WO2001007929A1

WO2001007929A1 - Satellite communication system and method

Info

Publication number: WO2001007929A1
Application number: PCT/GB2000/002787
Authority: WO
Inventors: Richard Wyrwas
Original assignee: Ico Services Ltd.
Priority date: 1999-07-22
Filing date: 2000-07-19
Publication date: 2001-02-01
Also published as: GB9917296D0; GB2352347A

Abstract

A satellite communications system and method, where first (10) and second (10') satellites can pass messages between a user terminal (44) and an earth station (38), and where the first (10) and second (10') satellite can transmit a broadcast signal to the user terminal (44) as the satellites (10, 10') move in orbit (12, 12') as indicated by arrow (46), comprises a two-stage means for measuring the position of the user terminal (44), on the surface of the earth (14), the first stage being, optionally, by means of Doppler shift measurements or propagation delay measurements between the first satellite (10) and the user terminal (44), and the second stage optionally, being the user terminal (44) reporting back to the earth station (38) the difference in the frequency that the user terminal (44) measured for each of the broadcast signals from the satellites (10, 10'), or the user terminal reporting back to the earth station (38) the difference in time of arrival between the broadcast signals from the satellite (10, 10').

Description

SATELLITE COMMUNICATION SYSTEM AND METHOD

Field of the invention

The present invention relates to satellite communication systems, and a method of operating such systems, in which a user terminal communicates with an earth station via a satellite and there is a requirement for the location of the user terminal to be known to the earth station.

Background It is known, in a satellite communications, for a user terminal, for example in the form of a mobile radio telephone handset, to communicate with an earth station, via a satellite, to establish a telephone call or to receive a telephone call, by means of the earth station linking into the terrestrial wire and cable system or into another radio telephone system.

One network known as the IRIDIUM™ satellite cellular system is described in EP-A-0 365 885 and US Patent No. 5 394 561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.

Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed with an orbital radius in the range of 10-20,000 km. Reference is directed to the ICO™ satellite cellular system described for example in GB-A-2 295 296. With this system, the satellite communications link does not permit communication between adjacent satellites. Instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to conventional land-based telephone networks. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM. Also simpler satellite communication techniques can be used than with a LEO network. Reference is directed to "New Satellites for Personal Communications", Scientific American, April 1998, pp. 60 - 67, for an overview of LEO/MEO satellite networks.

There are two main reasons for establishing the exact position of the user terminal on the surface of the earth. Firstly, in order to know how to direct a radio signal to or from a user terminal, when required, from particular satellite at a particular time, it is necessary to know the approximate location of the user terminal so that the appropriate beam from the appropriate satellite can be selected to cover the portion of the Earth's surface where the user terminal is located. Secondly, in a satellite communication system, in order that call barring, local billing or other restrictions based on the territory wherein the user terminal may be operated can be observed, it is necessary to determine the location of the user terminal with sufficient accuracy for the necessary restrictions to be imposed.

It is known to provide a user terminal where the individual terminal employs "Global Positioning by Satellite" (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed 27th March 1993.

Such systems require multiple frequency capability from the handset, together with enhanced complication of the handset, in order that the handset may be capable both of communications and of GPS measurements.

It is advantageous, therefore, to provide a system and method whereby the position, on the surface of the earth, of the user terminal or handset can be determined with sufficient accuracy for communication and fiscal purposes without undue complication of the handset or user terminal and without the necessity of the provision of or access to a separate satellite system concerned with GPS.

The present invention seeks to provide a solution to the problem of determining the location of a user terminal or handset, on the surface of the earth, when interacting with a satellite communication system, without the necessity to resort to a solution involving GPS and with sufficient accuracy for the operational and fiscal requirements of a satellite communication system.

In another system, it is merely necessary for the satellite to receive a transmission from the user terminal, in response to the earth station, via the satellite, requesting radio communication from the user terminal, for the earth station to use the delay in response from the user terminal for ranging purposes and to use the Doppler shift on the received frequency from the user terminal, together with a foreknowledge of the position and velocity of the satellite, to determine, with a certain degree of precision, the position of the user terminal on the surface of the earth.

Unfortunately, if the cost of the user terminal is to be kept at realistic limits, consistent with the trade in handheld mobile radio telephones, the accuracy of the crystal clock or other frequency source within the user terminal cannot be made consistent with sufficient accuracy of determination of the position of the user terminal, on the surface of the earth, for the functional and fiscal aspects connected with operation of a satellite telephone communication system.

Summary of the invention

The present invention seeks to provide a method and system whereby the position of a user terminal, on the surface of the earth, can rapidly be determined without multiple frequency capability in the user terminal and with a high degree of accuracy.

According to a first aspect, the present invention consists in a satellite communications system wherein a user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said system being characterised by; said earth station being operable to exchange signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; by said user terminal being operable to receive said broadcast signal from said first satellite and said second satellite; by said user terminal being operable to measure the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; by said user terminal being operable to report said difference in said values of said second range dependent characteristic to said earth station; and by said earth station being operable, thereafter, to analyse said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

According to a second aspect, the present invention consists in a method, for use in a satellite communications system wherein a user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said method being characterised by including the steps of: said earth station exchanging signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; said user receiving said broadcast signal from said first satellite and said second satellite; said user terminal measuring the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; said user terminal reporting said difference in said values of said second range dependent characteristic to said earth station; and said earth station analysing said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

According to a third aspect, the present invention consists in a user terminal, for use in a satellite communications system wherein said user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said user terminal being characterised by; said earth station being operable to exchange signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; by said user terminal being operable to receive said broadcast signal from said first satellite and said second satellite; by said user terminal being operable to measure the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; by said user terminal being operable to report said difference in said values of said second range dependent characteristic to said earth station; and by said earth station being operable, thereafter, to analyse said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

The various aspects of the invention may further provide that the first range dependent characteristic is the Doppler shift between the first satellite and the user terminal and that the second range dependent characteristic is the measured frequency of the broadcast signal received from the first satellite and from the second satellite. The various aspects of the invention may further provide that the first range dependent characteristic is the time delay for a radio signal between the first satellite and the user terminal and that the second range dependent characteristic is the time of arrival of the broadcast signal from the first satellite and the second satellite.

The various aspects of the invention may further provide that the earth station is further operable to measure the time delay for a radio signal between the user terminal and the first satellite, that the first range dependent characteristic is the Doppler shift between the first satellite and the user terminal, and that the second range dependent characteristic is the time of arrival of the broadcast signal from the first satellite and from the second satellite.

