WO2001011720A1 - Determination de la position d'un satellite - Google Patents

Determination de la position d'un satellite Download PDF

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Publication number
WO2001011720A1
WO2001011720A1 PCT/GB2000/003059 GB0003059W WO0111720A1 WO 2001011720 A1 WO2001011720 A1 WO 2001011720A1 GB 0003059 W GB0003059 W GB 0003059W WO 0111720 A1 WO0111720 A1 WO 0111720A1
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WO
WIPO (PCT)
Prior art keywords
satellite
user terminal
earth station
positional signal
time
Prior art date
Application number
PCT/GB2000/003059
Other languages
English (en)
Inventor
Richard Wyrwas
Original Assignee
Ico Services Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ico Services Ltd filed Critical Ico Services Ltd
Publication of WO2001011720A1 publication Critical patent/WO2001011720A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/1853—Satellite systems for providing telephony service to a mobile station, i.e. mobile satellite service
    • H04B7/18545—Arrangements for managing station mobility, i.e. for station registration or localisation
    • H04B7/18547—Arrangements for managing station mobility, i.e. for station registration or localisation for geolocalisation of a station
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
    • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
    • G01S5/14—Determining absolute distances from a plurality of spaced points of known location
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
    • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
    • G01S5/0205—Details
    • G01S5/021—Calibration, monitoring or correction

