver anywhere on Earth.
[0014] - GNSS satellites: satellites that are sending true (authentic) GNSS signals;
[0015] - GNSS receiver: A device that receives signals from GNSS satellites.
[0016] - MEO (Mid Earth Orbit) satellite; satellites that are orbiting the earth at an altitude of around 20.000 km - 30.000 km; GNSS satellites normally are in MEO.- LEO (Low Earth Orbit) satellite: A satellite that orbits the Earth at an altitude of less than 2000 km, as for example at 500 km. A low Earth orbit (LEO) satellite has an orbit around Earth with a period of 128 minutes or less;
[0017] - Observable: Measurements such as radial distance, signal power, and Doppler shifts, obtained from GNSS signals;.
[0018] - Track: A continuous measurement of an observable derived by a receiver from a GNSS signal while the receiver is in lock on the received signal.
[0019] - Threshold level: A predetermined value used to compare an observable with the predetermined value;
[0020] - Spoofer: A signal source (transmitter) located on earth which is transmitting false GNSS signals with the intent that a GNSS receiver of the false GNSS signal will accept it as an authentic signal to deceive the receiver into calculating incorrect position, velocity or time (PVT) data;
[0021] - ECEF (Earth-Centered Earth-Fixed Coordinates): The coordinates in an Earth-Centered Earth-Fixed (ECEF) system using Cartesian coordinates (X,Y,Z) to represent positions relative to the center of a reference ellipsoid.
[0022] A first aspect of the invention provides a method for detecting satellite signal spoofing, the method comprising the following steps:
[0023] a) Monitoring a plurality of signals of a Global Navigation Satellite System received from a plurality of GNSS signal sources, wherein the monitoring is conducted with at least one GNSS receiver which is coupled to at least one antenna located on at least one Low Earth Orbit satellite;
[0024] b) Tracking of at least one observable of the signals received by the GNSS receiver and obtaining a plurality of tracks from the plurality of signals, c) Determining a length of each track and Analyzing the length of the tracks by comparing the determined length of each track with a predetermined first threshold level,
[0025] d) Selecting each track having a length of the track which is lower than the first threshold level, to obtain at least one selected track or a plurality of selected tracks, e) Determining or Estimating of a position of the at least one LEO satellite or of positions of each LEO satellite of a plurality of LEO satellites which has received a selected track, to obtain a location of the signal source from which the signal of the selected tracks is received,f) Analyzing the selected tracks on a time-basis by a sliding window having a preselected time size below a second threshold level, and
[0026] g) Averaging the position of the at least one LEO satellite or of the position of each LEO satellite, which has received a selected track, within the sliding window, to improve the location of the signal source from which the signal of the selected tracks is received.
[0027] The method disclosed is for detecting satellite signal spoofing and enables to geolocate the source of spoofing signals on Earth. It is utilizing the already existing plurality of satellites in Low Earth Orbit (LEO), which all are normally equipped with an antenna and a GNSS receiver and thus are providing a proven opportunity to detect, classify and geolocate terrestrial GNSS interference. Emitter geolocation from LEO enables a worldwide coverage with a frequent refresh rate. Since LEO satellites have a high distance from terrestrial interference sources, they permit tracking of authentic GNSS signals transmitted by true (authentic) GNSS satellites and are thus able to capture precise time-tagged data from time-synchronized LEO-based receivers and a precise orbit determination, based on well-known orbit determination techniques including estimators and interpolators. The existing plurality of LEO satellites thus enables to get a worldwide overview on the location of terrestrial emitters, including GNSS spoofers.
[0028] To geolocate (potential) GNSS spoofers on earth, the method involves several steps to monitor, track, analyze, classify and determine the location of the source of real GNSS signals and false GNSS signals (spoofing signals) provided by spoofers and received by a GNSS receiver onboard of at least one LEO satellite.
[0029] The method starts by monitoring multiple signals from various GNSS signal sources, including real GNSS satellites and spoofers located on Earth, using at least one GNSS receiver, which is onboard of at least one LEO satellite and is connected to an antenna. The GNSS receiver onboard of the LEO satellite tracks observables from these signals and obtains multiple tracks. The length of each track is determined and compared to a predetermined first threshold level. Tracks shorter than this first threshold level are selected in step d of the method. The position of the LEO satellite that received the selected tracks is then determined or estimated to find the location of the signal source. The selected tracks are analyzed over time using a sliding window with a preselected time size below a second threshold level. The position of the at least one LEO satellite or the positions of allLEO satellites that received the selected tracks is averaged within the sliding window to improve the accuracy of the signal source location. Finally the number of false alarms may be reduced by applying a third threshold level (threshold 3) to the number of flagged GNSS satellites.
[0030] The advantages of the method include a cheap and reliable detection of spoofing activities by analyzing the locations of the signal sources from which the at least one LEO satellite is receiving GNSS signals. By tracking of at least one observable of the GNSS signals received by the LEO satellite, a plurality of tracks from the GNSS signals is obtained. Each obtained track has an individual length. The length of a track depends on the geometry between the location of the signal source from which the signal was transmitted and the receiving LEO receiver. In case the LEO satellite flies over an area where a spoofer transmits powerful GNSS spoofing signals from earth surface, these spoofing signals will typically be received by the LEO satellite only for a very short period of time during each orbit. This is due to the placement of the GNSS antenna and the receiver on board of the LEO satellite and the LEO satellite speed with respect to Earth (considering that a LEO satellite requires for example only 128 minutes to circle around the entire planet). In case the LEO satellite is receiving a true GNSS signal from a valid GNSS satellite, which is orbiting the Earth in MEO, the signals will be received by the LEO satellite for a much longer period of time during each orbit, due to the relative movement of the LEO satellite (in LEO) and the GNSS satellite (in MEO). Therefore, the length of signal tracks can be used to distinguish the sources of the signals received by the LEO satellite. Thus, signals which are providing short tracks may be attributed to signal sources located on earth (spoofers) and signals which are providing long tracks may be attributed to signal sources located in MEO (true GNSS satellites). By determining the length of the tracks and comparing the obtained length of each track with a predetermined threshold level (threshold 1), it is possible to select the signals which are likely coming from a spoofer located on earth. For this reason, in step d) of the method, each track having a length of the track which is lower than the first threshold level (threshold 1) is selected, to identify the tracks which are likely coming from a spoofer located on earth.
