WO2021032749A1

WO2021032749A1 - Detection of a decoy operation of a satellite signal receiver

Info

Publication number: WO2021032749A1
Application number: PCT/EP2020/073119
Authority: WO
Inventors: Loïc Davain
Original assignee: Safran Electronics & Defense
Priority date: 2019-08-22
Filing date: 2020-08-18
Publication date: 2021-02-25
Also published as: FR3100066A1; FR3100066B1

Abstract

A method for navigating based on satellite positioning data and on non-satellite positioning data, comprising the following steps of: • - computing a first reference navigation that is hybridised on the basis of non-satellite positioning data (30) and of satellite positioning data (20), the first reference navigation being recalibrated on the satellite positioning data; • - computing a second reference navigation on the basis of the non-satellite positioning data (30); • - comparing (40) an output of the first reference navigation and an output of the second reference navigation and deducing therefrom the existence or the absence of a decoy operation of the satellite signal receiver (20).

Description

DETECTION OF A LURRING OPERATION OF A SATELLITE SIGNAL RECEIVER

BACKGROUND OF THE INVENTION The present invention relates to the field of navigation and more precisely to positioning and navigation by means in particular of the reception of satellite signals transmitted by satellites belonging to a constellation of satellites distributed around the Earth. Positioning by satellites (or GNSS stands for “Global Navigation Satellite System”) is mainly implemented by the GPS, Galileo, GLONASS and BeiDou systems. The invention relates more particularly to inertial navigation aided by the reception of satellite signals.

Satellite positioning consists in receiving signals transmitted by satellites whose position is known and in deducing from the duration (or time of flight), between the transmission and the reception of each of the signals, a so-called pseudo-distance measurement between the satellite signal receiver (commonly, and sometimes incorrectly, called GPS receivers) and each of the satellites from which the signal has been received (each signal comprising an identifier of the satellite and the time of emission of the signal). Thus, it suffices to have the signals of four satellites to have the latitude, the longitude and the altitude of the receiver as well as an error on the measured times, but the positioning is all the more precise that large number of satellites whose signals have been taken into account by the receiver to calculate its position.

As a result, this positioning system, which is relatively precise, has spread widely and many vehicles are now equipped with a satellite signal receiver. Due to the falling costs of satellite signal receivers, most people also have cell phones of the type. smartphone (or mobile phone) themselves fitted with a satellite signal receiver.

In parallel with this development of satellite signal receivers, decoying devices have appeared to deceive these satellite signal receivers (we speak of "decoy" or "spoofing" of receivers). (Jn such a device comprises an electronic processing unit connected to a radiofrequency signal transmitter to transmit fraudulent signals having the characteristics of satellite signals. More precisely, the electronic processing unit is arranged to generate, from an initial position receiver of a satellite signal receiver, fraudulent signals which, when picked up by the satellite signal receiver, cause the satellite signal receiver to calculate an erroneous position. The actual initial position of the satellite signal receiver can be detected for example by means of a laser range finder or communicated by the vehicle on board the satellite signal receiver as required by certain navigation rules, in particular air and sea (ADS-B or AIS signals sent by vehicles to communicate its position to its neighbors).

For the fraudulent signals to be taken into account by the satellite signal receiver, it is not sufficient to transmit the fraudulent signals with a power greater than the original satellite signals. It is also necessary that the fraudulent signals have the same code phase and a Doppler effect being in the same range as those of the satellite signals previously received by the satellite signal receiver. If the first fraudulent signal received is consistent with the position calculated last by the satellite signal receiver and with the satellite signals received previously, and if the fraudulent signals subsequently received are consistent with each other, the fraudulent signals will be used by the satellite signal receiver as if they were real satellite signals and the error in the real position of the satellite signal receiver cannot not be detected.

Hybrid navigation systems are known which merge inertial positioning data originating from an inertial navigation unit and satellite positioning data originating from a satellite signal receiver. These navigation systems integrate one or more Kalman filters arranged so that the hybrid navigation is readjusted to the satellite positioning data. The Kalman filter is protected by an innovation test to detect outliers and reject them. However, if the fraudulent signals have sufficient consistency, then they can pass this innovation test and thus it is possible to cause the hybrid navigation to follow the decoyed position. Thus, in these systems, it is the satellite positioning data which make it possible to compensate for the errors of the inertial positioning data over the long term so that fraudulent signals would lead to a navigation error despite the hybridization of the satellite positioning data with inertial positioning data.

It is therefore understood that the implementation of such decoying devices can be prejudicial to the safety of a decoyed vehicle and possibly to that of vehicles moving in the same area as the decoyed vehicle.

