GB2180425A

GB2180425A - Navigation system and method

Info

Publication number: GB2180425A
Application number: GB08522721A
Authority: GB
Inventors: Rolf Johannessen; Francis Giles Overbury
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-09-13
Filing date: 1985-09-13
Publication date: 1987-03-25
Also published as: GB2180425B

Abstract

A mobile receiver determines its position from received hyperbolic terrestrial navigation system signals (e.g. Loran-C) and geostationary earth satellite signals, the latter being spectrum spread with a chipping code time locked to a standard such as the Loran-C signals, by time comparing the chipping code with the Loran-C signals. in the case of two Loran-C transmitters and one satellite the mobile receiver determines one line of position from the time comparison and another line of position from the received Loran-C signals assumption being made to the effect that the receiver is on the earth's surface. Integrating a terrestrial navigation system e.g. Loran-C with satellite signals in this manner effectively enhances the range of Loran-C without requiring high peak power pulses from the satellite. <IMAGE>

Description

SPECIFICATION Navigation system and method This invention relates to a navigation system and method based on a terrestrial navigation system, such as a marine navigation system, using fixed stations radiating pulsed transmissions.

Loran-C is a low frequency electronic position fixing system using pulsed transmissions at 100KHz. Loran-C chains consist of a master transmitting station and two or more secondary or slave transmitting stations. The system operates by measuring the difference in the time of arrival of pulses from the master and slave stations. These measurements are made by first matching the pulse envelopes (coarse time difference measurement) and then matching the phase of the 100KHz carrier within the envelope (fine time difference measurement).

Each station transmits groups of pulses at a specified interval known as the group repetition interval (GRI), the value of which is unique to each chain and is such that the transmissions from each station, with the assigned interval between them, can be accommodated within it. The selected GRI is also such as to avoid interference with other chains. Within any chain the transmissions of each slave station are retarded with respect to those of the master. This retardation ensures that signals from the various stations are aiways received in the same sequence anywhere within the coverage area. Hence the order of pulse reception within the coverage area of a four slave chain is master pulse group, first slave pulse group, second slave pulse group, third slave pulse group and fourth slave pulse group.The master pulse group consists of nine pulses and each slave group of eight pulses.

Groundwave ranges of 800 to 1200 nautical miles are typical, depending on transmitter power, receiver sensitivity and attenuation over the signal path. Loran-C pulses also propogate as skywaves which arrive after the groundwave and Loran-C has a significantly greater range than other hyperbolic navigation system, such as Decca, due in part to its ability to distinguish by time separation between ground and skywave signal components.

One disadvantage of Loran-C is that its low frequency transmissions limit its useful range to about 1000 nautical miles and because all of the stations are land based, and the number of station chains is limited, there are large areas of ocean where Loran-C receivers are unusable.

In our co-pending Application No.8501029 (Serial No ) (R. Johannessen 51) there is disclosed a means whereby electronic position fixing systems using land based transmitting stations can be utilised in ocean areas normally beyond the range of the land based stations. The method makes use of the fact that there are now numerous earth satellites in geosynchronous orbit. In the arrangement of the said co-pending Application electronic position fixing signals for hyperbolic navigation systems, e.g. Loran-C, are broadcast from a geostationary earth satellite and a land based electronic position fixing system provides distance measurement from two or more transmitting stations to the satellite whereby the satellite's position is determined accurately, the satellite actually broadcasts navigational signals as modified to take account of the satellite's position as determined.Thus Loran-C signals are impressed upon signals from satellites to enhance the range of Loran C.

It is an object of the present invention to provide an alternative means of extending the range of Loran-C by employing geostationary satellites.

According to one aspect of the present invention there is provided a method whereby a mobile receiver can determine its position comprising transmitting, for reception by the mobile receiver, hyperbolic terrestrial navigation system signals and geostationary earth satellite signals, the satellite signals being spectrum spread with a chipping code time locked to a standard, the mobile receiver determining at least one surface of position by time comparing a respective received chipping code with the received hyperbolic terrestrial navigation system signals.