[0031] In the next step of the method (step e) the position of the at least one LEO satellite which has received the signal of at least one of the selected tracks is determined, in order to geolocate the source of the received signal which has provided the short track. In step e) of the method, a geolocation of the source of the signal, from which the short track has been detected and selected in step d) is determined. For all events selected in step d) of the method, e.g. for all short tracks having a track length lower than the predefined threshold level (threshold 1), the location of thedetection (e.g. the geolocation of the source of the signal) is determined by first determining or estimating the current position of the LEO satellite which has received the selected (short) track and subsequently determining the geolocation of the signal source on Earth, based on the determined (current) position of the LEO satellite at the time of receiving the short track signal. The geolocation of the signal source can be expressed by a ground projection of the current orbital position of the LEO satellite which has received the short track signal at the time of receiving the signal on Earth, in particular by locating the signal source in an ECEF (x-y-z coordinate system).
[0032] If more than one LEO satellite has received the selected short tracks at the same time, it is advantageous to average the determined geolocation of the source of these tracks, in order to improve the geolocation of the signal source on Earth by averaging over the multiple events received by the different LEO satellites at the same time. To make sure that the plurality of the LEO satellites have received from the same signal source at the same time, a sliding window technique is used in step f) of the method for selecting the different events on a time-scale basis. Therefore, the selected fracks are analyzed on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2). All selected tracks being within the timewindow between zero and the second threshold level (threshold 2) are selected as potential spoofing signals. Finally, in the last step (step g) of the method) the (slightly) different geolocations of the current orbital position of each of the LEO satellites, which has received a selected short frack signal being within the sliding window, are averaged, to improve the geolocation of the signal source from which the signal of the selected tracks is received. The averaged geolocation is then attributed to the location of a spoofer on earth.
[0033] In a preferred implementation of the method, the first threshold level (threshold 1) can be selected by a user and preferably is selected by the user to be 100 seconds or lower, with a preference for 50 seconds or lower, and in particular 40 seconds. This introduces a specific parameter for the first threshold level used in the method for detecting satellite signal spoofing and ensures that only those events are considered as a (potential) spoofing event, which produce signal tracks having a short frack length. The preferred selection of the first threshold level (threshold 1) is taking into account the time needed for a LEO satellite for a full orbit, which for example is 128 minutes, and the time in which a LEO satellite is seeing a true GNSS signal from a true GNSS satellite orbiting in MEO. Since true GNSS signals are seen by a LEO satellite for time periods which are normally much longer than several minutes, selecting the first threshold level (threshold 1) to be lower than 100 seconds ensures that most of the true events (e.g. signals received be the LEO satellite from trueGNSS satellites) are excluded from to be a (potential) spoofing signal. The lower the selected first threshold level (threshold 1) is, the more likely it will be that an event producing a short track is caused by a spoofer located on earth and not by a true GNSS signal. However, if the first threshold level (threshold 1) is selected to be tool low, no signals may remain having a track length above the set first threshold level (threshold 1). Therefore, a preferred range for setting the first threshold level (threshold 1) is to be between 1 second and 100 seconds and more preferable to be within 10 seconds and 50 seconds. An appropriate selection of the first threshold level can enhance the accuracy and reliability of the spoofing detection method.
[0034] In a preferred embodiment of the method, which can preferably be used if the at least one LEO satellite or the plurality of LEO satellites has received GNSS signals from more than one (unique) GNSS satellite (orbiting in MEO), possible false alarms can be reduced by determining the number of unique GNSS satellites from which GNSS signals are received. The determined number of impacted unique GNSS satellites is compared in this preferred embodiment with a predetermined third threshold level (threshold 3). Each of the signal sources, from which the signal of the selected short fracks has been received, is attributed to a spoofing source, if the determined number of impacted unique GNSS satellites is above the third threshold level (threshold 3). By this way, selected events (e.g. selected short track signals), obtained by performing steps a) to g) of the method, which are actually based on a true (e.g. a correct or valid) GNSS signal received from a true GNSS satellite, can be excluded from the selected events (e.g. the selected short track signals) obtained in step g). This enables to avoid a false alarm by the selected event, which in fact is not a signal received from a spoofer.
[0035] The third threshold level (threshold 3) can be set globally (i.e. the same value for any given location on Earth). Alternatively, the setting of the third threshold level (threshold 3) can depend on the Latitude and Longitude of the detected location (e.g. the detected geolocation of a spoofer).
