GB2364191A

GB2364191A - Covert reference station

Info

Publication number: GB2364191A
Application number: GB0013080A
Authority: GB
Inventors: Timothy John Spriggs
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2000-05-30
Filing date: 2000-05-30
Publication date: 2002-01-16
Also published as: GB0013080D0

Abstract

A covert reference station 202 for a satellite-based navigation system, such as GPS, is used to generate local position error corrections without an accurate survey of the location of the reference station being necessary. The signal from each satellite has a carrier wave and includes coded data. A pseudorange from the reference station to each satellite is calculated from the time of flight of the corresponding satellite signal (210). The position of the reference station is established by measuring the phase of the carrier wave for each satellite signal (212). Knowing the position, true range measurements are generated (216) and compared with the pseudoranges for each satellite signal (218). The result is a set of local position error corrections (214), which allow a remote receiver unit 100 to correct for factors such as local atmospheric conditions. For covert operations, the reference station may be camouflaged and arranged only to transmit local error corrections when a valid interrogation signal is received.

Description

2364191 COVERT REFERENCE STATION The present invention relates to a covert

reference station In particular a covert differential Global Positioning System (GPS) reference s station.

Conventional GPS comprises a constellation of Earth-orbiting satellite vehicles (S Vs), GPS control segment and a plurality of GPS receiver units The S Vs each transmit GPS signals which are continually updated by a master control station Each GPS signal emitted by a given SV carries coded information The coded information includes the position and velocity of the given satellite vehicle (ephemeris data) and the time the GPS signal was transmitted.

In the normal (code measurement) mode of operation of GPS, the location of a given GPS receiver unit is determined relative to the constellation of S Vs As the position of each SV and the time of transmission of each GPS signal is known from the information in the GPS signal and as the time of arrival of each GPS signal can be recorded, a current estimate of the location (or 'position solution') of the given GPS receiver unit may be calculated The location of the given GPS receiver unit can be determined unambiguously provided the GPS signals from four or more satellites can be received at one time.

Each SV has a clock and each satellite clock is kept synchronised with every other satellite clock by the master control station Since the internal clock of the given GPS receiver unit must also be synchronised with the satellite clocks, there is provision for obtaining clock correction information from the GPS signal.

Typical military receiver position measurements are accurate to within a radius of thirteen metres with a confidence level of 95 % A significant source of error in the position measurements is due to time varying local phenomena, notably including atmospheric conditions.

A technique known as Differential GPS (DGPS) can be used to reduce the error in position measurement In addition to the component units of the standard GPS, DGPS requires at least one reference station.

The DGPS reference station is accurately surveyed so that the position is known precisely Range measurements with no correction for synchronisation errors between an SV transmitter and a remote GPS receiver unit are known as "pseudoranges" Since the GPS signals carry ephemeris data for the S Vs and the position of the reference station is known, the true ranges to each SV can be calculated The reference station then uses GPS in normal mode to establish pseudoranges to each SV for which a GPS signal can be received (each SV "in view") Only GPS signals from S Vs in view can be used in establishing the pseudoranges from the reference station In this way, the reference station can estimate the errors introduced by local phenomena, for example atmospheric conditions The reference station generates local error corrections by comparing the true ranges and the pseudoranges The reference station can then transmit the estimated error corrections to remote GPS receiver units.

The location of the remote GPS receiver units can then be calculated with a very much reduced error; the error can be more than halved The benefit of this reduced error becomes less pronounced with increasing distance between the remote GPS receiver units and the GPS reference station At distances within two hundred and fifty kilometres, the reduced error is significant; the error being limited to within a radius of seven metres with a 95 % confiden le l.

