Source: https://patents.google.com/patent/GB2324218A/en
Timestamp: 2019-05-22 16:56:25
Document Index: 584053092

Matched Legal Cases: ['arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 44', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14']

GB2324218A - Satellite acquisition in navigation system - Google Patents
Satellite acquisition in navigation system Download PDF
GB2324218A
GB2324218A GB9707213A GB9707213A GB2324218A GB 2324218 A GB2324218 A GB 2324218A GB 9707213 A GB9707213 A GB 9707213A GB 9707213 A GB9707213 A GB 9707213A GB 2324218 A GB2324218 A GB 2324218A
GB9707213A
GB9707213D0 (en
1997-04-09 Application filed by ICO Services Ltd filed Critical ICO Services Ltd
1997-04-09 Priority to GB9707213A priority Critical patent/GB2324218A/en
1997-05-28 Publication of GB9707213D0 publication Critical patent/GB9707213D0/en
1998-10-14 Publication of GB2324218A publication Critical patent/GB2324218A/en
1 2324218 IMPROVEMENT IN SATELLITE POSITION DETERMINATION The present
invention relates to position measuring systems and methods where a terminal, on the surface of the earth, receives signals from a plurality of navigational satellites which can be used. accurately, to measure the position of the terminal.
There are several systems, in use, where a terminal can receive signals from orbiting navigational satellites whereby the position of the terminal, on the surface of the earth, can be measured with accuracy extending down to a few metres. One such system is the NAVSTAR 174 GPS (Global Positioning by Satellite) system. A terminal will receive signals from any navigational satellites which it can hear. The navigational satellites broadcast to all terminals in their vicinity. Messages pass only from the navigational satellite to the terminal. The satellite transmits its identity, the exact time at which the broadcast was sent, and copious details concerning the orbit of the navigational satellite. The terminal notes the time that the data was received from the navigational satellite and stores the identity and details of the orbit. The terminal resolves its own clock inaccuracies by processing data from additional satellites. By calculating the precise location of the navigational satellite, from the orbit data, at the instant when the broadcast was sent, and by comparing this with the time the broadcast was received, to establish the delay between when the broadcast was sent and when it was received, the terminal is able to calculate the propagation delay 2 is between the navigational satellite and itself. The propagation delay represents a measured distance between the navigational satellite and the terminal at the instant the broadcast was made. By listening to many different navigational satellites, the terminal is able to triangulate between them to provide a positional fix.
Effective though the GPS system undoubtedly is, it contains several disadvantages. Firstly, in order to avoid signal collision, the navigational satellites broadcast on different channels, separated by appropriately orthogonal spreading sequences. In order, when first switched on, for the terminal, not having any idea of its location or the present position of any navigational satellite, to establish its position, it is necessary for the terminal to listen for all navigational satellites on all known navigational satellite channels. Given that the GPS system has many satellites, and that only a handful of the navigational satellites are required for a positional fix, this represents a gross inefficiency and can require a "cold" terminal to wait for up to thirty minutes for a sufficient number of satellite channels to be scanned and for reciver clock inaccuracies to be resolved. The present invention seeks to provide a system and method, and a terminal for use in the system, whereby the "from cold" start-up time of a terminal can be reduced from a potentially long period to just a short period.
Because of its military origins, it is preferred that a GPS terminal should not transmit for fear of revealing 3 the position of the terminal to an enemy. While the present invention seeks to incorporate the terminal within a satellite communications system, it nonetheless also seeks to provide a means whereby the start-up time of a cold terminal can be greatly reduced without any need for the terminal, in the satellite communications system, being required to transmit or being required to run orbit simulations for the navigational satellites.
The GPS terminal in common with all terminals for navigational satellite position fixing systems, is a powerful data processor. It has to assemble the rather large amount of orbital data for each navigational satellite whose broadcasts it can receive. Thereafter, the terminal is required to process all of the data from many navigational satellites in order to calculate its position. While this may not be a 4 problem for military systems where cost and size usually are not a problem, in the civilian market cost and size can be a paramount factor. This has the result that the terminal is often equipped only with a small microprocessor which, although quite capable of performing the necessary calculations and functions, takes a great deal of time to do so. The net result is a handheld terminal which, from cold, can take a very long time to present its user with a result. By incorporating the terminal within a communications network, the present invention seeks to relieve the terminal of this burden so that very simple and economic terminals can be employed.
The terminal may find itself in a position of poor reception, such as when its radio paths are blocked by buildings, geoghraphical features, and the like. Under these circumstances, it may not be possible for signals from a sufficient number of navigational satellites to be received for a position fix. The present invention seeks to provide means whereby the position of the terminal can be found provided the terminal has a view of, as a minimum, at least one navigational satellite and one communications satellie.
According to anther aspect, the present invention consists in a method for use in a navigational satellite positioning system wherein a terminal is operative to receive time and orbit information from a plurality of navigational satellites to calculate the position of the terminal, said method being characterised by the steps of employing a communications network to inform said terminal which of said plurality of navigational satellites are within range of said terminal, and by said terminal responding thereto to listen only for those of said plurality of navigational satellites that are within range 6 According to another aspect, the present invention consists in a terminal, for use in a navigational satellite positioning system wherein said terminal is operative to receive time and orbit information from a plurality of navigational satellites to calculate the position of said terminal, said terminal being characterised by includion in a communications network, operative to inform said terminal which of said plurality of navigational satellites are within range of said terminal, and by said terminal, in response thereto, being operative to listen only for those of said plurality of navigational satellites that are within range.
The invention and its embodiment further provide a system and method, and a terminal for use in the system, where the communications network provides an array of abutting radio beams, and where the communications network 7 periodically broadcasts, in each beam, information concerning which of the plurality of navigational satellites would be within range of a terminal in that beam. In the preferred embodiment, the communications network includes a satellite with 163 beams in an array. The invention can encompass other schemes. The provision of this feature allows the terminal to have a more or less comprehensive knowledge of what navigational satellites are available, without ever having to transmit to the either to a communications satellite or elsewhere.
The invention and embodiment further provide a system and method, and a terminal for use in the system, where the communications network comprises one or more communications 8 satellites, where the terminal transmits to the one or more communications satellites and where each of the one or more communications satellites can transmit to the terminal, each of the one or more communications satellites sending and receiving signals from an earth station, the earth station exchanging signals with the terminal through the one or more communications satellites and thereafter, analysing the signals to determine the position of the terminal on the surface of the earth. This feature ensures that the approximate position of the terminal can be measured, using the communication satellite or satellites, to allow the earth station more accurately to calculate which navigational satellites will be in view at the terminal.
The invention and embodiment, still further, provide a system and method, and a terminal for use in the system, where the communications satellite communicates with the 9 terminal using one out of a plurality of beams, each of said plurality of beams being interactive with a respective one out of a plurality of areas on the surface of the earth, any ambiguity of position of the terminal being resolved by observation of with which out of the plurality of beams the terminal exchanges the signals. All position determinations for the terminal provide a plural set of points where intersecting loci, on the surface of the earth, provide a plurality of possible points where the terminal could be. This provision allows a rapid and easy solution to resolution of any uncertainty. In the preferred embodiment, each communications satellite provides an array of 163 abutting radio beams projected onto the surface of the earth. The invention, however, can encompass numerous other schemes.
Still further, the invention and embodiment provide a system and method, and a terminal for use in the system, where the earth station is operative to send out a message, via each of the more than one communications satellite, and where the terminal is operative to return the message within a predetermined time of receipt of the message via each of 11 said more than one communications satellite, the earth station being operative thereby to calculate the propagation delay between the more than one communications satellites and the terminal. This feature provides a manner whereby the earth station can measure the propagation delay to establish a distance between each communications satellite and the terminal.
The invention and its embodiment, even further, provide a system and method, and a terminal for use in the system, where the terminal detects and records the time of arrival of broadcast messages from communications satellites which are no longer in sight and reports the previous broadcast messages to the earth station, the earth station using knowledge of the position of the communications satellites, no longer in sight, at the time of receipt of the 12 broadcast message by the terminal to assist in the calculation of the position of the terminal. By this means, assuming the terminal has not moved a significant distance since the receipt of the first broadcast message, the earth station can make a virtually instant approximation of the terminal's position based on a stored history, in the terminal, of those communications satellites it has already heard.
The invention and its embodiment also provide that the system and method, and a terminal for use in the system, include the terminal being capable of noting the 13 is apparent recorded time, reported by the terminal, between two know intervals and thereby correcting for drift and offset error in the timer in the terminal. The internal clock, in the terminal, for reasons of economy, is subject to systematic error, environmental drift and numerous other errors. The clock, in the terminal, will therefore, in all probability, show the wrong time and keep bad time. By noting the errors in the clock in the terminal, at least for a short while, the earth station can correct for them.
The invention and its embodiment also provide that the communications satellite sends, to the earth station, a 14 signal on a first generated frequency, and that the earth station sends a signal at a first known frequency to the communications satellite the communications satellite using an 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 measuring the first generated frequency and the first transposed frequency and deriving therefrom the doppler shift between earth station and the communications satellite and the error in the internal oscillator in the communications satellite. This simple process, exploiting the synthesis and transposition capacity of the communications satellite, allows the satellite to be "calibrated" by the earth station so that the satellite itself can be come an accurate source and relay of signals for the terminal.
The invention and its embodiment also include the earth station, after having derived the doppler shift between the earth station and the communications satellite and the error in the internal oscillator in the communications satellite, causing the communications satellite to send a signal at a second known frequency to the terminal, the terminal using an internal oscillator to transpose the signal of a second known frequency and return the transposed signal to the earth station, through the communications satellite, on a second transposed frequency, the terminal is sending, to the earth station, via the communications satellite, a signal on a second generated frequency, the earth station measuring the second transposed frequency and the second generated is frequency and deriving therefrom the doppler shift between the communications satellite and the terminal and also deriving the error in the internal oscillator in the terminal. Thus, the satellite having been "calibrated" by the earth station, the earth station compensates for the known "calibration" of the satellite to use the satellite to "calibrate" the terminal.
