GB2180426A

GB2180426A - Satellite navigation systems

Info

Publication number: GB2180426A
Application number: GB08522722A
Authority: GB
Inventors: Rolf Johannessen
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: GB2180426B

Abstract

An integrated communications and navigation system for mobile terminals comprising a plurality of navigation satellites and at least one geostationary communications satellite. The communications satellite transmits a signal time locked to a reference, such as a spread spectrum signal time locked to NAVSTAR, the plurality of navigation satellites being of the NAVSTAR constellation. Thus the communications satellite can act as a navigation satellite in order to increase the possible coverage of NAVSTAR, particularly in those regions where the PDOP is worse than 6.

Description

SPECIFICATION Satellite navigation systems This invention relates to satellite navigation systems and in particular to enhancement of the coverage provided by such systems.

In a satellite navigation system a receiver determines its position by one of the following, namely: determining pseudo-range to four different satellites; determining pseudo-range to three different satellites and carrying independent knowledge of time or altitude; determining range to three different satellites; or determining range to two different satellites and carrying independent knowledge of altitude and azimuthal ambiguity resolution information.

The NAVSTAR satellite navigation system as currently envisaged and comprising twenty one satellites has a PDOP (Position Dilution of Precision-this is a measure of the system accuracy, the higher the number the worse the accuracy) which in most places averages 2.5, but there are four latitude bands where a PDOP worse than 6 arises for part of the day and it is desirable to be able to improve navigation in those bands.

The Joint Tactical Information Distribution System (JTIDS) is a communications system which has the inherent capability of providing passive, high accuracy relative navigation. If some of the stations operating within the JTIDS net know their actual position and transmit the same, then all participants can derive their time position.

A number of civil systems are in use where a receiver designed for communications purposes can be utilised for navigation purposes.

Examples are the VHF Radio Lighthouse, CON SOL and the Japanese rotating pattern radio beacons.

According to one aspect of the present invention there is provided an integrated communications and navigation system for mobile terminals, the system comprising a plurality of navigation satellites and at least one geostationary communications satellite, which communications satellite transmits a signal time locked to a reference whereby the communications satellite can be used by a mobile terminal as a further navigation satellite.

According to another aspect of the present invention there is provided a method of navigating a mobile terminal involving the use of a plurality of navigation satellites and at least one geostationary communications satellite via which latter communications to and from the mobile terminal is achieved, and wherein for navigation the communications satellite transmits a signal time locked to a reference whereby it can be used by the mobile terminal as a further navigation satellite.

Embodments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a hand-shake protocol in which a transfer request is initiated by a satellite, and Figure 2 shows a hand-shake protocol in which a transfer request is initiated by a mobile (mobile terminal).

The present invention is concerned with a combined system in which communication is the primary function and is chargeable and in which navigation is an inbuilt secondary feature, probably free from specific charge, a suitable amount to cover its cost being built into the communication charges. The proposed system, which we have termed the Communication Navigation Satellite System (CONAT), does not alter the minimum number of satellites which are required, but at least one is used for communications, the rest could be communications or navigation only.

In the following the latter is assumed and that the navigation satellites are in particular from the NAVSTAR system.

A basic requirement of the CONAT system is that the communication satellite must emit a timing (or phase) signal with the precision and ambiquity appropriate for the navigation function, which timing must be locked to the timing of the navigation satellites or to some common reference.

It is thus proposed that the carrier with communications modulation is spectrum spread, that the chipping rate and code length be compatible with NAVSTAR and that the chipping clock be locked either to NAVSTAR or to Universal Time.

Using a chipping rate of 1.023MHz gives us a bit length of 960ft (approximately 300 metres) with a possible range accuracy of 0.1 bit length. If 1023 bits is used for the code length we set a basic ambiguity of 161 nautical miles.

By staying with the lower NAVSTAR chipping rate of 1.023MHz the spectrum occupied is limited. Frequency separation between the first nulls in the (sin x)/x spectrum is 2.046MHz. This minimises the linearity requirements in the satellite transponder if one is used as opposed to employing onboard processing.

