WO1999034232A1

WO1999034232A1 - Station transmitting correction signals to a navigation receiver

Info

Publication number: WO1999034232A1
Application number: PCT/FR1998/002852
Authority: WO
Inventors: Jean-Pierre Arethens
Original assignee: Sextant Avionique
Priority date: 1997-12-23
Filing date: 1998-12-23
Publication date: 1999-07-08
Also published as: FR2772928B1; FR2772928A1; CA2315899A1; RU2000120170A; EP1040363A1

Abstract

The invention concerns a station transmitting correction signals to a navigation receiver of the differential type determining the position of a mobile station based on the measurement of distances separating said mobile from at least three satellites or beacons. Each measurement is corrected by a correction signal, transmitted by the station, which represents the difference between the distance, measured at the station, from said station to the satellite concerned and the real value of said difference. The station comprises, for measuring the distances to the satellites, at least two sets of receiver equipment so arranged as to minimise the probability that the two sets of equipment simultaneously receive signals affected by multi-paths. Each receiver equipment (10, 12, 14) comprises a navigation receiver (201, 202, 203), for example of the differential type, designed to supply a signal representing said equipment measured position, and the station comprises means (16) for comparing with a threshold the distance separating each equipment measured position from said equipment real position and for controlling the transmission of correction signals based on the results of the comparisons. The threshold takes into account the importance given to each direction.

Description

CORRECTION SIGNAL TRANSMITTING STATION TO A RECEIVER

NAVIGATION

The invention relates to a station for transmitting correction signals to a mobile equipped with a differential type position determination receiver.

Positioning systems, in particular for the navigation of land, air or sea mobiles, generally call upon a constellation of satellites or beacons emitting signals which allow a user to determine, in real time, his position and, possibly , its speed. The most common positioning system is the GPS system ("Global Positioning System") which will be mainly referred to below without the invention being limited to this application.

The user's GPS receiver calculates the distance separating him from at least four satellites transmitting messages at known times, these times of transmission being included in the message transmitted by each satellite. The distance from the receiver to each satellite is determined by the product of the speed of light and the time taken for the signal to travel this distance. This duration is equal to the difference between the time of reception and the time of transmission. Speed is determined by Doppler effect, that is to say by measuring the difference between the frequency received and the transmission frequency of the carrier of the GPS signal.

The accuracy of the position measurements obtained with a GPS receiver is limited by the noise affecting the distance measurements to the different satellites due, on the one hand, to propagation delays, and, on the other hand, above all, to interference. voluntary. Thus, the accuracy on the position is 100 meters in a horizontal plane and 150 meters in the vertical direction. This precision is, in general, insufficient, especially when it comes to guiding a mobile such as an aircraft in landing procedure or a land vehicle, for example on the road.

However, the noises affecting the measurements are observable noises with weak dynamics which can be corrected. This is why a differential GPS system is used, calling on a fixed station, in particular on the ground, transmitting correction signals to the user.

Such a differential system takes advantage of the fact that, on the one hand, the position of the fixed station is known with precision and, on the other hand, that, at all times, the distance from the fixed station to each satellite is also known with precision. The ground station, close to the mobile, is equipped with a specialized receiver comprising elements allowing it: - the measurement of the distances from this station to each satellite on the basis of the information contained in the signals which it receives from the satellites as does a mobile GPS receiver,

- the calculation of the exact distances from the station to each satellite from the knowledge that we have of the positions of the station and of the satellites, and

- the calculation of the differences between the exact distances and the measured distances, and of a radio transmitter allowing it to communicate with the GPS receivers of mobiles moving nearby. This station transmits, to the mobile, a corrective signal which is made up, for each satellite, by the difference between the known distance and the measured distance. The differential receiver of the mobile can thus correct, for each satellite, the measurements which it performs by adding to each measurement the correction which has been transmitted to it.

With these corrections, the positioning accuracy of a mobile is approximately 2 meters in the horizontal direction and 3 meters in the vertical direction. In a ground station, however, reception can be disturbed by additional noise which cannot be observed by a single measuring device. These noises are attributable to ulti-paths, that is to say to reflections of the received signal (coming from the satellite). Indeed, the received signal can be reflected on the ground, on the relief or on buildings. This is why a ground station generally comprises several specialized receivers and means for identifying the affected reception (s) for multi-path (s). Identification of the faulty reception is obtained by comparing the correction signals supplied by the various specialized receivers. The latter are, of course, arranged in locations chosen to minimize the probability that these receivers are affected simultaneously with multipath errors. The identification of the receivers receiving signals affected by multi-paths is based on the fact that, the receivers on the ground being close to each other, in the absence of multi-paths, the corrections provided by each receiver are the same and that if one of the receivers receives a "multi-path" signal, the correction calculated by this receiver is different from the corrections calculated by the other receivers, not affected by multi-path errors.