[0036] The additional features of the preferred embodiment of the method add an additional layer of analysis to the method, considering that sometimes a short frack of the received signals may happen in a natural manner and is coming from a true GNSS satellite. Since the probability of several unique GNSS satellite experiencing the same effect in a natural manner at the same time is not very high (particularly if the latitude of the LEO projection on ground is not too high), those true signal events coming from a valid GNSS satellite can be filtered out by attributing them to the number of affected unique GNSS satellites. The more unique GNSS satellites are affected, the higher is theprobality that the selected short track signals are coming from a true GNSS satellite (and thus are valid GNSS signals) and not from a spoofer located on earth. The preferred embodiment of the methods is thus enabling the system to distinguish between legitimate signal sources and those that may be attempting to deceive or manipulate the GNSS system (spoofing sources). By attributing signal sources to spoofing sources based on the comparison of the affected number of unique GNSS satellites with the third threshold level, the method can effectively identify real threats and allows to take appropriate action to mitigate the risks associated with satellite signal spoofing.
[0037] In an implementation of the preferred embodiment of the method, the third threshold level is preselected and can be selected by a user. Preferably, the third threshold level (threshold 3) is an integer including zero. When the third threshold level (threshold 3) is selected to be zero, all events selected in steps a) to g) of the method are attributed to a spoofer (e.g. to come from a spoofing signal). Preferably the third threshold level is larger than 1 and particularly is between 2 and 5.
[0038] In a preferred implementation of the method, the analyzing of the selected tracks on a time-basis according to step f) of the method is conducted by a sliding window technique within a fixed time window. This implementation introduces a specific mechanism for analyzing the selected fracks on a time-basis using a sliding window technique within a fixed time window having a selectable time width. This technique is designed to ensure that all selected events occurred more or less at the same time, e.g. within the selected time window defined by the fixed time window. The fixed time window is preferably lower than 20 minutes and more preferably lower than 10 minutes. By setting these preferred time windows and by analyzing the selected tracks on a time-basis by a sliding window technique, the method ensures that only the selected tracks which have occurred within the fixed time window will be considered as (potential) spoofing signals and that all events outside of the fixed time window are excluded (and my be attributed to true GNSS signals). The sliding window technique allows for a continuous monitoring and averaging of the (current) positions of only those LEO satellites that have received the selected tracks (nearly) at the same time. This approach improves the detection of satellite signal spoofing by providing a more accurate determination of the geolocation of the signal source in the averaging step (step g) of the method), since only those events will be used for the averaging of the geolocation of the signal sources, which have been occurred (nearly) at the same time. The fixed time window mechanism thus brings an additional feature that enhances the spatial resolution of the geolocation, making the spoofing detection method more accurate.In a preferred implementation of the method, the following observables are obtained by at least one of the following measurements of the received signal or a combination thereof:
[0039] - a signal power measurement of the received signal,
[0040] - measurement of the ratio of a carrier power to a noise density of the received signal (C / NO),
[0041] - a carrier phase measurement of the received signal,
[0042] - a code phase measurement of the received signal,
[0043] - a pseudorange measurement of the received signal,
[0044] - a code pseudo-range measurement of the received signal or a carrier smoothing of code pseudoranges,
[0045] - a phase range measurement of the received signal,
[0046] - a doppler shift measurement of the received signal.
[0047] The disclosed method therefore offers a variety of different observables which can be measured by standard methods and data processing techniques. The preferred implementation provides specific mechanisms for obtaining at least one of or a plurality of the mentioned observables from the received signal, which is enhancing the method for detecting satellite signal spoofing. Further, the preferred implementation allows for a parallel measurement of a plurality of observables selected from the given list of observables. A parallel measurement of a plurality of different observables is improving the accuracy of the geolocation of the signal sources determined from the measured and selected tracks of the plurality of different observables. The accuracy of the geolocation of the signal sources can be improved by averaging over the determined locations form the selected tracks of the plurality of different observables.
[0048] Preferrably, abnormalities in the values of the observables can also be used to assist the method in identifying the authenticity of the received signals, improving the accuracy. For example, the following observables can be analyzed with regard to abnormalities:
[0049] Signal power measurement allows for the assessment of the strength of the received signal, which can be indicative of the source of the signal transmission. The measurement of the ratio of carrier power to noise density (C / NO) can provide insight into the quality of the signal, helping to identify any degradation that may suggest spoofing. Carrier phase measurement and code phase measurement may offer detailed information on the phase characteristics of the received signal, which may also be indicative for determining the authenticity of the signal source. Pseudorange measurement and code pseudo-range measurement or carrier smoothing of code pseudorangescontribute to the calculation of the distance between the LEO-based receiver onboard of the LEO satellite and the signal source, aiding in the verification of the location of the signal source. Phase range measurement further can refine the phase data of the received signals, and thus may enhance the distinguishing between true GNSS signals and spoofing signals. Doppler shift measurement captures the frequency changes in the received signal, which can also be used to distinguishing between true GNSS signals and spoofing signals. By leveraging these different and multiple measurements of observables, the method can perform a more thorough analysis of the received signals, thereby improving the detection of satellite signal spoofing.
[0050] In a further preferred implementation of the method, the method is providing a map or a globe showing a location of detected signal sources placed on Earth. This map or globe integrates the determined geolocation of detected spoofers into a coherent visual format which visually represents the location of spoofing signal sources placed on Earth, making the data more accessible and actionable for users. It can be used as a valuable tool to quickly identify and assess the positions of spoofing signal sources, facilitating better decision-making and situational awareness. For example, the map or globe can be used in the navigation of airplanes.
[0051] In a further preferred implementation of the method, the method is generating an alarm signal, which is indicating the detection of at least one spoofing signal. The alarm signal may be an electronic signal, an audible signal or a visual signal displayed on a screen or by a control lamp.