A basic combined system requires one communication satellite in geostationary orbit and operating in CDMA (Code Division Multiple Access). One of its channels has a carrier permanently present and modulated with orbital data, any spare capability can be used for one way data of general interest. The channel could be a calling channel to which the mobile, whose position is to be determined, is always tuned when in its idle state. For navigation the mobile locks an internal spreading code to that from the communications satellite and then calculates position using navigation satellites from the NAVSTAR constellation, and the communications satellite, which is act ing as an additional navigation satellite.When the mobile communicates with the communications satellite it uses a different channel with a different spreading code which is nevertheless locked to NAVSTAR timing or a common reference. Alternatively the different channel for communications may not be spread but has some timing signal locked to NAVSTAR or the common reference. In such a basic system, for the duration of the navigation the mobile will not be able to access any communications satellite transmissions.

A commercial communications satellite system raises revenue by charging for the communication services supplied, however if the satellite is used also for navigation it is possible, unless the navigation function involves two-way traffic with identification, for "pirates" to design suitable receivers to utilise the navigation information without paying for the communications service.

It would clarly be possible for a "pirate" to design receive only systems listening to NAVSTAR and to the CONAT and so gain the improved navigation capability without financially contributing to the navigation cost through communication.

A level of protection could be built in whereby the precision of the calculatable position by a particular mobile is made dependent on the extent of communication previously carried out by that mobile.

One possibility is to build a timing bias into the communications and navigation satellites which is slowly varying, for example on a week by week basis. Each time a communications message is exchanged, a hand-shake protocol is required to confirm (ACKnowledge) successful data transfer. Provided the response delay in the mobile is controlled on the nanosecond level, this can be utilised to transfer a pseudo-range measurement to a true range measurement. This enables clock difference to be controlled or corrected for.

Figs. 1 and 2 illustrate how this can be organised within a hand-shake protocol in, respectively, the case of a transfer request initiated by the communications satellite and the case where the data transmission is in the reverse direction. In each case the successful "ACK" signal is accompanied by a message from the satellite detailing the round trip delay "T," as measured by the satellite or its ground control station.

In Fig. 1 the satellite requests the allocation of a channel of the mobile, this is performed and an "ACK" signal transmitted. Data is then transmitted from the satellite to the mobile and at the end thereof an end of message (EOM) signal is responded to by a further "ACK" signal. The satellite then sends a "return to call channel" signal together with a value for T,, the round trip time from which the distance of the mobile from the communications satellite can be calculated. Since this satellite is geostationary the distance therebetween helps with identifying the position of the mobile, each communication resulting in an updated value for said distance, and improving range accuracy particularly when the PDOP is high.

In Fig. 2 the mobile requires to transmit data to the satellite. The mobile thus requests a channel and when a channel is allocated this is acknowledged with an "ACK" signal, and the data transmitted ending with an end of message signal which the satellite acknowledges. When the satellite has transmitted the data onwards, e.g. to a coastal earth station, it transmits a message delivered signal to the mobile which responds with an "ACK" signal.

The satellite responds this time with a return to call channel signal and indicates the round trip time To to the mobile.

An alternative possibility for protecting the revenue by control of the precision of the calculated position is such that each time a mobile is involved in a communications exchange it generates a small random number, say 5 bits. As part of its hand-shake sequence it transmits this number to the communications satellite. The final command from the satellite to the mobile then incorporates a code whose interpretation is "return to calling channel and note that my present clock bias is 2718AS plus the random number you sent me" (2718,uS being by way of example only).For pirate receivers to utilise this data they must themselves know what that random number was, which requires both the equipment to receive both up and down frequencies and the ability to pick up the paying mobile which at least is difficult in the case of ships beyond the horizon.

While all protection can be bypassed, the likelihood is brought down considerably by these techniques. Ambiguity separation may be raised beyond the 161 nautical miles referred to earlier either by lengthening the code further or by using the measurement of round trip time associated with the "ACK" function.

Thus the invention proposes the use of communications satellites operating in CDMA using spread spectrum time locked to NAVSTAR, or universal time, together with navigation satellites in a combined system (CONAT) which thus achieves better coverage, in other words integrated communication and navigation at a mobile terminal is made possible by transmission from a communication satellite of a spread spectrum signal time locked to NAVSTAR. Thus a mobile can now navigate using a mixture of NAVSTAR satellies and communications satellites and achieve better coverage. The use of an ACKnowledge function provides range ambiguity resolutions and/or safeguards revenue for the communications satellite operator. Such a combined system means that there can be commonality of the equipment at the mobile.The mobile user may be able to use a common antenna for navigation and low data rate communication. Several RF front-end components could be common, the frequency source may be shared and where the formats are compatible, some of the processing could be common.

