Source: https://patents.google.com/patent/EP0856957A2/en
Timestamp: 2019-09-22 04:44:30
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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 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14', 'arth 14']

EP0856957A2 - User terminal positioning system and method of employing external signals - Google Patents
User terminal positioning system and method of employing external signals Download PDF
EP0856957A2
EP0856957A2 EP98300587A EP98300587A EP0856957A2 EP 0856957 A2 EP0856957 A2 EP 0856957A2 EP 98300587 A EP98300587 A EP 98300587A EP 98300587 A EP98300587 A EP 98300587A EP 0856957 A2 EP0856957 A2 EP 0856957A2
EP98300587A
EP0856957A3 (en
1998-01-28 Application filed by ICO Services Ltd filed Critical ICO Services Ltd
1998-08-05 Publication of EP0856957A2 publication Critical patent/EP0856957A2/en
1999-04-14 Publication of EP0856957A3 publication Critical patent/EP0856957A3/en
It is known to provide a user terminal where the individual user terminal employs "Global Positioning by Satellite" (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed 27th March 1993. The GPS system tends to be very slow of access, requires a very sophisticated receiver of a costly nature, and the GPS satellites can often be totally inaccessible to the user terminal, in sufficient simultaneous numbers for a position determination to be achieved. In addition, the accuracy of the position determination is well in excess of what is actually required for satellite communications purposes.
Such systems require enhanced complication of the handset, in order that the handset may be capable both of communications and of GAS measurements.
Additionally, the invention provides a system and method wherein said indication of those out of said plurality of navigational satellites which are potentially within range of sending a signal to said user terminal are broadcast, by said communications satellite, to all of a plurality of user terminals within communications range of said communications satellite. The invention further provides a system and method wherein said user terminal is operative to commence a timing operation on receipt of a message from said earth station, wherein said user terminal is operative to terminate said timing operation on receipt of a signal from said autonomous source, wherein said user terminal is operative to employ the measured, elapsed time of said timing operation as said time of arrival of said signal at said user terminal, and wherein said earth station is operative to use the propagation delay between said earth station, via said communications satellite, to said user terminal, to deduce the true time of arrival of said signal at said user terminal.
Finally, the invention provides a system and method wherein said user terminal is operative to respond to plural signals from each of said plurality of navigational satellites and wherein said earth station is operative to incorporate each instance of receipt of a signal from each of said plurality of navigational satellites into said estimation of said position of said user terminal. The invention is further explained, by way of example, by the following description, taken in conjunction with the appended drawings, in which:
Figure 6 is a schematic view of the general situation where an earth station talks to a user terminal via the communications satellite to determine propagation delays between the user terminal and the communications satellite.
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 user terminal and the internal oscillator error in the user terminal.
Figure 10 shows how intersecting lines of measured doppler frequency shift and propagation delays may be used to measure the position of the user terminal on the surface of the earth.
Figure 14 shows the situation where the user 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 user 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 user terminal on the surface of the earth.
Figure 19 is a flow chart of the cativities of the user terminal in the overall scheme of the present invention. and
Attention is firstly drawn to figure 1,
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 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 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 present invention is concerned, the only requirement is that every part of the 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 shows the cone 16 at a minimum of 10 degrees elevation to the surface of the earth.
Figure 4 shows how the cones 16 of radio coverage mail interact with the surface of the earth to produce many types of different regions.
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 user terminals 44 which may be provided in a form not dissimilar to a hand held cellular radio telephone. It is to be understood that the user terminal 44 may also comprise more elaborate vehicle mounted equipment for use in land vehicles, ships and aircraft.
With the parameters mentioned in this preferred example, the communications satellite moves around its orbit 12 12', as indicated by a first arrow 46, with a velocity of 4.9kkm 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 0.47km 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 user terminal 44 is defined with reference to its distance along the ground track 48 and its distance along the cross track 54 with reference to the nadir 50.
Figure 6 is a schematic view of the general situation where an earth station 38 talks to a user terminal 44 or via the communications satellite 10.
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 user 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.
