HK1170571B

HK1170571B - Method and system for the geolocation of a radio beacon in a search and rescue system

Info

Publication number: HK1170571B
Application number: HK12111338.4A
Authority: HK
Inventors: T．卡尔梅特; M．莫内拉
Original assignee: 泰勒斯公司
Priority date: 2010-10-22
Filing date: 2012-11-09
Publication date: 2017-02-24

Abstract

Method for the geolocation of a device (101) transmitting a signal containing at least one message to a plurality of relay satellites (102a,102b,102c,102d,102e) in a medium earth orbit (MEO), visible from the said device, receiving the said message and transmitting it to processing means (203), characterized in that it comprises at least the following steps: o determination of the times of reception TRi of the said message by the said relay satellites, o determination of the pseudo-distances Di between the said device (101) and the said relay satellites, o searching for and acquiring a minimum number N of satellite radio navigation signals, o determination of the time lags between the transmission of the said radio navigation signals and their reception by the said device (101), o broadcasting by the said device (101) of these time lags in the said message, o determination of the position of the said device (101) from at least the said pseudo-distances, from the said time lags and from the positioning coordinates of the said relay satellites and of the said radio navigation satellites.

Description

Method and system for geo-location of radio beacons in search and rescue system

Technical Field

The present invention relates to a method and system for geo-locating a radio beacon using information transmitted within a search and rescue system associated with the radio beacon.

Background

The search and rescue System (SAR) consists of one or more constellations of satellites that receive alarm signals from radio beacons on the uplink. The signal is transmitted at an international distress frequency. The alarm signal is again transmitted to the ground station responsible for extracting the distress information from the alarm signal, and the distress information is then sent to the task control center.

The known SAR system is the global Cospas-Sarsat system, which is mainly used for detecting a boat, airplane or personal accident. The Cospas-Sarsat system uses, among other things, a constellation of low orbit satellites called LEOSAR (lowearth orbit search and rescue) to receive and send alarm messages to ground stations.

Another purpose of the SAR system is to locate the radio beacon transmitting the distress signal. For this purpose, the use of low orbit satellites makes it possible to position by Doppler-fizeau effect (Doppler-fizeau effect). A single satellite uses the frequency of arrival of the alert message data at several successive times within the time record during its movement. Since the frequency of arrival of the received signal is different at each instant, the location of the radio beacon can be derived therefrom.

However, the greatest disadvantage of positioning according to the doppler-fizeau effect is that the positioning time is too long, since a single satellite has to perform several successive measurements during its movement before the radio beacon position can be derived therefrom. Furthermore, for positioning applications that require very precise positioning, the frequency measurement is not of sufficient accuracy. Finally, in order to have sufficient relative velocity, doppler-fizeau measurements are mainly used for low orbit satellites, which has the following disadvantages: the service life of the satellite is short and the coverage of the small constellation is low (typically about 35% for 6 satellites).

The Cospas-Sarsat system was developed using a new constellation of satellites called MEOSAR (mediumearth orbits search and rescue) with higher orbits. These satellites are located in orbits mainly used by GNSS (global navigation satellite system) satellites such as those of the GPS or GALILEO system. This orbit is called the Medium Earth Orbit (MEO), corresponding to a spatial region between 2000 km and 35000 km. It is therefore possible to use the same satellites to benefit from both SAR alerting and GNSS geolocation functions. This possibility is foreseen in the first generation GALILEO satellites and the third generation GPS satellites in the coming years.

Figure 1 is an overview of such a system using GALILEO satellites. A radio beacon 101 communicates with a constellation of SAR satellites 102a, 102b, 102c, 102d, 102 e. At least one of the satellites 102c is also a GNSS satellite. Some of these satellites may also have only geolocation functionality. The radio beacon 101 transmits its distress information in an alert message to the SAR satellites 102b, 102c over uplinks 111, 114. The alarm message is then transmitted to the ground station 104 via the downlinks 112, 115. The positioning of the radio beacon 101 is mainly performed by using a GNSS receiver in the beacon, whereby the calculated position is transmitted via an uplink between the beacon and the satellite. The receiver receives positioning signals from at least four visible GNSS satellites and can derive its position therefrom by known means. The location is then transmitted along with the distress message via the downlink 111, 115 to the ground station 104, and the ground station 104 can then transmit the beacon location to the control center. The advantage of using medium orbit satellites is that at least one of these satellites is always in ground view, which ensures that confirmation of the reception of the alarm message by the ground station is provided.