The invention and its embodiment also provide a system and method, and a terminal for use in the system, including the communications network, preferably a satellite communications network, informing the terminal of the established approximate position for the terminal and, in response thereto, the terminal providing a user interpretable 16 indication of said established approximate position. In the preferred embodiment the user interpretable indication is chosen to be by means a visual display, preferably the same display used to provide other functions on the terminal. The invention, however, encompassed the user interpretable indication being audible, or by any other means or combination of means whereby information can be provided to a human being.
17 The invention and its preferred embodiment provide a system and method, and a terminal for use in the system, where the navigational satellite is a satellite in a constellation other than that occupied by the communications satellite, preferably but not exclusively a constellation comprising a plurality of navigational satellites. The invention also encompasses that the communications satellite can also serve as a navigational satellite.
Finally, the invention and its embodiment provide a system and method, and a terminal for use in the system, where the terminal commences a timing operation on receipt of a message from the earth station, terminates the timing operation on receipt of a signal from the navigational satellite, and employs the measured, elapsed time of the timing operation as the time of arrival of the signal at the terminal, the earth station is operative to using the propagation delay between the earth station and the terminal to deduce the true time of arrival of the signal, from the navigational satellite, at the terminal. This measure reduces any systematic errors in the clock at the terminal to simple, second order effects by taking only the difference in 18 is time between two events, rather than the absolute recorded time.
Figure 1 shows a planar constellation of communications satellites disposed about the earth.
Figure 2 illustrates how the communications satellites are disposed in orthogonal orbital planes.
Figure 3 shows the structure of the cone of radio coverage provided by each communications 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 communications 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 terminal via the communications satellite to determine propagation delays between the terminal and the communications satellite.
Figure 7 shows the geometry of doppler frequency shift measurement for the communications satellite.
Figure 8 is a schematic representation of the exchange of test signals between the earth station and the communications satellite to determine the relative doppler shift and internal oscillator error of the communications satellite.
19 Figure 9 is a schematic representation of how a calibrated communications satellite, according to figure 8, may, in turn, be used to determine the relative doppler shift between the communications satellite and terminal and the internal oscillator error in the terminal.
Figure 10 shows how intersecting lines of measured doppler frequency shiftand propagation delays may be used to measure the position of the terminal on the surface of the earth.
Figure 11 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 terminal has direct access to more than one communications satellite.
Figure 15 is a flow chart of the activities of the earth station when determining the position of the terminal on the surface of the earth employing one communications satellite, or more than one communications satellite, if available.
Figure 16 is a flow chart showing how the earth station can incorporate timed broadcasts in determining the position of the terminal on the surface of the earth.
Figure 17 is an expansion of figure 1 wherein the communications satellites are shown in their relationship to navigational or positioning satellites.
Figure 18 is a schematic diagram showing the total environment of the present invention.
Figure 19 is a flow chart of the cativities of the terminal in the overall scheme of the present invention.
Figure 20 is a flow chart of the overall activity of the earth station in relation to figure 19.
Figure 21 is a block diagram of the elements in different terminals which can be employed in and as the present invention.
Figure 22 is a combined flow chart of the activites of either of the terminals of figure 21, together with the corresponding activities of the communications system, when working out the position of either of the terminals.
and Figure 22 is a flow chart of that terminal, shown in figure 21, which can communicate, when it interacts to determine the position of the terminal, together with the corresponding activities of the communications network.
21 The description of the preferred embodiment commences with a description of the communications satellite system.
Figure 2 illustrates how the communications satellites 10 are disposed in orthogonal orbital planes.
The first orbit 12 of figure 1 is supplemented by a second orbit 12, having communications 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 22 at 45 degrees to the equator 24 and having planes which are orthogonal (at 90 degrees) to each other.
In the example shown, the communications satellites 10 orbit above the surface of the earth 14 at an altitude of 10 355km. Those skilled in the art will be aware that other orbital heights and numbers of communications 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 communications satellites 10. In particular, the orthogonality of the orbits ensures that the communications satellites 10 of the second orbit 12' provides radio coverage for the third types of area 22 of no radio coverage for the communications satellites in the first orbit 12, and the communications satellites 10 in the first orbit 12 provide radio coverage for those areas 22 of the third type where the communications satellites 10 of the second orbit 12, provide no radio coverage. By such an arrangement, it is ensured that every point, on the surface of the earth 14, has, at least, one communications satellite 10 10', visible at all times.
It will become clear that, although the two orbits 12, 12, are here shown to be of the same radius, the embodiment of the invention as hereinbefore and hereinafter described will function with orbits 12, 12, of different radii. Equally, there may be more than two orbits 12, 12 So far as the embodiment of the present invention is concerned, the only requirement is that every part of the 23 surface of the earth 14 is in receipt of radio coverage from at least one communications satellite 10 at all times.
Figure 3 shows the structure of the cone 16 of radio coverage provided by each communications 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 divided into a plurality of spot beams 30, by means of a corresponding plurality of directional antennae on the communications satellite 10. The communications satellite 10 is intended for mobile radio telephone communications and each of the spot beams 30 corresponds, roughly, to the equivalent of a cell in a cellular radio telephone network. In figure 3, the cone of radio coverage 16 is distorted due to the geometry of the map of the earth's surface provided. 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.
24 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 communications satellite, second areas 20 where there is radio coverage by two communications satellites, and even fourth areas 32 where coverage is provided by three or more communications 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 communications satellite 10 above the surface of the earth.
The communications 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, an 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 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 terminal 44 may also comprise more elaborate vehicle mounted equipment for use in land vehicles, ships and aircraft.
With the parameters mentioned in this preferred example, the communications satellite moves around its orbit 12 121, 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 communications satellite, is known as the nadir 50.
At the same time the earth 14 is rotating, at its equator with a velocity of 0.47k:m per second, as indicated by a third arrow 52. Directions, relative to the ground track 48, at 90 degrees thereto, are termed crosstrack as indicated by a fourth arrow 54. Hereinafter, the position of the 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 terminal 44 or via the communications satellite 10.
26 The earth station 38 communicates with the communications 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 communications 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.
Implicit in figure 6, but not specifically shown, is the fact that communications satellite 10 contains its own precise oscillator, conveniently in the form of a crystal oscillator, which the communications satellite 10 uses for converting the frequencies of incoming and outgoing signals and for use as a frequency reference when synthesising frequencies. Likewise, the terminal 44 contains its own internal synthesised oscillator, working from a master oscillator, preferable a crstal oscillator, for converting frequencies of incoming sinals and synthesising the frequencies of outgoing signals.
27 Equally, 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 exact location, on the surface of the earth 14, of the earth station 38, is known with great precision. Likewise, the parameters or the orbit 12 12, of the communications satellite 10 and its position in that orbit, at any instant, are also known with great precision. The uncertain element, which is the purpose of the present invention to resolve, is the position of the terminal 44 on the surface of the earth 14.
Not previously mentioned, is the fact that the terminal 44 transmits on the terminal uplink 64 to the subscriber antenna 42 and similarly receives on the terminal downlink link 62 from the subscriber antenna 42. The communications satellite 10 will only be in communication with one earth station 38 at a time, but may be in communication with a great many terminals 44. Each terminal will be in one particular spot beam 30 of the plurality of spot beams shown in figure 3.
The communications satellite 10 will be moving relative to the surface of the earth 14, and therefore relative to the earth station 38 and to the terminal 44, as indicated in a fifth arrow 66. Likewise, the surface of the earth 14 will be moving relative to the orbit 12 12' of the 28 is communications satellite 10 as generically indicated by a sixth arrow 68.
Propagation delay is measured between the earth station 38 and the terminal 44 to establish the propagation delay between the terminal and the communications satellite 10. The earth station 38 sends out a signal on the uplink radio link 58 to the communications satellite 10 which is, in turn, sent to the terminal 44 via the terminal downlink 62 Upon receipt of the signal from the earth station 38, the terminal waits for a predetermined period and then sends its own message, via the 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 earth station 38 began to receive the response message from 29 the terminal 44 from the downlink radio link 60. The earth station controller 56 knows the propagation delay times for signals, through the communications satellite 10, from the uplink radio link 58 onto the terminal downlink 62 and, correspondingly, the propagation delay through the communications satellite 10 between the 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 terminal 44 before it responds to the received message from the earth station 38. These propagation delays and the predetermined delay of the 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 communications 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 over all 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 communications satellite 10 from the overall propagation delay, the propagation delay between the terminal 44 and the communications satellite 10 may,precisely, be measured. This means that, since the propagation is entirely at the speed of light, the linear distance between the communications satellite 10 and the terminal 44 is known. According to the propagation delay, the terminal may exist on any point of a spherical surface centred on the communications satellite 10. Because the spherical surface intersects the surface of the earth 14, and the terminal 44 is on the surface of the earth, the location of the 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 communications satellite 10.
Figure 7 shows the geometry of doppler frequency shift measurement for the communications satellite 10- As the communications satellite 10 moves as indicated by a 7th arrow 70, the change in frequency of a radio signal sent from the communications satellite 10 and the perceived frequency of a radio signal received by the communications satellite 10 from a fixed source such as the terminal 44, depends upon the cosin of the angle between the communications satellite 10 and the recipient of a transmitted radio signal from the communications satellite or the source of a transmitted radio signal to the communications 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 communications satellite 10 at their collective apex, extending towards infinity, and having, as 31 their collected axis 74, the direction of the motion of the communications 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 communications satellite for radio frequencies receivers or sources which the communications satellite 10 is approaching. It is to be understood that a corresponding set of coaxial cones 72 extend "behind" the communications satellite, having the same apex and axis. The doppler shift "in front" of the communications satellite 10 is shown by an increase in frequency. The doppler shift "behind" the communications satellite 10 is provided by a corresponding decrease in frequency.