It is known from document US-A-2009/0254278 a hybrid satellite / inertial navigation system which provides hybrid operational navigation (operational in this meaning that it is this which is used to steer the vehicle) which is readjusted to the satellite navigation with a low gain (or “low authority”). It follows that, if the operational navigation is effectively affected by the decoy, it is nevertheless more robust to the decoy than a hybrid navigation of the “full authority” type. However, the precision of operational navigation is degraded in the absence of decoy due to the low gain adjustment.

Document US-A-2018/088241 describes a method for detecting a decoy of a satellite signal receiver. This method consists in comparing a series of positions supplied by purely inertial navigation and a series of positions supplied by purely satellite navigation at the same instants, and in calculating a corresponding autocorrelation function. OBJECT OF THE INVENTION

The object of the invention is in particular to detect a decoy operation.

SUMMARY OF THE INVENTION

To this end, according to the invention there is provided a navigation method by means of a satellite signal receiver on board a vehicle comprising an electronic navigation unit connected to the satellite signal receiver and to a non-satellite positioning unit to calculate operational navigation hybridized from non-satellite positioning data and satellite positioning data by applying an innovation test. The process comprises the steps of:

- calculate a first reference navigation, hybridized from non-satellite positioning data and satellite positioning data without applying an innovation test, the first navigation of reference being readjusted to the satellite positioning data;

- calculate a second reference navigation from the non-satellite positioning data;

- Compare an output from the first reference navigation and an output from the second reference navigation and deduce therefrom an existence or an absence of decoy operation of the satellite signal receiver.

Thus, the method according to the invention makes it possible to detect an exposure of a satellite signal receiver to a decoy operation. It is therefore possible to alert the user to the existence of this decoy operation, to detect the end of exposure to fraudulent signals and to restore the nominal navigation performance relatively quickly. It will be understood that the method of the invention makes it possible to have both precise operational navigation in the absence of decoy (hybrid operational navigation of the “full authority” type), and detection of any decoy attempt.

Preferably, the method comprises the step of determining a difference between the speed resulting from the first reference navigation and the speed resulting from the second reference navigation and of comparing this difference with a predetermined threshold.

This makes it possible to detect a decoy in a very simple and very reliable manner.

Alternatively, or in combination with the comparison of the speeds, the method comprises the steps of estimating, from the first reference navigation, at least one error estimate of at least one inertial sensor of the non-satellite positioning unit and comparing the estimate to a predetermined threshold. This makes it possible to detect a decoy in a very simple and very reliable manner.

The subject of the invention is also an electronic navigation unit programmed to implement the above method.

Other characteristics and advantages of the invention will emerge on reading the following description of a particular and non-limiting embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the accompanying drawings, among which:

FIG. 1 is a schematic view of a device for implementing the method of the invention during a decoy operation.

DETAILED DESCRIPTION OF THE INVENTION With reference to the figure, the invention is here described in application to an aircraft 1 equipped with a navigation system generally designated as 10 comprising a satellite signal receiver 20 and an inertial positioning unit 30 which are connected to an electronic navigation unit 40.

The satellite signal receiver 20 is arranged, in a manner known per se, to receive positioning satellite signals transmitted by satellites of a constellation of satellites S of at least one satellite positioning system (GNSS) such as GPS, Galileo, GLONASS and BeiDou and to calculate, from these satellite signals, satellite positioning data such as a pseudo-distance, a phase measurement, a latitude, a longitude, an altitude, and a time error. The inertial unit 30 comprises an inertial measurement unit comprising inertial sensors, here conventionally three accelerometers arranged along the axes of a measurement frame and three gyrometers arranged to measure rotations of this measurement frame with respect to a reference frame. The inertial measurement unit 30 further comprises, in a manner known per se, a processing unit arranged to determine inertial positioning data, such as position, attitude and speed data, from the signals of measurement produced by inertial sensors.

The electronic navigation unit 40 comprises one or more processors and a memory containing at least one program containing instructions implementing the method of the invention. In particular, the electronic navigation unit 40 is programmed to calculate navigations using the positioning data.

During the execution of this program, the electronic navigation unit 40 calculates a hybridized operational navigation from the inertial positioning data and the satellite positioning data. Hybrid navigation can be based on loose coupling in position (and / or speed) or tight coupling in pseudo-distance (and / or delta range). To carry out the hybridization, the program uses a Kalman filter which comprises a bank of filters and which is protected by an innovation test aimed at verifying the consistency of the satellite positioning data between them. The innovation test is known in itself and can detect and reject outliers.

Operational navigation is used for steering the vehicle to cause the vehicle to follow a predetermined route.

The method of the invention aims to detect a decoying operation during which a decoying device D, here on the ground, knowing the real position of the aircraft 1, transmits fraudulent satellite signals intended to be received by the satellite signal receiver 20 and taken into account in the calculation of the hybridized navigation instead of the authentic satellite signals to bring the aircraft 1 on a real trajectory different from that indicated by the navigation system. The structure and operation of the decoying device D are known per se and will not be further described here.