Not previously mentioned, is the fact that the user terminal 44 transmits on the user terminal uplink 64 to the subscriber antenna 42 and similarly receives on the user terminal downlink link 62 from the subscriber antenna 42. The 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 user terminals 44. Each user terminal will be in one particular spot beam 30 of the plurality of spot beams shown in figure 3.
The communications satellite 10 will be moving relative to the surface of the earth 14, and therefore relative no the earth station 38 and to the user 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 communications satellite 10 as generically indicated by a sixth arrow 68.
The signals exchanged between the earth station 38 and the communications satellite 10, in common with the signals exchange between the user terminal 44 and the communications satellite 10, all enjoy a propagation delay and a frequency shift, due to the motion of the communications satellite 10 relative to the earth station 38 and to the user terminal 44 caused by the doppler effect. The present invention in part concerns itself with means of employing the doppler shift in frequencies, due to the motion of the communications satellite 10, and measurement of the propagation delay, to determine the position of the user terminal 44 on the surface of the earth 14.
Propagation delay is measured between the earth station 38 and the user terminal 44 to establish the propagation delay between the user 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 user terminal 44 via the user terminal downlink 62. Upon receipt of the signal from the earth station 38, the user terminal waits for a predetermined period and then sends its own message, via the user terminal uplink 64 and the downlink radio link 60, back to the earth station 38. The earth station controller 56 notes the elapse of time from the instant that the earth station 38 began to transmit the message on the uplink radio link 58 and the instant when the earth station 38 began to receive the response message from the user terminal 44 from the downlink radio link 60. The earth station controller 56 knows the propagation delay times for signals, through the communications satellite 10, from the uplink radio link 58 onto the user terminal downlink 62 and, correspondingly, the propagation delay through the communications satellite 10 between the user terminal uplink 64 and the downlink radio link 60. Equally, the earth station controller 56 knows, with precision, the predetermined elapsed time employed by the user terminal 44 before it responds to the received message from the earth station 38. These propagation delays and the predetermined delay of the user terminal 44 are subtracted, by the earth station controller 56, from the overall elapsed time to determine the actual propagation delay of the radio wave via the various links 58, 60, 62, 64 in the return journey of the message from and to the earth station 38. The radio wave propagates always at the speed of light, which is constant. Because the position of the earth station 38, on the surface of the earth, is precisely known, and because the position of the 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 user 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 user terminal 44 is known. According to the propagation delay, the user terminal may exist on any point of a spherical surface centred on the communications satellite 10. Because the spherical surface intersects the surface of the earth 14, and the user terminal 44 is on the surface of the earth, the location of the user terminal 44 may be inferred as being on the line intersection of the spherical surface of the earth 14 and the sphere of measured distance centred on the 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 user 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 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 user terminal downlink 62. The user terminal 44 then returns the signal via the user terminal uplink 64, once again to be converted in frequency by the internal oscillator of the 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 link 60 signal and deduces the doppler frequency shift, at the user terminal 44, resulting from the motion of the communications satellite 10 as indicated by the 5th arrow 66.
The earth station 38 sends a signal of know frequency f(1) 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) = (1+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) = (1+d)f(sent) Thus, if the earth station sends a frequency f(1) 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(1)(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)(1+m).
The output of the frequency changer 76 is thus: 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 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(1) 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(2f(1) + d2f(1)) - mdf(2) - f(2)m The second order terms d2f(1) and mdf(2) are insignificant compared to the other terms, and can be ignored.
Thus f(R1) = f(1)-f(2)+d(2f(1)+(2)- mf(2)) The communications 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(1)-f(2)+d(2f(1)-f(2))-mf(2) and f(R2) = f3(1+d+m) Now, f(1), 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 user 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 user terminal 38.
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 user 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 communications satellite, and once again works out the two equations in two unknowns to solve for the communications satellite 10 to user terminal 44 doppler shift and to solve for the proportional error in the master oscillator in the user terminal 44.
Figure 10 shows how measurement of Doppler frequency shift and delays can be used to locate a user terminal 44 on the surface of the earth 14.