[0052] In a preferred implementation of the method, the method is carried out by a computer. In a further aspect, the disclosure refers to a computer program product comprising instructions which, when the program is executed by the computer, cause the computer to carry out the method. Preferably, the computer is placed on earth and receives the data for conducting the method by the LEO satellites.
[0053] The term "computer program product" refers to a software package that can be stored on a medium and executed by a computer. The instructions contained within the program are designed to implement the method, which is a series of steps or operations that the computer performs.
[0054] The computer program product may comprise a frontend and a backend section. The frontend section is implementing a graphical user interface (GUI), which in particular is displaying the map or globe provided by the method. The computer program product can provide a global coverageand timely detection of spoofing activities, which may be used for example for airline dispatch services that rely on accurate GNSS data for flight planning.
[0055] The frontend section may comprise an application programming interface (API) implemented to generate the alarm signal which is indicating the detection of at least one spoofing signal. The alarm signal may be provided by the API as an electronic signal, an audible signal or a visual signal.
[0056] The backend section is implemented to receive and to process the data of the at least one LEO satellite or the plurality of LEO satellites. These data in particular include data obtained by the GNSS receiver onboard of the LEO satellite(s). The data received and processed in the backend section may also include external data received from other sources, as for example of data providers or satellite constellation providers. These data are received from and processed in the backend section and may be transmitted as data reports and in the form of the map or globe provided by the method to the frontend section.
[0057] All threshold levels used in the method (Threshold 1, Threshold 2, Threshold 3) can be set manually by a user. Preferably, when the method is implemented to be conducted on a computer, the computer is providing a graphical user interface (GUI) which is set up for inputting parameters by the user. The GUI may be implemented in particular for receiving the threshold levels (threshold 1, threshold 2, threshold 3) selected by the user.
[0058] The threshold levels can also be defined using Machine Learning (ML) supervised or unsupervised methods, based on curated datasets. In particular, the threshold levels “Threshold 2” and “Threshold 3” can be defined using ML supervised methods.
[0059] In the preferred embodiment of the method, in which the number of affected unique GNSS satellites is determined and false alarms are filtered out by defining the third threshold level (Threshold 3), the selection of a suitable value for the third threshold level (Threshold 3) may be provided by the algorithm by supervised or unsupervised ML methods.
[0060] Accordingly, also in steps e) to g) of the method, supervised ML methods, using ML regression techniques to estimate the geolocation of spoofers, may be utilized.A further aspect of the invention provides a Global Navigation Satellite System (GNSS) comprising:
[0061] - a plurality of GNSS signal sources (4, 5),
[0062] - at least one Low Earth Orbit (LEO) satellite (3);
[0063] - at least one antenna (2) arranged on the at least one Low Earth Orbit (LEO) satellite (3),
[0064] - at least one GN S S receiver ( 1 ) onboard of the at least one Low Earth Orbit (LEO) satellite (3), wherein the GNSS receiver (1) is coupled to the at least one antenna (2) for receiving GNSS signals from the plurality of GNSS signal sources (4, 5), - and at least one processor adapted to perform the following steps: a) Monitoring the GNSS signals received from the plurality of GNSS signal sources (4, 5)
[0065] b) Tracking of at least one observable of the GNSS signals for obtaining a plurality of fracks from the GNSS signals,
[0066] c) Determining a length of each track and Analyzing the length of the tracks by comparing the determined length of each track with a predetermined first threshold level (threshold 1),
[0067] d) Selecting each track having a length of the frack which is lower than the first threshold level (threshold 1), to obtain at least one selected frack or a plurality of selected tracks,
[0068] e) Determining or Estimating of a position of the at least one LEO satellite (3) or of positions of each LEO satellite (3) of a plurality of LEO satellites (3) which has received a selected frack, to obtain a location of the signal source from which the signal of the selected tracks is received,
[0069] f) Analyzing the selected tracks on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2), and g) Averaging the position of the at least one LEO satellite (3) or of the position of each LEO satellite (3) which has received a selected frack within the sliding window, to improve the location of the signal source from which the signal of the selected tracks is received.
[0070] In this aspect of the invention the Global Navigation Satellite System (GNSS) includes multiple GNSS signal sources and at least one Low Earth Orbit (LEO) satellite or preferably a plurality of LEO satellites, which are comprising at least one antenna positioned on the LEO satellite and atleast one GNSS receiver onboard of the LEO satellite. The GNSS receiver is connected to the antenna to receive signals from different GNSS signal sources, which may include a plurality of true (authentic) GNSS satellites, in particular GNSS satellites in Mid-Earth Orbit (MEO), and spoofers placed on earth. The GNSS receiver includes a processor that performs the steps of the method by processing the received signals to detect and analyze spoofing activities.
[0071] In a preferred embodiment of the Global Navigation Satellite System, each LEO satellite is containing an antenna, which is extending in a longitudinal direction and is positioned at an angle with respect to the Zenith. The angle with respect to the Zenith is preferably larger than 0 degrees and particularly falls within the range of 30 degrees to 180 degrees relative to the Zenith.
[0072] This orientation of the antennas on the LEO satellites ensures that the GNSS signals of true GNSS satellites orbiting in MEO and also the false GNSS signals (spoofing signals) transmitted by spoofers located on earth can be received by the antennas. The preferred orientation of the antennas on the LEO satellites allows for a more effective capture of signals from various directions, thereby enabling to receive even low-power signals from GNSS satellites orbiting in MEO and also low-power spoofing signals from earth.