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 communications satellite 10, using its own internal oscillator, translates the frequency of the signal and provides it on the terminal downlink 62. The terminal 44 then returns the signal via the terminal uplink 64, once again to be converted in frequency by the internal oscillator of the communications 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 32 link 60 signal and deduces the doppler frequency shift, at the terminal 44, resulting from the motion of the communications satellite 10 as indicated by the Sth 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 communications satellite 10 to calibrate the errors and doppler shift experienced between the earth station 38 and the communications satellite 10.
The earth station 38 sends a signal of know frequency f(l) on the uplink radio link 58 to the communications satellite 10. The communications satellite 10 has an internal master oscillator which controls all of the synthesised frequencies used by communications satellite 10.
If the master oscillator has a proportional error m, then any frequency, synthesised using the master oscillator, in the communications satellite, is proportionally in error, so that:
f(actual) = (l+m)f(intended) Likewise, the communications 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 communications satellite 10, or from the communications satellite 10 to the earth station 38:
f(received) = (l+d)f(sent) 33 Thus, if the earth station sends a frequency f(l) on the uplink radio link 58 to the communications satellite 10, because of doppler shift the communications satellite receives a frequency f(received at communications satellite) = f(l)(1+d) Now, the communications 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 communications satellite 10 synthesises an intended frequency f(2) to be subtracted from frequency of the signal received at the communications satellite 10 from the earth station 38. The intended frequency f(2) is subject to the proportional error in the master oscillator on the communications satellite 10, and so becomes f(2)(l+m).
f (1) (1+d) f (2) (1+m) and this is sent, back to the earth station 10, via the subscriber antenna 44. But the communications 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(R1), is given by 34 f (R1) = (1+d) (f (1) (1+d) - f (2) (1+m)) The earth station controller 56 measures f(R1) with extreme precision. Thus, f(R1), f(I) and f(2) are all known numbers, but m and d are unknown. Expanding the expression for f(R1) we obtain f (R1) = (f (1) - f (2)) + d (2 f (1) + d 2 f (1)) - 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(R1) = f(l)f(2)+d(2f(l)+(2)- mf(2)) is The communi'cations 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 communications satellite 10. Thus, the actual frequency sent on the downlink radio link 60 becomes:
(1+m) f 3) Since the communications 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) = (1+d) (1+m) f (3) thus f(R2) = f(3) +df(3)+mf(3)+mdf(3) The second order term mdf(3) is very small compared to the other terms and can be ignored. This leaves the following equations.
f(R1) = f(l)-f(2)+d(2f(l)-f(2))-mf(2) and f(R2) = f3(l+d+m) Now, f(l), f(2) and f(3) are precisely know numbers and f(R1) 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 terminal 44.
The earth station 38 and the earth station controller 56 first 'calibrate, the communications satellite 10 as described with reference to figure 8. Being able to predict the behaviour the communications satellite 10, the earth station 38 effectively moves its point of operation from the surface of the earth 14 and places it at the communications satellite 10. The communications satellite 10 will show a different doppler shift with respect to the earth station 38 than it displays with respect to the terminal 38.
36 is The subscriber antenna 42 and the frequency changer 76 are shown twice in the communications satellite 10 simply to indicate that two paths exist, where the earth station 38 receives signals from the terminal 44 via the communications satellite 10 and the earth station 38 sends signals to the terminal 44 via the communications satellite 10.
A moment of reflection will show that precisely the same method has been used by the earth station 38, extended via the calibrated, communications satellite 10, to measure the errors of the terminal 44, as the earth station 38 used to 'calibrate' the communications 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 37 communications satellite, and once again works out the two equations in two unknowns to solve for the communications satellite 10 to terminal 44 doppler shift and to solve for the proportional error in the master oscillator in the terminal 44.
Figure 10 shows how measurement of Doppler frequency shift and delays can be used to locate a 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 communications 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 milliseconds, corresponding to the position, on the surface of the earth 14, where the terminal 44 might be situated. Likewise, the figures adjacent to the delay 38 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 terminal 44 to the communications 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 communications satellite communications system which holds the example given as the preferred embodiment of the present invention.
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 single Doppler frequency shift measurement as described with reference to figure 8 and 9, it is possible to estimate the position of the 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 terminal 44 might be situated. This is resolved by noting which of the plurality of spot beams 30 received the signal from the terminal 44.
39 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.
Figure 11 shows a surprising result. If no correction is made for the movement of the earth 14 relative to the nadir 50 of the communications satellite 10, or of the orbital velocity of the communications satellite 10 relative to the earth, the actual position of the terminal 44, as shown in figure 11, relative to the communications 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 is 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 hunndreth. 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(1) 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(1) 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 41 a relatively small error so that, interpreting the graphs of figure 11, 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 beams 30 depending upon the beam in which the 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 communications 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 foregoing description applies to those areas 18, as shown in figures 1 and 4, as having single radio coverage from a communications satellite 10. The following
42 description applies to those areas 20, shown in figures 1 and 4, where there is multiple radio coverage from the communications satellite 10.
Figure 14 shows the situation where the terminal 44, on the surface of the earth 14, has radio coverage from more than one communications satellite 10 101. Ideally, the two communications satellites 10 101 should both be visible to the terminal 44 and to a single earth station 38. However, it is possible that a communications satellite 101 may be visible of the terminal 44 but not the single earth station 38. Alternatively, the other communications satellite 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 communications satellite 10 101 and the terminal may be exchanged for one of the earth stations 38 to act as a master in determining the position of the terminal 44 on the surface of the earth 14.
If more than one communications satellite 10 101 is visible, or has been visible in the near past, instead of executing a doppler ranging operation as described with reference to figures 7, 8, 9, 10, 11 and 12, a simple time delay measurement is executed as described with reference to figures 6, 10, 11 and 13. An earth station 38 38' sends a signal to each of the communications satellites 10 10' and, as previously described, and measures the propagation delay between the communications satellite 10 101 and the terminal 44.
43 As earlier described with reference to figure 6, the delay measurements generate, as the possible position of the terminal 44 relative to the communications satellite 10, a spherical surface, centred on each of the communications satellites 10 10' which intersect with each other, and with the surface of the earth 14, to give a unique location for the terminal 44 on the surface of the earth 14, subject to beam identity ambiguity resolution, hereinbefore described. If the terminal is assumed to be on the surface of the earth, only two communications satellite propagation delays are necessary for absolute location of the terminal. If more than 3 communications satellites 10 101 are so used, the terminal 44 may be absolutely located in space, also allowing for altitude variations on the surface of the earth 14. It is to be noted, with reference to the description of figure 10, that the delay contours 82 are considerably more accurate, particularly at extreme range from the nadir 50 along the ground track as indicated by the horizontal lines of 78, than are the doppler contours 84. Accordingly, the method of measurement of the position of the terminal 44 on the surface of the earth 14 describe with reference to figure 14 is more accurate.
Accordingly, the embodiment of the invention initially concerns itself with, in what manner, the position of the terminal 44 is to be determined on the surface of the earth 14 using one or more communications satellites 10 101 Where only one communications satellite 10 is visible, the ranging method shown in figure 10 is employed. When more 44 than one communications satellite is visible, the position determined method described in relation to figure 14 is employed. These techniques are used, initially, to gain a rough estimation of the position of the terminal 44.
Attention is now drawn to figure 15 which shows the activity of the earth station controller 56 in that one of the earth stations 38 38' which executes the rough estimation position determination for the terminal 44 using the communications satellite 10 101.
In a first operation 96 the earth station 98 listens for a request of some kind of the terminal 44. If a first test 98 fails to detect a call from the terminal 44, control is passed back to the first operation 96. If the first test 98 determines that the earth station 38 has been polled by the terminal 44, control is passed to a second operation 98. The second operation 98 sends a transmission, via the communications satellite 10, to the terminal 44 as described with reference to figure 6, 9 and 10. It is to be presumed that the operation of figure 8, where the communications satellite is -calibrated", has already been executed. If the operation described with reference to figure 8 has not been executed, the second operation 100 executes the necessary calibration of the communications satellite 10.
The second operation 100 also analyses the results from the doppler frequency shift measurement and from the time delay measurement based on one mutual transmission between the earth station 38 and the terminal 44 to give a 1 1 guess as to the position of the terminal 44 on the surface of the earth 44.
The earth station 38, having made an approximate estimate of the position of the terminal 44, on the surface of the earth, is then in a position to determine whether or not the terminal 44 will be visible to more than one communications satellite 10. If a second test 102 decides that only one communications satellite is visible, control passes to a third operation 104 which determines which one out of the plurality of spot beams 30 is occupied by the terminal 44. This information may also be known by the earth station 38 based on whi.ch of the spot beams 30 the signal from the terminal 44 was received.
Control passes from the third operation 104 to a fourth operation 106 where, with reference to figure 12 on its associated description, depending upon which spot beam 30 is occupied by the terminal 44, the optimum number of samples by message exchange is executed. This gives the greatest provision in position determination as described with reference to figure 11.
When the fourth operation 106 has performed its necessary function, control passes to a fifth operation 108 where delay measurements are made, as described with reference to figure 6, for the optimum number of samples for delay measurement as described with reference to figures 11 and 14.
The fourth 106 and fifth operations 108 may be conducted simultaneously, the number of sampling instance 46 being the larger of which ever is greater for doppler shift or delay measurement as shown as reference to figures 12 and 13 for a particular spot beam 30, and the result being analysed for the lesser number only up to the smaller number required, later results being discarded.
The sum of the function of the fourth operation 106 and the fifth operation 108 is to give the best estimate, based on the style of position analysis described with reference to figure 10 where spheres of constant time delay and cones of constant doppler shift intersect the surface of the earth 14.