Definitions

  • the present invention relates to satellite communications systems wherein an earth station is operative to communicate with a plurality of user terminals on the surface of the earth, by sending signals to an orbiting satellite, the satellite relaying signals from the earth station to the user terminals and from the user terminals to the earth station.
  • Satellite communications network is described in EP-A-0 365 885 and US Patent No. 5 394 561 (Motorola), which makes use of a constellation of so- called low earth orbit (LEO) satellites, that have an orbital radius of 780 km.
  • LEO low earth orbit
  • Mobile user terminals such as telephone handsets establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.
  • Alternative schemes which make use of so-called medium earth orbit
  • Satellite communications are based on a commercial and regulatory knowledge of the position of the user terminal on the surface of the earth. It is known to determine the position of a user terminal, at registration with a satellite or instigation of a call, by means of signal exchanges which consume time, full channel bandwidth and power from the satellite. More of a problem is the fact that an individual user terminal gains individual attention from the satellite. Equally, it is known for a user terminal to enter quiescent or dormant mode, where only occasional activity is required to maintain registration. This time could otherwise be spent in a useful way. The present invention seeks to improve upon the manner in which the position of a user terminal is determined.
  • the present invention comprises a satellite communications system in which a satellite is operable to relay traffic between an earth station and a user terminal, within an area of radio coverage of the satellite, by means of a first antenna pattern, said earth station comprising a positional signal source operable to transmit a positional signal to said satellite, and said satellite being operable to relay said positional signal to said area of radio coverage using a second antenna pattern.
  • the invention can provide means for one satellite to cooperate in the simultaneous and continuous determination of position for all user terminals within its coverage.
  • the present invention comprises a method of operating a satellite communications system in which a satellite is operable to relay traffic between an earth station and a user terminal, within an area of radio coverage of the satellite, by means of a first antenna pattern, the method including the steps of: transmitting a positional signal from the earth station to the satellite, and relaying the positional signal from the satellite to said area of radio coverage using a second antenna pattern.
  • an earth station in a satellite communications system in which a satellite is operable to relay traffic between the earth station and a user terminal within an area of radio coverage of the satellite by means of a first antenna pattern, the earth station comprising a positional signal source operable to transmit a positional signal to the satellite and further comprising means for configuring the satellite to relay the positional signal to said area of radio coverage using a second antenna pattern.
  • a user terminal for use in a satellite communications system in which a satellite is operable to relay traffic between an earth station and the user terminal using a first antenna pattern, the user terminal being operable to receive a positional signal from the earth station relayed via the satellite using a second antenna pattern.
  • the invention further provides a system and method wherein the satellite comprises a common phased antenna array, and wherein the satellite is operable, simultaneously, to employ the common phased antenna array to generate both the first antenna pattern and the second antenna pattern.
  • the invention further provides a system and method wherein the first antenna pattern comprises a plurality of spot beams.
  • the invention further provides a system and method wherein the second antenna pattern is a global beam.
  • the invention further provides a system and method wherein the positional signal comprises an indication of the time of emission of the relayed positional signal from the satellite and an indication of the position of the satellite.
  • the invention further provides a system and method wherein the positional signal source is operable to monitor the relayed positional signal, to calculate the correct delay between the earth station and the satellite by knowledge of the position of the earth station and of the satellite, thereby to calculate any error in the indicated time in the positional signal, and to adjust the positional signal to bring the any error towards zero.
  • the invention further provides a system and method wherein the user terminal is operative to provide a user terminal status report, to the earth station, indicative of the user terminal's current estimate of the user terminal's position and the time, currently on an internal clock in the user terminal, and wherein the earth station is operative to employ the current estimate of the user terminal's position and a knowledge of the position of the satellite and the earth station to compensate for the path delay between the earth station and the user terminal to provide, on a corrective return signal to the user terminal, an indication of the correct clock time, the user terminal responding thereto by resetting the internal clock to the indicated correct time.
  • the invention further provides a system and method wherein the earth station is operative to compare consecutive values of the time, currently on said internal clock in said user terminal to derive a correction factor, reflecting the drift of the internal clock, and is operative to include the correction factor in the corrective return signal, the user terminal responding thereto by applying the correction factor to time measurement by said internal clock.
  • Figure 1 shows a constellation of communications satellites in an orbit about the earth
  • Figure 2 shows a pair of crossed orbits
  • Figure 3 shows the pattern of spot beams, generated by the satellite, on the surface of the earth
  • Figure 4 shows a view, from the earth, of the or a satellite from the earlier Figures;
  • Figure 5 shows a schematic diagram of the signal paths and processing elements of the satellite of Figure 4;
  • Figure 6 shows the user terminal antenna array in greater detail
  • Figure 7 shows a cross-sectional view of an individual antenna element of Figure 6;
  • Figure 8 is a block diagram showing the manner of antenna pattern formation
  • Figure 9 illustrates the polar diagram resulting from simultaneous generation of the spot beams of Figure 3 and a global beam;
  • Figure 10 is a schematic diagram of the elements in the earth station
