WO2009130304A1 - Procédé de synchronisation entre récepteur et satellite - Google Patents

Procédé de synchronisation entre récepteur et satellite Download PDF

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Publication number
WO2009130304A1
WO2009130304A1 PCT/EP2009/054960 EP2009054960W WO2009130304A1 WO 2009130304 A1 WO2009130304 A1 WO 2009130304A1 EP 2009054960 W EP2009054960 W EP 2009054960W WO 2009130304 A1 WO2009130304 A1 WO 2009130304A1
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WO
WIPO (PCT)
Prior art keywords
data
time
signal
satellite
receiver
Prior art date
Application number
PCT/EP2009/054960
Other languages
English (en)
Inventor
Ty Lewis
Fredrik Lindstrom
Original Assignee
Nordnav Technologies Ab
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 Nordnav Technologies Ab filed Critical Nordnav Technologies Ab
Publication of WO2009130304A1 publication Critical patent/WO2009130304A1/fr

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Classifications

    • 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
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G01S19/258—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS relating to the satellite constellation, e.g. almanac, ephemeris data, lists of satellites in view
    • 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
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • 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
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
    • G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/42—Determining position

Definitions

  • the present invention relates to a method of synchronising a receiver with a signal transmitted by a remote transmitter on-board a satellite.
  • Satellite positioning systems known as “Global Navigation Satellite Systems” (GNSS) are well known throughout the world for providing accurate positioning of personnel and vehicles on land, sea and in the air. These systems rely upon obtaining extremely accurately timed signals from a number of satellites, each signal including an accurate transmission time together with data describing the orbit of the satellite in question. This information is used by a receiver to calculate the position of the person or vehicle in question to an accuracy of a few metres.
  • GNSS Global Navigation Satellite Systems
  • GNSS receivers One specific limitation upon GNSS receivers is the time that is required for a receiver to achieve synchronisation with a particular satellite. This may take upon to 6 seconds in good signal reception conditions and longer in poor reception conditions. The 6 second delay is caused by the format of the transmitted signal from each satellite. Specifically, the time stamp of the signal which is known as the Time of Week (TOW) is only transmitted every 6 seconds (once per subframe) and thus with known methods it may take 6 seconds to receive this signal before synchronisation can be achieved, the average time being 3 seconds.
  • TOW Time of Week
  • a method of synchronising a receiver with a signal transmitted by a remote transmitter on- board a satellite the signal comprising the repeated transmission of time data defining the transmission time and orbit data relating to the orbit of the satellite, the method comprising:- receiving the transmitted signal at the receiver; obtaining reference data at least a part of which corresponds to the data within the transmitted signal; estimating a receipt time relating to the transmitted signal; selecting a predicted signal based upon the reference data and the estimated receipt time; comparing the predicted and received signals; and, determining the time synchronisation of the received signal based upon the comparison and the time data.
  • the transmitted signal typically comprises data transmitted at a predetermined transmission rate. This allows the calculation of the timing of each transmitted bit to be established by reference to the time data and the number of data bits which have been transmitted since that particular time data.
  • the time data comprises a "time stamp” and is preferably Time of Week (TOW) data.
  • the orbit data may take a known format according to GNSS standards. It typically gives the precise position of the satellite at any given time.
  • the TOW may therefore be used to calculate the precise position of the satellite at the time corresponding to the TOW.
  • the orbit data preferably take the form of position data, such as data according to accepted astronomical or GNSS conventions.
  • the orbit data may therefore take the form of ephemeris data defining the orbit of the satellite.
  • a "constellation" of satellites is provided to give sufficient geographical coverage for users and to provide a sufficient number of signals (of which four are needed for positioning) to enable a user position to be calculated.
  • the orbit data for any satellite may further comprise almanac data defining the position of the transmitter and each of the additional transmitters as a function of time.
  • the almanac data is of lower accuracy than the ephemeris data but does provide the orbital positions of all satellites in the constellation at any given time.
  • the number of satellites is such that the almanac data may be transmitted over a significantly longer period by transmitting only part of the almanac data with each repeat of the ephemeris data.
  • the reference data allows synchronisation to be established without waiting for the TOW signal to be received.
  • Such reference data may be obtained from a number of potential sources, although in each case it should be bit true and therefore identical to at least part of the signal being transmitted by the satellite.
  • the reference data may comprise bit true time data and/or orbit data which has been transmitted previously by the transmitter on-board the satellite itself, or which has been transmitted by one of the other satellites in the constellation, when available. This data could be obtained from a local source computer which has had previous access to a bit true signal. It should be noted that the reference data requires an exact time (such as a TOW) associated with it to allow calculation of the predicted signal to be received at the receiver. This is needed to ensure synchronisation.
  • the reference data includes the time data and the orbital data for that particular time data. This is most advantageously provided in an identical format to that transmitted by the satellite allowing a relatively simple generation of the predicted signal for comparison purposes.
  • the reference data may also be provided by an assistance server.
  • This is a computer server which is networked by an appropriate network protocol such as the Internet.