[0073] The at least one processor of the system may be onboard of a LEO satellite and in particular can be integrated in the GNSS receiver onboard of one of the LEO satellites. In this embodiment of the system, the LEO satellite, in which the processor is integrated, is providing the final result of the method, as for example a list of detected spoofers and their geolocation or a map or globe indicating the locations of detected spoofers. The data including the result of the method may be transmitted by the LEO satellite to earth, where it can be received for example by a control station placed on earth. Preferrably, the at least one processor is integrated in a computer placed on earth. In this preferred embodiment of the system, the raw (unprocessed) GNSS data, received by the GNSS receiver onboard of the LEO satellite, is transmitted to earth and received there by the computer, which is comprising a processor for processing the raw GNSS data in accordance with the method of the invention.
[0074] BRIEF DESCRIPTION OF THE DRAWINGSThe foregoing advantageous features and objectives of the invention will be described in detail and further features and advantageous technical effects will be made apparent from the detailed description of preferred embodiments given below with reference to the Figures, in which
[0075] Fig- 1 is a schematical representation of the orbits of a LEO satellite and a GNSS satellite in MEO sending true GNSS signals, which are received by the LEO satellite;
[0076] Fig. 2 is a schematical representation of the orbit of a LEO satellite and the location of a spoofer which is located on earth and is sending a spoofing signal, which is received by the LEO satellite;
[0077] Fig. 3 is a schematical representation of the LEO satellite, the spoofer located on earth and the GNSS satellite shown in Figures 1 and 2, wherein the spoofer is sending a spoofing signal and the GNSS satellite is sending a true GNSS signal, and these signals are received by an antenna and processed by a GNSS receiver onboard of the LEO satellite;
[0078] Fig. 4 is a schematical representation of the orientation of the antenna onboard of the LEO satellite of Figure 3
[0079] Fig. 5 is a schematical representation of a computer system for detecting satellite signal spoofing including a flowchart showing the steps of a preferred embodiment of the method according to the invention implemented in the computer system;
[0080] Fig. 6 is an example of a temporal progression of a plurality of tracks of observables determined in the method according to the invention from signals received by a GNSS receiver onboard of a LEO satellite;
[0081] Fig. 7 is an example of a map provided by the method according to the invention based on the plurality of tracks of observables shown in Figure 7, wherein the map shows a geolocation of a detected spoofer;
[0082] Fig. 8 is a schematic diagram of an example of a system for detecting satellite signal spoofing, in which the method according to the invention is implemented as a computer program product, which is processing GNSS data received by aGNSS receiver onboard of a LEO satellite in a backend and is providing signal spoofing information to a user system in a frontend of the computer program product.
[0083] DETAILED DESCRIPTION
[0084] Referring to FIGURES 1 and 2, the orbit of a LEO satellite 3 is shown. The LEO satellite 3 is orbiting the Earth in an orbit of for example 128 minutes. As shown in Figure 1, a GNSS satellite 4 is orbiting the earth in MEO. The GNSS satellite 4 is sending true GNSS signals, which are received by the LEO satellite 3. The GNSS satellite 4 is thus a signal source transmitting GNSS signals towards the Earth which can be received by the LEO satellite 3. Due to the relative (orbital) movement of the LEO satellite 3 and the GNSS satellite 4, the transmission of the GNSS signals transmitted by the GNSS satellite 4 has a relative large transmission cone 8, as shown in Figure 1.
[0085] As shown in Figure 2, a further GNSS signal source 5, which is transmitting spoofing signals, is placed on earth. The further signal source 5 is thus designated in the following as a “spoofer” 5. Since the spoofer 5 is placed on Earth, the spoofing signals transmitted by the spoofer 5 have a relative low transmission cone e, which is lower than the transmission cone 8 of the GNSS signal transmitted by the GNSS satellite 4.
[0086] FIGURES 1 and 2 are showing a simplified constellation which is used in the method according the invention for detecting satellite spoofing. In reality, a plurality of LEO satellites 3 and a plurality of GNSS satellites 4 is orbiting the earth and also a plurality of spoofers 5 may be placed on earth. The method of the invention is used to geolocate the spoofers 4 on Earth. The simplified constellation of the LEO satellite 3, the GNSS satellite 4 in MEO and the spoofer 5 on Earth is also shown schematically in FIGURE 3.
[0087] Each of the LEO satellites 3 is comprising an antenna 2 and a GNSS receiver 1 onboard of the satellite, as schematically shown in the detailed view of the LEO satellite 3 in the box of FIGURE 3. The LEO satellite 3 is receiving the true GNSS signals transmitted by the GNSS satellite 4 and also the spoofing signals transmitted by the spoofer 5 with the antenna 2.In FIGURE 4, the orientation of the antenna 2 on the LEO satellite 3 is shown schematically. The antenna 2 is extending in a longitudinal direction which is directed at a predefined angle P with the ZENITH. The angle generally can be any angle between 0° and 180°. Preferably, the angle p is larger than 0° and in particular is between 30° and 180°, most preferably between 30° and 150°. If the antenna 2 is directly pointing to ZENITH (with an angle P of 0°), the antenna 2 would perfectly be directed towards the GNSS satellites 4 orbiting in MEO, but would not receive low-powered spoofing signals of the spoofers 5. To enable the antenna 2 to receive the spoofing signals from the spoofers 5 and as well the true GNSS signals of the GNSS satellites 4, it is therefore preferable to orient the antenna 2 within the preferred angle range of 30° to 180° to ZENITH.