In order to detect such a decoy operation, the electronic navigation unit 40 is furthermore arranged to execute decoy detection processes, processes which are here advantageously combined.

According to a first detection process, the electronic navigation unit 40 calculates two other navigations, and more precisely hybridized navigation and non-hybridized navigation, namely:

a first reference navigation, hybridized from the inertial positioning data and the satellite positioning data, the first reference navigation being readjusted to the satellite positioning data;

- a second reference navigation only at from inertial positioning data.

The first reference navigation is not an operational navigation: it is only used to detect the decoy. To force the first reference navigation to be sensitive (or prone) to deception, the innovation test is disabled.

The second reference navigation can result from the implementation of a non-readjusted Kalman filter. The implementation of a Kalman filter not readjusted by the second navigation also makes it possible to develop a consistent quality indicator.

The electronic navigation unit 40 then compares an output of the first reference navigation and an output of the second reference navigation and deduces therefrom an existence or an absence of decoy operation of the satellite signal receiver.

In the first reference navigation, the Kalman filter is configured to readjust the navigation on the satellite data so that, in the event of deception, the Kalman filter will produce an abnormal modeling of the errors of the inertial sensors. Thus, the first reference navigation is forced to follow the satellite data even if they are erroneous. Having the first reference navigation and the second reference navigation makes it possible to compare the dynamics of inertial navigation with the dynamics of hybrid navigation which is forced to follow the satellite data.

The electronic navigation unit 40 is arranged to determine a difference between the speed resulting from the first reference navigation and the speed resulting from the second reference navigation and to compare this deviation with a predetermined threshold. The predetermined threshold is equal to a multiple of a standard deviation calculated from a law of distribution of the speed deviations, the multiple preferably being 4.

According to a second decoy detection process, the electronic navigation unit 40 is arranged to estimate from the first reference navigation at least one error estimate of at least one inertial sensor of the non-satellite positioning unit and compare the estimate to a predetermined threshold. The electronic navigation unit 40 is here arranged to estimate for the first reference navigation at least one gyrometric drift and to compare the estimated gyrometric drift with a predetermined threshold.

In this case, three gyrometric drifts are estimated here:

- two horizontal fins;

- a drift of course.

For each of these three drifts, the predetermined threshold is equal to a multiple of a standard deviation calculated from a distribution law of the drift, the multiple preferably being equal to 4.

It should be noted that the use of a threshold not fixed but based on the standard deviations calculated by the hybridization filters makes it possible to improve the sensitivity of the detection of decoys by taking into account the current quality of the calculated navigations, and by the same by taking into account the natural characteristics of a navigation not readjusted (period of Schuler, oscillation 24h). According to a third detection process, the hybrid operational navigation (which implements an innovation test) is monitored in order to ensure that the innovation test does not return a rejection rate higher than a predetermined threshold representative of an abnormality.

When one of the detection processes reveals a decoy operation, it returns an alert. It will be noted that the detection processes are executed simultaneously by the same computer program so that all the detection processes are active simultaneously and independently.

The method of the invention, in this particular embodiment, combines the results of the detection processes to assess the credibility of the threat.

Thus, the electronic navigation unit 40 can be programmed to issue a credible threat alert as soon as one of the detection processes has identified a deviation. The alert can have different levels depending on whether:

- the second detection process has issued an alert for at least one of the monitored inertial sensors;

- the second detection process issued an alert simultaneously for several of the monitored inertial sensors;

- the second detection process issued an alert simultaneously for all the monitored inertial sensors;

- hybrid operational navigation (which implements an innovation test) shows an abnormal rejection rate. The electronic navigation unit 40 is arranged to establish a score of the threat which increases by 1 each time one of the above criteria is satisfied.

The score can therefore be between 1 and 5. and;

We could consider that:

- a score equal to 1 or 2 indicates an unlikely threat

- a score equal to 3 or 4 indicates a potential threat;

- a score equal to 5 indicates the credible presence of a threat.

It should be noted that it is important to keep an operational navigation separate from the two reference navigations because the satellite signal receiver may be subject to a decoy operation for several tens of minutes: it would therefore not be possible to rely solely on it. inertial positioning data to ensure navigation.

Of course, the invention is not limited to the embodiment described but encompasses any variant coming within the scope of the invention as defined by the claims.

In particular, the vehicle's navigation system may be different from that described.

The vehicle can be equipped with several inertial units each providing inertial navigation. Provision can be made to use each of these inertial navigations as reference navigation for the detection of decoys: there will therefore be as many distinct detection processes which will be combined to ensure consolidated detection. Alternatively, only part of the inertial navigations can be used as reference navigation. As a further variant, it is possible to use an average of all or part of these inertial navigations to form a reference navigation for the detection of decoys.