In Figure 10, the horizontal axis 78 corresponds to measurement in the direction of the second arrow 48 of figure 5 along the ground track. The vertical axis 80 corresponds to measurement along the cross track as indicated by the fourth arrow 54 in figure 6.
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 user terminal 44 might be situated. Likewise, the figures adjacent to the delay contours 82 indicate the particular delay in milliseconds, for that particular delay contour 82 and that was the particular position on the surface of the earth 14. Various figures are shown in degrees, being the angle of elevation from the user terminal 44 to the 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.
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 user terminal 44 on the surface of the earth 14 at that point where its particular delay contour 82 and Doppler contour 84 cross.
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 user 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 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 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 user terminal 44 is found, for each of these spot beams 30, for doppler shift measurements, according to the preferred embodiment illustrating the present invention. It will be seen that numbers of optimum measurements range from 90 to 42. If other sampling rates and communications satellite orbital heights are chosen, other optimum numbers of measurement apply.
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 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 user terminal 44, on the surface of the earth 14, has radio coverage from more than one communications satellite 10 10'. Ideally, the two communications satellites 10 10' should both be visible to the user terminal 44 and to a single earth station 38. However, it is possible that a communications satellite 10' may be visible of the user 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 10' and the user terminal may be exchanged for one of the earth stations 38 to act as a master in determining the position of the user terminal 44 on the surface of the earth 14.
If more than one communications satellite 10 10' 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 10' and the user terminal 44.
As earlier described with reference to figure 6, the delay measurements generate, as the possible position of the user 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 user terminal 44 on the surface of the earth 14, subject to beam identity ambiguity resolution, hereinbefore described. If the user terminal is assumed to be on the surface of the earth, only two communications satellite propagation delays are necessary for absolute location of the user terminal. If more than 3 communications satellites 10 10' are so used, the user 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 user terminal 44 on the surface of the earth 14 describe with reference to figure 14 is more accurate.
Accordingly, the invention initially concerns itself with, in what manner, the position of the user terminal 44 is to be determined on the surface of the earth 14 using one or more communications satellites 10 10'. Where only one communications satellite 10 is visible, the ranging method shown in figure 10 is employed. When more 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 user 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 user terminal 44 using the communications satellite 10 10'.
In a first operation 96 the earth station 98 listens for a request of some kind of the user terminal 44. If a first test 98 fails to detect a call from the user 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 user terminal 44, control is passed to a second operation 98. The second operation 98 sends a transmission, via the communications satellite 10, to the user 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.
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 user 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.
The fourth 106 and fifth operations 108 may be conducted simultaneously, the number of sampling instance 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.
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 for which of the plurality of spot beams 30 for each communications satellite the user 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 user 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 user terminal 44 as it co-operates with the earth station 38 in yet a further alternative for locating the user 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 user 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 38' can instruct different communications satellites 10' to provide a broadcast message. Each earth station 38' 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 38' from which the broadcast message originated. As an alternative, the broadcast message can also include indication from which earth station it originated.
Returning once again to figure 16, the user terminal, in an 11th 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 user 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 user terminal 44 keeps a record of the last several communications satellites 10 to be heard.
If the seventh test 132 detects that there is only one communications satellite visible, control passes to a thirteenth operation 134 where the user terminal 44 responds to delay and doppler measurements as indicated with reference to figures 6 to 13. The user 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 user terminal 44 on the surface of the earth 14. Control next passes to a fourteenth operation 136 where the user 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 positioning 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 user 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 user terminal 44 responds to a propagation delay measurement from each of the communications satellites 10 10' as described with reference to figures 14 and 15. The user 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.
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 positioning satellites 142 are disposed of in another orbit 144 about the earth 14.
The positioning 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 positioning 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.
The another orbit 144 given in this example, is at approximately 20 thousand kilometres from the surface of the earth 14 and allows the positioning satellites 142 to circle the earth in 12 hours. Again, it will be apparent that positioning satellites 142 may be employed in orbits of any height for the present invention to function.