The various aspects of the invention may further provide that the earth station is further operable to measure the Doppler shift between the first satellite and the user terminal, that the first range dependent characteristic is the time delay for a radio signal between the user terminal and the first satellite, and that the second range dependent characteristic is the measured frequency of the broadcast signal from the first satellite and from the second satellite.

The invention may further provide a system, method and user terminal where the earth station is operable to send the message via the first satellite an optimum number of times, dependently upon the estimated position of the user terminal with respect to the first satellite, and to take the average of the respective first range dependent characteristics derived therefrom.

The invention may further provide a system, method and user terminal where the first satellite is operable to send, to the earth station, a signal on a first generated frequency, where the earth station is operable to send a signal at a first known frequency to the first satellite, and where the first satellite is operable to use a respective internal oscillator to transpose the signal of a first known frequency and return the transposed signal to the earth station on a first transposed frequency, the earth station being operable to measure the first generated frequency and the first transposed frequency to derive therefrom the Doppler shift between the earth station and the first satellite, and the error in the internal oscillator in the first and satellite.

The invention may further provide a system, method and user terminal where the earth station is operable, after having derived the Doppler shift and the error in the internal oscillator in the first satellite, to cause the first satellite to send a signal, at a second known frequency, to the user terminal, where the user terminal is operable to use an internal oscillator to transpose the signal of a second known frequency and return the transposed signal to the earth station, through said first satellite, on a second transposed frequency, where the user terminal is operable to send, to the earth station, via the first satellite, a signal on a second generated frequency, where the earth station is operable to measure the second transposed frequency and the second generated frequency, and where the earth station is operable to derive therefrom the Doppler shift between the first satellite and the user terminal and to derive the error in the internal oscillator in the user terminal.

The invention may further provide a system, method and user terminal where the first satellite and the second satellite are in the same orbit.

The invention may further provide a system, method and user terminal where the first satellite and the second satellite are in different orbits.

Brief description of the drawings The invention is further explained, by way of example, by the following description, taken in conjunction with the appended drawings, in which: Figure 1 shows a planar constellation of satellites disposed about the earth; Figure 2 illustrates how the satellites are disposed in orthogonal orbital planes; Figure 3 shows the structure of the cone of radio coverage provided by each satellite;

Figure 4 shows how the cones of radio coverage, shown in Figure 3 may interact with the surface of the earth to produce many types of different regions; Figure 5 is a view, from above, of a satellite above the surface of the earth, illustrative of the various motions relative to the earth;

Figure 6 is a schematic view of the general situation where an earth station talks to a user terminal via the satellite to determine propagation delays between the user terminal and the satellite;

Figure 7 shows the geometry of Doppler frequency shift measurement for the satellite;

Figure 8 is a schematic representation of the exchange of test signals between the earth station and the satellite to determine the relative Doppler shift and internal oscillator error of the satellite;

Figure 9 is a schematic representation of how a calibrated satellite, according to

Figure 8, may, in turn, be used to determine the relative Doppler shift between the satellite and user terminal and the internal oscillator error in the user terminal; Figure 10 shows how intersecting lines of measured Doppler frequency shift and propagation delays may be used to measure the position of the user terminal on the surface of the earth;

Figure 1 1 is a graph showing the derivation of the optimal number of samples for best estimation of position; Figure 12 is a chart showing, for the particular preferred embodiment, the derived optimal number of samples for Doppler frequency shift averaging;

Figure 13 is a chart showing, for the particular preferred embodiment, the derived optimal number of samples for propagation delay averaging;

Figure 14 shows the situation where the user terminal has direct access to more than one satellite;

Figure 15 is a flowchart of the activities of the user terminal and the earth station in measuring the position of the user terminal, according to the present invention;

Figure 16 shows the situation where the first range dependent characteristic is the Doppler shift between the first satellite and the user terminal, and where the second range dependent characteristic is the difference in receipt times for a broadcast signal, at the user terminal, from the first and second satellites; and Figure 17 shows the situation where the first range dependent characteristic is the propagation delay between the user terminal and the first satellite, and the second range dependent characteristic is the difference in received frequency for a broadcast signal from the first satellite and from the second satellite.

Detailed description

Figure 1 shows a planar constellation of satellites disposed about the earth. The plurality of satellites 10 are evenly disposed around a circular orbit 12 above the surface of the earth 14. Each of the satellites 10 is designed to provide radio communications with apparatus on the surface of the earth 14 when the individual satellite 10 is more than 10 degrees above the horizon. Each satellite 10 therefore provides a cone 16 of radio coverage which intersects with the surface of the earth 14.

Three types of coverage areas as provided on the earth's surface. A first type of area 18 is one which has radio coverage from only one satellite 10. A second type of area 20 is an area where there is radio coverage from more than one satellite 10. Finally, a third type of area 22 receives radio coverage from none of the satellites 10 in the orbit 12 shown.

Figure 2 illustrates how the satellites 10 are disposed in two orthogonal orbital planes. The first orbit 12 of Figure 1 is supplemented by a second orbit 12' having satellites 10 disposed there about in a similar manner to that shown in Figure 1. The orbits 12' are orthogonal to one another, each being inclined at 45 degrees to the equator 24 and having planes which are orthogonal (at 90 degrees) to each other.

In the example shown, the satellites 10 orbit above the surface of the earth 14 at an altitude in the region of 10 500km which generally corresponds to the ICO™ system. Those skilled in the art will be aware that other orbital heights and numbers of satellites 10 may be used in each orbit 12, 12'. This configuration is preferred because the example provides global radio coverage of the earth 14, even to the north 26 and south 28 poles, with a minimum number of satellites 10. In particular, the orthogonality of the orbits ensures that the satellites 10 of the second orbit 12' provides radio coverage for the third types of area 22 of no radio coverage for the satellites in the first orbit 12, and the satellites 10 in the first orbit 12 provide radio coverage for those areas 22 of the third type where the satellites 10 of the second orbit 12' provide no radio coverage.

It will become clear that, although the two orbits 12, 12' are here shown to be of the same radius, the system will function with orbits 12, 12' of different radii. Also, there may be more than two orbits 12, 12 '.

Figure 3 shows the structure of the cone 16 of radio coverage provided by each satellite 10. For convenience, the radio coverage cone 16 is shown centred, on a map of the earth, at latitude 0 degrees at longitude 0 degrees. The cone 16 of radio coverage is formed of a plurality of spot beams 30 that are individually produced by means of a corresponding plurality of directional antennae on the satellite 10. The satellite 10 is intended for mobile radio telephone communications and each of the spot beams 30 corresponds to a cell in a cellular radio telephone network. In Figure 3, the cone of radio coverage 16 is distorted due to the projection geometry of the map. Figure 3 also shows the extent of interaction of the cone 16 of radio coverage down to the edges of the cone 16 being tangential to the earth's surface, that is, to the point where the cone 16 represents a horizontal incidence at its edges, with the surface of the earth. By contrast, Figure 1 shows the cone 16 at a minimum of 10 degrees elevation to the surface of the earth.