However, the use of a positioning receiver disposed in a radio beacon has disadvantages in that a positioning process is complicated and the beacon is lost. In particular, in order to locate itself, a GNSS receiver must first search for at least four visible geolocation satellites. For example, to calculate the first point, decoding the GPS signal would take 30 seconds to 1 minute. This non-negligible processing time directly affects the autonomy of the beacons.

Disclosure of Invention

The invention has the following objects in particular: complexity and loss of radio beacons are reduced by using the functionality of the alarm system to determine position location directly without using or limiting the use of GNSS receivers. One of the objects of the invention is to also reduce the lock time before a position fix is determined. The joint use of SAR systems and GNSS systems is one assumption in order to utilize all available resources in an optimal way.

The invention relates to a method for geo-locating a device that transmits a signal comprising at least one message to a plurality of relay satellites in medium earth orbit visible from said device, receives said message and transmits it to a processing device, characterized in that it comprises at least the following steps:

○ determining the time of receipt T of the message by the relay satellite_Ri，

○ by solving equation set T_Ri＝D_i/c+T_eDetermining a pseudorange D between the device and the relay satellite_iWhere c is the propagation speed of the transmission signal, T_eIs the time the device transmits the message, i is from 1 to the number of visible satellites N_visThe change is that the number of the first and second,

○ at least from said pseudorange D_iAnd the location coordinates of the relay satellite determine the location of the device.

In a variant embodiment of the invention, the relay satellite is part of a constellation of SAR alarm and rescue systems.

In a variant embodiment of the invention, the signal is a distress signal comprising an alarm message.

In a variant embodiment of the invention, the method further comprises:

○ searching and acquiring step, searching and acquiring N satellite radio navigation signals by means of a receiving device included in said apparatus, N being at least equal to 2+ m-N_visWhere m is the number of spatial coordinates of the device, N_visIs the number of relay satellites visible from the device,

determining the time lag between the transmission of the radionavigation signals and the reception of these signals by the device,

these time lags are broadcast by the device in the message,

○ determine the position of the device by additionally solving the equation T_Rj(GNSS)＝D_j(GNSS)/c+T_ej(GNSS), wherein T_Rj(GNSS) is the time of reception, T, of the beacon receiving the radio-navigation signal transmitted by the GNSS satellite j_ej(GNSS) is its transmission time, D_j(GNSS) is the pseudorange between a beacon and a GNSS satellite j.

In a variant embodiment of the invention, the number of relay satellites in view is N_visStrictly less than 1+ m, the steps of searching for and acquiring satellite radio-navigation signals are started.

In a variant embodiment of the invention, the transmission time T of the message is_eMeasured by the device and transmitted in a message to the relay satellites which transmit them to the processing device in their order when the number of relay satellites in view of the device is N_visStrictly less than 1+ m, the minimum number N of radio navigation signals searched is reduced by one.

In a variant embodiment of the invention, the method further comprises measuring the reception frequencies of said signals transmitted by said device to said relay satellites, and further determining the position of said device by means of these measurements and the transmission frequencies of said signals.

In a variant embodiment of the invention, the position information of the device is transmitted to the relay satellite together with the message and then to said processing device.

In a variant embodiment of the invention, the positioning coordinates of the relay satellites and/or of the radionavigation satellites are determined from the ephemeris of these satellites.

The invention also relates to a system for the geographical positioning of a device, characterized in that it comprises a device capable of transmitting a message; a plurality of relay satellites in medium earth orbit capable of receiving and broadcasting said messages; and, a processing device capable of determining a location of the apparatus; said system being suitable for implementing the method according to the invention.

In a variant embodiment of the invention, the processing device is located at a distance from the relay satellite or is installed in the relay satellite.

In a variant embodiment of the invention, said relay satellite further comprises means for transmitting radio navigation signals.

In a variant embodiment of the invention, the apparatus is a radio beacon, the signal is a distress signal containing an alarm message, and the processing device transmits the alarm message to a control center.