Returning to the second test 102, it has been detected that there is just not a single communications satellite 10, control is passed to a fourth test 114 which determines if there is more than one communications satellite 10 10'present. If the fourth test 114 detects that there is a plurality of communications satellites 10 10'available, control passes to a seventh operation 116 where the earth station 38, via the earth station controller 56, determines 47 for which of the plurality of spot beams 30 for each communications satellite the terminal 44 is accessible. Thereafter, control passes to an eighth operation 118 where the earth station 38 exchanges the optimum number of radio bursts for each communications satellite 10 according to figure 6 and its associated description, and according to figures 10 and 13 and their associated descriptions. once the position of the terminal 44 has been approximately determined by the eighth operation 118, control passes to the sixth operation 110 and thereafter as earlier described, back to the first operation 96.
Figure 16 shows the activity of the terminal 44 as it co-operates with the earth station 38 in yet a further 48 alternative for locating the terminal 44 in the surface of the earth 14.
The individual communications satellites 10, at periodical intervals, send out broadcast messages, on all of the spot beams 30, intended to be received by all terminals 44. The broadcast message, from each communications satellite, originates originally, from an earth station 38 and contains information which identifies from which communications satellite the broadcast message is emanated. The time of transmission of the broadcast message is accurately known because, as described with reference to figure 6, the earth station 38 is aware of the precise distance between itself and the communications satellite 10. Equally, as shown in figure 14, different earth stations 381 can instruct different communications satellites 10' to provide a broadcast message. Each earth station 381 is aware of the position of the communications satellite 10 at all times and will also be aware of the identity of the earth station 38 381 from which the broadcast message originated. As an alternative, the broadcast message can also include indication from which earth station it originated.
In any event, it is merely necessary to note the time of arrival of a broadcast message at a terminal 44, and to know from which communications satellite 10 it originated, in order, effectively, to do a ranging "propagation delay" measurement on the terminal 44 from the communications satellite 10. Once again, a sphere of fixed delay, in terms of distance, describes the potential locus of the terminal 44 49 about the central communications satellite 10, and the terminal 44 can lie on the line of intersection of the sphere centred on the communications satellite 10, with the surface of the earth 14.
Returning once again to figure 16, the terminal, in an llth operation 124, listens for the broadcast messages from the communications satellites 10 until a fifth test 126 detects that a communications satellite has been heard. Control then passes to a 12th operation 128 where the terminal, using an internal clock, notes and stores the instant of receipt of the message from the communications satellite 10 together with the identity of the particular communications satellite 10 from which the message originated. The terminal 44 keeps a record of the last several communications satellites 10 to be heard.
1 ' If the seventh test 132 detects that there is only one communications satellite visible, control passes to a thirteenth operation 134 where the terminal 44 responds to delay and doppler measurements as indicated with reference to figures 6 to 13. The terminal 44 also sends, to the earth station 38 the list of times and identities of heard communications satellites 10 which was accumulated by the 12th operation 128.
The earth station controller 56 then combines all of these measurements and will know the position of the terminal 44 on the surface of the earth 14. Control next passes to a fourteenth operation 136 where the terminal 44 proceeds with whatever activity is required of it, which, as will later be described, can include message receipt from one or more of a plurality of navigational satellites, and establishement and execution of phone calls, until an eighth test 138 detects that the activity is over and passes control back to the eleventh operation 124 where the terminal 44 listens for messages from the communications satellites 10.
If the seventh test 132 detects that more than one communications satellite present, control passes to a fifteenth activity 140 where the terminal 44 responds to a propagation delay measurement from each of the communications satellites 10 101 as described with reference to figures 14 and 15. The terminal 44 also reports, to the earth station 38, the contents of the list accumulated in the twelfth operation 128 during the time of receipt and identity of communications satellite broadcast messages.
51 At this point, the earth station 38 with which the terminal 44 is interactive will have sufficient information to determine the position of the terminal 44 along the surface of the earth 14.
Equally, in the thirteenth activity 134, if the combined propagation delay and Doppler frequency shift measurement produces a location which roughly corresponds to the location resulting from intersection of the spheres of constant delay as determined from the list of broadcast receipt times and communications satellite identities as 52 is collected by the twelfth operation 128, and this latter determination is more accurate, then the earth station 38, through its earth station controller 56, can opt to use the latter determination.
The terminal 44 comprises an internal clock. This clock, of course, has relative inaccuracies. The earth station 38, in combination with the earth station controller 56, possess a very accurate clock. In order for the earth station 38 properly to use the list gathered by the 12th operation 128, it is necessary to correct the errors in the clock on the terminal. This is very simply done. The earth station 38, at a first known instant, requests the terminal 44 to indicate the time, on its clock to the earth station 38. The earth station 38 knows the propagation delay between itself and the terminal 44. The time of response, by the terminal 44, is thus very accurately known. Having noted what time the terminal clock believes it to be, the earth station 38 and the earth station controller 56 wait for a 53 predetermined period, perhaps one second, and request that the terminal 44 once again tells the earth station 38 what time the terminal thinks it is. The earth station 38 thus has two readings from which the rate of drift of the clock on the terminal 44 and the accumulated timing error can be determined. The earth station 38, with the earth station controller 56, can thus extrapolate using the known drifts and errors, the times recorded in the list generated by the twelfth operation 128. The corrected times are then compared with the known times of transmission from each communications satellite 10 of the particular broadcast messages. The earth station controller 56 can then calculate the propagation delay between each communications satellite and the terminal. Since the position of each communications satellite is accurately known, it is possible to determine the range of the terminal 44 from the particular communications satellite which did the broadcasting.
Thus far, the description of the invention has been restricted to the initial phase where the communications satellite 10 or satellites 10 101 have been used to make an
54 is estimate of the position of the terminal 44. The following description now passes the the manner in which information, received by the terminal 44 from one or more of a plurality of navigational satellites, is used signigicantly to enhance the accuracy of the determination of the position of the terminal 44 on the surface of the earth 14.
Figure 17 is an expansion upon figure 1. Communications satellites 10 are disposed in an orbit 12 about the earth 14 giving cones 16 a radio coverage. Members of an extra constellation of navigational satellites 142 are disposed of in another orbit 144 about the earth 14.
The navigational satellites 142, chosen to illustrate this element of the present invention, are selected to be those employed in the global positioning system (GPS) provided by Navstar and as described in their service and signal specification published, in its second edition, on June 2nd 1995. In this system, the constellation comprises 24 navigational satellites 142 disposed in six orbits 144, around the earth 14, there being 4 satellites 142 in each orbit 144 and each of the orbits 144 being inclined to the equator at an angle of 55 degrees.
Figu re 18 is a schematic diagram illustrating the environment in which the present invention is practised.
In addition, the terminal 44 is operative to receive transmissions from navigational satellites 142. The terminal 44 is operative to relay information concerning transmissions from the navigational satellites 142 to the earth station 38 and its associated control of 56. The earth station 38 is also aware of the orbital positions and 56 is behaviour of the navigational satellites 142 either by direct transmission or by prior knowledge of the system of the navigational satellites 142 in their constellation.
While two navigational satellites 142 have been shown in figure 18, it is to be understood that the terminal 44 may, at any instant, be unable to see any navigational satellites 142, just one navigational satellite 142, or many navigational satellites 142 at the same time. The situation shown in figure 18 is not restrictive.
As a variation on the theme of receiving a signal from a known source in a known position, transmitted at a known time, the terminal 44 can, in place of the navigational satellites 142, instead employ a fixed service radio station 143, at a known position on the surface of the earth. Such stations provide a low frequency signal giving the time of the transmission and are used, among other applications, for running selfadjusting clocks. The low frequency signal, typically in the range 10khz to 100khz, is ducted across the surface of the earth 14 rather than reflected from the ionised layers of the atmosphere. Because of this, the propagation speed of the low frequency radio wave is known and the distance from the fixed radio station 143 can be measured by measuring the delay in receipt of its time signal.
Again, as a variation on the navigational satellites 142, in the embodiment of the invention the GPS satellites 142 can be replaced by any other navigational satellites. In particular, there is an alternative navigational satellite 57 system, provided by the former USSR (now the Russian federation), which can be used in exactly the same manner as hereinafter described. All that is required is simply that the satellites 142 have known orbits and transmit signals, to be picked up by the terminal, at a known or determinable time.
As a further variation on the theme of the navigational satellites 142, it is not even necessary that the navigational satellites 142 be navigational satellites. Other satellite communications systems do and will exist. Eachcommunications system has satellites whose orbital parameters which can be known or measured by the earth station 38 56. Satellite communications systems periodically poll or interrogate terminals or other equipment on the ground 14. It is simply necessary for the terminal 44 of the present invention to detect a polling signal or other transmission from an alternative satellite communications system, and for the earth station 38 56 to know the time of the transmission and the position of the source, for the present invention to be practised using signals from an alternative satellite communications system. Such alternative satellites are characterised by being in a different constellation from the communications satellites 10 101 of the present invention, which definition covers all forms of satellites 142 which can be used by the present invention.
All of the sources 142 143 of navigational signals hereinbefore and hereinafter described are autonomous. That 58 is to say, the time and nature of the signal they send is not under any form of control from the earth station 38 56 or the terminal. This is in contrast to the signals exchanged between the terminal 44 and the earth station 38 via the communications satellite 10.
Figure 19 shows a flow chart of the activity of the terminal 44 within the scope of the present invention.
Entry is to a sixteenth activity 146 which corresponds to the activities otherwise shown in the flow charts of figures 15 and 16. In this manner, an approximate estimation is made of the position of the terminal 44 on the surface of the earth. If there is only one communication satellite 10 visible to the terminal 44, the combination of a delay measurement and a Doppler shift measurement are used to estimate the position of the terminal 44. If more than one communication satellite 10 is available, a combination of propagation delays between the terminal 44 and the communications satellite 10 is used.