It is to be observed, that because of the curvature of the earth, the spot beams 30 are of near uniform, slightly overlapping circular shape at the centre whereas, at the edges, the oblique incidence of the spot beams 30 onto the surface of the earth 14 causes considerable distortion of shape. Figure 4 shows how the cones 16 of radio coverage may interact with the surface of the earth to produce many types of different regions.

As discussed with reference to Figure 1 , numerous cones or radio coverage 16 may overlap to produce first areas 18 where there is radio coverage by only one satellite, second areas 20 where there is radio coverage by two satellites, and even fourth areas 32 where coverage is provided by three or more satellites. It is to be understood that each of the cones 16 of radio coverage represented in Figure 4 is divided, as shown in Figure 3, into its own independent set of spot beams 30.

Figure 5 is a view, from above, of a satellite 10 above the surface of the earth. The satellite 10 comprises solar panels 34 for power supply, a downlink antenna 36 for sending bulk telephone traffic to one of a plurality of earth stations 38, and uplink antenna 40 for receiving general traffic from the earth stations 38, and a subscriber antenna 42 which provides the plurality of spot beams 30, shown in Figure 3, intended to provide communications with user terminals 44 which may be provided in a form not dissimilar to a hand held cellular radio telephone. It is to be understood that the user terminal 44 may also comprise more elaborate vehicle mounted equipment for use in land vehicles, ships and aircraft.

With the parameters mentioned in this example, the satellite moves around its orbit 12 12', as indicated by a first arrow 46, with a velocity of 4.9km per second. Ignoring for the moment the rotation of the earth 14, the spot beams 30 also move across the surface of the earth 14 with a similar velocity along a ground track as indicated by a second arrow 48. The point immediately beneath the satellite, is known as the nadir 50.

At the same time the earth 14 is rotating, at its equator with a velocity of 0.47km per second, as indicated by a third arrow 52. Directions, relative to the ground track 48, at 90 degrees thereto, are termed cross-track as indicated by a fourth arrow 54. Hereinafter, the position of the user terminal 44 is defined with reference to its distance along the ground track 48 and its distance along the cross track 54 with reference to the nadir 50.

Figure 6 is a schematic view of the general situation where an earth station 38 talks to a user terminal 44 or via the satellite 10.

The earth station 38 further comprises an earth station controller 56 which controls the activity of the earth station 38. The earth station 38 is located at a first point on the surface of the earth 14 and the user terminal 44 may be at any other point on the surface of the earth within range of the satellite 10 when the satellite 10 is in range of the earth station 38.

The earth station 38 communicates with the satellite 10 via an uplink radio link 58, via the uplink antenna 40 of Figure 5, using frequencies in the band 5150 to 5250 megahertz. The earth station 38 receives signals from the satellite 10 via the downlink antenna 36 of Figure 5 on a downlink radio link 60 using signals in the frequency range 6975 to 7075 megahertz.

The user terminal 44 receives signals from the satellite 10 via a user terminal downlink 62 using frequencies in the range 2170 to 2200 megahertz. The user terminal 44 sends messages and signals to the satellite 10 via a user terminal uplink 64 operating in the frequency band 1980 to 2010 megahertz. These frequencies are merely exemplary and those skilled in the art will be aware from the following description, that numerous other frequencies for the uplinks and downlinks could be used.

Implicit in Figure 6, but not specifically shown, is the fact that satellite 10 contains its own precise oscillator, conveniently in the form of a crystal oscillator, which the satellite 10 uses for converting the frequencies of incoming and outgoing signals and for use as a frequency reference when synthesising frequencies. Likewise, the user terminal 44 contains its own internal synthesised oscillator, working from a master oscillator, preferably a crystal oscillator, for converting frequencies of incoming signals and synthesising the frequencies of outgoing signals.

The earth station 38 and the earth station controller 56 between them contain, or have access to, extremely precise frequency references and time references. These references may actually be contained within the earth station 38 and the earth station controller 56, or may be derived from elsewhere via a land line or other service.

The location of the earth station 38, on the surface of the earth 14 is known with great precision. Likewise, the parameters or the orbit 12, 12' of the satellite 10 and its position in that orbit, at any instant, are also known with great precision. The uncertain element, is the position of the user terminal 44 on the surface of the earth 14.

The user terminal 44 transmits on the user terminal uplink 64 to the subscriber antenna 42 and similarly receives on the user terminal downlink link 62 from the subscriber antenna 42. The satellite 10 will only be in communication with one earth station 38 at a time, but may be in communication with a great many user terminals 44. Each user terminal will be in one particular spot beam 30 of the plurality of spot beams shown in Figure 3.

The satellite 10 will be moving relative to the surface of the earth 14, and therefore relative to the earth station 38 and to the user terminal 44, as indicated in a fifth arrow 46. Likewise, the surface of the earth 14 will be moving relative to the orbit 12 12' of the satellite 10 as generically indicated by a sixth arrow 68.

The signals exchanged between the earth station 38 and the satellite 10, in common with the signals exchange between the user terminal 44 and the satellite 10, all undergo a propagation delay and a Doppler frequency shift, due to the motion of the satellite 10 relative to the earth station 38 and to the user terminal 44. The present invention in part concerns itself with means of employing the Doppler shift in frequencies, due to the motion of the satellite 10, and measurement of the propagation delay, to determine the position of the user terminal 44 on the surface of the earth 14.