  • Figure 1 1 illustrates how the earth station, user terminal and other satellites interact
  • Figure 12 is a flowchart of the activities of the positional signal source, in the earth station, as it automatically corrects for any system delays to deliver a perfect positional signal
  • Figure 13 is illustrative of the exchange of user terminal status reports directed towards the earth station and corrective signals, directed towards the user terminal, whereby accumulated timing errors and drift rates are corrected in the internal clock of the user terminal;
  • Figure 14 is a flowchart showing the activities of the earth station and of the user terminal in the situation shown in Figure 13.
  • Figure 1 shows a planar constellation of satellites disposed about the earth.
  • the plurality of satellites 10 are evenly disposed around a circular orbit 12 above the surface of the earth 14.
  • Each of the satellites 10 is designed to provide radio communications with apparatus on the surface to the earth 14 when the individual satellite 10 is more than 10 degrees above the horizon.
  • Each satellite 10 therefore provides a cone 16 of radio coverage which intersects with the surface of the earth 14.
  • the surface of the earth has three types of areas.
  • a first type of area 18 is one which has radio coverage from only one satellite 10.
  • a second type of area 20 is an area where there is radio coverage from more than one satellite 10.
  • a third type of area 22 receives radio coverage from none of the satellites 10 in the orbit 12 shown.
  • Figure 2 illustrates how the satellites 10 are disposed in orthogonal orbital planes.
  • the first orbit 12 of Figure 1 is supplemented by a second orbit 12' having satellites 10 disposed there about in a similar manner to that shown in Figure 1.
  • the orbits 12' are orthogonal to one another, each being inclined at 45 degrees to the equator 24 and having planes which are orthogonal (at 90 degrees ) to each other.
  • the satellites 10 orbit above the surface of the earth 14 at an altitude in the region of 10 500km which generally corresponds to the ICOTM system.
  • ICOTM ICOTM
  • other orbital heights and numbers of satellites 10 may be used in each orbit 12, 12'.
  • This configuration is preferred because the example provides global radio coverage of the earth 14, even to the north 26 and south 28 poles, with a minimum number of satellites 10.
  • the orthogonality of the orbits ensures that the satellites 10 of the second orbit 12' provides radio coverage for the third types of area 22 of no radio coverage for the satellites in the first orbit 12, and the satellites 10 in the first orbit 12 provide radio coverage for those areas 22 of the third type where the satellites 10 of the second orbit 12' provide no radio coverage.
  • the two orbits 12, 12' are here shown to be of the same radius, the system will function with orbits 12, 12' of different radii. Also, there may be more than two orbits 12, 12 '.
  • Each satellite 10 is in bidirectional communication with an earth station 1 1 on the surface of the earth 14 and within the cone of radio coverage 16 of the satellite 10.
  • the satellite 10 is in potential bidirectional communication with a plurality of user terminals 13 (only one shown), also on or near the surface of the earth 14, and anywhere within the cone of radio coverage 16 of the satellite 10.
  • the satellite 10 acts as a simple relay, whereby traffic from the earth station 1 1, such as telephone calls and, as will later be described, slow data, is directed to the user terminal (s) 13 and traffic from the user terminal(s) 13 is directed to the earth station 11.
  • traffic from the earth station 1 1 such as telephone calls and, as will later be described, slow data
  • traffic from the user terminal(s) 13 is directed to the earth station 11.
  • Only one earth station 1 1 is shown, it is to be understood that a sufficient number and distribution of earth stations 1 1 is provided so that all satellites 10 have at least one earth station 1 1 within their respective cones of radio coverage 16.
  • Figure 3 shows the structure of the cone 16 of radio coverage provided by each satellite 10.
  • the radio coverage cone 16 is shown centred, on a map of the earth, at latitude 0 degrees and 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 satellite 10.
  • the 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.
  • 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.
  • Figure 1 shows the cone 16 at a minimum of 10 degrees elevation to the surface of the earth.
  • the spot beams 30 are of near uniform, slightly overlapping circular shape at the centre whereas, at the edges, the oblique incidences of the spot beams 30 onto the surface of the earth 14 causes considerable distortion of shape.
  • Figure 4 is a view, from the surface of the earth 14 showing an orbiting satellite 10 in greater detail.
  • the satellite 10 comprises a body 32 on which solar panels 34 are mounted on rotating yokes 36.
  • the body 32 of the satellite 10 also supports uplink antennas 38 and downlink antennae 40 whereby the satellite 10 can communicate with an earth station 1 1 for communication and control purposes.
  • the uplink antennas 38 in the example given, provide a reception path for the satellite 10 to receive bulk traffic and command signals, sent from the earth station 1 1 on a frequency of, for example, 5 GHz .
  • the downlink antenna 40 sends bulk traffic and commands from the satellite 10 to the earth station 1 1 on a frequency of, for example, 7 GHz.
  • the uplink antenna 38 and the downlink antenna 40 are both fairly wide beam so that the earth station 1 1 can make contact with the satellite 10 over the whole time it is within sight of the earth station 1 1. Those skilled in the art will appreciate that other frequencies can be employed by the uplink 38 and downlink 40 antennas.
  • the satellite 10 comprises a transmission antenna array 42 and a reception antenna array 44 whereby the satellite 10 can maintain contact with user terminal(s) 13, which can, for example, be vehicle mounted or resemble cellular telephone handsets, on the surface of the earth 14.
  • the transmission array 42 operates on a frequency band of, for example, 2170 to 2200 MHz and the reception array 44 operates on a frequency band of, for example, 1980 to 2010 MHz.
  • the bandwidth, each way of 30MHz is, for telephony purposes, split into channels of 25KHz width and spacing, and is used to carry telephone traffic and operational data/commands between the earthbound user terminal(s) 13 and the satellite 10.
  • the satellite 10 relays the traffic and operational commands/data to the earth station 11 via the uplink antenna 38 and the downlink antenna 40.