The terminal, as described in figure 19, cooperates with the earth station 38 and its associated controller 56 to achieve the measurements hereinbefore described.
59 Having estimated the position the terminal 44, on the surface of the earth 14, control passes to a seventeenth activity 148 where the terminal 44, listening to the communications satellite 10, notes the time that a broadcast transmission is received from the earth station 38. It is to be understood that, under normal circumstances, the clock, within the terminal, is well below any standard of accuracy necessary to make any meaningful measurement with navigational satellites 142. However, the present invention provides that even a poor quality clock in the terminal 44 may be used in combination with signals from navigational satellites 142 to achieve realistic results when interacting with navigational satellites 142.
Control then passes to a twentieth activity 154 where the terminal 44 extracts the very minimum information from the signal from the navigational satellite 142, namely the identity of the particular navigational satellite of 142 of which the satellite was received Control then passes to a twenty-first activity 156 where the terminal 44 calculates the elapsed time, otherwise the time delay, between receipt of the communications satellite broadcast, executed in the seventeenth activity 148 and the noted time of receipt of the signal from the navigational satellite 142. Although the clock in the terminal is inherently of poor regulation, because differences are merely a secondary effect, the error in measuring the time delay in the receipt of the broadcast in the seventeenth activity 148 and the time of navigational satellites 142 signal receipt in the nineteenth activity 152 is very small. These are simply second order effects. By starting a timing operation on receipt of the broadcast message from the earth station 38, and by ending the timing operation when the signal from the navigational satellite 142 is received, and noting the elapsed time between the two events, the terminal 44 creates a record which is meaningful to the earth station 38 and its controller 56 and which the earth station 38 56 can use, together with the knowledge of the propagation delay, via the communications satellite 10, between the terminal 44 and the earth station 38, and, at the earth station 38, an accurate knowledge of the real time, to determine, with considerable accuracy, the instant at which 61 is the signal, from the navigational satellite 142, arrived at the terminal 44.
Figure 20 is a flow chart of the activity of the earth station 38 and its associated controller 56 in response to the various activities of the terminal 44 shown in figure 19.
Entry is to a twenty-third activity 160 where the earth station 38 cooperates, as otherwise illustrated with reference to figures 15 and 16, to establish, using the communications satellite 10 alone, an approximate estimation of the position of the terminal 44 on the surface of the earth 14.
Having thus approximately established the position of the terminal 44 on the surface of the earth, control passes to a twenty-fourth activity 162 where the earth station 38 and its associated controller 56 receive, if they are available, any messages from the terminal 44 indicating the identity of any navigational satellite 142 which may have sent a signal to the terminal 144 and the delay between the receipt, by the terminal 44, of the broadcast from the earth station 38 and the time when the terminal 44 receives the 62 signal from the navigational satellite 142. If no navigational satellite 142 was ever visible to the terminal 44, then, of course, the twenty- fourth activity 162 has nothing to do. However, the twenty-fourth activity 162 will note the delays and identities any and all of the navigational satellites 142 from which the terminal 44 may have received signals.
Having made the calculation for each signal from a navigational satellite 142, control passes to a twenty-sixth 63 activity 166. The earth station 38 and its associated controller 56 have available the parameters of the constellation of the navigational satellite 142. This includes information concerning the exact position of each navigational satellite 142 at any instant, and the instant at which a particular navigational satellite 142 will have sent its signal. This is derived either from an internal reference in the control of 56, by active on - line information from a control centre for the navigational satellites 142, or, as shown in figure 18, by interpreting signals from the navigational satellites 142 by direct reception. That is to say, the earth station 38 and its controller 56 can directly interact with the navigational satellites 142 to monitor their positions and times of sending of any signals, for comparison with the results from the terminal 44. It does not matter in which manner this information was obtained. It is simply sufficient that the information is available.
Having determined, from the identity of each satellite, the necessary physical parameters for the constellation of navigational satellites 142, control passes to a twenty-seventh activity 168 where the propagation delay from each position satellite 142 to the terminal 44 is calculated. Thereafter a twenty-eighth activity 170 uses knowledge of the actual positions of the navigational satellites 142 at the instant of their sending their original signals to the terminals 44, and of the calculated propagation delay between each one and the terminal 44, to 64 calculate the loci and their intersections whereat the terminal 44 can be situated.
Control then passes to a twenty-ninth activity 172 where a best estimate of the position of the terminal 44 on the surface of the earth 14 is made based on the available measurements. For example, if only one communication satellite 10 was visible and no navigational satellites 142, the combination of Doppler shift measurement and propagation delay measurement to the terminal 44 for a single communications satellite is accepted, but with a very low confidence level. If two communications satellite's 10 10, were visible, but no navigational satellites 142, the combination of delay measurements is used to estimate the position of the terminal 44 on the surface of the earth 14 but with a low confidence level. if either two communications satellite 10 and one navigational satellite 142 were simultaneously accessible, or only one communication satellite 10 and one navigational satellite 142 were simultaneously visible, the estimated position of the terminal 144 on the surface of the earth 14, as calculated, is given a medium confidence level.
While figures 19 and 20 show various activities occurring in sequence, it is to be appreciated that information, regarding the position satellites 142, can be gathered before, during and after execution of the sixteenth activity 146 and the twenty-third activity 160. Equally, further information from as yet unheard navigational satellites 142 can be added at any time to improve the position estimation for the terminal 44. Likewise, the terminal 44 can store results, gained from receiving signals from navigational satellites 142 before the earth station 38 interrogated the terminal 44 for service, thereby making for a very rapid and accurate determination of positions.
66 In the example given above, the estimation of the position of the terminal 44 has been made employing just one delay measurement from each navigational satellite 142. It is to be appreciated that the present invention also encompasses the position of the terminal 44 being estimated on the basis of plural measurements from each navigational satellite 142.
As an additional feature, in the event that the earth station 38 56 can establish the position of the terminal with a high degree of certainty, the earth station 38 56 can pass an indication of the measured position of the terminal 44, to the terminal 44, using either a message to be displayed by the terminal 44 or by a synthesised au-dio message, so that the terminal 44 becomes useable as a position location device despite not having the inherent capacity so to do.
Notwithstanding the activities, apparatus and method hereinbefore described, the description now passes to the invention as claimed, in the operation of which the
67 description hereinbefore provided constitutes, in whole or in part, a vital, exemplary operational element.
Figure 21 shows block diagrams of two generic types of terminal which are applicable to the embodiment of the present invention.
A first style of terminal 44A comprises a GPS receiver 174 and a communications satellte receiver 176. The GPS receiver can be replaced by a receiver capable of interacting with any other satellite navigation receiver. The GPS receiver 174 receives signals from navigational satellites 142 and analyses those signals to determine the exact position of the terminal 44 44A 44B on the surface of the earth 14. The first style of terminal also comprises a communications satellite receiver 176 which is operative to receive radio signals from the communications satellites 10 and to interpret the signals therefrom. The satellite communications receiver 176 passes instructions to the navigational satellite receiver 174 to enable the navigational satellite receiver 174 better to acquire the necessary information from the navigational satellites 142.
68 Within the scope of the embodiment of the invention, it envisaged that the first 44A and second 44B styles of terminal can be simple units, dedicated only to position determination. It is also envisaged that the elements of the terminals 44A 44B can be part of other apparatus, such as a user terminal as earlier described. Nonetheless, the two styles of terminal 44A 44B shown in figure 21 constitute the minimum for the practise of the invention as claimed.
Figure 22 shows a combined flow chart of the activites of the communications satellite system 10 38 and of either the first style of terminal 44A or the second style of terminal 44B, when acquiring the necessary data from the navigational satellites 142. For the flow chart in figure 22, it is to be understood that the communications satellite tranceiver 178 in the second style 44B of terminal functions in a receiveonly mode.
69 Control then passes to a thirty-second activity 184 where the earth station 38 causes the communications satellite 10 to supply the indication of accessible satellites to the messages broadcast in each spot beam 30. The encoded information is sent out, from time to time, on a regular basis. Each terminal 44 44A 44B, in a spot beam 30, and receiving the communications satellite 10 broadcast. will be able to derive therefrom indication of which navigational satellites 142 are accessible.
The act of broadcast in a spot beam 30, indicated by broken line, transfers control to a thirty-third activity 186 where the communications satellite receiver 176 receives the broadcast from the communications satellite 10. Control then passes to a thirty-fourth activity 188 where the communications satellite receiver 176 decodes, from the broadcast, the identities of the navigational satellites 142 which can be heard. The communications satellite receiver 176 then indicates these identities to the navigational satellite receiver 174. In a thirty-fifth activity 190 the navigational satellite receiver 174 receives the indication of accessible navigational satellites 142 from the communications satellite receiver 176 and responds thereto by limiting its atempted listening for navigational satellites 142 to those indicated. Thus, the navigational satellite receiver 174 avoids the need to listen for a long time before it can detetermine its position. Having received enough signals from navigational satellites 142, the navigational satellite receiver 174, in a thirty-sixth activity 190, calculates the position of the terminal 44 44A 44B and displays it to the user. The indication can be either on a visual display, such as a liquid crystal display screen, or audible, such as synthesised speech, or both. In addition, the indication can be provided in the form of digital data for use in another system such as a computer, or a control system, or a navigational system, or the like.
Return of control to the thirty-third actvity 186 ensures that the terminal 44 44A 44B periodically updates itself.
The approach used in the activities of figure 22 is preferred for military purposes since the necessary speed-up of the positiioning satellite receiver 174 is achieved without the necessity for transmission by the terminal 44 44A 44B of any kind.
In addition to indication of the appropriate navigational satellites 142, the communications satellite 10 can also broadcast indication of the position, on the surfcae of the earth, of each spot beam 30, so that the terminal 44 44A 44B, even if access to the navigational satellites 142 fails, can have an approximate idea of its position and can display it, if required, this approximate indication being almost instantaneous.