Figure 18 is a schematic diagram illustrating the environment in which the present invention is practised.
While two positioning satellites 142 have been shown in figure 18, it is to be understood that the user terminal 44 may, at any instant, be unable to see any positioning satellites 142, just one positioning satellite 142, or many positioning 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 user terminal 44 can, in place of the positioning 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 self-adjusting 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.
Figure 19 shows a flow chart of the activity of the user 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 user terminal 44 on the surface of the earth. If there is only one communication satellite 10 visible to the user terminal 44, the combination of a delay measurement and a Doppler shift measurement are used to estimate the position of the user terminal 44. If more than one communication satellite 10 is available, a combination of propagation delays between the user terminal 44 and the communications satellite 10 is used.
The user terminal, as described in figure 19, co-operates with the earth station 38 and its associated controller 56 to achieve the measurements hereinbefore described.
Having estimated the position the user terminal 44, on the surface of the earth 14, control passes to a seventeenth activity 148 where the user 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 user terminal, is well below any standard of accuracy necessary to make any meaningful measurement with positioning satellites 142. However, the present invention provides that even a poor quality clock in the user terminal 44 may be used in combination with signals from positioning satellites 142 to achieve realistic results when interacting with positioning satellites 142.
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 user terminal 44 shown in figure 19.
Entry is to a twenty-third activity 160 where the earth station 38 co-operates, 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 user terminal 44 on the surface of the earth 14.
Having made the calculation for each signal from a positioning satellite 142, control passes to a twenty-sixth activity 166. The earth station 38 and its associated controller 56 have available the parameters of the constellation of the positioning satellite 142. This includes information concerning the exact position of each positioning satellite 142 at any instant, and the instant at which a particular positioning 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 positioning satellites 142, or, as shown in figure 18, by interpreting signals from the positioning satellites 142 by direct reception. That is to say, the earth station 38 and its controller 56 can directly interact with the positioning satellites 142 to monitor their positions and times of sending of any signals, for comparison with the results from the user terminal 44. It does not matter in which manner this information was obtained. It is simply sufficient that the information is available.
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 positioning satellites 142 can be added at any time to improve the position estimation for the user terminal 44. Likewise, the user terminal 44 can store results, gained from receiving signals from positioning satellites 142 before the earth station 38 interrogated the user terminal 44 for service, thereby making for a very rapid and accurate determination of positions.
A satellite communications system where an earth station is operative to exchange messages with a user terminal through a communications satellite in movement relative thereto, and to exchange messages with said communications satellite, to measure the position of the user terminal, said system being characterised by: said user terminal being operative to receive a signal, sent from a known position at a known time from a known autonomous source; said user terminal being operative to note the time of arrival of said signal; said user terminal being operative to communicate said time of arrival to said earth station via said communications satellite; said earth station being operative to calculate the distance between said autonomous source and said user terminal; and said earth station being operative to incorporate said calculated distance in the estimation of said position of said user terminal, otherwise derived by said exchanging messages between said earth station and said user terminal via said communications satellite.
A system, according to claim 1, wherein said autonomous source, in said signal, is operative to provide indication of its identity, wherein said user terminal is operative to detect said identity, and wherein said user terminal is operative to convey, to said earth station, indication of said identity.
A system, according to claim 1 or claim 2, wherein said autonomous source is operative, in said signal, to provide indication of the time of origin of said signal from said autonomous source, and wherein said user terminal is operative to convey, to said earth station, indication of said time of origin of said signal from said autonomous source.
A system, according to claim 1, 2 or 3, wherein said autonomous source is a satellite in a constellation other than that occupied by said communications satellite.
A system, according to claim 4, wherein said autonomous source is a navigational satellite.
A system, according to claim 1, 2 or 3, wherein said autonomous source is a terrestrial, low frequency time station.
A system, according to claim 5 wherein said navigational satellite is one of a constellation comprising a plurality of navigational satellites.
A system, according to claim 7, wherein said user 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 user terminal, concerning any of said plurality of navigational satellites from which a signal can be received by said user terminal.