According to another aspect of the present invention there is provided a navigation system whereby a mobile receiver can determine its position comprising means for transmitting, for reception by the mobile receiver, hyperbolic terrestrial navigation signals and geostationary earth satellite signals, the satellite signals being spectrum spread with a chipping code time locked to a standard, the mobile receiver including means for determining at least one surface of position by time comparing a respective received chipping code with the received hyperbolic navigation signals.

An embodiment of the invention will now be described with reference to the accompanying drawing which shows schematically an arrangement comprising two Loran-C transmitters and one geostationary satellite.

The Loran-C system derives position by timing the arrival of the leading edge of a high energy narrow pulse. Ambiguities are sufficiently far apart not to pose a problem. Clearly it is not feasible to transmit through a communications satellite, such as INMARSAT, a pulse at 100KHz with high peak power. However, it is possible to occupy one INMARSAT channel and transmit on it data defining the satellite position and defining Loran-C timing relative to some external standard like univer sal time, and then to spread the spectrum with a code which is time locked to that same external standard.

This principle is indicated in the drawing which shows the operation for a system using one geostationary satellite with two Loran-C transmitters. A similar arrangement may be organised for two satellites and one Loran-C transmitter.

In the drawing there is shown a geostationary satellite 1 which receives signals from a coastal earth station (CES) 2 and transmits signals for receipt by a mobile receiver 3 (assumed to be on the earth's surface) whose position is to be determined. Two Loran-C transmitting stations 4 and 5, station 4 being a master and station 5 a slave, also transmit signals for receipt by the mobile receiver 3.

The coastal earth station 2 is controlled by a hybrid system control station 6 which has a receiver 7 for L-band signals, for example at 1540MHz, transmitted by the satellite and a receiver 8 for Loran-C signals at 100KHz. The hybrid system control station 6 computes data regarding the satellite's position, the means for determining said position is not shown in the figure and the relative time for code division multiplexing (CDM) relative to Loran-C, which it transmits to the satellite 1 by the CES 2. The spectrum of communications via the satellite is spectrum spread with a chipping code which is time locked to the terrestrial Loran-C signals. The mobile receiver 3 comprises a receiver 9 for Loran-C signals at 100KHz, a receiver 10 for the L-band spectrum spread signals transmitted by the satellite at 1540MHz, for example, and a computer 11.The Loran-C receiver 9 provides one LOP (line of position) from the received Loran-C signals and timing information. The L-band receiver 10 provides the data, regarding satellite position and relative time for CDM relative to Loran-C, transmitted to it via the satellite 1 from the CES 2. The one LOP, the two items of timing information and the said data are input to the computer. From the received timing information and in dependence on the received data the computer calculates the other "line of position", which is not a line in the usual sense but rather a surface owing to the involvement of a satellite, by time comparing the chipping code received from the satellite with the signals received from the Loran-C transmitters 4 and 5.Thus one LOP and one surface are obtained and hence the computer can output the position of the mobile receiver 3 assuming it to be on the earth's surface.

The satellite is thus used as a pseudo Loran-C transmitter in a manner which overcomes the problem of high peak power.

The spreading of the signal requires to be achieved in a manner which achieves an acceptably high accuracy and an acceptably low interference to other users. The possibilities of achieving this involve either a low chip rate to confine the spectrum to a narrow frequency range away from other users, or a high chip rate covering a large portion of other users but at a low spectral density.

In the case of one Loran-C transmitter and two geostationary satellites, at the mobile receiver the received Loran-C signals will only provide timing information, the position will be determined from the spread spectrum satellite information by time comparing the received chip codes with the received Loran-C signals and employing the data transmitted to the satellites. Each satellite will have either a unique carrier or a unique code (chipping rate).