7 1 Attention is now drawn to figure 23 which shows a flow chart of the activities of the communications satellite system 38 10 and of the terminal 44B when an alternative embodiment of the invention is provided. Since this embodiment requires the terminal 44B to send signals to the communications satellite 10, it is to be understodd that onl terminals of the second style 44B are suitable for this approach.
In a thirty-seventh activity 194 in the terminal 44B, and a thirty-eighth activity 196 in the earth station 38, the satellite system 10 38 and the terminal 44B cooperate according to any or all of the methods described with reference to figure 1 to 20, by exchanging messages using the communications satellite tranceiver 178, to measure the position of the terminal 44B on the surface of the earth 14 to the accuracy permitted using the communications satellite 10 or plural communications satellites 10. The earth station 38 uses the information gained to calculate the position of the terminal 44B and then, in a thirty-ninth activity 198, using a knowledge of the time, and the orbits and positions of each of the navigational satellites 142, calculates, for that terminal 44B alone, the best accessible navigational satellites 142. In a fortieth activity 200, the earth station 38 causes the communications satellite 10 to transmit to the terminal 44B indication of for which navigational satellites 142 the navigational satellite receiver 174 should listen. The message is decoded by the receiving portion of the communications satellite transceiver 178 in a forty-first 72 activity and passed to the navigational satellite receiver 174 which, in a forty-second activity 204 responds by listening only for the indicated navigational satellites 142, thus improving the time required to receive sufficient data from the navigational satellites 142 for a precise measurement of the position of the terminal 44B to be made. once there is sufficient data, in a forty-third activity 206 the navigational satellite receiver 174 calculates the position of the terminal 44B and causes its display in any of the ways indicated in the description of figure 22. In addition, the fortieth activity 200 can equally well inform the terminal 44B of the estimated position using the communications satellite 10 or satellites 10 in the thirtyeighth activity 196, which the terminal 44B, if required, can display to give a near instantaneous approxiomation to the position of the terminal 44B.
1. A navigational satellite positioning system wherein a terminal is operative to receive time and orbit information from a plurality of navigational satellites to calculate the position of the terminal, said system being characterised by including a communications network, operative to inform said terminal which of said plurality of navigational satellites are within range of said terminal, and by said terminal, in response thereto, listening only for those of said plurality of navigational satellites that are within range.
2. A system, according to claim 1, wherein said terminal is operative to exchange messages with said communications network to establish an approximate position for said terminal, wherein said communications network, in response to a knowledge of said approximate position, is operative to calculate which of said plurality of navigational satellites are within range of said terminal, and wherein said communications network is operative to inform said terminal which of said plurality of navigational satellites are within range of said terminal.
3. A system, according to claim 1 or claim 2 wherein said communications network provides an array of abutting radio beams, and wherein said communications network is operative periodically to broadcast, in each beam, information concerning which of said plurality of navigational satellites would be within range of a terminal in that beam.
74 is 4. A system, according to claim 1,2 or 3 wherein said communications network is operative to calculate the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, wherein said communications network is operative to inform said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, and wherein said terminal, in response thereto, is operative to adjust its frequency of reception to accommodate said approximate doppler shift for each of said plurality of navigational satellites that is within range. 5. A system, according to claim 2, wherein said communications network comprises one or more communications satellites, wherein said terminal is operative to transmit to said one or more communications satellites and wherein each of said one or more communications satellites is operative to transmit to said terminal, each of said one or more communications satellites being operative to send and receive signals from an earth station, said earth station being operative to exchange signals with said terminal through said communications satellites and thereafter, being operative to analyse said signals to determine the position of said terminal on the surface of the earth. 6. A system according to claim 5 wherein, when said one or more communications satellites comprises just one communications satellite, said exchanged messages between said terminal and said earth station are adapted to measure the doppler shift due to motion of said one communications satellite relative to said terminal and to measure the radio propagation delay between said communications satellite and said terminal.
9. A system according to claim 8 wherein each of said one or more communications satellites is each operative to provide a broadcast messages at predetermined times, said terminal is operative to measure and recording the time of arrival of each broadcast message, said terminal is operative to report back to said earth station said time of arrival of each received broadcast message, said earth station is operative to compare the reported time of arrival of said each broadcast message with said predetermined times to calculate the propagation delay between said terminal and each of said one or more communications satellites, and said 76 earth station is operative to calculate the position of said terminal, relative to each of said one or more communications satellites, based on a knowledge of the actual position of each of said one or more communications satellites at said predetermined times.
10. A system according to claim 8 or claim 9 wherein said earth station is operative to send out a message, via each of said more than one communications satellite, and where said terminal is operative to return a message within a predetermined time of receipt of said message via each of said more than one communications satellite, said earth station being operative thereby to calculate the propagation delay between said more than one communications satellites and said terminal.
12. A system according to claim 8, 9, 10 or 11 wherein said terminal is operative to detect and record the time of arrival of broadcast messages from communications satellites which are no longer in sight and to report said previous broadcast messages to said earth station, said earth station using knowledge of the position of said communications satellites, no longer in sight, at the time of receipt of the 77 broadcast message by said terminal to assist in the calculation of the position of said terminal.
13. A system according to claim 8, 9, 10 or 11 wherein said terminal is operative to detect and record the time of arrival of broadcast messages from navigational satellites which are no longer in sight and to report said previous broadcast messages from said navigational satellites to said earth station, said earth station using knowledge of the position of said navigational satellites, no longer in sight, at the time of receipt of the broadcast message from said navigational satellites by said terminal to assist in the calculation of the position of said terminal.
14. A system according to claim 8, 9, 10, 11 12 or 13 wherein said terminal is operative to note the apparent recorded time, reported by said terminal, between two know intervals and is operative thereby to correct for drift and offset error in the timer in said terminal.
15. A system according to claim 6 or 7 wherein said earth station is operative to exchange said messages with said terminal a first optimum number of times to establish, by averaging, said doppler shift and a second optimum number of times to establish, by averaging, said propagation delay, said first and second optimum number of times being dependent upon the estimated position of said terminal.
16. A system according to any of claims 5 to 15 wherein said communications satellite is operative to send, to said earth station, a signal on a first generated frequency, and 78 is wherein earth station is operative to send a signal at a first known frequency to said communications satellite and wherein said communications satellite is operative 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 operative to measure said first generated frequency and said first transposed frequency and to derive therefrom the doppler shift between said earth station and said communications satellite and the error in the internal oscillator in said communications satellite. 17. A system according to claim 16 wherein said earth station is operative, after having derived said doppler shift and said error in said internal oscillator in said communications satellite, to cause said communications satellite to send a signal at a second known frequency to said terminal, and wherein said terminal is operative 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 communications satellite, on a second transposed frequency, and wherein said terminal is operative to send, to said earth station, via said communications satellite, a signal on a second generated frequency, said earth station being operative to measure said second transposed frequency and said second generated frequency, and operative to derive therefrom the doppler shift between said communications satellite and said 79 terminal and to derive the error in the internal oscillator in said terminal. 18. A system, according to claims 5 to 17, wherein said earth station is operative to calculate the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, wherein said earth station, via said communications satellite, is operative to inform said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, and wherein said terminal, in response thereto, is operative to adjust its frequency of reception to accommodate said approximate doppler shift for each of said plurality of navigational satellites that is within range. 19. A system, according to claims 1, 2, 3 or 4, wherein said communications network comprises a terrestrial radio telephone network. 20. A system, according to claim 2, according to claims 5 to 18, or according to claim 19 when dependent upon claim 2, wherein said communications network is operative to inform said terminal of said established approximate position for said terminal and, in response thereto, said terminal is operative to provide a user interpretable indication of said established approximate position. 21. A system, according to claim 20, wherein said user interpretable indication includes use of a visual display.
22. A system, according to claim 2, or according to claim 3 or claim 4 when dependent upon claim 2, or according to claims 5 to 18, or according to claims 20 or 21, or according to claim 19 when dependent upon claim 2, wherein said terminal is operative to receive a signal, sent from a known position at a known time from a known navigational satellite; said terminal is operative to note the time of arrival of said signal; said terminal is operative to communicate said time of arrival to said earth station; said earth station is operative to calculate the distance between said known navigational satellite and said terminal; and said earth station is operative to incorporate said calculated distance in the estimation of said position of said terminal.
23. A system, according to claim 22, wherein said known navigational satellite, in said signal, is operative to provide indication of its identity, wherein said terminal is operative to detect said identity, and wherein said terminal is operative to convey, to said earth station, indication of said identity.
24. A system, according to claim 22 or claim 22, wherein said navigational satellite is operative, in said signal, to provide indication of the time of origin of said signal from said navigational satellite, and wherein said terminal is operative to convey, to said earth station, indication of said time of origin of said signal from said known navigational satellite.
25. A system, according to claim 22, 23 or 24, wherein said navigational satellite is a satellite in a constellation other than that occupied by said communications satellite.
81 is 26. A system, according to claim 25 wherein said navigational satellite is one of a constellation comprising a plurality of navigational satellites.
27. A system, according to claim 26, wherein said terminal is operative to respond to any of said plurality of navigational satellites from which a signal can be received and wherein said earth station is operative to respond to information, received from said terminal, concerning any of said plurality of navigational satellites from which a signal can be received by said terminal.
28. A system, according to any of claims 22 to 27, wherein said terminal is operative to commence a timing operation on receipt of a message from said earth station, wherein said terminal is operative to terminate said timing operation on receipt of a signal from said navigational satellite, wherein said terminal is operative to employ the measured, elapsed time of said timing operation as said time of arrival of said signal at said terminal, and wherein said earth station is operative to use the propagation delay between said earth station and said terminal to deduce the true time of arrival of said signal at said terminal.