A system, according to claim 8, wherein said earth station, on the basis of said measurement of said position of said user terminal by said exchange of messages between said earth station and said user terminal via said communications satellite, is operative to indicate, to said user terminal, those out of said plurality of navigational satellites which are potentially within range of sending a signal to said user terminal, and wherein said user terminal is responsive thereto to restrict potential reception of signals to signals originating from those navigational satellites, indicated by said earth station.
A system, according to claim 9, wherein said indication of those out of said plurality of navigational satellites which are potentially within range of sending a signal to said user terminal are broadcast, by said communications satellite, to all of a plurality of user terminals within communications range of said communications satellite.
A system, according to any of the preceding claims, wherein said user terminal is operative to commence a timing operation on receipt of a message from said earth station, wherein said user terminal is operative to terminate said timing operation on receipt of a signal from said autonomous source, wherein said user terminal is operative to employ the measured, elapsed time of said timing operation as said time of arrival of said signal at said user terminal, and wherein said earth station is operative to use the propagation delay between said earth station and said user terminal to deduce the true time of arrival of said signal at said user terminal.
A system, according to claim 7, 8, 9 or 10, or according to claim 11 when claim 11 is dependent upon claim 7, 8, 9 or 10, wherein said user terminal is operative to respond to plural signals from each of said plurality of navigational satellites and wherein said earth station is operative to incorporate each instance of receipt of a signal from each of said plurality of navigational satellites into said estimation of said position of said user terminal.
A system, according to claim 7, 8, 9 or 10, or according to claim 11 when claim 11 is dependent upon claim 7, 8, 9 or 10, wherein said user terminal is operative to respond to plural signals from each of said plurality of navigational satellites and wherein said earth station is operative to incorporate each instance of receipt of a signal from each of said plurality of navigational satellites into said estimation of said position of said user terminal .
A method for operating a satellite communications system where an earth station is operative to exchange messages with a user terminal through a communications satellite in movement relative thereto, and to exchange messages with said communications satellite, to measure the position of the user terminal , said method including the steps of: said user terminal receiving a signal, sent from a known position at a known time from a known autonomous source; said user terminal noting the time of arrival of said signal; said user terminal communicating said time of arrival to said earth station via said communications satellite; said earth station calculating the distance between said autonomous source and said user terminal; and said earth station incorporating said calculated distance in the estimation of said position of said user terminal, otherwise derived by said exchange of messages between said earth station and said user terminal via said communications satellite.
A user terminal configured to perform a method according to claim 14.
EP98300587A 1997-02-01 1998-01-28 User terminal positioning system and method of employing external signals Withdrawn EP0856957A3 (en)
EP0856957A2 true EP0856957A2 (en) 1998-08-05
EP0856957A3 EP0856957A3 (en) 1999-04-14
EP98300587A Withdrawn EP0856957A3 (en) 1997-02-01 1998-01-28 User terminal positioning system and method of employing external signals
WO2000014566A1 (en) * 1998-09-09 2000-03-16 Qualcomm Incorporated Accurate range and range rate determination in a satellite communications system
EP1200850A1 (en) * 1999-07-12 2002-05-02 Eagle Eye, Inc. Fast acquisition position reporting system
WO2013180772A3 (en) * 2012-05-03 2014-01-30 Raytheon Company Global positioning system (gps) and doppler augmentation (gdaug) and space location inertial navigation geopositioning system (spacelings)
WO2000011809A1 (en) * 1998-08-25 2000-03-02 Ico Services Limited Leo mobile satellite communication system with access to base station according to position of the mobile
EP1200850A4 (en) * 1999-07-12 2004-05-12 Eagle Eye Inc Fast acquisition position reporting system
WO2001007929A1 (en) * 1999-07-22 2001-02-01 Ico Services Ltd. Satellite communication system and method
EP3067715A1 (en) * 2012-05-03 2016-09-14 Raytheon Company Space location geopositioning system (spacelings)
US6031489A (en) 2000-02-29
Designated state(s): AT BE CH DE DK ES LI
1999-05-06 16A New documents discovered after completion of the ep-search report