In two of the embodiments of the present invention, propagation delay is measured between the earth station 38 and the user terminal 44. The earth station 38 sends out a signal on the uplink radio link 58 to the satellite 10 which is, in turn, sent to the user terminal 44 via the user terminal downlink 62. Upon receipt of the signal from the earth station 38, the user terminal waits for a predetermined period and then sends its own message, via the user terminal uplink 64 and the downlink radio link 60, back to the earth station 38. The earth station controller 56 notes the elapse of time from the instant that the earth station 38 began to transmit the message on the uplink radio link 58 and the instant when the earth station 38 began to receive the response message from the user terminal 44 from the downlink radio link 60. The earth station controller 56 knows the propagation delay times for signals, through the satellite 10, from the uplink radio link 58 onto the user terminal downlink 62 and, correspondingly, the propagation delay through the satellite 10 between the user terminal uplink 64 and the downlink radio link 60. Equally, the earth station controller 56 knows, with precision, the predetermined elapsed time employed by the user terminal 44 before it responds to the received message from the earth station 38. These propagation delays and the predetermined delay of the user terminal 44 are subtracted, by the earth station controller 56, from the overall elapsed time to determine the actual propagation delay of the radio wave via the various links 58, 60, 62, 64 in the return journey of the message from and to the earth station 38. The radio wave propagates always at the speed of light, which is constant. Because the position of the earth station 38, on the surface of the earth, is precisely known, and because the position of the satellite 10 in its orbit 12 12' is also precisely known, the sum of the propagation delays on the uplink radio link 58 and the downlink radio link 60 can be precisely calculated. The earth station controller 56 is already aware of the overall elapsed time for the propagation of the message along the radio paths 58, 60, 62, 64. By subtracting the calculated delay on the radio path 58 60 between the earth station 38 and the satellite 10 from the overall propagation delay, the propagation delay between the user terminal 44 and the satellite 10 may be precisely measured. This means that, since the propagation is entirely at the speed of light, the linear distance between the satellite 10 and the user terminal 44 is known. According to the propagation delay, the user terminal may exist on any point of a spherical surface centred on the satellite 10. Because the spherical surface intersects the surface of the earth 14, and the user terminal 44 is on the surface of the earth, the location of the user terminal 44 may be inferred as being on the line intersection of the spherical surface of the earth 14 and the sphere of measured distance centred on the satellite 10.

Figure 7 shows the geometry of Doppler frequency shift measurement for the satellite 10. As the satellite 10 moves as indicated by a 7th arrow 70, the change in frequency of a radio signal sent from the satellite 10 and the perceived frequency of a radio signal received by the satellite 10 from a fixed source such as the user terminal 44, depends upon the cosine of the angle between the satellite 10 and the recipient of a transmitted radio signal from the satellite or the source of a transmitted radio signal to the satellite 10. Accordingly, if we plot those regions in space for pre-determined Doppler frequency changes, there is obtained a series of coaxial cones 72 having the satellite 10 at their collective apex, extending towards infinity, and having, as their collected axis 74, the direction of the motion of the satellite 10 as indicated by the 7th arrow 70. Figure 7 shows the cones 72 extending only for a finite distance. It is to be understood that the cones 72 are of infinite extension. Likewise, Figure 7 has only shown the cones "in front" of the satellite for radio frequencies receivers or sources which the satellite 10 is approaching. It is to be understood that a corresponding set of coaxial cones 72 extend "behind" the satellite, having the same apex and axis. The Doppler shift "in front" of the satellite 10 is shown by an increase in frequency. The Doppler shift "behind" the satellite 10 is provided by a corresponding decrease in frequency.

Where the cones 72 cut the surface of the earth 14, for a particular Doppler frequency shift, defines a further line along which the user terminal 44 may be located. Referring again to Figure 6, a Doppler frequency shift measurement is executed by the earth station 38 providing a signal of known frequency on the uplink radio link 58. The satellite 10, using its own internal oscillator, translates the frequency of the signal and provides it on the user terminal downiink 62. The user terminal 44 then returns the signal via the user terminal uplink 64, once again to be converted in frequency by the internal oscillator of the satellite 10 and sent back to the earth station 38 via the downlink radio link 60. The earth station controller 56 measures the frequency of the downlink radio link 60 signal and deduces the Doppler frequency shift, at the user terminal 44, resulting from the motion of the satellite 10 as indicated by the 5th arrow 66.

Figure 8 is a schematic diagram of the manner in which the earth station 38 and the earth station controller 56 interact with the satellite 10 to calibrate the errors and Doppler shift experienced between the earth station 38 and the satellite 10.

The earth station 38 sends a signal of known frequency f(l) on the uplink radio link 58 to the satellite 10. The satellite 10 has an internal master oscillator which controls all of the synthesised frequencies used by satellite 10. If the master oscillator has a proportional error m, then any frequency, synthesised using the master oscillator, in the satellite, is proportionally in error, so that:

f(actual) = (l+m)f (intended)

Likewise, the satellite 10 is moving with respect to the earth station 38, thus introducing a proportional Doppler shift, let us call it d, so that, no matter whether the signal goes from the earth station 38 to the satellite 10, or from the satellite 10 to the earth station 38:

f(received) = (l+d)f(sent)

Thus, if the earth station sends a frequency f(l) on the uplink radio link 58 to the satellite 10, because of Doppler shift the satellite receives a frequency f(received at satellite) = f(l)(l+d)

Now, the satellite employs a frequency changer 76 to convert the signal, received from the earth station 38 to a frequency suitable for use via the subscriber antenna 42. In order so to do, the satellite 10 synthesises an intended frequency f(2) to be subtracted from frequency of the signal received at the satellite 10 from the earth station 38. The intended frequency f(2) is subject to the proportional error in the master oscillator on the satellite 10, and so becomes

f(2)(l+m).

The output of the frequency changer 76 is thus:

f(l)(l+d) - f(2)(l+m)

and this is sent, back to the earth station 10, via the subscriber antenna 44. But the satellite 10 is moving, and thus imparts a further Doppler shift. Thus, the frequency, received by the earth station 38 from the subscriber antenna 42, let us call it f(Rl), is given by

f(Rl) = (l+d)(f(l)(l+d) - f(2)(l+m)) (1 )

The earth station controller 56 measures f(Rl) with extreme precision. Thus, f(Rl), f(l) and f(2) are all known numbers, but m and d are unknown. Expanding the expression for f(Rl) we obtain

f(Rl) = (f(l ) - f(2)) + d(2f(l ) + d²f(l )) - mdf(2) - f(2)m

The second order terms d²f(l) and mdf(2) are insignificant compared to the other terms, and can be ignored. Thus: f(Rl) = f(l)-f(2)+d(2f(l)+(2)- mf(2))

The satellite 10 synthesises a third signal, with frequency f(3), which it sends via the downlink radio link 60 to the earth station 38. The third signal f(3) is subject to the proportional error of the master oscillator in the satellite 10. Thus, the actual frequency sent on the downlink radio link 60 becomes:

(l+m)f 3

Since the satellite 10 is moving, the signal on the downlink radio link 60 is also subject to Doppler shift. The frequency, f(R2), received at the earth station 38 on the downlink radio link 60 is thus given by:

f(R2) = (l+d)(l +m)f(3) thus: f(R2) = f(3) +df(3)+mf(3)+mdf(3) (2)

The second order term mdf(3) is very small compared to the other terms and can be ignored. This leaves the following equations.

f(Rl) = f(l )-f(2)+d(2f(l)-f(2))-mf(2) (1 ) and f(R2) = f3(l+d+m) (2)

Now, f(l), f(2) and f(3) are precisely know numbers and f(Rl) and f(R2) are accurately measured and thus known. This reduces the equations to being two simultaneous equations in two unknowns, namely m and d, which can thus be solved for the unknowns.