  • the solar panels 34 are, for example, automatically steered to face the sun and so power the satellite 10, and the satellite 10 describes 360 degree roll, pitch and yaw in each orbit of the earth to ensure that the transmission array 42 and the reception array 44 always face the earth 14 and that the solar panels are always able to face the sun to extract maximum power.
  • the steering of the solar panels 34 and the orbital rotations of the satellite 10 do not form part of the present invention, but are here given by way of example to provide completeness of the system description.
  • Figure 5 is a schematic block diagram of the internal functions of an exemplary satellite 10.
  • the satellite 10 comprises a forward path 46 which conducts signals from the earth station 1 1 uplink antenna 38 to the user terminal downlink array 42.
  • a backward path 48 conducts signals from the user terminal receiving array 44 to the earth station 11 downlink antenna 40.
  • Signals from each of the elements in the user terminal receiving array 44 are amplified by a corresponding plurality of low noise amplifiers 50 and then frequency converted to an intermediate frequency by a corresponding plurality of frequency changers 52.
  • the intermediate frequency output from each frequency changer 52 is them converted from an analogue to a digital signal by a corresponding plurality of analogue to digital converters 54.
  • the digital outputs of the analogue to digital converters 54 are provided as input to a multiplexing unit 56 which, in turn, provides input to a backward path digital beam formation network 58 whose function will be described in more detail hereafter, but which, essentially, extracts the 129 signals received from the individual elements in the receiving array 44 and converts them into the equivalent of 163 spot beams 30.
  • the 163 signals are then each passed through an equivalent number of respective return path bandpass filters 59, each effectively 150kHz wide (having an edge allowance for doppler shift).
  • the 163 filtered equivalent spot beam 30 signals are then provided to a multiplexer 60 which provides one signal for each of the elements in the earth station 1 1 downlink antenna 40, then a corresponding number of digital to analogue converters 62, in turn, drive a corresponding number of intermediate frequency to C-Band frequency changers.
  • the output from each intermediate frequency to C Band frequency changer 64 drives a corresponding C Band power amplifier 66, each driving a respective element in the earth station 1 1 downlink antenna 40.
  • the satellite simply relays signals from the user terminals 13 s, on the surface of the earth 14, to the earth station 1 1 , elsewhere on the surface of the earth 14.
  • the forward path 46 has signals, received from the earth station 11 , entering at the uplink earth station antenna 38, each antenna element having its received signal amplified by forward link low noise amplifiers 68 and converted to an intermediate frequency by a forward path front end frequency changer 70.
  • Each signal is then provided as input to a respective forward path analogue to digital converter 72 where the digital output is provided as input to a forward path demultiplexer 74 providing input to forward path bandpass filters 76 ( each with the same usable 150 KHz width as the return path bandpass filters 59) and a forward path digital beam formation network 78, (corresponding to the return path digital beam formation network 58).
  • signals are each fed to a forward path multiplexer 80 and thence to a respective forward path digital to analogue converter 82 whose analogue outputs are provided to a forward path rear end frequency changer 84 which converts the intermediate frequency of the forward path to the frequency used in the user terminal downlink 42.
  • forward path power amplifiers 85 drive each of the individual elements in the user terminal downlink antenna 42.
  • the forward path thus acts as a transparent relay for signals from the earth station 1 1 to the user terminal(s) 13.
  • a central, controlling processor 88 controls all of elements in the satellite
  • Figure 6 is a more detailed view of the user terminal downlink array 42 and the user terminal uplink array 44 of the satellite 10.
  • Each of the user terminal uplink array 44 and the user terminal downlink array 42 comprise a plurality of individual elements 86, arranged in a pattern.
  • the invention is equally applicable to different numbers of spot beams 30 and different numbers and layouts of elements 86.
  • the elements 86 are individually driven to create a pattern of spot beams, on the surface of the earth, rather like the cells of a cellular phone network, whereby terrestrial users with handsets or other equipment may communicate with the satellite 10.
  • FIG. 7 is a cross sectional view of an antenna element 86.
  • Each antenna element 86 comprises a circular cross section cylinder 88, made of a radio reflective material, and closed at its proximal end to the satellite 10 by an end wall 90.
  • a feed line 92 is connected to a dipole element 94 which is spaced from resonant parasitic elements 96.
  • antenna elements 86 are already known in the art.
  • the antenna element 86 is shown merely by way of example as a type of antenna element 86 which may be individually driven, in a known pattern of spacing, when forming an array of spot beams or any other pattern of radio signals to be projected towards, or received from, the surface of the earth 14.
  • Figure 8 is a block diagram of the manner in which the individual antenna elements 86 may be electronically phased to produce the pattern shown in Figure 3. It is to be appreciated that the pattern shown in Figure 3 is merely one of many possible beam patterns for the spot beams 30 for which the present invention is applicable. The spot beams 30 may be fewer or more in number.
  • the example of Figure 8 is shown in terms of the forward path 46 of Figure 5. It is to be appreciated that exactly the same technique is applied to the return path 48 of Figure 5.
  • An input feed 98 comprises signals, which are to be fed to an individual element 86 in the user terminal downlink array 42 , and corresponding to one of the plural outputs of the forward path bandpass filter 76, having passed through forward path demultiplexer 74 and the forward path analogue to digital converter 72, which are in the form of a stream of binary words or binary digits representative of the instant amplitude of demultiplexed analogue input to the forward path analogue to digital converter 72.
  • the input feed 98 is provided as input to a first fast fourier transformer 100 which converts the stream of binary digits or binary words into a further stream of binary digits or binary words representative of the amplitude of the elements of the frequency spectrum of the input feed 98.