  • the assistance server may provide the transmitted data for one or more satellites. This can be communicated to the receiver via a suitable data connection including wirelessly using electromagnetic radiation. A mobile telephone or satellite telephone network may be used for this purpose.
  • a server is in communication with a receiver which receives the transmitted signals of one or more satellites and these are then forwarded over the Internet for use.
  • the receiver may be provided with an internal (on-board) clock which is synchronised to a time of week signal and is set as a receiver clock once an accurate position has been found upon a previous occasion. At later times the internal clock may be assumed to keep reasonable time and therefore provide a good estimate of the currently received time of week for the signal (the receipt time).
  • the reference data and the transmitted signal are each typically subdivided into time partitions. Therefore the transmitted signal preferably comprises time divisions as superframes, each superframe comprising a number of frames and each frame comprising a number of subframes. Typically each subframe is divided into words and each word into bits. Each subframe typically comprises time data as Time Of Week data acting as a time stamp. In order to enable any time ambiguities to be resolved, the estimate of the receipt time preferably has an accuracy allowing resolution to the nearest frame.
  • the accuracy of the receipt time determines the number of data bit comparisons that are needed to achieve synchronisation. It is therefore advantageous to estimate the accuracy of the receipt time and thereby use this to set the size of the range within the predicted data which is to be used in the comparison with the received transmitted data signal. The smaller the number of data bit comparisons the shorter the time delay before synchronisation is achieved.
  • the predicted signal is constructed from the reference data and is essentially a time-shifted version (by forward calculation) of the reference data based upon what the reference data would have been at the receipt time.
  • the comparison between the predicted and the transmitted data is typically a bit comparison.
  • the protocol used is that of a code division multiple access (CDMA) signal having a "chip rate" of preferably 1.023 MHz, where the 1023 bit code repeats every millisecond.
  • CDMA code division multiple access
  • This protocol is that presently used by the Global Positioning System (GPS) and typically the present invention is contemplated as being used in the GPS system.
  • GPS Global Positioning System
  • the invention may equally be used in other satellite systems such as Galileo, Compass, Quasi Zenith Satellite System (QZSS), and so on.
  • the invention finds further advantage when used with a method of synchronising a receiver which waits for the time data to be transmitted.
  • the method is typically largely computer-implemented and therefore the invention contemplates the embodiment of the methods discussed herein as a computer program product comprising computer program code for performing some or all of the method steps of any of the first or second aspects of the invention when such code is executed upon a computer.
  • the invention also extends to a satellite receiver adapted to receive a signal from a remote transmitter on-board a satellite the receiver being further adapted to perform the method according to first or second aspects of the invention.
  • Figure 1 is a flow diagram of the example method
  • Figure 2 shows the content of the reference signal and predicted signal
  • Figure 3 illustrates the bit comparison of the method.
  • GPS global positioning system
  • the method described uses "assistance data" (reference data) from a second source so as to provide high speed synchronisation with the satellite transmission. This must be valid bit true data which is identical to that being transmitted by the satellite.
  • the ephemeris data of the satellite that describing its orbit
  • the method also relies upon an accuracy of less than ⁇ 15 seconds in the GPS time of the receiver for the purposes of clock aiding (to ensure resolution to a specific frame in the satellite transmission as is now discussed).
  • a GPS signal comprises a series of "frames" (also called “pages”), each of 30 seconds in length. Each frame is divided into five subframes of 6 seconds. Each subframe comprises a preamble followed by the TOW data.
  • the TOW is the GPS time according to the satellite's clock at the time of transmission from the satellite. This is extremely accurate since each satellite has an on-board atomic clock. Thus the TOW is transmitted every 6 seconds.
  • the ephemeris data is the same in each frame, but the almanac data is different between frames. It takes 25 consecutive frames to transmit the full almanac data of all satellites in the constellation (this being known as a "superframe").
  • the normal method for establishing synchronisation between a GPS receiver and a satellite transmission is to decode the data received from the satellite until the Time of Week (TOW) signal is obtained.
  • the TOW is obtained by first waiting for the preamble sequence (8 bits in word 1 of a subframe) and then decoding the complete first and second words (word 1 and word 2).
  • Word 2 contains the TOW count.
  • a normal time of week synch for the worst case might take up to six seconds after data bits have started to be decoded (since a subframe is six seconds long). Of course this time is longer in the event that there is a weak signal.
  • the known method of establishing a receiver position involves performing this method for each of the 4 satellites that are required for positioning.
  • this time period for acquiring synchronisation can be shortened significantly if data other than the TOW itself are used to establish the synchronisation. It is possible to obtain bit true data for the ephemeris and almanac from various sources and this allows knowledge of what data should be received from a particular satellite within a particular sub- frame. Since much of this data is actually repeated (particularly the ephemeris), an estimate of the GPS time at the receiver is needed to within the nearest frame. This can be estimated using an accurate previous GPS TOW.
  • a source which may be an assistance server or indeed may be any GPS satellite (the signals differ between satellites only by one data bit), provided the GPS time was determined for that signal, thus giving a particular data bit in a subframe and the frame number of the superframe.
  • the ephermis data are Kepler elements which describe the orbit of the satellite in question. These data are updated every two hours. Taking the ephemeris data, all that is then needed is the GPS TOW (i.e. the time) to give the exact position of the satellite at that GPS time. The position is calculated according to a known algorithm which provides the position and velocity vectors of the satellite.