The corrections calculated by the receivers are therefore compared two by two and if the difference between two corrections exceeds a predetermined -threshold, neither of the two corrections respondents are not transmitted. On the other hand, if the difference between the corrections does not exceed the threshold, the correction transmitted to the mobile is one of the corrections or the average of the corrections measured. This type of multi-path identification is difficult to adjust. In particular, the determination of the threshold is the result of a compromise between contradictory requirements: if the threshold is too high, the corrections sent to the mobile will not always be correct, and if the threshold is too low, the station will too often eliminate the emission of correction signals sent to the mobile. In addition, the comparisons between corrections are all made with respect to the same threshold, that is to say that all the directions are assigned the same weight, while some might require greater precision than others. By "direction" is meant here the direction of the right joining the station to a satellite.

So far, attempts have been made to refine the comparison criterion using increasingly complex statistical studies.

One can consider, to ensure the reliability of the correction signal emitted by a ground station, to equip this ground station with a differential navigation receiver of the same type as that of a mobile. This navigation receiver placed on the ground in a known position and receiving, in addition to the signals from the constellation of satellites, the correction signals delivered by the ground station must deliver, when the ground station operates correctly, a fairly precise estimate of his position. Monitoring the accuracy of this position estimate, which can be done by comparing the distance between the estimated position and the actual position to a threshold, can be used to trigger an alarm and prevent the ground station from transmitting its corrections if the threshold is exceeded. However, this results in a deactivation of the ground station when it may have only one the only specialized receiver affected by a multi-path phenomenon and have with its other specialized receivers sets of valid corrections that could have been transmitted. In other words, this precautionary measure does not make it possible to identify the defective correction (s) the removal of which would ensure continuity of service from the ground station.

The invention aims to remedy these drawbacks. Its subject is a ground station transmitting correction signals for satellite navigation receivers comprising:

- at least two reception equipment located in different geographical locations chosen so as to minimize the probability that these two reception equipment simultaneously receive multi-path signals and each provided with distance estimation means delivering from the signals from the navigation satellites, estimates of the distances separating them from the navigation satellites,

means for determining the correction signals comparing the sets of distance estimates supplied by the reception equipment and the exact distances separating the reception equipment from the navigation satellites deduced from the knowledge of the geographical locations of the reception equipment and the knowledge of the positions of the navigation satellites in their orbits, and drawing from these comparisons the correction signals, and

- transmission means for communicating said correction signals to the navigation receivers of the mobiles operating in the vicinity of the station.

This station is remarkable in that it further comprises

- at each of its reception equipment, localization means delivering an estimate of position geographic from the distance estimates with respect to the navigation satellites provided by the distance estimation means, said location means constituting with the distance estimation means a navigation receiver,

means for identifying reception equipment affected by multi-path phenomena operating by comparing the distance separating the estimate of geographical position supplied by each reception equipment with its known geographical position from a threshold, and

selection means for rejecting the sets of distance estimates with respect to the navigation satellites delivered by the reception equipment identified by the identification means as providing a position estimate too far from their real position.

Thus, to identify multi-routes, the parameter used is a difference in geographic position and not a parameter that does not take direction into account. Under these conditions, the threshold naturally takes into account the importance given to each direction.

The rejection of corrections will therefore be less frequent than in known systems without the reliability of the corrections being altered.

Furthermore, the station according to the invention is not very sensitive to synchronization faults between the time bases of the receivers, unlike the known stations, since it does not require a comparison between data, necessarily synchronized, supplied by two receivers.

Preferably, the navigation receivers which determine the position are differential type navigation receivers of the type which are usually used in mobiles. In this way, the cost of each receiver can be low because it is mass produced for mobiles. It can also be observed that, although the receiver of the ground station according to the invention performs operations more complex than the receivers usually used in such stations, its cost is nevertheless reduced, due to this possibility of mass production.