  • the output of the first fast fourier transformer 100 is provided as input to an adjuster 102 which is controlled by the controlling processor 88.
  • the adjuster 102 scales the individual binary words to adjust the amplitude of the individual frequency components indicated by the output of the first fast fourier transformer 100 and adjusts the phase thereof by digitally delaying or advancing (relatively) binary words or binary digits.
  • the output of the adjuster 102 is then fed to a second fast fourier transformer 104 which performs the inverse transformation converting the signal back into a stream of binary digits representative of a signal in the time domain.
  • the output of the second fast fourier transformer 104 is fed as input to the forward path digital to analogue converter 82 which converts the input stream of binary digits or binary words into a continuous analogue output which is provided as drive (via the forward path tail end frequency changer 84 and the forward path power amplifier 85) to an individual antenna element 86 in the user terminal downlink array 42.
  • any beam pattern within the capability of the numbers and disposition of the elements 86 in an antenna array 42, 44 can be created.
  • Just how any particular antenna radiation pattern can be achieved is known in the art, and described in "Reference Data For Radio Engineers", ISBN 0-672-20678-1 , published in its fifth (1968) revision in its fifth printing (1973) by Howard W. Sams & Co. Inc, and in the extensive bibliography published therein.
  • Figure 9 shows the polar diagram of the user terminal downlink array 42 and the user terminal uplink array 44. These are achieved by the processor 88 instructing the forward path digital beam formation network 78 and the return path digital beam formation network 58 with the correct parameters for the achievement of the desired polar diagram for each frequency group emanating, respectively, from the forward path bandpass filters 76 and return path bandpass filters 59.
  • One of the 150kHz reserved frequency blocks in the forward path bandpass filter 76 is elected to form a global beam 108 for transmission from the user terminal downlink array 42.
  • the global beam 108 fills the entire cone of radio coverage 16 of the satellite 10.
  • another reserved 150kHz frequency block in the return path bandpass filter 59 is elected to form the global beam 108 for reception by the user terminal uplink array 44.
  • the "global beam" 108 is in fact two beams, preferably (but not of necessity) identical to each other, there being one for transmission to user terminals 13 on the earth 14 and another for reception of signals from user terminals 13 on the earth.
  • Figure 9 shows how a dual service can be obtained.
  • a global beam 108 having a frequency allocation for traffic from the earth station 1 1 to the earth 14 and another frequency allocation for traffic from the earth 14 to the earth station 1 1.
  • the earth station 1 1 can use the forward path 46 global beam reserved frequency allocation to pass any kind of traffic to be transmitted by the global beam 108 on the user terminal downlink array 42. Likewise, the earth station 1 1 can receive any kind of traffic within the return path global beam reserved frequency allocation from the user terminal uplink array 44. All this time, the earth station 1 1 can carry on the business of normal telephone traffic, using the spot beams 30 in the normal way and employing all the other, non-reserved frequency blocks present in the forward path bandpass filter 76 and the return path bandpass filter 59.
  • the traffic capacity, created through the global beam 108 is totally independent of all other traffic and can be co-ordinated by the earth station in an independent manner.
  • FIG 10 shows the earth station 1 1 according to the present invention.
  • a radio frequency transmitter/receiver 1 18 feeds radio signals to, and receives radio signals from, the dish antenna 120, which is pointed at and tracks the satellite 10.
  • An earth station controller 122 passes traffic signals to, and receives traffic signals from, the transmitter/receiver 1 18. In addition, the earth station controller 122 specifies to the transmitter/receiver 1 18 on what frequency signals are to be transmitted to the satellite 10 and identifies received radio signals by their frequency.
  • An interface switch 124 provides an interface between the earth station 1 1 and the global terrestrial telephone network 126, thereby enabling telephone and other calls to be placed through the satellite 10 and the earth station 1 1.
  • the earth station 11 further comprises a positional signal source 128 which generates positional signals, based on the ephemera of the communications satellite 10 (known to the earth station 1 1) for use on the global beam 108.
  • the positional signal source 128 sends its positional signals to the earth station controller 122 so that the positional signals are sent through the global beam 108.
  • any user terminal 13 can receive positional signals, based on the position and time of sending of the communications satellite 10, anywhere within the global beam 108 (which covers the cone of coverage 16 of the satellite 10), and on a frequency within the range of the user terminal downlink array 44, which the user terminal 13 is designed to receive.
  • the earth station 1 1 positional signal source 128 knows the distance, and hence, time delay between the earth station 1 1 and the satellite 10, and can thus provide a transmission, to be made, by the satellite 10, which is emitted, from the satellite 10, at a known time with a correct timestamp to indicate the time of emission from the satellite 10.
  • the positional signal source 128 instructs the earth station controller 122 to transmit the positional signal on a frequency, corresponding to accommodation within the band of reserved frequencies in the forward path bandpass filter 76.
  • the positional signal, generated by the positional signal source 128 indicates sufficient information for the user terminal 13 to know the instant position of the communications satellite 10, and the time of emission of the positional signal from the satellite 10.
  • Figure 1 1 shows how the positional signals are used by the earth station 11 and the user terminal 13.
  • the earth station 1 1 has the positional signal source 128 send its positional signal 130 to the communications satellite 10. This signal is returned 132 to the user terminal 13 by the global beam 108 within the cone 16 of radio coverage of the communications satellite 10. Other communications satellites 10B, 10C, within the field of view of the user terminal 13, and each in contact with an earth station 1 1 , are doing exactly the same thing.
  • the user terminal 13 thus has at least one positional signal for position fixing at any one time, and probably more than one positional signal from more than one communications satellite 10, 10B, IOC.