Drawings

Other features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, in which:

figure 1 is a diagram of an alarm system with a constellation of satellites in MEO orbit,

fig. 2 is a block diagram of a geolocation system in accordance with the present invention.

Detailed Description

Fig. 2 is a block diagram of a geolocation system of radio beacons according to the present invention. The beacon 101 includes a device that generates and transmits an alarm signal containing distress information. It also comprises a device for receiving radio-navigation signals, for example signals transmitted by GNSS satellites. The radio beacon 101 communicates with at least one relay satellite 102, the relay satellite 102 comprising a first device 201 for receiving an alarm signal and transmitting said signal to a ground station 203. The relay satellite 102 also comprises a second radio-navigation device 202 able to transmit GNSS signals to the terrestrial beacon 101. The receiving apparatus 201 and the radio navigation apparatus 202 may be located on two different satellites. The ground station 203 receives the messages contained in the alarm signals and transmits them to a control center, not shown. The programmable device 204 makes it possible to generate an acknowledgement for the beacon 101. The remote controller 205 is used to control the satellite.

In the following description, a relay satellite refers to a satellite including the device for receiving an alarm signal 201, for example, a satellite compatible with the SAR system.

One benefit of using satellites in MEO orbits is that several satellites are likely to be seen at the same time from a point on the ground at any time. In contrast, for alarm systems using a low-orbit (LEO) constellation, often only one satellite is visible, which results in the use of a geolocation method based only on doppler information resulting from the motion of this single satellite.

As described above, when an accident occurs, the radio beacon 101 transmits an alarm signal at the distress frequency to all visible satellites including the device 201 that receives this signal. The alarm signal is broadcast, that is, the alarm signal is transmitted to all satellites listening to the frequencies in distress. The advantage of broadcasting is that it does not require an initial search procedure to determine which satellites are visible to the beacon. When the alarm message is received by satellite i, the latter is able to record the time of reception T according to its internal clock_Ri. Based on this information, the time T can be determined_eAnd time T_RiAnd the distance D between the beacon and the satellite_iAssociated expression of where T_eTransmission time, T, for transmission of the alarm message by the radio beacon_RiFor receiving messages for satellitesReceiving time, the expression is as follows:

T_Ri＝D_i/c+T_e(1)

where c is the propagation speed of the transmission signal and i varies from 1 to the number of visible satellites. Because of the time T_RiAnd T_eNot measured by the same clock, and there may be time asynchronism between the radio beacon clock and the satellite clock, so the term pseudorange D is used_i"rather than" true distance, "which is obtained after an estimate of the time shift between the two clocks.

The satellite positions are known by themselves or by the ground station 203. Expression (1) contains four unknowns, the three spatial coordinates of the beacon 101 and the transmission time T of the alarm message_e. Thus, if at least four satellites are visible from the beacon, the system of equations obtained can be solved to obtain an accurate position fix of the beacon therefrom.

In the case of two-dimensional positioning of beacons only, then expression (1) contains only three unknowns and only three satellites in view are needed. In general, the number of satellites required is equal to the number of unknowns contained in expression (1). If m is the number of coordinates of the radio beacon, the number of visible satellites required to obtain the location of the radio beacon is equal to 1+ m.

In a variant embodiment of the invention, the radio beacon may also comprise a transmission time T in the transmitted alarm message_e. In this case, the fourth unknown to be removed corresponds to the time at T due to the lack of synchronization between the satellite clock and the beacon clock_eAnd a reception time T_RiThe time uncertainty in between.

Thus, by using the invention with four visible satellites containing only devices for receiving alert messages, it is possible to completely dispense with the use of GNSS receivers in radio beacons, which brings significant advantages in terms of complexity, autonomy and processing time.

The time of reception measurements are made on the satellite and transmitted, together with the alarm message and possibly the satellite position, to the ground station, which is responsible for the necessary processing to solve a set of equations with four unknowns so that the position of the beacon 101 can be obtained. For this purpose, the ground station 203 comprises a device for processing the received information. Alternatively, the system of equations may also be solved by retransmitting the beacon's positioning information directly to the payload on the satellite of the ground station.

In variant embodiments of the invention, the measurement of the reception time may be replaced completely or partially by a doppler frequency measurement. Because the velocity of the satellite in the MEO orbit is lower than the velocity of the satellite in the low LEO orbit, however, the accuracy associated with these doppler measurements is also lower.