In a further alternative embodiment the spread spectrum chipping rate is locked not to Loran-C but to some other standard like Universal Time and the telemetred data enables the receiver to relate Loran-C also to the same time standard.

Claims

1. A method whereby a mobile receiver can determine its position comprising transmitting, for reception by the mobile receiver, hyperbolic terrestrial navigation system signals and geostationary earth satellite signals, the satellite signals being spectrum spread with a chipping code time locked to a standard, the mobile receiver determining at least one surface of position by time comparing a respective received chipping code with the received hyperbolic terrestrial navigation system signals.

2. A method as claimed in claim 1 wherein the chipping code is time locked to the hyperbolic terrestrial navigation system signals.

3. A method as claimed in claim 2 involving the use of one geostationary earth satellite and two Loran-C transmitters, which latter provide said hyperbolic terrestrial navigation system signals, the mobile receiver determining one surface of position from said time comparison and a line of position from the received Loran-C signals, it being assumed the mobile receiver's position is on the earth's surface.

4. A method as claimed in claim 3 wherein the two Loran-C transmitters comprise a master and a slave transmitter of one Loran-C chain.

5. A method as claimed in claim 3 or claim 4 wherein data is transmitted to the satellite by a coastal earth station for transmission to the mobile receiver, which data concerns the position of the satellite and relative time for code division multiplexing relative to the Loran-C signals.

6. A method as claimed in claim 2 involving the use of two geostationary earth satellites and one Loran-C transmitter, which latter provides said hyperbolic terrestrial navigation system signals, each satellite having a respective chipping code, the mobile receiver determining its position by respective time comparisons for each chipping code with the received Lo ran-C signals.

7. A method as claimed in claim 1, wherein the chipping code is time locked to universal time and the satellite signals enable the mobile receiver to relate the hyerbolic terrestrial navigation signals to universal time.

8. A navigation system whereby a mobile receiver can determine its position comprising means for transmitting, for reception by the mobile receiver, hyperbolic terrestrial navigation signals and geostationary earth satellite signals, the satellite signals being spectrum spread with a chipping code time locked to a standard, the mobile receiver including means for determining at least one surface of position by time comparing a respective received chipping code with the received hyperbolic navigation signals.

9. A navigation system as claimed in claim 8 wherein the chipping code is time locked to the hyperbolic terrestrial navigation signals.

10. A navigation system as claimed in claim 9, and including two Loran-C transmitters and one geostationary earth satellite, the two Loran-C transmitters comprising said hyperbolic terrestrial navigation signals transmitting means, the mobile receiver being such as to determine one surface of position from said time comparison and a line of position from signals received from the two Loran-C transmitters, it being assumed that the mobile receiver is on the earth's surface.

11. A navigation system as claimed in claim 10, wherein the two Loran-C transmitters comprise a master and a slave transmitter of one Loran-C chain.

12. A navigation system as claimed in claim 10 or claim 11, and including a coastal earth station for the transmission of data to the satellite for transmission to the mobile receiver, which data concerns the position of the satellite and relative time for code division multiplexing relative to the Loran-C signals.

13. A navigation system as claimed in claim 8 and including two geostationary earth satellites and one Loran-C transmitter, which latter provides said hyperbolic terrestrial navigation system signals, each satellite having a respective chipping code, the mobile receiver being such as to determine its position by respective time comparison for each chipping code with the received Loran-C signals.

14. A navigation system as claimed in claim 8, wherein the chipping code is time locked to universal time and the satellite signals enable the mobile receiver to relate the hyperbolic terrestrial navigation signals to universal time.

15. A method whereby a mobile receiver can determine its position substantially as herein described with reference to the accompanying drawing.

16. A navigation system substantially as herein described with reference to the accompanying drawing.

17. A receiver for operation in the system or method of any preceding claim, comprising means for determining at least one surface of position by time comparing a respective received chipping code with the received hyperbolic navigation signals.