[0088] The signals received by the antenna 2 are provided to the GNSS receiver 1, which is processing the signals to obtain GNSS data. These GNSS data are transmitted by the LEO satellite 3 to Earth and can be received by a control station 6 placed on earth. In the control station 6, a receiver for receiving the signals of the LEO satellite 3 and a computer 11 for processing the received signals are installed. On the computer, a computer program product according the invention is implemented. The computer program product implemented on the computer in the control station 6 is conducting the method for detecting satellite spoofing according the invention.
[0089] In a preferred embodiment of a system 10 for detecting satellite signal spoofing, which is schematically shown in the flowchart of FIGURE 5, the following steps are conducted for geolocating at least one spoofer 5 or a plurality of spoofers 5 by a processor 12, which may be included in the computer 11 placed in the control station 6:
[0090] STEP a) Receiving GNSS data obtained by monitoring a plurality of signals received from a plurality of GNSS signal sources 4, 5, wherein the monitoring is conducted with at least one GNSS receiver 1 which is coupled to at least one antenna 2 located on at least one LEO satellite 3;
[0091] STEP b) Tracking of at least one observable of the signals received by the GNSS receiver 1 and obtaining a plurality of fracks from the plurality of signals,
[0092] STEP c) Determining a length of each track and Analyzing the length of the fracks by comparing the determined length of each track with a predetermined first threshold level (threshold 1),STEP d) Selecting each track having a length of the track which is lower than the first threshold level (threshold 1), to obtain at least one selected track or a plurality of selected tracks,
[0093] STEP e) determining or estimating the position of the at least one LEO satellite 3 or of the position of each LEO satellite 3 which has received a selected track, and determining a location of the signal source from which the signal of the selected tracks is received, based on the determined or estimated position of the at least one LEO satellite 3 or of each LEO satellite 3 which has received a selected track;
[0094] STEP f) Analyzing the selected tracks on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2), and
[0095] STEP g) Averaging the position n of the at least one LEO satellite 3 or of the position of each LEO satellite 3, which has received a selected track, within the sliding window, to improve the location of the signal source from which the signal of the selected tracks is received.
[0096] Examples of tracks of two different observables of the signals received by the GNSS receivers 1 of two LEO satellites A and B (according to STEP b) are shown in the diagrams of FIGURE 6. Therein, Pseudorange estimates and C / NO estimates of signals received by the GNSS receivers 1 of the two LEO satellites A, B are shown over time during a (short) section of the orbits of the LEO satellites 3. As can be seen from Figure 6, most of the tracks have a rather long track length over time, e.g. the signals from which the tracks are evaluated are seen by the LEO satellite 3 for a rather long time. A rather long track length is indicating that the signal from which this track is obtained is likely transmitted by a true GNSS satellite 4 from MEO. The reason why signals from true GNSS satellites 4 are causing long tracks of observables becomes clear from Figure 1, where the large transmission cone 8 of the GNSS satellites 4 orbiting in MEO is shown. In Figure 6, one track of the two observables having a rather short frack length is also shown. The fracks with the rather short length is shown in the diagrams of Figures 6 in a box designated as “short track”. It can be seen that the short fracks of the two observables Pseudorange and C / NO shown in the box of each diagram are observed simultaneously (e.g. at the same time) and that the length of these tracks is much shorter than the lengths of all other tracks shown in the diagrams of Figure 6. Regarding therather short track seen in Figure 6 at the beginning of the diagram it is to be noted that the beginning of this track is before the start of the diagram of Figure 6 and therefore actually is much longer than shown in Figure 6.
[0097] The short length of the tracks shown in the box is an indication for the source of the signal to be a spoofer 5 placed on earth, because a signal source placed on earth is producing much shorter tracks of observables. The reason is that the LEO satellite 3 due to its orbital movement over the location of a spoofer 5 placed on earth only sees the spoofing signals only for a short time during its orbital movement. This is visually demonstrated in Figure 2 by the smaller transmission cone e, which is much smaller than the transmission cone 8 of the signals transmitted by GNSS satellites 4 shown in Figure 1.
[0098] Therefore, in STEP d) of the method, the tracks having a short track length are selected, by comparing their track length with the first threshold level (threshold 1), because these short tracks may be attributed to a spoofing signal transmitted by a spoofer 5 placed on Earth.
[0099] In STEP e) of the method, the position of the at least one LEO satellite 3 or of each LEO satellite 3 which has received a short track selected in STEP d), are determined. Determining of a position of the at least one LEO satellite or of the position of each LEO satellite which has received a selected track preferably is based on determining a current orbital position of the LEO satellite at the time of receiving a signal comprising the selected track. Based on the determined position of the LEO satellites 3 at the time of receiving the signal, from which the selected short fracks are resulting, the location of the signal source is determined and a geolocation of the signal source on Earth is performed, for example by determining the coordinates of the signal source in an ECEF coordinate system.
[0100] To improve the geolocation of the signal source obtained in STEP e), the selected (short) fracks are evaluated on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2) in STEP f) of the method. This ensures that the observed events took place nearly at the same time, e.g. within the time window defined by length of the sliding window, which is defined by the second threshold level (threshold 2). The events, which have been occurred within the time window defined by the sliding window then are used in STEP g) of the method for an improvement of the geolocation of the signal sources by averaging the geolocations obtained by each selected event (e.g. a selected frack of an observable).In the averaging step (STEP g), the geolocations (e.g. a point on a map or globe or a point in an ECEF coordinate system) obtained from different LEO satellites 3 are spatially averaged to merge the geolocations of the signal source obtained from each LEO satellite 3 into one single point. It is ensured therein that the signal source observed by the different LEO satellites 3 is the same and was observed by the different LEO satellites 3 close in time or even at the same time by the sliding window technique used in STEP f) of the method.