An alternative technique for improving the accuracy of position measurements relies on measurements of the phase of the carrier wave of the GPS signals transmitted by GPS satellites rather than the coded information present in the GPS signal In effect, a GPS receiver counts the s number of cycles of the carrier wave there are between the satellite and the GPS receiver The phase measurement technique can take some time to resolve the initial ambiguity in the count of cycles but once a correct count has been established position measurement errors can be reduced to less than one metre with 95 % confidence The term 'ambiguity' here refers to 1 o the difficulty in obtaining a correct count of the integer number of cycles which have elapsed; the phase, or fractional remainder, can be recovered immediately but a coarse estimate of the position must be found before the integer count can be established with any degree of accuracy Carrier phase differential positioning currently requires a GPS reference station within the order of tens of kilometres of the GPS receiver unit.

In military applications of GPS position measurement, for example position measurement for targeting purposes, it is necessary to reduce the error in position to less than the thirteen metre error margin obtainable in conventional GPS operation Straightforward use of the DGPS technique to reduce the error is unlikely to be successful in military applications since an accurately surveyed DGPS reference station deployed near an enemy would have only a limited lifespan Indeed, if the necessary preliminary surveying activity is observed, the reference station may come under attack before the DGPS system can go into service.

It is therefore an object of the invention to obviate or at least mitigate the aforementioned problems.

In accordance with the present invention, there is provided a covert reference station device having:

a housing; a receiver means capable of receiving both coded information and carrier phase information transmitted in each of a plurality of satellite signals, each satellite signal having a carrier wave and each satellite signal 'including coded information; a radio transceiver which transmits error corrections to at least one remote receiver unit; and a processing means; wherein receiver means, the radio transceiver and the processing lo means are all shock-mounted within the housing.

Advantageously, the receiver means includes a first receiver for receiving coded information transmitted in a given satellite signal and a second receiver for measuring the phase of the carrier wave of the given satellite signal.

Preferably, the first receiver is a first GPS receiver, the second receiver is a second GPS receiver, the satellite signal is a GPS signal and the remote receiver unit is a remote GPS receiver unit.

Preferably, the second receiver uses the phase of the carrier wave of the given satellite signal to establish the position of the covert reference station device; the first receiver is used to calculate pseudoranges for each of a plurality of satellite signals; the processing means calculates true ranges from the coded information carried in each satellite signal using the established position of the covert reference station device and compares the pseudoranges with the true ranges to give local code measurement corrections; and the radio transceiver transmits the local code measurement corrections to the at least one remote receiver unit.

The housing is preferably camouflaged Alternatively, the housing may be disguised as an unexploded bomb.

The covert reference station device may be arranged to be deployed by an aircraft in a similar manner to a bomb without significant damage.

A covert differential GPS reference station can thus be dropped into an area and still work out its own position using carrier-phase and other s measurements which it can then use to provide local code measurement corrections to remote GPS receiver units, for instance the GPS receiver units carried by bomber aircraft or the GPS receiver units embedded in effector weapons.

The remote receiver unit may be carried by a mission platform, for lo example a bomber aircraft Alternatively, the remote receiver unit may be carried by a weapon, for instance an effector weapon, a bomb or a missile.

Advantageously, the reference station does not begin to transmit local code measurement corrections until receipt of a valid interrogation signal.

The concept of the reference station not transmitting until interrogated is advantageous since it reduces the likelihood of early detection of the device.

The reference station device is preferably fitted with a self-destruct mechanism The reference station device may be fitted with a commanded destruct mechanism.

The destruct mechanism may obliterate the entire reference station device or simply render the electronics thereon useless.

The reference station is advantageously fitted with anti-tampering apparatus and/or apparatus for safe decommissioning.

According to a second aspect of the present invention there is provided, a global navigation system including a covert reference station as above, a plurality of satellite vehicles, a master control station and at least one remote receiver unit, wherein the covert reference station receives satellite signals from each of the satellite vehicles and transmits error correction signals to the at least one remote receiver unit.