In practice, particularly in urban environments where there are numerous obstacles, the number of visible satellites may be less than four. In the case where at most three relay satellites are visible, the invention makes possible the measurements necessary to complete the geolocation of the beacon by making supplementary measurements on the radio navigation signals received by the beacon.

It is known that radio navigation signal receivers use measurements on signals from at least four satellites in order to determine information related to their positioning. For each satellite, the time of reception of a signal by a beacon is related to the time of transmission of the signal by the satellite and the distance of the satellite from the receiver. The spatial coordinates of the satellite are transmitted in the signal and the receiver must therefore fully demodulate the signal.

In the case of the present invention, the satellite position is known by the ground station 203, either by directly transmitting this information with an alarm message for the satellite to perform this function, or by means of ephemeris. Thus, the radio-navigation signals received by the beacon do not require complete demodulation, but only an estimate of the difference between the satellite transmission time and the beacon reception time. The estimate is calculated by detecting and receiving a time record of the correlation peak in the signal. For example, in GALILEO signals, the correlation peak, also known as "Pilot tone ", occurs every 4 ms. The time recording of the correlation peak makes it possible to obtain an item of time lag information between the time of transmission of the GNSS satellite and the time of reception of the beacon. There is still time ambiguity because the satellite clock and beacon clock are not synchronized. Removing the ambiguity is not necessary, since then the unknown quantity T_eCan be determined directly under the time reference of the satellite clock. This information is then transmitted with the alarm message to the relay satellite 102 and subsequently to the ground station 203, making it possible to determine beacon positioning in combination with the time of reception measurement of the satellite alarm message.

Then, the equation set (1) is completed by the following equation:

T_Rj(GNSS)＝D_j(GNSS)/c+T_ej(GNSS)(2)

wherein, T_Rj(GNSS) is the time of reception, T, of the beacon receiving the radio-navigation signal transmitted by the GNSS satellite j_ej(GNSS) is its transmission time, D_j(GNSS) is the pseudorange between a beacon and a GNSS satellite j.

In a variant embodiment of the invention, the complete part of the radio-navigation signal for a sufficient time to measure the correlation peak can be transmitted by the radio beacon on the uplink together with its final destination ground station, the ground station being responsible for making the measurements. These variables constitute solutions that are easier to implement for beacons, but the amount of data that is transmitted again on the uplink is more expensive, which can be limited from the point of view of the available bandwidth of the uplink.

In practice, the system includes five unknowns, which are the three spatial coordinates of the beacon, the transmission time of the alert message, and the time ambiguity on the time record of the correlation peak of the radio navigation signal. In this case, the present invention can be realized by the following configuration: three relay satellites and two radio navigation satellites, or two relay satellites and three radio navigation satellites. If only one relay satellite is visible, the radio beacon will have to search for four radio navigation satellites, as in conventional GNSS systems, but without the need to fully demodulate the signals in order to obtain the accurate position of the satellite as mentioned before.

In a variant embodiment of the invention, the transmission time of the alarm message may be transmitted by the beacon together with said message in a unit corresponding to its internal clock. The system then has only four unknowns. The present invention can be realized by the following configuration: three relay satellites and one radio navigation satellite, or two relay satellites and two radio navigation satellites, or one relay satellite and three radio navigation satellites.

More generally, if N is the number of visible relay satellites of the radio beacon, the radio beacon must perform a minimum of 5-N searches on the radio navigation signal if the transmission time of the warning message is not transmitted, and the radio beacon must perform 4-N searches if the transmission time of the warning message is transmitted.

The above-described embodiments of the present invention are based on the SAR system as an example. The invention also applies to any system for collecting data by satellite, in which the data is broadcast from a radio beacon to a receiving device on the satellite and retransmitted to a ground station, without departing from the scope of the invention. In addition, the invention applies in the same way to any device equivalent to a radio beacon, comprising on the one hand the means for generation of messages, recording of times and transmission to the satellites and on the other hand the means for receiving and processing radio-navigation signals from the satellites.

In short, the method and system according to the invention have the advantage of reducing the complexity of the processing operations to be taken to locate the radio beacon, and also of reducing the processing time that must be spent before the first measurement point is determined.