Figure 9 is a schematic view of how the earth station 38 measures the proportional Doppler shift error and master oscillator error on the user terminal 44. The earth station 38 and the earth station controller 56 first "calibrate" the satellite 10 as described with reference to Figure 8. Being able to predict the behaviour the satellite 10, the earth station 38 effectively moves its point of operation from the surface of the earth 14 and places it at the satellite 10. The satellite 10 will show a different Doppler shift with respect to the earth station 38 than it displays with respect to the user terminal 38.

The subscriber antenna 42 and the frequency changer 76 are shown twice in the satellite 10 simply to indicate that two paths exist, where the earth station 38 receives signals from the user terminal 44 via the satellite 10 and the earth station 38 sends signals to the user terminal 44 via the satellite 10.

Firstly, the earth station 38 sends a signal on the uplink 58 which is transposed by the frequency changer 76 and sent down on the user terminal downlink 62 to the user terminal 44. The user terminal 44 makes a measurement of the signal on the user terminal downlink 62, transposes its frequency by a nominal fixed amount and resends the transposed signal on the user terminal uplink 64 to the satellite 10 via the subscriber antenna 42 to be transposed via the mixer 76 and sent, via the downlink radio link 60, to the earth station 38 where the earth station controller 56 makes an accurate frequency measurement. The user terminal 44 also makes an independent transmission, via the satellite, as described, at a nominal frequency, known to the earth station 38 and its controller 56.

It will be understood that the same methodology can be used by the earth station 38, extended via the "calibrated" satellite 10, to measure the errors of the user terminal 44, as the earth station 38 used to "calibrate" the satellite. There has been one loop - back frequency measurement, and one independent signal at a nominal synthesised frequency. The earth station controller 56 corrects for the "calibration" of the satellite, and once again works out the two equations in two unknowns to solve for the satellite 10 to user terminal 44 Doppler shift and to solve for the proportional error in the master oscillator in the user terminal 44. Figure 10 shows how measurement of Doppler frequency shift and delays can be used to locate a user terminal 44 on the surface of the earth 14. In Figure 10, the horizontal axis 78 corresponds to measurement in the direction of the second arrow 48 of Figure 5 along the ground track. The vertical axis 80 corresponds to measurement along the cross track as indicated by the fourth arrow 54 in Figure 6. Only one quadrant is shown. It is to be understood that the pattern, as shown, is symmetrical about the axes in all four quadrants.

The delay measurements, described with reference to Figure 6, create a series of delay contours 82, approximating to circles centred on the nadir 50 which corresponds to the point 00 in Figure 10. Whereas the delay contours 82 represent the intersections of spheres of constant delay centred on the satellite, Doppler contours 84 represent the lines of intersection of the plurality of coaxial cones 72 described in relation to Figure 7. The Figures given for the Doppler contours relate to the Doppler shift, in kHz, corresponding to the position, on the surface of the earth 14, where the user terminal 44 might be situated. Likewise, the Figures adjacent to the delay contours 82 indicate the particular delay in milliseconds, for that particular delay contour 82 and that was the particular position on the surface of the earth 14. Various Figures are shown in degrees, being the angle of elevation from the user terminal 44 to the satellite 10 if it were in that location. Figure 10 extends out to a minimum elevation of 10 degrees, which, in this instance, is the operational minimal of the satellite communications system.

Also shown in Figure 10, overlaid, are some of the spot beams 30 described with reference to Figure 3 and 4. It is to be understood that spot beams 30 fill the entirety of the four quadrants. Only a few spot beams 30 have here been shown to avoid undue cluttering and complication of Figure 10.

Essentially, on the basis of a single delay measurement as described with reference to Figure 6, and a Doppler frequency shift measurement as described with reference to Figure 8 and 9, it is possible to estimate the position of the user terminal 44 on the surface of the earth 14 at that point where its particular delay contour 82 and Doppler contour 84 cross.

Because there exist 4 quadrants, there is a degree of ambiguity in determining which of the four quadrants the user terminal 44 might be situated. This is resolved by noting which of the plurality of spot beams 30 received the signal from the user terminal 44.

It is to be observed, in Figure 10, that the Doppler contours 84 are in fact drawn as a pair of lines rather than a single line. This is to represent the proportional error in the measurement. Close to the nadir 50, the lines in the Doppler contour 84 are close together indicating a small positional error. By contrast, at large distances along the ground track shown by the horizontal axis 78, the pairs of lines in the Doppler contours 84 become wider apart indicating a greater error. By contrast, although the delay contours 82 are also pairs of lines indicating an uncertainty, in the accuracy of the measurement, the pairs of lines in the delay contours are much closer together.

Embodiments of the invention, as will later be described, permit the measurement of the following parameters, in effect, using a pair of satellites 10, and a difference measuring technique comprising:

A) Propagation delay between a first satellite 10 and a user terminal 44, and thereafter effective Doppler frequency shift between the user terminal 44 and first and second satellites 10 by user terminal 44 reported measurement of the difference in received frequency of a common frequency broadcast signal from each of the first and second satellites 10, having first established the Doppler shift between the user terminal and one of the two satellites, or

B) Propagation delay between the first satellite 10 and the user terminal 44 by the means described above, and thereafter a delay (distance) measurement from the second satellite 10 by the user terminal 44 noting and reporting the difference in the time of receipt of a simultaneously transmitted broadcast from the first and second satellites 10, or C) Doppler shift measurement between the first satellite 10 and the user terminal 44, using the technique described above, and thereafter Doppler shift measurement between the user terminal 44 and the second satellite 10 by means of the user terminal 44 reporting the difference in received frequency of a common frequency broadcast signal from the first and second satellites 10, or

D) Doppler shift measurement between the first satellite 10 and the user terminal 44, using the technique described above, and thereafter delay (distance) measurements between the user terminal 44 and the first and second satellites 10 by means of the user terminal 44 reporting the time difference of receipt of a simultaneously broadcast signal from each of the first and second satellites 10, having first established the propagation delay between the user terminal and one of the satellites.

In each of the four preferred embodiments (A to D), a similar diagram to that shown in Figure 10, will be generated, having crossing contours 82 84 . Figure 10 shows Doppler contours 84 crossing delay contours 82. The four embodiments (A to D) allow for Doppler contours 84 to superimpose upon other Doppler contours 84, and delay contours 82 to superimpose upon other delay contours 82, from first and second satellites 10. The principle by which position of the user terminal 44 can be worked out is exactly the same as shown in Figure 10. The user terminal 44 has a location co-incident with crossing contours 82 84.