[0101] Specific values for the threshold levels “threshold 1” and “threshold 2” may be selected by a user of the system. For providing predefined values for the threshold levels “threshold 1” and “threshold 2”, the user may input the selected values over a GUI of the computer 11. The processor 12 of the computer 11 is using the predefined values for the threshold levels “threshold 1” and “threshold 2”in performing the method steps, in particular in STEPS c), d) and f). Typical values for the first threshold level (threshold 1) are between 10 seconds and 50 seconds for the second threshold level (threshold 2) Typical values are between 1 Minute and 20 Minutes.
[0102] In the case that a plurality of GNSS signals from several GNSS satellites 4 is received by the GNSS receiver 1 onboard of the LEO satellite 3, it is preferable to conduct after step g) an additional step in the method:
[0103] STEP h) Determining the number of impacted GNSS satellites, from which GNSS signals are received by the GNSS receiver 1, comparing the determined number with a predetermined third threshold level (threshold 3), and attributing only those signal sources, from which the signal of the selected fracks has been received, to a spoofing source 5, if the determined number of impacted GNSS satellites is above the third threshold level (threshold 3).
[0104] By this additional STEP h), possible false alarms generated by conducting STEPS a) to g) can be filtered out. The idea on this filtering of possible false alarms is that sometimes short fracks may happen in a natural manner (e.g. caused by a true GNSS satellite), but it will be unlikely that true GNSS signals emitted by several unique GNSS satellite 4 experiencing the same effect in a natural manner at the same time. This is particularly the case, if the latitude of the projection of the (current) orbital position of the LEO satellite on earth is not too high at the moment of detecting the event.The conduction of STEP h) can be considered as an optional step in the method. By setting the third threshold level (threshold 3) to zero, all events obtained by performing STEPS a) to g) are attributed to a spoofer 5 as the signal source.
[0105] The data generated by the processor 12 can be provided in the form of a list including the detected spoofers 5 and their location. The data may also be visualized in the form of a map or a globe indicating the determined geolocations of detected spoofers 5. An example of a map showing the geolocation of a spoofer 5 detected on the basis of the tracks of observables shown in Figure 6 is shown in FIGURE 7. Therein, a hexagon is shown which is indicating the location of the detected spoofer 5. The list, map or globe preferably includes a time-stamp to indicate the time of detection of the spoofers 5 included therein.
[0106] The map or globe generated by the processor 12 can be displayed on a display 13 connected to the computer 11. The display 13 can be integrated in the computer 11 or it can be remote from the computer.
[0107] FIGURE 8 shows a schematic representation of the computer program executed by the processor 12 of the system 10 shown in Figure 5. Figure 8 shows a schematic diagram of an example of a system for detecting satellite signal spoofing, in which the method according to the invention is implemented as a computer program product, which is set up to provide information on GNSS signal spoofing to a user system. The computer program product is processing (raw) GNSS data received by the GNSS receiver 1 onboard of a LEO satellite 3 in a backend and is providing signal spoofing information to the user system in a frontend of the computer program product. The user system shown in Figure 8 is an airline dispatch system, which is connected for data exchange with the computer system 10 and in particular with the frontend of the computer program product. The frontend of the computer program product may be integrated directly into the user system. In the backend of the computer program product, the GNSS data provided by the LEO satellites 3 are received and processed by the processor 12 of the computer system 10, in accordance with the method of the invention. The geolocation of the spoofers 5 detected in the backend can be displayed on the display 13 of the system 10 and / or it can be provided to the user system for displaying.
[0108] It will be appreciated that although specific embodiments have been described for purpose of illustration, various modifications can be made without deviating from the scope of the disclosure.It is possible for example to integrate the processor 12, on which the method can be conducted as a computer-implemented method, in (one of) the LEO satellites 3 in particular in the GNSS receiver 1 onboard of (one of) the LEO satellites 3. Further, it is possible to set up the computer program product to generate a list of the detected spoofers with their locations and / or an alert signal indicating the detection of a spoofer 5, instead of or in addition to providing a map or globe indicating the geolocation of detected spoofers.
Claims
Claims1. A method for detecting satellite signal spoofing, the method comprising:a) Monitoring a plurality of signals of at least one Global Navigation Satellite System (GNSS) received from a plurality of GNSS signal sources (4, 5), wherein the monitoring is conducted with at least one GNSS receiver (1) which is coupled to at least one antenna (2) located on at least one Low Earth Orbit (LEO) satellite (3);b) Tracking of at least one observable of the signals received by the GNSS receiver (1) and obtaining a plurality of tracks from the plurality of signals,c) Determining a length of each track and analyzing the length of the tracks by comparing the determined length of each track with a predetermined first threshold level (threshold 1),d) Selecting each track having a length of the track which is lower than the first threshold level (threshold 1), to obtain at least one selected track or a plurality of selected tracks,e) Determining or estimating a position of the at least one LEO satellite (3) or of positions of each LEO satellite (3) of a plurality of LEO satellites (3) which has received a selected track, to obtain a location of the signal source from which the signal of the selected tracks is received, f) Analyzing the selected tracks on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2), andg) Averaging the position of the at least one LEO satellite (3) or of the position of each LEO satellite (3), which has received a selected track, within the sliding window, to improve the location of the signal source from which the signal of the selected tracks is received.i2. The method according to claim 1, wherein the plurality of GNSS signal sources is comprising a plurality of GNSS satellites (4) and / or at least one spoofer (5).
3. The method according to claim 1 or 2, wherein a number of impacted GNSS satellites, related to each averaged position of a LEO satellite (3) which has received a selected track, is determined.