Input/output facilities may also be shared.

Thus the user obtains both the navigation and communications functions for a price which would be less than two mobile systems separately. The combined system results in improved PDOP for a NAVSTAR based system where a geostationary satellite is visible since the choice of satellites is increased and a better relative geometry becomes possible. The combined system also means that there is less dependence on low elevation satellites.

The PDOP of 2.5 uses a 5" elevation mask angle. In urban areas low elevation angles are difficult to achieve. The availability of a geostationary satellite will help in this respect.

Claims

1. An integrated communications and navigation system for mobile terminals, the system comprising a plurality of navigation satellites and at least one geostationary communications satellite, which communications satellite transmits a signal time locked to a reference whereby the communications satellite can be used by a mobile terminal as a further navigation satellite.

2. A system as claimed in claim 1, wherein the communications satellite signal is a timing signal.

3. A system as claimed in claim 2, wherein the communications satellite signal is a spread spectrum signal time locked to the timing of the navigation satellites.

4. A system as claimed in claim 3, wherein the spread spectrum signal is a carrier with communications modulation, the chipping rate and code length being compatible with NAVSTAR navigation satellites which comprises said navigation satellites, and the chipping clock being locked to NAVSTAR or universal time.

5. A system as claimed in claim 1 wherein for navigation a mobile terminal locks an internal spreading code to a spreading code from the communications satellite and calculates its position using a number of the navigation satellites and the communications satellite.

6. A system as claimed in any one of the preceding claims wherein the precision of its position as calculated by a particular mobile terminal is dependent on the extent of communication previously carried out by that mobile terminal via the communications satellite.

7. A system as claimed in claim 6 wherein upon communications exchange between the communications satellite and the particular mobile terminal a hand-shake protocol is employed to acknowledge successful data transfer and to control or correct for clock difference.

8. A system as claimed in claim 7 wherein following an acknowledge signal a signal indicative of the round trip time between the mobile terminal and communications satellite as determined by the satellite or its ground control station is supplied to the mobile terminal whereby the resolve range ambiguity.

9. A system as claimed in claim 6 wherein upon communications exchange between the communications satellite and the particular mobile terminal the mobile terminal generates a random number which it transmits to the communications satellite in a hand-shake sequence and wherein the communications satellite subsequently employees the random number in its clock bias.

10. An integrated communications and navigation system for mobile terminals substantially as herein described with or without referencr to Fig. 1 or Fig. 2 of the accompanying drawings.

11. A method of navigating a mobile terminal involving the use of a plurality of navigation satellites and at least one geostationary communications satellite via which latter communications to and from the mobile terminal is achieved, and wherein for navigation the communications satellite transmits a signal time locked to a reference whereby it can be used by the mobile terminal as a further navigation satellite.

12. A method as claimed in claim 11, wherein the signal is a spread spectrum signal time locked to the timing of the navigation satellites.

13. A method as claimed in claim 12, wherein the spread spectrum signal is a carrier with communications modulation, the chipping rate and code length being compatible with NAVSTAR navigation satellites which comprise said navigation satellites, and the chipping clock being locked to NAVSTAR or universal time.

14. A method as claimed in any one of claims 11 to 13, including the step of controlling the precision of mobile terminal position calculation in dependence on the extent of communications previously carried out via the communications satellite.

15. A method as claimed in claim 14 including the step of employing a hand-shape protocol to acknowledge successful data transfer, between the communications satellite and the mobile terminal, and to control or correct for clock difference.

16. A method as claimed in claim 15, wherein following an acknowledgement of successful data transfer the communications satellite transmits a signal to the mobile terminal indicative of round trip time therebetween whereby its possible to resolve range ambiquity.

17. A method as claimed in claim 14 including the step of the mobile terminal gener ating a random number which it transmits to the communications satellite in a hand-shake sequence during communications exchange, the communications satellite subsequently employing the random number in its clock bias.

18. A method of navigating a mobile terminal substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

19. A mobile communications terminal adapted to operate in the system or method according to any preceding claim.