As earlier described with reference to Figure 6, an example of the way crossing contours can be used is given based on a two-satellite delay determination. The delay measurements generate, as the possible position of the user terminal 44 relative to the satellite 10, a spherical surface, centred on each of the satellites 10, 10' which intersect with each other, and with the surface of the earth 14, to give a unique location for the user terminal 44 on the surface of the earth 14, subject to ambiguity resolution, hereinbefore described. If the user terminal is assumed to be on the surface of the earth, only two satellite propagation delays are necessary for absolute location of the user terminal within limits required for the operation of a telephone system. Returning, by way of example, to Figure 10, attention is drawn to the relatively large errors on the Doppler contours 84 at great distance along the ground track. In order to overcome the rather large errors in the Doppler contours 84 at great distances along the ground track as indicated by reference 78, an averaging process in undertaken, which is also applicable to the propagation delay measurements as above described.

Figure 1 1 shows a surprising result. If no correction is made for the movement of the earth 14 relative to the nadir 50 of the satellite 10, or of the orbital velocity of the satellite 10 relative to the earth, the actual position of the user terminal 44, as shown in Figure 1 1 , relative to the satellite 10, steadily increases with time as shown by the solid line 86. Each measurement of the Doppler shift and of the delay takes a predetermined period. Accordingly, the positional error as shown by the solid line 86 increases steadily with the number of measurements made.

The positional error, as measured, falls, by well known statistical principles, by the root of the sum of the squares. For example, if a hundred samples are taken, the average error falls to one tenth. If ten thousand samples are taken, the average error falls to one hundredth. If a million samples are taken, the average error falls to one thousandth, and so on. Broken line 88 indicates the falling rate of measured positional error against the number of samples.

The dotted line 90 represents the sum of the broken line 88 and the solid line 86 indicating the actual positional error against the number of samples. It is to be noted that there is a minimum region 92 where the measured positional error is at its least, fewer numbers of measurement producing a greater measured positional error, and greater numbers of measurements also producing a greater measured position error. It is to be observed that the minimum region 92 is quite flat and there are a range of values N(l) to N(2) between which the measured positional error is more or less at a minimum. An optimum number of numbers of measurements may thus be selected between the numbers N(l) and N(2) which will give the best positional estimation. The exact number of optimum measurements depends very much upon the initial measurement error. Returning, briefly, to Figure 10, the slope of the broken line 88 representing the improvement of positional error in terms of the number of measurements taken, being a square root, it is to be observed that the delay contour lines 82 start off with a relatively small error so that, interpreting the graphs of Figure 1 1 , a relatively small number of measurements would be required to produce an optimum number of measurements. Conversely, the Doppler contours 84, along the ground track is indicated by the horizontal axis 78 are relatively large so that the slope of the broken line 88 is relatively shallow, demanding a relatively large number of measurements to achieve a best estimation of positional error.

Figure 12 is a first quadrant indication of the optimal number of measurements to be taken for each of the spot beam 30 depending upon the beam in which the user terminal 44 is found, for each of these spot beams 30, for Doppler shift measurements, according to the preferred embodiment illustrating the present invention. It will be seen that numbers of optimum measurements range from 90 to 42. If other sampling rates and satellite orbital heights are chosen, other optimum numbers of measurement apply.

Likewise, Figure 13 shows the optimum number of bursts or samples for each of the spot beams 30 for delay measurements as described with reference to Figure 6. Surprisingly, the optimum number of samples ranges from 201 near the nadir along the cross track as indicated by the vertical lines 80 and drops to surprising low values at the periphery of the spot beams 30.

The embodiments (A to D) of the present invention apply to those areas 20, shown in Figures 1 and 4, where there is multiple radio coverage from the satellite 10.

Figure 14 shows the situation where the user terminal 44, on the surface of the earth 14, has radio coverage from more than one satellite 10, 10'. The first satellite 10 and the second satellites 10' are both visible to the user terminal 44 and to a single earth station 38. However, it is possible that a satellite 10, 10' may be visible of the user terminal 44 but not the single earth station 38. Alternatively, one or other of the satellites 10', 10 will be visible to another earth station 38 '. This is not a problem since both earth stations 38 38' may be joined by a ground communication line 94 where data, derived from the satellites 10, 10' and the user terminal may be exchanged for one of the earth stations 38 to act as a master in determining the position of the user terminal 44 on the surface of the earth 14. Thus, only one of the satellites actually needs to be visible to the earth station 38, the Doppler shift, or the effective propagation delay between the user terminal 44 and the invisible satellite 10 being reported back to the earth station 38 by means of the difference reports generated by the user terminal 44 in response to difference measurements on the broadcast signal. The minimum requirement is thus that one satellite 10 be visible to the earth station 38 and both satellites be visible to the user terminal 44.

The present invention concerns itself with, in what manner, the position of the user terminal 44 is to be determined on the surface of the earth 14 where at least two satellites 10 are visible. When more than one satellite is visible, the position is determined by the method described in relation to Figure 14.

Figure 15 is a flowchart of the activities of the earth station 38 (in the left-hand portion) and of the user terminal 44 (in the right-hand portion) when performing a user terminal 44 position determination when at least two satellites 10, 10' are visible to the user terminal 44, and in accordance with the present invention

In a first operation 96 the earth station 38 sends the necessary messages to measure the first range dependent property, and, in a second operation 98, the user terminal 44 co-operates with the first operation 98. As a first option, the first range dependent property is the propagation delay between the user terminal 44 and the first satellite, and is measured in accordance with the description given above with respect to Figures 6, 1 1 and 13.

In a second option, the first range dependent property is the Doppler shift between the first satellite 10 and the user terminal 44, and is measured in accordance with the description, given above in relation to Figures 7, 8, 9, 1 1 and 12.

Having established Doppler contours 84 or propagation delay contours from the first satellite 10, with regard to the first satellite 10, and as illustrated in Figure 10, a third operation 100 has the earth station 38 cause the first satellite 10 and the second satellite 10' send a broadcast message (BCCH) to the user terminal, for preference, but not of necessity, employing the appropriate spot beam 30 from each satellite 10, 10', wherein each satellite 10, 10' has received signals from the user terminal 44.