The invention uses the features of both the alarm system and the radio navigation system in combination, the satellites of both systems being located in the same medium earth orbit, which makes it possible to see from a point on the ground a sufficient number of satellites to determine the items of positioning information by calculating pseudoranges.

Claims

1. A method of geolocation of a device (101), said device (101) transmitting signals containing at least one message to a plurality of relay satellites (102a, 102b, 102c, 102d, 102e) located on medium earth orbits visible from said device, said relay satellites receiving said message and transmitting said message to a processing device (203), comprising at least the steps of:

○ by solving equation set T_Ri＝D_i/c+T_eDetermining a pseudorange D between the device (101) and the relay satellite_iWhere c is the propagation speed of the transmitted signal, T_eIs the time at which the device (101) transmits the message, i is from 1 to the number N of relay satellites visible from the device (101)_visThe change is that the number of the first and second,

○ at least from said pseudorange D_iAnd the location coordinates of the relay satellites determine the position of the device (101),

○ searching and acquiring step, searching and acquiring N satellite radio navigation signals, N being at least equal to 2+ m-N, by means of a receiving device comprised in said apparatus (101)_visWhere m is the number of spatial coordinates of the device (101), N_visIs the number of relay satellites visible from the device (101),

determining the time lag between the transmission of the radionavigation signals and the reception of these signals by the device (101),

these time lags are transmitted by the device (101) in the message,

○ the position of the device (101) is determined by additionally solving the equation T_Rj(GNSS)＝D_j(GNSS)/c+T_ej(GNSS), wherein T_Rj(GNSS) is the time of reception, T, of the beacon receiving the radio-navigation signal transmitted by the GNSS satellite j_ej(GNSS) is its transmission time, D_j(GNSS) is the pseudorange between a beacon and a GNSS satellite j.

2. Method of geolocation of a device (101) according to claim 1 characterized in that the relay satellites (102a, 102b, 102c, 102d, 102e) are part of the constellation of search and rescue systems.

3. Method of geolocation of a device (101) according to one of claims 1 and 2 characterized in that said signal is a distress signal containing an alarm message.

4. Method for the geolocation of a device (101) according to claim 1 characterized in that it consists inNumber of visible relay satellites N_visStrictly less than 1+ m, the steps of searching for and acquiring satellite radio-navigation signals are started.

5. Method for the geolocation of a device (101) according to claim 4, characterized in that said message has a transmission time T_eMeasured by the device (101) and transmitted in a message to the relay satellites, which transmit them to the processing device (203) in their order, when the number N of relay satellites in view of the device_visStrictly less than 1+ m, the minimum number of radio navigation signals searched is reduced by one.

6. Method of geolocation of a device (101) according to claim 5 characterized in that it further comprises measuring the reception frequency of said signals transmitted by said device (101) to said relay satellites, further determining the position of said device (101) from these measurements and the transmission frequency of said signals transmitted by said device (101) to said relay satellites.

7. Method of geolocation of a device (101) according to claim 1 characterized in that the location information of the device (101) is transmitted to a relay satellite together with the message and then to said processing device (203).

8. Method of geolocation of a device (101) according to claim 1 characterized in that the location coordinates of the relay satellites and/or of the radionavigation satellites are determined from the ephemeris of these satellites.

9. A system for geolocation of a device (101), characterized in that it comprises, a device (101) capable of transmitting messages; a plurality of relay satellites (102a, 102b, 102c, 102d, 102e) located in medium earth orbit and capable of receiving and broadcasting said messages; and, a processing device (203) capable of determining a location of the apparatus; the system is adapted to implement the method according to one of claims 1 to 8.

10. The system for geolocation of device (101) according to claim 9, characterized in that said relay satellites (102a, 102b, 102c, 102d, 102e) are part of a constellation of SAR alarm and rescue systems.

11. System for the geolocation of a device (101) according to one of claims 9 and 10, characterized in that said processing device (203) is distant from or installed in said relay satellite.

12. The system for the geolocation of a device (101) according to claim 11 characterized in that said relay satellites further comprise means to transmit radionavigation signals.

13. The system for geolocation of a device (101) according to claim 12 wherein said device is a radio beacon, said signal containing said message is a distress signal containing an alarm message, said processing device (203) transmitting said alarm message to a control center.