In accordance with a third aspect of the present invention there is provided a method of operating a covert reference station device having a receiver means, a radio transceiver and a processing means; the method including:

receiving a plurality of satellite signals on the receiver means, each satellite signal having a carrier wave and including coded information; measuring the distance travelled by each satellite signal from the 1 o time of arrival at the receiver means and from the coded information and generating a pseudorange for each satellite signal; measuring the phase of the carrier wave for each satellite signal; repeatedly estimating the position of the covert reference station device using the corresponding measured phases of the carrier waves of the plurality of satellite signals; calculating the true range corresponding to each satellite signal from the known position of the covert reference station device and from the coded information; comparing the pseudorange to the true range and generating local code measurement corrections; and using the radio transceiver to transmit the local code measurement corrections to remote receiver units.

Advantageously, the last step above includes: receiving an interrogation signal and transmitting the local code measurement corrections on receipt of a valid interrogation signal.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:

FIGURE 1 shows a conventional differential GPS configuration; FIGURE 2 shows a covert differential GPS configuration including a first embodiment of a covert reference station device according to the present invention; and FIGURE 3 shows a second embodiment of a covert reference station device according to the present invention.

Throughout the following discussion identical reference signs correspond to like features.

Figure 1 shows a conventional differential GPS configuration as 1 o described above The position of reference station 102 is known precisely through an accurate survey The reference station 102 has a GPS receiver 110, a radio transmitter 114 and a computer 120 The computer calculates "pseudorange estimates" (step 116) which are the true ranges to each SV in view, given the known position of the reference station 102 and provided the ephemeris data from the S Vs are correct The GPS receiver 110 makes code phase measurements from the GPS signals that can be received resulting in pseudorange measurements The pseudorange measurements are compared to the true ranges output by the computer 120 (step 118).

The result of the comparison is used to calculate the necessary error reducing correction The corrections are then transmitted by the radio transceiver 114 for reception by a remote GPS receiving unit 100.

The remote GPS receiving unit 100 comprises a radio receiver 104 for obtaining the corrections for local code measurements, a GPS receiver 106 for receiving code from S Vs in view and a computer 108 for calculating corrected pseudoranges and for deriving precise position solutions for navigation The GPS receiver 106 generates pseudoranges for each SV in view and the corrections received by the radio receiver 104 are applied to the pseudoranges to give pseudoranges that are corrected for local errors.

Figure 2 illustrates a covert differential GPS configuration in accordance with the present invention, which has a deployable reference station device 202 and a mobile GPS receiver unit 100, identical to that in Figure 1 The deployable reference station device 202 is shock-mounted in a housing (not shown) and comprises a first GPS receiver 210 capable of making code measurements, a second GPS receiver 212 capable of making carrier phase measurements, a radio transceiver 214 and a computer 220.

io The housing (not shown) is capable of being dropped by an aircraft in a similar manner to a bomb.

The shock-mounted reference station device 202 may be carried and delivered as part of a genuine bombing raid Alternatively, the aircraft can specifically deploy the reference station device 202 by pretending to be aborting a mission and jettisoning a payload including the reference station device 202 In either circumstance the reference station device 202 will be disposed at some distance from prime targets to reduce the likelihood of detection or tampering Potentially, the reference station device 202 may be disposed in enemy territory The reference station device 202 might be camouflaged or disguised as a device to be avoided, for example an unexploded bomb or sub-munition In a further covert operations measure, the reference station device 202 is arranged to start transmitting only when a valid interrogation signal is received A valid interrogation may include the transmission of an identification code and/or the use of encryption.

It will be understood that the deployment of a reference station device may be achieved by methods other than dropping from aircraft, for instance a reference station device may be sent out to a suitable location on a surface vehicle, for example a truck or a boat, and left in a concealed place Alternatively a reference station device could be carried by hand to a suitable location.

GPS measurement and tracking operations can begin even before the reference station device 202 is safely on the ground Once in position and in normal operation, the reference station device 202 will first obtain a conventional GPS position estimate from code measurements, using the first GPS receiver 210 Bearing in mind that the reference station device 202 is stationary, these code measurements can then be improved upon using carrier phase measurements made using the second GPS receiver lo 212 Due to the errors in the system, the position solution is not static.