Figure 16 shows a first alternative whereby the position of the user terminal 44 can be determined when the first range dependent property is the Doppler shift between the user terminal 44 and the first satellite 10. In order to establish the position of the user terminal 44 with respect to the time delays between the user terminal and the first and second satellites 10, 10', based on differences in broadcast message receipt times from the first and second satellites 10, 10', it is necessary to establish a baseline for one of the satellites 10, 10' so that the difference can have an absolute meaning for the necessary spherical interaction between the radio waves from the satellites 10, 10' with the spherical surface of the earth 14 , so that an eighth operation 1 10 has the earth station 38 send the necessary messages to the user terminal 44, and a ninth operation 1 12 has the user terminal co-operate with the earth station 38 in the eighth operation 1 10, for a baseline to be drawn between the user terminal 44 and the first satellite 10, so that differential measurements can have an absolute meaning. Figure 17 shows a second alternative where the first range dependent characteristic is the propagation delay between the user terminal 44 and the first satellite 10. Here, a tenth operation 1 14 has the earth station 38 send the necessary messages for the Doppler shift between the first satellite 10 and the user terminal 44 to be measured, and an eleventh operation 1 16 has the user terminal 44 co-operate with the earth station 38 to achieve the necessary measurement.. This achieves a baseline measurement of Doppler shift whereby the difference in Doppler shift for a broadcast received frequency difference measurement can have absolute meaning with regard to contour 84 lines for Doppler shift on the surface of the earth 14.

Looking again at Figure 15, as a first a first option, relating to Figure 17, the broadcast signal is sent on the same known frequency from the first satellite 10 and from the second satellite 10'. A fourth operation 102 then has the user terminal 44 receive the broadcast message from each of the first 10 and second 10' satellites. A fifth operation 106 then has the user terminal 44 measure the frequency of the received broadcast signal from the first 10 and the second 10' satellites using its internal oscillator, calculate the difference there-between, and report the difference in measured frequency back to the earth station 38. This has the property, effectively, of allowing measurement of the Doppler shift between the user terminal 44 and either the first 10 and/or the second 10' satellite.

Again, with reference to Figure 15, as a second option, with regard to Figure 16, the broadcast signal is sent at the same, known instant from the first satellite 10 and the second satellite 10'. The fourth operation 102 again receives the broadcast signals from each of the first 10 and second 10' satellite and the fifth operation 106 has the user terminal 44 note the time of arrival, on the user terminal's 44 internal clock, of each broadcast signal, calculate the difference there-between, and report the difference in the time of arrival back to the earth station 38. This has the effect of allowing the calculation of the propagation delay between the user terminal and either the first 10 or/or the second satellite 10'. It can be seen that the options for the first 96 and second 98 operations, together with the options for the third 100, fourth 102 and fifth 106 operations, by combination, provide the descriptions of the preferred embodiments (A, B, C and D) given hereinbefore.

After the fifth operation 106, a sixth operation 104 has the earth station 38 receive the difference report and a seventh operation 108 has the earth station calculate the position of the user terminal 44 on the surface of the earth 14 according to contour crossing, as discussed with reference to Figure 10, having used the difference report to calculate either a Doppler shift of a propagation delay and resolving any ambiguity as described hereinbefore.

By using a difference report, the absolute accuracy of the internal oscillator in the user terminal, or the accumulated timekeeping errors and drift in the internal clock of the user terminal 44, are subject only to the very small errors concurrent in clock rate or oscillator precision during the time to measure the frequency or time of arrival of the two received broadcast signals. An improved accuracy in position determination is thereby achieved, despite having a low precision time or frequency reference within the user terminal 44.

Although the invention has been described generally in relation to the ICO™ system, it will be appreciated that it could be equally well applied to any of the satellite mobile telecommunications networks described in Scientific American supra.

Also, whilst the user terminals UT have been described herein as mobile telephone handsets, it will be understood that they may be semi-mobile e.g. mounted on a ship or aircraft. The UT may also be stationary e.g. for use as a payphone in a geographical location where there is no terrestrial telephone network.

Claims

1. A satellite communications system wherein a user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said system being characterised by; said earth station being operable to exchange signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; by said user terminal being operable to receive said broadcast signal from said first satellite and said second satellite; by said user terminal being operable to measure the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; by said user terminal being operable to report said difference in said values of said second range dependent characteristic to said earth station; and by said earth station being operable, thereafter, to analyse said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

2. A system according to claim 1 wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal and wherein said second range dependent characteristic is the measured frequency of the broadcast signal received from said first satellite and from said second satellite.

3. A system according to claim 1 , wherein said first range dependent characteristic is the time delay for a radio signal between said first satellite and said user terminal and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and said second satellite.

4. A system according to claim 1 , wherein said earth station is further operable to measure the time delay for a radio signal between said user terminal and said first satellite, wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal, and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and from said second satellite.

5. A system according to claim 1 , wherein said earth station is further operable to measure the Doppler shift between said first satellite and said user terminal, wherein said first range dependent characteristic is the time delay for a radio signal between said user terminal and said first satellite, and wherein said second range dependent characteristic is the measured frequency of the broadcast signal from said first satellite and from said second satellite.

6. A system according to claim 1, 2, 3, 4 or 5 wherein said earth station is operable to send said message via said first satellite, to measure said first range dependent characteristic, an optimum number of times, dependently upon the estimated position of said user terminal with respect to said first satellite, and to take the average of the respective first range dependent characteristics derived therefrom.

7. A system according to claim 2, or according to claim 4, 5, or according to claim 6, when claim 6 is dependent upon claim 2, 4 or 5, wherein said first satellite is operable to send, to said earth station, a signal on a first generated frequency, and wherein said earth station is operable to send a signal at a first known frequency to said first satellite, and wherein said first satellite is operable to use a respective internal oscillator to transpose said signal of a first known frequency and return the transposed signal to said earth station on a first transposed frequency, said earth station being operable to measure said first generated frequency and said first transposed frequency and to derive therefrom the Doppler shift between said earth station and said first satellite, and the error in the internal oscillator in said first and satellite.

8. A system according to claim 7 wherein said earth station is operable, after having derived said Doppler shift and said error in said internal oscillator in said first satellite, to cause said first satellite to send a signal, at a second known frequency, to said user terminal, wherein said user terminal is operable to use an internal oscillator to transpose said signal of a second known frequency and return the transposed signal to said earth station, through said first satellite, on a second transposed frequency, wherein said user terminal is operable to send, to said earth station, via said first satellite, a signal on a second generated frequency, wherein said earth station is operable to measure said second transposed frequency and said second generated frequency, wherein said earth station is operable to derive therefrom the Doppler shift between said first satellite and said user terminal and to derive the error in the internal oscillator in said user terminal.

9. A system according to any of the preceding claims wherein said first satellite and said second satellite are in the same orbit.