29. A system, substantially as described, with reference to the appended drawings.
30. A method for use in a navigational satellite positioning system wherein a terminal is operative to receive time and orbit information from a plurality of navigational satellites to calculate the position of the terminal, said method being characterised by the steps of: including said 82 terminal in a communications network; employing said communications network to inform said terminal which of said plurality of navigational satellites are within range of said terminal, and by said terminal, in response thereto, listening only for those of said plurality of navigational satellites that are within range.
31. A method, according to claim 30, including the steps of said terminal exchanging messages with said communications network to establish an approximate position for said terminal, said communications network, in response to a knowledge of said approximate position, calculating which of said plurality of navigational satellites are within range of said terminal, and said communications network is informing said terminal which of said plurality of navigational satellites are within range of said terminal.
32. A method, according to claim 30 or claim 31 including the steps of said communications network providing an array of abutting radio beams, and said communications network periodically broadcasting, in each beam, information concerning which of said plurality of navigational satellites would be within range of a terminal in that beam.
33. A method, according to claim 30, 31 or 32 including the steps of said communications network calculating the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, said communications network informing said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, and said 83 terminal, in response thereto, adjusting its frequency of reception to accommodate said approximate doppler shift for each of said plurality of navigational satellites that is within range.
34. A method, according to claim 31, including said communications network employing one or more communications satellites, said terminal transmitting to said one or more communications satellites, each of said one or more communications satellites transmitting to said terminal, each of said one or more communications satellites sending and receiving signals from an earth station, and said earth station exchanging signals with said terminal through said one or more communications satellites and, thereafter, analysing said signals to determine the position of said terminal on the surface of the earth.
35. A method according to claim 34 where, when said one or more communications satellites comprises just one communications satellite, said method includes the step of said exchanging messages between said terminal and said earth station to measure the doppler shift due to motion of said one communications satellite relative to said terminal and to measure the radio propagation delay between said communications satellite and said terminal.
36. A method according to claim 35 including the steps of said communications satellite communicating with said terminal using one out of a plurality of beams, each of said plurality of beams being interactive with a respective one out of a plurality of areas on the surface of the earth, and 84 is resolving ambiguity of position of said terminal by observing with which out of said plurality of beams said terminal exchanges said signals.
37. A method according to claim 34 where, when said one or more communications satellites comprises more than one communications satellite, includes the steps of said exchanged signals between said earth station and said terminal are being employed to measure the propagation delay between each of said more than one communications satellites and said terminal.
38. A method according to claim 37 including the steps of each of said one or more communications satellites providing a broadcast messages at predetermined times, said measuring and recording the time of arrival of each broadcast message, said terminal reporting back to said earth station said time of arrival of each received broadcast message, said earth station comparing the reported time of arrival of said each broadcast message with said predetermined times to calculate the propagation delay between said terminal and each of said one or more communications satellites, and said earth station calculating the position of said terminal, relative to each of said one or more communications satellites, based on a knowledge of the actual position of each of said one or more communications satellites at said predetermined times.
39. A method according to claim 37 or claim 38 including the steps of said earth station sending out a message, via each of said more than one communications satellite, said S is terminal returning a message within a predetermined time of receipt of said message via each of said more than one communications satellite, and said earth station calculating the propagation delay between said more than one communications satellites and said terminal.
40. A method according to claim 39 including the steps of said earth station sending said message via said each of said more than one communications satellites an optimum number of times, dependently upon the estimated position of said terminal with respect to said each of said more than one communications satellites, and taking the average of the propagation delays derived therefrom.
41. A method according to claim 37, 38, 39 or 40 including the steps of said terminal detecting and recording the time of arrival of broadcast messages from communications satellites which are no longer in sight and reporting said previous broadcast messages to said earth station, said earth station using knowledge of the position of said communications satellites, no longer in sight, at the time of receipt of the broadcast message by said terminal to assist in the calculation of the position of said terminal.
42. A method according to claim 37, 38, 39 or 40 including the steps of said terminal detecting and recording the time of arrival of broadcast messages from navigational satellites which are no longer in sight and reporting said previous broadcast messages from said navigational satellites to said earth station, said earth station using knowledge of the position of said navigational satellites, no longer in 86 sight, at the time of receipt of the broadcast message from said navigational satellites by said terminal to assist in the calculation of the position of said terminal.
43. A method according to claim 37, 38, 39, 40 41 or 42 including the steps of said earth station noting the apparent recorded time, reported by said terminal, between two known intervals and thereby correcting for drift and offset error in the timer in said terminal.
44. A method according to claim 35 or 36 including the steps of said earth station exchanging said messages with said terminal a first optimum number of times to establish, by averaging, said doppler shift and a second optimum number of times to establish, by averaging, said propagation delay, said first and second optimum number of times being dependent upon the estimated position of said terminal.
45. A method according to any of claims 34 to 44 including the steps of said communications 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 communications satellite, said communications satellite using an internal oscillator to transpose said signal of a first known frequency and returning the transposed signal to said earth station on a first transposed frequency, and said earth station measuring said first generated frequency and said first transposed frequency and deriving therefrom the doppler shift between said earth station and said communications satellite and the 87 error in the internal oscillator in said communications satellite.
46. A method according to claim 45 including the steps of said earth station, after having derived said doppler shift and said error in said internal oscillator in said communications satellite, causing said communications satellite to send a signal at a second known frequency to said terminal, said terminal using an internal oscillator to transpose said signal of a second known frequency and returning the transposed signal to said earth station, through said communications satellite, on a second transposed frequency, said terminal sending, to said earth station, via said communications satellite, a signal on a second generated frequency, said earth station measuring said second transposed frequency and said second generated frequency, and deriving therefrom the doppler shift between said communications satellite and said terminal and deriving the error in the internal oscillator in said terminal.
47. A method, according to claims 34 to 36, including the steps of said earth station calculating the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, said earth station, via said communications satellite, informing said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, and said terminal, in response thereto, adjusting its frequency of reception to accommodate said approximate 88 doppler shift for each of said plurality of navigational satellites that is within range.
48. A method, according to claims 30, 31, 32 or 33, wherein said communications network comprises a terrestrial radio telephone network.
49. A method, according to claim 31, according to claims 34 to 47, or according to claim 48 when dependent upon claim 31, including the steps of said communications network informing said terminal of said established approximate position for said terminal and said terminal, in response thereto, providing a user interpretable indication of said established approximate position.
50. A method, according to claim 49, wherein said user interpretable indication includes use of a visual display.
51. A method, according to claim 31, or according to claim 32 or claim 33 when dependent upon claim 31, or according to claims 34 to 47, or according to claims 49 or 50, or according to claim 48 when dependent upon claim 31, including the steps of said terminal receiving a signal, sent from a known position at a known time from a known navigational satellite; said terminal noting the time of arrival of said signal; said terminal communicating said time of arrival to said earth station; said earth station calculating the distance between said known navigational satellite and said terminal; and said earth station incorporating said calculated distance in the estimation of said position of said terminal.
89 52. A method, according to claim 51, wherein said known navigational satellite, in said signal, is operative to provide indication of its identity, said method including the steps of said terminal detecting said identity, and said terminal conveying, to said earth station, indication of said identity. 53. A method, according to claim 51 or claim 52, including the steps of said navigational satellite providing indication, in said signal, of the time of origin of said signal from said navigational satellite, and said terminal conveying, to said earth station, indication of said time of origin of said signal from said known navigational satellite. 54. A method, according to claim 51, 52 or 53, wherein said navigational satellite is a satellite in a constellation other than that occupied by said communications satellite. 55. A method, according to claim 54 wherein said navigational satellite is one of a constellation comprising a plurality of navigational satellites. 56. A method, according to claim 55, including said terminal responding to any of said plurality of navigational satellites from which a signal can be received and said earth station responding to information, received from said terminal, concerning any of said plurality of navigational satellites from which a signal can be received by said terminal. 57. A method, according to any of claims 51 to 56, including said terminal commencing a timing operation on receipt of a message from said earth station, said terminal terminating said timing operation on receipt of a signal from said navigational satellite, said terminal employing the measured, elapsed time of said timing operation as said time of arrival of said signal at said terminal, and said earth station using the propagation delay between said earth station and said terminal to deduce the true time of arrival of said signal at said terminal.
58. A method, substantially as described, with reference to the appended drawings.
59. A terminal, for use in a navigational satellite positioning system wherein said terminal is operative to receive time and orbit information from a plurality of navigational satellites to calculate the position of said terminal, said terminal being characterised by includion in a communications network, operative to inform said terminal which of said plurality of navigational satellites are within range of said terminal, and by said terminal, in response thereto, being operative to listen only for those of said plurality of navigational satellites that are within range.
60. A terminal, according to claim 59, operative to exchange messages with said communications network to establish an approximate position for said terminal, said communications network. in response to a knowledge of said approximate position, is operative to calculate which of said plurality of navigational satellites are within range of said terminal, said communications network being operative to inform said terminal which of said plurality of navigational satellites are within range of said terminal.
91 61. A terminal, according to claim 59 or claim 60, for use in a system wherein said communications network provides an array of abutting radio beams, and wherein said communications network is operative periodically to broadcast, in each beam, information concerning which of said plurality of navigational satellites would be within range of a terminal in that beam.
62. A terminal, according to claim 59, 60 or 62, for use in a system wherein said communications network is operative to calculate the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, and wherein said communications network is operative to inform said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, said terminal, in response thereto, beingoperative to adjust its frequency of reception to accommodate said approximate doppler shift for each of said plurality of navigational satellites that is within range.
63. A terminal, according to claim 60, for use in a system wherein said communications network comprises one or more communications satellites, said terminal being operative to transmit to said one or more communications satellites, each of said one or more communications satellites being operative to transmit to said terminal, each of said one or more communications satellites being operative to send and receive signals from an earth station, said earth station being operative to exchange signals with said terminal 92 through said communications satellites and thereafter, being operative to analyse said signals to determine the position of said terminal on the surface of the earth.
