Method and system for the geolocation of a radio beacon in a search and rescue system
A method for the geolocation of a device transmitting a signal containing at least one message to a plurality of relay satellites in a medium earth orbit, visible from said device, receiving said message and transmitting it to processing means, comprises at least the following steps: determination of the times of reception of the message by the relay satellites; determination of the pseudo-distances between the device and the relay satellites; searching for and acquiring a minimum number N of satellite radio navigation signals; determination of the time lags between the transmission of the radio navigation signals and their reception by the said device; broadcasting by the device of these time lags in the message; and, determination of the position of the device from at least the pseudo-distances, from the time lags and from the positioning coordinates of the relay satellites and of the radio navigation satellites.
1. A method of geolocation of a device transmitting a signal containing at least one message to a plurality of N vis relay satellites visible from said device, the method comprising:
determining times of reception T Ri of said message by the N vis relay satellites,
determining, from said times of reception T Ri and with respect to a time T e of transmission of the message by the device, the pseudo-distances D i between said device and each of said relay satellites N vis
searching for and acquiring a number N of satellite radio navigation signals, N being at least equal to 2+m−N vis where m is a number of space coordinates of the device and N vis is the number of relay satellites visible from the device,
determining an information on a time lag between the time of transmission of each radio navigation signal j and the time of reception of each radio navigation signal j T Ri by the device,
determining, from said information, the pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites, and
determining the position of the device from the pseudo-distances D j between the device and each of the N vis relay satellites and from the pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites.
2. The geolocation method according to claim 1 , wherein the relay satellites are part of a constellation of a search and rescue system.
3. The geolocation method according to claim 1 , wherein said signal is a distress signal containing an alerting message.
4. The geolocation method according to claim 1 ,
wherein said pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites are determined by the additional solving of the equation
T Rj (GNSS)= D j (GNSS)/ c+T ej (GNSS),
where T Rj (GNSS) is the time of reception, by the device, of the radio navigation signal transmitted by the radio navigation satellite j, T ej (GNSS) is the time of transmission by the radio navigation satellite of the radio navigation signal and D j (GNSS) is the pseudo-distance between the device and the radio navigation satellite.
5. The geolocation method according to claim 1 , wherein the time of transmission T e of said message is measured by said device and transmitted in the message to said relay satellites, the minimum number N of searched radio navigation signals then being reduced by one when the number N vis of relay satellites visible from said device is strictly less than 1+m.
6. The geolocation method according to claim 1 , further comprising:
measurements of the reception frequency of said signal transmitted by the said device to said relay satellites,
determination of the position of said device being carried out from the measurements and from the transmission frequency of said signal.
7. The geolocation method according to claim 1 , wherein the positioning coordinates of the relay satellites and/or of the radio navigation satellites are determined from the ephemeredes of the satellites.
8. The geolocation method according to claim 1 further comprising transmitting said information with said message.
9. The geolocation method according to claim 1 , wherein said pseudo-distances Di between the device and each of the N vis relay satellites are determined by solving the system of equations
T Ri =D i /c+T e ,
where c is the speed of propagation of the transmitted signal, T e is the time of transmission of the message by the device and i varying from 1 to the number N vis of visible satellites.
10. The geolocation method according to claim 1 , wherein said pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites are determined by solving the following equations:
T Rj (GNSS)= D j (GNSS)/ c+T ej (GNSS),
where T Rj (GNSS) is the time of reception, by the device, of the radio navigation signal transmitted by the radio navigation satellite j, T ej (GNSS) is the time of transmission by the radio navigation satellite of the radio navigation signal and D j (GNSS) is the pseudo-distance between the device and the radio navigation satellite j.
11. The geolocation method according to claim 1 , wherein the relay satellites are in a medium earth orbit (MEO).
12. A system for the geolocation of a device able to transmit a message to a plurality of relay satellites, said system comprising:
a first module for determining the times of reception T Ri of the message by each of a number N vis of relay satellites,
a second module for determining, from said times of reception T Ri and with resect of a time T e of transmission of the message by the device, the pseudo-distances D i between the device and each of the N vis relay satellites,
a third module for searching for and acquiring a number N of satellite radio navigation signals, N being at least equal to 2+m−N vis where m is a number of space coordinates of the device and N vis is the number of relay satellites visible from the device,
a fourth module for determining an information on the time lag between the time of transmission of each radio navigation signal j and the time of reception T Ri , by the device, of each radio navigation signal j, and transmitting said information to a fifth module for determining, from said information, the pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites,
a sixth module for determining the position of the device from the pseudo-distances D i between the device and each of the N vis relay satellites and from the pseudo-distances D j (GNSS) between the device and each of the N radio navigation satellites.
13. The system according to claim 12 , wherein the relay satellites are part of a constellation of an SAR alerting and rescue system.
14. The system according to claim 12 , wherein said first module, said second module, said third module, said fourth module, said fifth module, and said sixth module are located in a ground station.
15. The system according to claim 12 , wherein said relay satellites comprise means of transmission of a radio navigation signal.
16. The system according to claim 12 , wherein said device is a radio beacon and said signal is a distress signal containing an alerting message.
17. The system according to claim 12 , wherein the first module is located on each relay satellite, the second module is located in a ground station, the third and fourth module are located in the device, the fifth and sixth modules are located in said ground station.
18. The system according to claim 12 , wherein the first module is further configured to transmit the times of reception T Ri of the message by each of a number N vis of relay satellites to a ground station, the first and the second module are located in a ground station, the third and fourth module are located in the device, the fifth and sixth modules are located in said ground station.