The non-satellite navigation unit can be an inertial unit but also another type of unit such as an optical navigation unit operating from the position of the stars or landmarks.

For an application to a satellite receiver arranged to receive satellite signals from the satellites of several satellite positioning systems (GNSS), provision will be made to carry out a decoy detection for each of these systems.

The predetermined speed threshold may be different from that mentioned above and for example equal to:

a multiple of a standard deviation calculated from a law of distribution of the speed deviations, the multiple preferably being 3;

a predetermined value for the speed difference, preferably approximately 3 meters per second.

The electronic navigation unit can be arranged to estimate for the first reference navigation at least one accelerometric bias and to compare the estimated accelerometric bias with a predetermined threshold.

The process can include the steps of:

- determining a difference between the speed resulting from the first reference navigation and the speed resulting from the second reference navigation, and comparing this difference with a predetermined threshold; estimating from the first reference navigation at least one accelerometric bias and comparing it with a predetermined threshold;

estimating from ir from the first reference navigation at least one gyrometric drift and comparing it with a predetermined threshold; and the step of issuing an alert in the event that one of the predetermined thresholds is exceeded. It is possible, but not mandatory, to assign a minimum score to the first threshold crossing and to raise it each time another threshold is exceeded, the probability of existence of a decoy operation being proportional to the number of exceedances.

The process can include the steps of:

- estimate from the first reference navigation of the errors of the sensors of the non-satellite positioning unit;

- compare the error estimate of each sensor with a predetermined threshold;

- issue an alert based on the number of threshold overruns, the probability of existence of a decoy operation being proportional to the number of overruns.

Although the combination of the detection processes is extremely efficient, the invention applies to the use of only one of these navigation processes, or two or more in combination.

The program can implement one or more Kalman filters.

The scoring system may be different from that described. In the event that N detection processes are set implementation, we can predict that the score varies from 1 to N with:

- a score lower than N / 3 to indicate a not very credible threat

- a score between N / 3 and 2.N / 3 to indicate a potential threat;

- a score greater than 2.N / 3 to indicate the credible presence of a threat.

Other rating choices are possible to limit the risk of false alarms or non-detection.

Claims

1. Method of navigation by means of a satellite signal receiver on board a vehicle comprising an electronic navigation unit linked to the satellite signal receiver and to a non-satellite positioning unit for calculating hybrid operational navigation from non-satellite data positioning and satellite positioning data by applying an innovation test, the method comprising the steps of:

- calculate a first reference navigation, hybridized from the non-satellite positioning data and the satellite positioning data without applying an innovation test, the first reference navigation being readjusted to the satellite positioning data;

- calculate a second reference navigation from the non-satellite positioning data; - Compare an output from the first reference navigation and an output from the second reference navigation and deduce therefrom an existence or an absence of decoy operation of the satellite signal receiver.

2. The method of claim 1, comprising the step of determining a difference between the speed resulting from the first reference navigation and the speed resulting from the second reference navigation and comparing this difference with a predetermined threshold.

3. Method according to claim 2, wherein the predetermined threshold is equal to a multiple of a standard deviation. calculated from a law of distribution of speed differences, the multiple preferably being 3.

4. Method according to claim 2, in which the predetermined threshold is equal to a multiple of a standard deviation calculated from a law of distribution of the speed deviations, the multiple preferably being 4.

5. The method of claim 2, wherein the predetermined threshold is equal to a predetermined value of the speed difference, preferably approximately 3 meters per second.

6. Method according to any one of the preceding claims, comprising the steps of estimating from the first reference navigation at least one error estimate of at least one inertial sensor of the non-satellite positioning unit and comparing it. 'estimate at a predetermined threshold.

7. The method of claim 6, comprising the steps of estimating for the first reference navigation at least one accelerometric bias and comparing the estimated accelerometric bias with a predetermined threshold.

8. The method of claim 6, comprising the steps of estimating for the first reference navigation at least one gyrometric drift and comparing the estimated gyrometric drift with a predetermined threshold.

9. The method of claim 8, wherein the estimated gyrometric drift is a horizontal drift.

10. The method of claim 8, wherein the estimated gyrometric drift is a heading drift.

11. The method of claim 7 or 8, wherein the predetermined threshold is equal to a multiple of a standard deviation calculated from a distribution law of error, the multiple preferably being equal to 4.

12. Method according to any one of the preceding claims, comprising the steps of:

- compare the error estimate of each sensor with a predetermined threshold;

13. Navigation system comprising an electronic navigation unit programmed to implement a method according to any one of the preceding claims.