  • the positional signals are received within the frequency band which the user terminal 13 is designed to receive.
  • user terminal 13 is adapted to receive a standard GPS signal from a standard GPS satellite 136, thereby ensuring that the user terminal 13, on nearly all occasions, has a sufficient number of GPS or generated positional signals in just the time it takes to acquire only one GPS satellite 136. Equally, the user terminal 13 can rely solely upon the communications satellites 10, 10B, IOC.
  • the signal from the earth station positional signal source can be in narrow band FM, AM, TDMA, CDMA, or any other format that the user terminal 13 is adapted to decode.
  • the user terminal 13, during gaps in operation, is adapted to listen to the global beam 108 and calculate the position of the user terminal 13, on the surface of the earth 14, according to well known principles of triangulation and distance plotting.
  • the user terminal 13 can then simply display its position, can relay its position to the earth station 1 1 at registration or commencement of traffic, or can transmit its position to the earth station 1 1 , using the global beam return path 48.
  • Figure 12 shows a flowchart, employed by the earth station 1 1 in order to compensate, among other things, for delays in the forward path 46 of the satellite 10.
  • the various processes and elements in the forward path 46 impose delays on any signal, presented at the earth station uplink antenna 38 for re-transmission through the user terminal downlink antenna 42.
  • the forward link bandpass filter 76 will introduce a group delay on any signal passing therethrough.
  • the forward path digital beam formation network 79, forward path digital to analogue converter 82, and, indeed, all of the components in the forward path impose their individual delays which are summed as the overall delay of the forward path 46.
  • the positional signal source 128 involves a feedback path whereby the delays through the satellite 10, and any other incidental delays, are subject to compensation.
  • the positional signal source 128 also comprises a global beam receiver 141 which monitors the signal from the global beam 108.
  • Figure 12 is indicative of the manner in which the positional signal source 128 uses the signal, received from the global beam 108, to compensate for variation in delays through the satellite 10, and which may be experienced through the atmospheric path and earth station 1 1.
  • the positional signal source 128 monitors the signal from the global beam 108 and notes the time of arrival of the positional signal, in the same manner as the positional signal would be interpreted by the user terminal 13.
  • the positional signal source 128 knows, exactly, the position, on the surface of the earth 14, of the receiving apparatus of the global beam receiver 141. Equally, the positional signal source knows, within the limits of measurement, as relayed thereto by a monitoring station, the position of the satellite 10. The positional signal source 128 is therefore equipped to know the true value of the delay which should be experienced between the earth station 1 1 and the satellite 10.
  • the positional signal source 128, receiving data from the global beam receiver 141 compares the time stamp on the signal from the global beam 108 with the value which should pertain. This represents the timing error. It can be positive or negative, depending upon conditions.
  • a third operation 146 then makes the appropriate correction. This can be a simple adjustment to the time stamp so that a true indication of the time of emission of the positional signal from the satellite 10 is provided, or, equally, could be a physical advancement or retardation of the instant at which the positional signal generator 128 provides its signal for transmission.
  • Figure 13 shows how the present invention compensates for and corrects any errors in the clock of the user terminal 13.
  • the user terminal 13 comprises, for example, a crystal oscillator which is used as the reference for all frequency synthesis for radio reception and transmission, and also serves as the master timing source for an internal clock that the user terminal 13 employs to keep a running time.
  • a timing error of one microsecond represents up to about a 300 metre error in determined position. It can be seen that the few parts per million error in the crystal oscillator, over time, can represent a very considerable ground error.
  • the user terminal 13 periodically reports back to the earth station 1 1. This occurs naturally in a cellular telephone environment, as the user terminal 13 moves from spot beam 30 to spot beam 30, or on first registration, or for a host of other reasons when the user terminal is either in active or dormant mode. Those skilled in the art will be aware of the occasions when this occurs.
  • the spot beam environment is used, in the example, though there is no reason why the invention should not include use of the global beam 108 for the message exchange hereinafter described.
  • the user terminal 13 sends, along with any incidental other information during its contact with the earth station 1 1 , a user terminal status report 148 through the satellite 10 (being one of the satellites 10, 10A, 10B shown in Figure 1 1) where the user terminal 13 reports:
  • the positional signal source is therefore able to give an accurate time to the user terminal 13, or, at least, more accurate than the previous running time on the user terminal clock. If this is the first contact by the earth station 1 1 with the particular user terminal 13, the earth station (via the positional signal source 128) is unable to measure the drift rate of the user terminal 13 internal clock, and so reports back a null result.
  • the user terminal 13 responds by correcting its internal clock to the time reported back by the earth station 1 1. The user terminal then proceeds to estimate its position, as earlier described, using the corrected time, until the next occasion when it has need to contact the earth station 1 1.
  • the user terminal 13 On next contact with the earth station 1 1 , the user terminal 13 once again reports 148 its current time and its improved positional estimate.
  • the whole process, described with reference to Figure 13, repeats, but this time the earth station 1 1, via the positional signal source 128, is able to estimate the rate of drift of the clock within the user terminal.
  • the earth station 1 1 therefore reports 150 to the user terminal 13 the correct time for the user terminal (as best guess on position) and the estimated rate of drift of the user terminal internal clock.
  • the user terminal 13 responds by correcting its clock, and, in all further timing operations, applying the known rate of drift when active in the manner described with reference to Figures 1 to 12.