However, the reference station device 202 is not moving with respect to the ground (nor therefore is the second GPS receiver 212) and as a result, the reference station device 202 must lie in the intersection of all the uncertainties surrounding successive position solutions.

is Once a precise position solution has been discovered, the reference station device 202 can measure the pseudoranges for the S Vs in view, using code measurements received by the first GPS receiver 210 True range measurements are calculated (step 216) from the established position solution for the reference station and from the ephemeris data received in the GPS signal, by the computer 220 The computer 220 then compares the pseudoranges with the true range measurements to obtain the differential GPS corrections (step 218).

When an aircraft making a precision attack approaches, the mobile GPS unit on board the aircraft will interrogate the reference station device 202, the interrogation signal being received by the radio transceiver 214.

Upon valid interrogation, the reference station device 202 will start transmitting the differential GPS corrections to the weapon system, again using the radio transceiver 214 The corrections can be used either by a mission platform, in this case, a bomber aircraft, and/or by an effector weapon, for example a missile or sub-munition, to improve upon targeting accuracy.

Although the reference station device 202 has a physical presence that might be detected and although it is possible that electromagnetic emissions due to internal circuitry may betray the reference station device 202, the reference station device 202 is effectively covert until the transmissions of corrections start It may be possible to use the reference station device 202 several times before the reference station device 202 is 1 o finally detected and put out of commission Advantageously the reference station device 202 is fitted with a self-destruct mechanism Alternatively the reference station device 202 is fitted with a commanded destruct mechanism Either mechanism may obliterate the reference station device or simply render the electronics thereon useless The reference station may also be fitted with anti-tampering apparatus and apparatus for safe decommissioning.

It will be understood that although the preceding description refers to a covert GPS reference station 202 having two GPS receivers, the reference station of the invention is not intended to be so limited The operations of both GPS receivers (a first GPS receiver for receiving the coded information carried by GPS signals and a second GPS receiver for receiving carrier phase information) can be performed by a single hybrid GPS receiver.

Figure 3 shows an embodiment of a covert reference station device having such a hybrid GPS receiver Both the coded information carried by the GPS signals and the phase of the carrier wave are received at a hybrid receiver means 304 The receiver means 304 outputs coded information to a coded information processor 310 and outputs the phase of the carrier wave to a carrier phase processor 312 The coded information processor 310 generates a conventional GPS position estimate from code measurements just as the first GPS receiver 210 does in Figure 2.

Similarly, the carrier phase processor 312 can then improve upon the code measurements just as the second GPS receiver 212 does A computer 220 then calculates true range measurements from the established position, compares the true ranges with pseudoranges generated by the coded information processor 310 and generates corrections for local code measurements As in Figure 2, the corrections are transmitted by the 1 o transceiver 214 to a remote GPS receiver unit 100.

In addition, the present invention is not intended to be limited to the GPS system but rather to any satellite-based navigation system, for example GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema or Global Navigation Satellite System), where both the phase of the satellite signal and the data coded into the satellite signal can be received by a covert reference station for generation of local position error corrections or wherein there are alternative means for deriving true ranges.

Furthermore the reference station can be arranged to use the signals of more than one satellite-based navigation system simultaneously or one at a time, for example using both GPS signals and GLONASS signals to achieve a more accurate location fix.

Claims

CLAIMS:

1 A covert reference station device having:

a housing; a receiver means capable of receiving both coded information and carrier phase information transmitted in each of a plurality of satellite signals, each satellite signal having a carrier wave and including coded information; a radio transceiver which transmits error corrections to at least one remote receiver unit; and a processing means; wherein the receiver means, the radio transceiver and the processing means are all shock-mounted within the housing.

2 A covert reference station device according to Claim 1, wherein the receiver means includes a first receiver for receiving coded information transmitted in a given satellite signal and a second receiver for measuring the phase of the carrier wave of the given satellite signal.