  • the almanac data is effectively lower accuracy ephemeris data for the present satellite and for each of the other satellites in the constellation.
  • the GPS receiver When the GPS receiver is powered on from a dormant state, if it is provided with reference data describing bit true ephemeris data for the satellites (or for particular satellites which may be "visible” in an assumed location) and if it is provided with a coarse estimate of the present GPS time (accurate to within a few seconds) then synchronisation can be performed more rapidly in many cases than the known method.
  • the coarse time will give a range of possible places in the frame that is presently being broadcast by a satellite. This range will be smaller the better the current time can be estimated.
  • the receiver calculations are in respect of the present signal samples being received from the satellite through the ADC of the receiver.
  • the time at the receiver can therefore include other factors such as the use of a long aerial and so on. However, these factors can be taken into account when calculating the exact time of receipt at the receiver, as will be appreciated by a person of ordinary skill in the art.
  • Figure 1 is a flow diagram setting out the main steps of the method.
  • the receiver is initialised, involving a power up from a dormant state.
  • the receiver acquires satellite transmission information in the form of reference data. This is obtained from an Internet URL via a wired or wireless connection.
  • the reference data contains bit true data which is being presently transmitted by the satellites in the GPS constellation.
  • the receiver acquires a signal from a particular satellite and begins decoding and sampling the signal.
  • the receiver generates an estimate of the present GPS TOW of the particular signal being received. This may be obtained from an on-board receiver clock or it might be obtained from the reference data signal of step 101.
  • step 104 an estimate is generated of the present inaccuracy in the time estimate of step 103. This will depend upon the source of the reference signal, how old the data is within the reference signal and whether or not the estimate is based upon a clock within the receiver.
  • Figure 2 gives an example of the data format and the relative timing. In the upper part of Figure 2 the reference data are illustrated. The vertical axis of
  • Figure 2 can be thought of as a time axis with a downward direction indicating an increase in time.
  • the reference data represents data relating to a TOW in the past (an "old GPS time") by a time period which may be anything from seconds to more than an hour.
  • the estimate of the present time from step 103 is illustrated in the arrow at the centre right of Figure 2, this therefore showing the difference between the time of the reference and the estimate of the present
  • the coarse time estimate will lead to a "guess" that the current transmitted data being received from the satellite is from page 8, subframe number 3 and 22 bits into word 9 of that subframe.
  • the coarse time uncertainty must not be large enough to create frame ambiguity, meaning it must be known to better than ⁇ 15 seconds.
  • the uncertainty in the time estimate is evaluated in this case (step 104) as around 4.5 seconds. Since it is known that the reference data repeats, the illustrated subframes shown with respect to frame (page) 8 comprise a predicted signal based upon the reference data at the "old" GPS time.
  • the receiver therefore selects a part (range) of the reference data signal for comparison with the data presently being received.
  • the receiver therefore predicts the signal that will be received by constructing this from the reference data (since the reference data signal repeats).
  • the received bits within the satellite signal must be compared to the bit true data in words 6-10 of subframe 3 and words 1-3 in subframe 4 of frame 8 (only the start of word 3 is shown in the example) of the predicted signal. This occurs at step
  • the GPS TOW of the reference data in Figure 2 is 300006 seconds, referencing to the first bit of subframe 2 in frame 2. If the current coarse time relating to the data presently being decoded is estimated at 304697.25 seconds it would mean that 4691.25 seconds has elapsed, that is 6 super frames, 6 frames, 1 subframe, 8 words and 22.5 bits into word 9.
  • step 107 the reference data are used to calculate the exact TOW of each bit being decoded. This is possible since the reference data contains the old TOW and the number of bits (that is, superframes, frames, subframes, words and bits) between that time and the presently decoded data is known, with each bit having a time length of exactly one millisecond. This is due to the CDMA transmission standard applied to GPS satellite signals.
  • the reference data used in the comparison simply has to meet the requirements of being identical to the signal received from the satellite and that each bit has an exactly known GPS TOW.
  • the signal which best meets these requirements is a satellite signal which has been transmitted previously at a precisely known time or which is one taken from an assistance server on the Internet (as in the present example).
  • an assistance server on the Internet as in the present example.
  • Such servers are known for providing GPS satellite data, these being essentially connected to ground stations which receive the satellite signals, the ground stations being at precise locations.
  • the time to obtain TOW synchronisation is a function of simply the time it takes to decode the data needed for performing the unique bit matching.
  • a further advantage of this method is that the TOW synching can be made on more words than just words 1 and 2, making it more robust if the signal is around the data decoding threshold. It can also be applied using sections of data (discontinuous) as long as the gap between them is known.
  • the method can be used to further advantage in conjunction with the known method of decoding the satellite signal until the TOW signal is received, since in some cases this will be received when or shortly after decoding begins.
  • the two methods may be used in parallel to minimise the time before synchronisation is achieved.
  • the length of the data to compare depends on the robustness wanted but about 30 bits is feasible. To decrease the amount of comparisons needed and increase the robustness one can also demand that the decoded data has to pass the parity on a word basis before comparing it with the assumed bit true data.