  • This process is applied repeatedly, until the position of the user terminal 13, as measured, is iterated down to within all other residual measurement error limits.
  • the rate of drift in the user terminal clock is constantly measured and corrected, so that temperature, voltage and other slow changes are compensated, and the user terminal clock is constantly corrected with increasing accuracy.
  • the earth station 1 1 can create a running average of the measured position of the user terminal 13 so as to reduce further the measurement errors (usually by a factor of the square root of the number of samples).
  • the invention encompasses the user terminal 13, at manufacture, having its clock drift rate measured and that measured rate being provided in the read-only-memory of the user terminal to provide an initial guess at drift rate, for use when no measured rate is available from an earth station 1 1.
  • Figure 14 is a flowchart of the activities, briefly described in relation to Figure 13. The left hand portion of the chart reflects the activities of the user terminal 13. The right hand portion of the chart reflects the activities of the earth station 1 1 and the positional signal source 128.
  • the user terminal listens to the positional signals in the global beams 108 of the satellites 10. It constantly updates its estimated position.
  • a first test 154 detects that, for whatever reason, the user terminal 13 has contact with the earth station 1 1 , when control is passed to a fifth operation 156 where the user terminal 13 sends a user terminal status report 148 to the earth station 13, reciting the current time on the user terminal clock and the position at which the user terminal 13 believes itself to be.
  • the earth station 1 1 positional signal source 128 receives and decodes the user terminal status report 148 and passes control to a seventh operation 160 where the positional signal source 128 calculates the distance between the earth station 1 1 and the user terminal 13 based on the round trip between the earth station 1 1 , the satellite 10 and the position of the user terminal 13 as provided by the user terminal status report 148. This calculated distance is turned into an equivalent delay, based on propagation being at the speed of light.
  • An eighth operation 162 then calculates the clock drift rate for the user terminal 13. If this is the first encounter by the earth station 1 1 with the particular user terminal 13, the positional signal source 128 has no prior data for the particular user terminal, in which case the eighth operation 162 creates a null result which, when sent back to the user terminal 13, allows the user terminal to employ either its factory pre-set value, or any earlier value it may have received from another earth station 1 1. If the earth station has received a prior user terminal status report 148, the eighth operation 162 compares two consecutive values of reported time, assumes the user terminal 13 has not moved, and notes the true elapse of time from the positional signal source's 128 own, highly accurate atomic clock, either sited on the earth station 1 1 or available by incoming line. The eighth operation 162 then calculates a correction ratio whereby the user terminal 13, when making subsequent elapsed time calculations, by applying the correction ratio, can achieve a significantly better estimation of true clock time.
  • a ninth operation 164 then transmits the correct time and the correction ratio for the user terminal 13 internal clock drift rate, to the user terminal 13, via the corrective return signal 150.
  • the sending of the correct time to the user terminal 13 is based on the latest estimate of the position of the user terminal, and the consequent delay, calculated by the seventh operation 160.
  • the correct time signal can be advanced and sent so that it arrives at the user terminal 13 at the instant when the indicated time is correct. Equally, the correct time signal can be so timed that the indicated correct time occurs a predetermined period after its receipt. These are just options under the invention. Other means can be used whereby the user terminal 13 is made aware of the correct time at its last estimated position.
  • An eleventh operation 168 then has the user terminal 13 correcting the time on its internal clock to the indicated correct time.
  • the user terminal 13 also applies the drift rate correction ratio so that, starting at the indicated correct time, and applying the drift rate correction ratio to the actual output of its internal clock, when interacting with the global beams 108, as in the fourth operation 152, the time used in estimating the user terminal 13 position is much more accurate.
  • a twelfth operation 170 then has the user terminal complete whatever business it has with the earth station 1 1, which caused the first test 154 to detect contact with the earth station 1 1. This could be registration, re-registration, spot beam 30 transfer, or the initiation of a call.
  • the earth station 1 1 co-operates.
  • the user terminal then passes control back to the fourth operation 152 where the user terminal 13 seeks improved positional estimates.
  • the user terminal 13 has a constantly updated and corrected internal clock, which more and more nearly, at each exchange of the user terminal status report 148 and the corrective return signal 150, approaches the accuracy and value of the standard clock available to the positional signal source 128.
  • the estimated position of the user terminal 13 improves, because of the improved accuracy of its internal clock.
  • the drift rate of the internal clock in the user terminal 13 is constantly monitored, and any alteration in the drift rate is automatically compensated, both in the short term and the long term.
  • the user terminal 13 is therefore able to provide, to the user, a constantly updated and improving indication of position, as well as being able to report its position, with good accuracy, to the earth station 1 1 at the commencement of a call on the telephone system.
  • the invention has been described generally in relation to the ICOTM system, it will be appreciated that it could be equally well applied to any of the satellite mobile telecommunications networks described in Scientific American supra.
  • the user terminals UT have been described herein as mobile telephone handsets, it will be understood that they may be semi-mobile e.g. mounted on a ship or aircraft.
  • the UT may also be stationary e.g. for use as a payphone in a geographical location where there is no terrestrial telephone network.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Astronomy & Astrophysics (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Radio Relay Systems (AREA)