64. A terminal, according to claim 63, for use in a system wherein, when said one or more communications satellites comprises just one communications satellite, said exchanged messages between said terminal and said earth station are adapted to measure the doppler shift due to motion of said one communications satellite relative to said terminal and to measure the radio propagation delay between said communications satellite and said terminal 65. A terminal, according to claim 64, for use in a system wherein said communications satellite is operative to communicate with said terminal using one out of a plurality of beams, each of said plurality of beams being interactive with a respective one out of a plurality of areas on the surface of the earth, ambiguity of position of said terminal being resolved by observation of with which out of said plurality of beams said terminal exchanges said signals.
66. A terminal, according to claim 63, for use in a system wherein, when said one or more communications satellites comprises more than one communications satellite, said exchanged signals between said earth station and said terminal are adapted to measure the propagation delay between each of said more than one communications satellites and said terminal.
67. A terminal, according to claim 66, for use in a system wherein each of said one or more communications 93 satellites is each operative to provide a broadcast messages at predetermined times, said terminal being operative to measure and record the time of arrival of each broadcast message, said terminal being operative to report back to said earth station said time of arrival of each received broadcast message, said earth station being operative to compare the reported time of arrival of said each broadcast message with said predetermined times to calculate the propagation delay between said terminal and each of said one or more communications satellites, and said earth station being operative to calculate the position of said terminal, relative to each of said one or more communications satellites, based on a knowledge of the actual position of each of said one or more communications satellites at said predetermined times.
68. A terminal, according to claim 66 or claim 67, for use in a system wherein said earth station is operative to send out a message, via each of said more than one communications satellite, said terminal being operative to return a message within a predetermined time of receipt of said message via each of said more than one communications satellite, said earth station being operative thereby to calculate the propagation delay between said more than one communications satellites and said terminal.
69. A terminal, according to claim 68, for use in a system wherein said earth station is operative to send said message via said each of said more than one communications satellites an optimum number of times, dependently upon the 94 is estimated position of said terminal with respect to said each of said more than one communications satellites, and to take the average of the propagation delays derived therefrom 70. A terminal, according to claim 66, 67, 68 or 69, operative to detect and record the time of arrival of broadcast messages from communications satellites which are no longer in sight and to report said previous broadcast messages to said earth station for said earth station to use knowledge of the position of said communications satellites, no longer in sight, at the time of receipt of the broadcast message by said terminal to assist in the calculation of the position of said terminal.
71. A terminal, according to claim 66, 67, 68 or 69, operative to detect and record the time of arrival of broadcast messages from navigational satellites which are no longer in sight and to report said previous broadcast messages from said navigational satellites to said earth station, for said earth station, using knowledge of the position of said navigational satellites, no longer in sight, at the time of receipt of the broadcast message from said navigational satellites by said terminal, to assist in the calculation of the position of said terminal.
72. A terminal, according to claim 66, 67, 68, 69, 70 or 71, for use in a system wherein said earth station is operative to note the apparent recorded time, reported by said terminal, between two know intervals and is operative thereby to correct for drift and offset error in the timer in said terminal.
73. A terminal, according to claim 64 or 65, for use in a system wherein said earth station is operative to exchange said messages with said terminal a first optimum number of times to establish, by averaging, said doppler shift and a second optimum number of times to establish, by averaging, said propagation delay, said first and second optimum number of times being dependent upon the estimated position of said terminal. 74. A terminal, according to any of claims 63 to 73, for use in a system wherein said communications satellite is operative to send, to said earth station, a signal on a first generated frequency, and wherein earth station is operative to send a signal at a first known frequency to said communications satellite and wherein said communications satellite is operative 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 operative to measure said first generated frequency and said first transposed frequency and to derive therefrom the doppler shift between said earth station and said communications satellite and the error in the internal oscillator in said communications satellite. 75. A terminal, according to claim 74, for use in a system wherein said earth station is operative, after having derived said doppler shift and said error in said internal oscillator in said communications satellite, to cause said communications satellite to send a signal at a second known 96 is frequency to said terminal, said terminal being operative 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 communications satellite, on a second transposed frequency, and said terminal being operative to send, to said earth station, via said communications satellite, a signal on a second generated frequency, said earth station being operative to measure said second transposed frequency and said second generated frequency, and operative to derive therefrom the doppler shift between said communications satellite and said terminal and to derive the error in the internal oscillator in said terminal.
76. A terminal, according to claims 63 to 175, for use in a system wherein said earth station is operative to calculate the approximate doppler frequency shift for each of said plurality of navigational satellites that are within range, wherein said earth station, via said communications satellite, is operative to inform said terminal of said approximate doppler shift for each of said plurality of navigational satellites that is within range, said terminal, in response thereto, being operative to adjust its frequency of reception to accommodate said approximate doppler shift for each of said plurality of navigational satellites that is within range.
77. A terminal, according to claims 59, 60, 61 or 62, for use in a system wherein said communications network comprises a terrestrial radio telephone network.
97 78. A terminal, according to claim 60, according to claims 63 to 76, or according to claim 77 when dependent upon claim 60, for use in a system wherein said communications network is operative to inform said terminal of said established approximate position for said terminal, said terminal, in response thereto, being operative to provide a user interpretable indication of said established approximate position. 79. A terminal, according to claim 78, wherein said user interpretable indication includes use of a visual display. 80. A terminal, according to claim 60, or according to claim 61 or claim 62 when dependent upon claim 60, or according to claims 63 to 76, or according to claims 78 or 79, or according to claim 77 when dependent upon claim 60, operative to receive a signal, sent from a known position at a known time from a known navigational satellite; said terminal being operative to note the time of arrival of said signal; said terminal being operative to communicate said time of arrival to said earth station; said earth station being operative to calculate the distance between said known navigational satellite and said terminal; and said earth station being operative to incorporate said calculated distance in the estimation of said position of said terminal 81. A terminal, according to claim 80, for use in a system wherein said known navigational satellite, in said signal, is operative to provide indication of its identity, 9 8 is said terminal being operative to detect said identity and to convey, to said earth station, indication of said identity.
82. A terminal, according to claim 80 or claim 81, for use in a system wherein said navigational satellite is operative, in said signal, to provide indication of the time of origin of said signal from said navigational satellite, said terminal being operative to convey, to said earth station, indication of said time of origin of said signal from said known navigational satellite.
83. A terminal, according to claim 80, 81 or 82, for use in a system wherein said navigational satellite is a satellite in a constellation other than that occupied by said communications satellite.
84. A terminal, according to claim 83, for use in a syetm wherein said navigational satellite is one of a constellation comprising a plurality of navigational satellites.
85. A terminal, according to claim 84, operative to respond to any of said plurality of navigational satellites from which a signal can be received, for use in a system wherein said earth station is operative to respond to information, received from said terminal, concerning any of said plurality of navigational satellites from which a signal can be received by said terminal.
86. A system, according to any of claims 80 to 85, operative to commence a timing operation on receipt of a message from said earth station, operative to terminate said timing operation on receipt of a signal from said 99 navigational satellite, and operative to employ the measured, elapsed time of said timing operation as said time of arrival of said signal at said terminal, for use in a system wherein said earth station is operative to use the propagation delay between said earth station and said terminal to deduce the true time of arrival of said signal at said terminal.
87. A terminal, substantially as described, with reference to the appended drawings.
GB9707213A 1997-04-09 1997-04-09 Satellite acquisition in navigation system Withdrawn GB2324218A (en)
GB9707213A GB2324218A (en) 1997-04-09 1997-04-09 Satellite acquisition in navigation system
DE1998601388 DE69801388D1 (en) 1997-04-09 1998-03-19 Localization for satellite terminal
EP19980302065 EP0871300B1 (en) 1997-04-09 1998-03-19 Satellite terminal position determination
US09/055,294 US6072430A (en) 1997-04-09 1998-04-06 Satellite terminal position determination
JP13586598A JPH1174827A (en) 1997-04-09 1998-04-09 Satellite terminal and its position decision system
GB9707213D0 GB9707213D0 (en) 1997-05-28
GB2324218A true GB2324218A (en) 1998-10-14
ID=10810544
GB9707213A Withdrawn GB2324218A (en) 1997-04-09 1997-04-09 Satellite acquisition in navigation system
US (1) US6072430A (en)
EP (1) EP0871300B1 (en)
JP (1) JPH1174827A (en)
DE (1) DE69801388D1 (en)
GB (1) GB2324218A (en)
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FR2802037B1 (en) * 1999-11-12 2006-08-04 Motorola Inc Method and apparatus for the conservation of the integrity of a GPS assists
DE4415083A1 (en) * 1994-04-29 1995-11-02 Bosch Gmbh Robert A method for selecting signals of navigation satellites
1997-04-09 GB GB9707213A patent/GB2324218A/en not_active Withdrawn
1998-03-19 EP EP19980302065 patent/EP0871300B1/en not_active Expired - Lifetime
1998-03-19 DE DE1998601388 patent/DE69801388D1/en not_active Expired - Lifetime
1998-04-06 US US09/055,294 patent/US6072430A/en not_active Expired - Fee Related
1998-04-09 JP JP13586598A patent/JPH1174827A/en not_active Withdrawn
GB2358977B (en) * 2000-01-10 2003-07-30 Motorola Inc Method and apparatus for assisted GPS integrity maintenance
DE69801388D1 (en) 2001-09-27
EP0871300A2 (en) 1998-10-14
EP0871300B1 (en) 2001-08-22
US6072430A (en) 2000-06-06
EP0871300A3 (en) 1999-03-31
JPH1174827A (en) 1999-03-16
GB9707213D0 (en) 1997-05-28
US20100292920A1 (en) 2010-11-18 Navigation Services Based on Position Location Using Broadcast Digital Television Signals
2002-05-29 WAP Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1)