Abstract

La présente invention concerne la synchronisation d'un récepteur avec un signal émis par un émetteur éloigné se trouvant à bord d'un satellite. Le signal comprend l'émission répétée de données temporelles définissant l'heure d'émission et de données d'orbite se rapportant à l'orbite du satellite. Le procédé consiste à faire recevoir par le récepteur le signal émis, à extraire des données de référence dont une partie au moins correspond aux données se trouvant à l'intérieur du signal émis, et à évaluer une heure de réception se rapportant au signal émis. Pour sélectionner un signal prédit, on se base sur les données de référence et l'heure de réception estimée, et on en fait la comparaison avec le signal reçu. On se base alors sur les données de comparaison et les données temporelles pour déterminer la synchronisation de l'heure du signal reçu.
PCT/EP2009/054960 2008-04-24 2009-04-24 Procédé de synchronisation entre récepteur et satellite WO2009130304A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0807524.4 2008-04-24
GB0807524A GB2459333A (en) 2008-04-24 2008-04-24 Method of synchronizing a receiver with a signal transmitted by a remote transmitter on-board a satellite

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WO2009130304A1 true WO2009130304A1 (fr) 2009-10-29

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PCT/EP2009/054960 WO2009130304A1 (fr) 2008-04-24 2009-04-24 Procédé de synchronisation entre récepteur et satellite

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
RU2444027C2 (ru) * 2010-03-12 2012-02-27 Общество с ограниченной ответственностью "Спирит Корп" Приемник спутниковых навигационных сигналов с блоком быстрого и высокочувствительного поиска

Citations (5)

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US5812087A (en) * 1997-02-03 1998-09-22 Snaptrack, Inc. Method and apparatus for satellite positioning system based time measurement
US6346911B1 (en) * 2000-03-30 2002-02-12 Motorola, Inc. Method and apparatus for determining time in a GPS receiver
US6788249B1 (en) * 2003-07-23 2004-09-07 Snaptrack Incorporated System for setting coarse GPS time in a mobile station within an asynchronous wireless network
US20050174284A1 (en) * 2004-02-06 2005-08-11 Charles Abraham Method and apparatus for processing satellite positioning system signals to obtain time information
US20070063894A1 (en) * 2005-09-19 2007-03-22 Yiming Yu GPS receiver using stored navigation data bits for a fast determination of GPS clock time

Family Cites Families (2)

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US5798732A (en) * 1996-09-19 1998-08-25 Trimble Navigation Limited GPS receiver having a fast time to first fix
US6459407B1 (en) * 2001-09-10 2002-10-01 Nokia Mobile Phones Cross-correlation system for time recovery in network-assisted GPS positioning

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5812087A (en) * 1997-02-03 1998-09-22 Snaptrack, Inc. Method and apparatus for satellite positioning system based time measurement
US6346911B1 (en) * 2000-03-30 2002-02-12 Motorola, Inc. Method and apparatus for determining time in a GPS receiver
US6788249B1 (en) * 2003-07-23 2004-09-07 Snaptrack Incorporated System for setting coarse GPS time in a mobile station within an asynchronous wireless network
US20050174284A1 (en) * 2004-02-06 2005-08-11 Charles Abraham Method and apparatus for processing satellite positioning system signals to obtain time information
US20070063894A1 (en) * 2005-09-19 2007-03-22 Yiming Yu GPS receiver using stored navigation data bits for a fast determination of GPS clock time

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
RU2444027C2 (ru) * 2010-03-12 2012-02-27 Общество с ограниченной ответственностью "Спирит Корп" Приемник спутниковых навигационных сигналов с блоком быстрого и высокочувствительного поиска

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GB0807524D0 (en) 2008-06-04
GB2459333A (en) 2009-10-28

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