Abstract

Système de détermination de la position d'un satellite conçu pour être mis en application dans un environnement de télécommunications et possédant une station terrestre (11) générant un signal de position relayé par le satellite (10) vers tous les terminaux d'utilisateurs (13) situés à l'intérieur de la couverture (16) du satellite au moyen d'un seul faisceau global (108) généré simultanément sur le réseau d'antennes (42, 44) de phase commune servant à générer également des faisceaux ponctuels (30) pour les communications par satellite. La station terrestre (11) comprend également un système servant à corriger le signal de position et consistant, au moment de l'émission satellite et de la position ponctuelle du satellite, à contrôler le faisceau global, à relever toutes erreurs de retard entre le satellite (10) et la station terrestre (11) et à régler le signal temporel dans le signal de position afin de corriger l'erreur. L'invention concerne également un procédé répétitif servant à corriger l'horloge interne du terminal d'utilisateur (13), ce qui permet de corriger toute déviation, d'établir une heure exacte et, par conséquent, d'améliorer l'évaluation de position du terminal d'utilisateur (13).
PCT/GB2000/003059 1999-08-10 2000-08-08 Determination de la position d'un satellite WO2001011720A1 (fr)

Applications Claiming Priority (2)

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GB9918874A GB2353159A (en) 1999-08-10 1999-08-10 Position determination in multi-beam satellite
GB9918874.0 1999-08-10

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WO2001011720A1 true WO2001011720A1 (fr) 2001-02-15

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EP2224611A1 (fr) 2009-02-27 2010-09-01 Astrium Limited Appareil de compensation
US8914040B2 (en) 2009-09-24 2014-12-16 Telefonaktiebolaget L M Ericsson (Publ) Method and arrangement in a telecommunication system

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997005710A1 (fr) * 1995-07-28 1997-02-13 International Mobile Satellite Organization Procede et dispositif de positionnement d'un faisceau etroit
WO1998025358A2 (fr) * 1996-12-06 1998-06-11 International Mobile Satellite Organization Procede et dispositif d'attribution de largeur de bande
EP0851330A2 (fr) * 1996-12-12 1998-07-01 ICO Services Ltd. Méthode et système pour la commande d'un satellite
EP0871300A2 (fr) * 1997-04-09 1998-10-14 ICO Services Ltd. Determination de position pour terminal d'un système satellitaire

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Publication number Priority date Publication date Assignee Title
JPH0461503A (ja) * 1990-06-29 1992-02-27 Mitsubishi Electric Corp フェーズドアレーアンテナ
JP2997610B2 (ja) * 1993-07-07 2000-01-11 三菱電機株式会社 移動体衛星通信システム
US5589834A (en) * 1994-04-22 1996-12-31 Stanford Telecommunications, Inc. Cost effective geosynchronous mobile satellite communication system
US6020847A (en) * 1996-04-25 2000-02-01 Twr Inc. Geolocation method and apparatus for satellite based telecommunications system

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997005710A1 (fr) * 1995-07-28 1997-02-13 International Mobile Satellite Organization Procede et dispositif de positionnement d'un faisceau etroit
WO1998025358A2 (fr) * 1996-12-06 1998-06-11 International Mobile Satellite Organization Procede et dispositif d'attribution de largeur de bande
EP0851330A2 (fr) * 1996-12-12 1998-07-01 ICO Services Ltd. Méthode et système pour la commande d'un satellite
EP0871300A2 (fr) * 1997-04-09 1998-10-14 ICO Services Ltd. Determination de position pour terminal d'un système satellitaire

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GB2353159A (en) 2001-02-14

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