4. The method according to claim 3, wherein the determined number of impacted GNSS satellites (4) is compared with a predetermined third threshold level (threshold 3), and each of the signal sources, from which the signal of the selected tracks has been received, is attributed to a spoofer (5), if the determined number of impacted GNSS satellites is above the third threshold level (threshold 3).
5. The method according to claim 4, wherein the third threshold level (threshold 3) is an integer including zero, wherein the third threshold level (threshold 3) preferably is larger than 1.
6. The method according to any one of the preceding claims, wherein determining of a position of the at least one LEO satellite (3) or of the position of each LEO satellite (3) which has received a selected track is comprising a geolocation of the LEO satellite (3) within an Earth-Centered Earth-Fixed (ECEF) coordinate system or a determination of a ground projection of the LEO satellite (3) on Earth from a geolocation of the LEO satellite (3).
7. The method according to any one of the preceding claims, wherein determining of a position of the at least one LEO satellite (3) or of the position of each LEO satellite (3) which has received a selected track is based on determining a current orbital position of the LEO satellite (3) at the time of receiving a signal comprising the selected track.
8. The method according to any one of the preceding claims, wherein analyzing the selected tracks on a time-basis is conducted by a sliding window technique within a fixed time window.
9. The method according to claim 8, wherein the fixed time window has a time size which is lower than 20 minutes and preferably lower than 10 minutes.
10. The method according to any one of the preceding claims, wherein the first threshold level (threshold 1) is 100 seconds or lower, preferably 50 seconds or lower and in particular 40 seconds.
11. The method according to any one of the preceding claims, wherein the first threshold level (threshold 1) is between 5 seconds and 100 seconds and preferably between 10 seconds and 50 seconds.
12. The method according to any one of the preceding claims, wherein the observables are obtained by at least one of the following measurements of the received signal or a combination thereof:- a signal power measurement of the received signal,- measurement of the ratio of a carrier power to a noise density of the received signal (C / N0),- a carrier phase measurement of the received signal,- a code phase measurement of the received signal,- a pseudorange measurement of the received signal,- a code pseudo-range measurement of the received signal or a carrier smoothing of code pseudoranges,- a phase range measurement of the received signal,- a Doppler shift measurement of the received signal.
13. The method according to any one of the preceding claims, wherein the method is providing a list, a map or a globe showing a location of signal sources (5) placed on Earth.
14. The method according to any one of the preceding claims, wherein the method is generating an alarm signal indicating a detection of at least one spoofing signal.
15. The method according to any one of the preceding claims, wherein the method is carried out by a computer (11).
16. A computer program product comprising instructions which, when the program is executed by a computer (11), cause the computer (11) to carry out the method of any one of claims 1 to 15.
17. A Global Navigation Satellite System (GNSS) comprising:- a plurality of GNSS signal sources (4, 5),- at least one Low Earth Orbit (LEO) satellite (3);- at least one antenna (2) arranged on the at least one Low Earth Orbit (LEO) satellite (3),- at least one GNSS receiver (1) onboard of the at least one Low Earth Orbit (LEO) satellite (3), wherein the GNSS receiver (1) is coupled to the at least one antenna (2) for receiving GNSS signals from the plurality of GNSS signal sources (4, 5),- and a processor (12) adapted to perform the following steps:a) Monitoring the GNSS signals received from the plurality of GNSS signal sources (4, 5)b) Tracking of at least one observable of the GNSS signals for obtaining a plurality of tracks from the GNSS signals, c) Determining a length of each track and analyzing the length of the tracks by comparing the determined length of each track with a predetermined first threshold level (threshold 1),d) Selecting each track having a length of the track which is lower than the first threshold level (threshold 1), to obtain at least one selected track or a plurality of selected tracks,e) Determining or Estimating of a position of the at least one LEO satellite (3) or of positions of each LEO satellite (3) of a plurality of LEO satellites (3) which has received a selected track, to obtain a location of the signal source from which the signal of the selected tracks is received, f) Analyzing the selected tracks on a time-basis by a sliding window having a preselected time size below a second threshold level (threshold 2), andg) Averaging the position of the at least one LEO satellite (3) or of the position of each LEO satellite (3) which has received a selected track within the sliding window, to improve the location of the signal source from which the signal of the selected tracks is received.
18. The Global Navigation Satellite System according to the preceding claim, wherein the plurality of GNSS signal sources (4, 5) is comprising a plurality of GNSS satellites in Mid-Earth Orbit (MEO) and the Global Navigation Satellite System is further comprising a plurality of LEO satellites (3), wherein each of the LEO satellites (3) is containing at least one antenna (2) and a GNSS receiver (1) onboard of the LEO satellite (3).
19. The Global Navigation Satellite System according to any one of claims 17 or 18, wherein each Low Earth Orbit (LEO) satellite (3) is containing an antenna (2) extending in a longitudinal direction, which on each Low Earth Orbit (LEO) satellite (3) is positioned in an angle (P) with respect to the Zenith, wherein the angle (P) is larger than 0°.
20. The Global Navigation Satellite System according to claim 19, wherein the antenna (2) is positioned at an angle (P) with respect to the Zenith between 30° and 180°.
21. The Global Navigation Satellite System according to any one of claims 17 to 20, wherein the at least one processor (12) is onboard of a LEO satellite (3) and in particular is integrated in the GNSS receiver (1) onboard of a LEO satellite (3).
22. The Global Navigation Satellite System according to any one of claims 17 to 21, wherein at least one processor (12) is integrated in a computer (11) placed on earth.