When such a differential navigation receiver is used, the corrections received by each receiver are corrections which come from another receiver belonging to another reception equipment of the same ground station. It is therefore not necessary to provide specialized receivers to make the corrections. Other characteristics and advantages of the invention will appear with the description of some of its embodiments, this being carried out with reference to the attached drawings in which the single figure is a diagram of a ground station for GPS differential system according to the invention.

In the example shown in the single figure, the ground station comprises three reception equipment, respectively 10, 12 and 14, and an assembly 16 for processing and transmission. The assembly 16 receives the signals supplied by the reception equipment 10, 12 and 14 and, on the basis of these signals, determines the correction signals to be transmitted to the mobile equipped with a differential navigation receiver (not shown). Conventionally, the transmission from the station to the mobile is carried out by radio in the VHF band.

The reception equipment 10, 12, 14 are identical to each other. They are arranged in locations close to each other. These locations are selected in such a way that the equipment cannot all be simultaneously subjected to a multipath disturbance.

The equipment 10, 12 and 14 being identical to each other, we will only describe one of them, namely that of reference 10. The equipment 10 comprises, on the one hand, a 20ι_ differential type navigation receiver and, on the other hand, a circuit 22 i for calculating the corrections.

The receiver 20] _ is practically identical to a conventional receiver which is usually installed inside a mobile. It thus comprises an input 24 _] _ for a correction signal and an output 26 ^ delivering, in succession, the raw measurements of the durations (measured by this receiver, without taking into account of correction signal) of signal paths of each satellite to this receiver. The raw measurement of a journey time consists in determining the instant of emission of the signal by the satellite - this instant being transmitted by the satellite

(i.e. contained in the transmitted signal) and to subtract it from the instant of reception. The correction calculation circuit 22 _] _ also receives, on an input 30] _, information representing the time taken by a signal moving at the speed of light to travel the distance separating the satellite from the receiver on the ground. This time does not constitute the result of a measurement but it is provided by a central computer (not shown) having in memory the evolution of the positions of each satellite as a function of time. Of course, the data supplied on the input 30ι_ of the circuit 22 _j _ and on the input 28 _] _ (connected to the output 26 _] _ of the receiver 20ι_) are in synchronism and correspond, at each instant, to the same satellite.

The circuit 22 - establishes the difference between the time applied to the input 30 _] _ and the time applied to the input 28] _ and provides on its output 32 _] _ the difference signal which constitutes a correction signal. This signal is transmitted, on the one hand, to circuit 16 and, on the other hand, to input 242 of receiver 2O2 of reception equipment 12.

The input 24 ^ of the receiver 20 _] _ is connected to the output 323 of the correction circuit 223 of the equipment 14. It allows the receiver 20-, _^ to receive a correction signal which it uses to correct the raw measurements journey times before using them, in a manner known per se, for triangulation operations with a view to estimating the geographical position which it delivers on its output 34 _x . The accuracy of this position estimate is of the order of 2 meters in the horizontal plane and 3 meters in the vertical plane.

Likewise, the output 322 of the correction circuit 222 of the equipment 12 is connected to the correction input 243 of the receiver 2O3 of the equipment 14.

The processing circuit 16 includes in memory the geographical position of each receiver. This processing circuit 16 determines, for each receiver 10, 12, 14, the distance DPj separating the measured position from the actual position. This circuit 16 compares these distances (or deviations) DPj_ with a threshold e. If all the deviations DP _] _, DP2 and DP3 are less than the threshold, the processing circuit 16 commands the transmission of correction signals (one correction signal per direction) to the navigation receiver of a mobile. Each correction signal is equal to the average, for example arithmetic, of the correction signals supplied, for the same direction, by each of the circuits 22 ^, 222 ^and 223. As a variant, the correction signal transmitted to the mobile is the one of the three corrections.

Of course, the correction signals are transmitted in sequence and correspond, at each instant, to the correction to be made in the direction of a determined satellite. If one of the receivers, for example the 2O2 receiver, receives a signal affected by multipaths, while the other two receivers receive the signals correctly, the difference DP2 is greater than the threshold e and the correction signal on the output 322 is excessive, which results in an inaccurate measurement by the 2O3 receiver. Under these conditions, the position deviation DP3 is also greater than the threshold. On the other hand, the 2O3 receiver performs a correct measurement. As a result, the correction signal supplied on output 323 has a correct value. Therefore, the position signal supplied on the output 34ι_ of the receiver 20τ_ is correct and, thus, the deviation DP ^ is less than the threshold e.