10. A system according to claims 1 to 8 wherein said first satellite and said second satellite are in different orbits.

1 1. A method for use in a satellite communications system wherein a user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said method being characterised by including the steps of: said earth station exchanging signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; said user receiving said broadcast signal from said first satellite and said second satellite; said user terminal measuring the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; said user terminal reporting said difference in said values of said second range dependent characteristic to said earth station; and said earth station analysing said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

12. A method according to claim 1 1 wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal and wherein said second range dependent characteristic is the measured frequency of the broadcast signal received from said first satellite and from said second satellite.

13. A method according to claim 1 1, wherein said first range dependent characteristic is the time delay for a radio signal between said first satellite and said user terminal and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and said second satellite.

14, A method according to claim 1 1 , including the further step of said earth station is measuring the time delay for a radio signal between said user terminal and said first satellite, wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal, and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and from said second satellite.

15. A method according to claim 1 1 , including the further step of said earth station measuring the Doppler shift between said first satellite and said user terminal, wherein said first range dependent characteristic is the time delay for a radio signal between said user terminal and said first satellite, and wherein said second range dependent characteristic is the measured frequency of the broadcast signal from said first satellite and from said second satellite.

16.. A method according to claim 11, 12, 13, 14 or 15 including the steps of: said earth station sending said message via said first satellite, to measure said first range dependent characteristic, an optimum number of times, dependently upon the estimated position of said user terminal with respect to said first satellite; and said earth station taking the average of the respective first range dependent characteristics derived therefrom.

17. A method according to of claim 12, or according to claim 14, or 15, or according to claim 16, when claim 16 is dependent upon claim 12, 14 or 15, including the steps of: said first satellite sending, to said earth station, a signal on a first generated frequency; said earth station sending a signal at a first known frequency to said first satellite; said first satellite using an internal oscillator to transpose said signal of a first known frequency and return the transposed signal to said earth station on a first transposed frequency; said earth station measuring said first generated frequency and said first transposed frequency; and said earth station deriving therefrom the Doppler shift between said earth station and said first satellite, and the error in the internal oscillator in said first and satellite.

18. A method according to claim 17, including the steps of: said earth station, having derived said Doppler shift and said error in said internal oscillator in said first satellite, causing said first satellite to send a signal, at a second known frequency, to said user terminal; said user terminal using an internal oscillator to transpose said signal of a second known frequency and return the transposed signal to said earth station, through said first satellite, on a second transposed frequency; said user terminal sending, to said earth station, via said first satellite, a signal on a second generated frequency; said earth station measuring said second transposed frequency and said second generated frequency; and said earth station deriving therefrom the Doppler shift between said first satellite and said user terminal and deriving the error in the internal oscillator in said user terminal.

19. A method according to claiml l , 12, 13, 14, 15, 16, 17 or 18, for use in a system wherein said first satellite and said second satellite are in the same orbit.

20. A method according to claim 1 1, 12, 13, 14, 15, 16, 17 or 18, for use in a system wherein said first satellite and said second satellite are in different orbits.

21. A user terminal for use in a satellite communications system wherein said user terminal is operable to transmit to first and second satellites, wherein each of said first and second satellites is operable to transmit to said user terminal, wherein each of said first and said second satellites is operable to provide a broadcast signal for reception by said user terminal, and wherein each of said first and second satellites is operable to send and receive signals from an earth station: said user terminal being characterised by; said earth station being operable to exchange signals with said user terminal through said first satellite to measure a first range-dependent characteristic between said first satellite and said user terminal; by said user terminal being operable to receive said broadcast signal from said first satellite and said second satellite; by said user terminal being operable to measure the difference in values between a second range dependent characteristic in said broadcast signal, received from said first satellite and said second satellite; by said user terminal being operable to report said difference in said values of said second range dependent characteristic to said earth station; and by said earth station being operable, thereafter, to analyse said first range dependent characteristic, and the difference between the values of said second range dependent characteristic, to determine the position of said user terminal on the surface of the earth.

22. A user terminal according to claim 21 , for use in a system wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal and wherein said second range dependent characteristic is the measured frequency of the broadcast signal received from said first satellite and from said second satellite.

23. A user terminal according to claim 21 , for use in a system wherein said first range dependent characteristic is the time delay for a radio signal between said first satellite and said user terminal and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and said second satellite.

24. A user terminal according to claim 21 , for use in a system wherein said earth station is further operable to measure the time delay for a radio signal between said user terminal and said first satellite, wherein said first range dependent characteristic is the Doppler shift between said first satellite and said user terminal, and wherein said second range dependent characteristic is the time of arrival of said broadcast signal from said first satellite and from said second satellite.

25. A user terminal according to claim 21 , for use in a system wherein said earth station is further operable to measure the Doppler shift between said first satellite and said user terminal, wherein said first range dependent characteristic is the time delay for a radio signal between said user terminal and said first satellite, and wherein said second range dependent characteristic is the measured frequency of the broadcast signal from said first satellite and from said second satellite.

26 . A user terminal according to claim 21, 22, 23, 24 or 25, for use where said earth station is operable to send said message via said first satellite an optimum number of times, dependently upon the estimated position of said user terminal with respect to said first satellite, and to take the average of the respective first range dependent characteristics derived therefrom.

27. A user terminal according to of claim 22, or according to claim 24 or 25, or according to claim 26 when claim 26 is dependent upon claim 22, 24 or 25, for use where said first satellite is operable to send, to said earth station, a signal on a first generated frequency, and wherein earth station is operable to send a signal at a first known frequency to said first satellite, and where said first satellite is operable to use an internal oscillator to transpose said signal of a first known frequency and return the transposed signal to said earth station on a first transposed frequency, said earth station being operable to measure said first generated frequency and said first transposed frequency and to derive therefrom the Doppler shift between said earth station and said first satellite, and the error in the internal oscillator in said first and satellite.

28. A user terminal according to claim 27, for use where said earth station is operable, after having derived said Doppler shift and said error in said internal oscillator in said first satellite, to cause said first satellite to send a signal, at a second known frequency, to said user terminal, said user terminal being operable to use an internal oscillator to transpose said signal of a second known frequency and return the transposed signal to said earth station, through said first satellite, on a second transposed frequency, said user terminal being operable to send, to said earth station, via said first satellite, a signal on a second generated frequency, said earth station being operable to measure said second transposed frequency and said second generated frequency, and said earth station being operable to derive therefrom the Doppler shift between said first satellite and said user terminal and to derive the error in the internal oscillator in said user terminal.

29. A user terminal according to claim 21 , 22, 23, 24, 25, 26, 27 or 28, for use where said first satellite and said second satellite are in the same orbit.

30. A user terminal, according to claim 21 , 22, 23, 24, 25, 26, 27, or 28, for use where said first satellite and said second satellite are in different orbits.

31. A ground station configured for performance of a method as claimed in claim 1 1.