3 A covert reference station device according to Claim 2, wherein the first receiver is a first GPS receiver, the second receiver is a second GPS receiver, the satellite signal is a GPS signal and the remote receiver unit is a remote GPS receiver unit.

4 A covert reference station device according to Claim 2, wherein the first receiver is a first GLONASS receiver, the second receiver is a second GLONASS receiver, the satellite signal is a GLONASS signal and the remote receiver unit is a remote GLONASS receiver unit.

A covert reference station device according to Claims 2, 3 or 4, wherein the second receiver uses the phase of the carrier wave of the given satellite signal to establish the position of the covert reference station device; the first receiver is used to calculate pseudoranges for each of a plurality of satellite signals; the processing means calculates true ranges from the coded information carried in each satellite signal using the established position of the covert reference station device and compares the pseudoranges with the true ranges to give local code measurement corrections; and the radio transceiver transmits the local code measurement corrections to the at least one remote receiver unit.

6 A covert reference station device according to any one of Claims 1 to 5, wherein the housing is camouflaged.

7 A covert reference station device according to any one of Claims 1 to 5, wherein the housing is disguised as an unexploded bomb.

8 A covert reference station device according to any one of Claims 1 to 5, wherein the housing is disguised as an unexploded submunition.

9 A covert reference station device according to any one of Claims 1 to 8, wherein the covert reference station device is arranged to be deployed by an aircraft in a similar manner to a bomb without significant damage.

A covert reference station device according to any one of the preceding claims, wherein the remote receiver unit is carried by a mission platform.

11 A covert reference station device according to Claim 10, wherein the mission platform is a bomber aircraft.

12 A covert reference station device according to any one of the preceding claims, wherein the remote receiver unit is carried by a weapon.

13 A covert reference station device according to Claim 12, wherein the weapon is an effector weapon.

14 A covert reference station device according to any one of the preceding claims, wherein the radio transceiver does not begin to transmit local code measurement corrections until receipt of a valid interrogation signal.

A covert reference station device according to any one of the preceding claims, further including a self-destruct mechanism.

16 A covert reference station device according to any one of the preceding claims, further including a commanded destruct mechanism.

17 A covert reference station device according to Claims 15 or 16, wherein the destruct mechanism is capable of obliterating the entire reference station device.

18 A covert reference station device according to Claims 15 or 16, wherein the destruct mechanism renders the electronics thereon useless.

19 A covert reference station device according to any one of the preceding claims, further including anti-tampering apparatus.

A covert reference station device according to any one of the preceding claims, further including apparatus for safe decommissioning.

1 o 21 A global navigation system including a covert reference station as claimed in any one of Claims 1 to 20, a plurality of satellite vehicles, a master control station and at least one remote receiver unit, wherein the covert reference station receives satellite signals from each of the satellite vehicles and transmits error correction signals to the at least one remote receiver unit.

22 A method of operating a covert reference station device having a receiver means, a radio transceiver and a processing means; the method including:

i) receiving a plurality of satellite signals on the receiver means, each satellite signal having a carrier wave and including coded information; ii) measuring the distance travelled by each satellite signal from the time of arrival at the receiver means and from the coded information and generating a pseudorange for each satellite signal; iii) measuring the phase of the carrier wave for each satellite signal; iv) repeatedly estimating the position of the covert reference station device using the corresponding measured phases of the carrier waves of the plurality of satellite signals; v) calculating the true range corresponding to each satellite signal from the known position of the covert reference station device and from the coded information; vi) comparing the pseudorange to the true range and generating local code measurement corrections; and vii) using the radio transceiver to transmit the local code measurement corrections to remote receiver units.

23 A method as claimed in Claim 22, wherein step vii) includes:

receiving an interrogation signal; and transmitting the local code measurement corrections on receipt of a valid interrogation signal.

24 A covert reference station device substantially as hereinbefore described with reference to Figures 2 and 3.

A global navigation system substantially as hereinbefore described with reference to Figures 2 and 3.

26 A method of operating a global navigation system substantially as hereinbefore described with reference to Figures 2 and 3.