Thus, only the receiver 20 ^ provides a correct position signal, that is to say corresponding to a measured position which is separated from the actual position by a distance below the threshold e.

It is understood that a receiver providing a correct measurement necessarily provides a correct correction signal and the correction signal which it receives is also correct. Under these conditions, the processing circuit 16 controls the transmission to the mobile of a correction signal corresponding either to the average of the correction signals on the outputs 32 _] _ and 323 or to one of these two signals.

If all receivers receive multi-path signals, the three DP-j_ signals will be greater than the threshold. In this case, the circuit 16 prohibits the emission of a correction signal to the mobile.

If the processing circuit 16 detects only one gap DPj_ position, eg DP2 exceeds the threshold ^* e, while the other two gaps _DP] _ and DP3 are lower than the threshold th, the circuit 16 emits only the correction from circuit 223 because the fact that the position measured by the equipment 12 deviates too much from the actual position means that the correction signal sent on output 322 ^is inaccurate or that the correction signal received on input 242 ^is inaccurate.

Of course, the invention is not limited to a ground station comprising three reception equipment. It may include more. In this case, the periods during which no correction signal is sent to the mobile may be less frequent.

It is also possible to use two reception equipment instead of three. However, in this case, the blocking periods, that is to say the failure to transmit correction signals, may be more frequent than with three reception devices as described above. The system according to the invention thus allows a discrimination of multi-paths which overcomes the synchronism faults between receivers because, unlike previously known systems, there is no use of a synchronous comparison of the corrections in each direction. Furthermore, the criterion of discrimination is linked to the desired precision and not to the precision in each direction. In other words, compared to a conventional system, the invention will have less frequent blocking periods.

Claims

1. Station transmitting correction signals to a differential type navigation receiver which determines the position of a mobile from the measurement of the distances separating the mobile from at least three satellites or beacons, each measurement being corrected by a signal of correction, issued by the station, which represents the difference between the distance, measured at the station, from this station to the satellite concerned and the actual value of this same distance, said station comprising: - at least two reception equipment (10, 12, 14) located in different geographical locations chosen so as to minimize the probability that these two reception devices simultaneously receive multi-path affected signals, and each provided with distance estimation means delivering from signals from navigation satellites, estimates of the distances between them and the navigation satellites,

means for determining the correction signals comparing the sets of distance estimates provided by the reception equipment and the exact distances separating the reception equipment from the navigation satellites deduced from the knowledge of the geographical locations of the reception equipment and the knowledge of the positions of the navigation satellites in their orbits and drawing from these comparisons the correction signals, and

- transmission means for communicating said correction signals to the navigation receivers of the mobiles operating in the vicinity of the station. said station being characterized in that it further comprises:

- at each of its reception equipment, localization means delivering, from the distance estimates relative to the navigation satellites provided by the distance estimation means, a position estimate, said location means constituting with the distance estimation means a navigation receiver (20 _1; 20 ₂ , 20 ₃ ),

means for identifying reception equipment affected by multi-path phenomena operating by comparing the distance separating the position estimate provided by each reception equipment with its known geographical position from a threshold, and

2. Station according to claim 1, characterized in that the means for determining the correction signals are located in part (22- _L , 22 ₂ , 22 ₃ ) in each reception equipment (10, 12, 14) where they generate locally a set of correction signals, the different sets of correction signals coming from the different reception equipment being subsequently merged to obtain the correction signals to be transmitted.

3. Station according to claim 1 or 2, characterized in that each navigation receiver is of the differential type.

4. Station according to claims 2 and 3, characterized in that the correction signals produced within a reception equipment are used by the differential navigation receiver of another reception equipment of the station.

5. Station according to claim 4, characterized in that the means for determining the correction signals operate by means of different sets of correction signals not rejected by the selection means following an alert by the identification means

6. Station according to claim 4, characterized in that the means for determining the correction signals operate by selecting one of the sets of correction signals not rejected by the selection means following an alert by the means of 'identification.

7. Station according to claim 4, characterized in that, when the difference between actual position and measured position exceeds the threshold for reception equipment, the means for determining the correction signals discard the sets of correction signals from this receiving equipment and the correction signal set which is applied to its differential navigation receiver.

8. Station according to any one of the preceding claims, characterized in that the number of reception equipment is at least equal to three.

9. Station according to any one of claims 3 to 8, characterized in that the differential navigation receiver is of the same type as that used on board the mobile.

10. Application of the station according to any one of the preceding claims to a differential type GPS system.