WO2009130304A1

WO2009130304A1 - Method of Receiver and Satellite Synchronisation

Info

Publication number: WO2009130304A1
Application number: PCT/EP2009/054960
Authority: WO
Inventors: Ty Lewis; Fredrik Lindstrom
Original assignee: Nordnav Technologies Ab
Priority date: 2008-04-24
Filing date: 2009-04-24
Publication date: 2009-10-29
Also published as: GB0807524D0; GB2459333A

Abstract

A method is provided of synchronising a receiver with a signal transmitted by a remote transmitter on-board a satellite. The signal comprises the repeated transmission of time data defining the transmission time and orbit data relating to the orbit of the satellite. The method comprises receiving the transmitted signal at the receiver; obtaining reference data at least a part of which corresponds to the data within the transmitted signal; and estimating a receipt time relating to the transmitted signal. A predicted signal is selected based upon the reference data and the estimated receipt time, this being compared with the received signal. The time synchronisation of the received signal is then determined based upon the comparison and the time data.

Description

Method of Receiver and Satellite Synchronisation

Field of the Invention The present invention relates to a method of synchronising a receiver with a signal transmitted by a remote transmitter on-board a satellite.

Background to the Invention

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.

The increasing use of GNSS places ever increasing demands upon the speed of calculation of an accurate position solution for the user and the poor signal reception conditions under which a position solution may be obtained.

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.

This delay is limiting upon the speed with which synchronisation can be achieved. It is desirable to reduce this time to obtain synchronisation with a satellite signal. Summary of the Invention

In accordance with a first aspect of the invention we provide 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 applicants have realised that synchronisation can be achieved significantly more rapidly than known methods by comparing the data received in the transmitted signal with reference data received at a known time. This method actually allows the synchronisation to be achieved without receiving the time signal itself from the satellite which has not been possible with prior methods. The method is also suitable for low signal conditions since the data comparison formed does not require continuous data and therefore is not reliant upon a particular section of the transmitted signal being received.

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. As will be appreciated, in general 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. As will be appreciated, in GNSS systems 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. When a number of additional transmitters are provided on-board further satellites 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. Typically 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. Preferably 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. Typically such 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.

It is also important in the method to provide a reasonably accurate estimate of the time data of the received signal. This might be assumed to be approximately the same as the time of transmission of the signal by the satellite (the difference likely to be about 70 milliseconds). A reasonable estimate of the time of week at the receiver may therefore be provided using the assistance server. It may be possible to include estimated delays imposed by the routing of the data over the Internet. Alternatively, or additionally, 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. Clearly there are a minimum number of comparisons that are required from a statistical perspective (such are more than 25 bits, and preferably around 30 bits) to ensure that the received signal is indeed the expected part of the repeating reference data.

A number of different data transmission protocols are envisaged by the present invention. It is preferred however that 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. 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. However, the invention may equally be used in other satellite systems such as Galileo, Compass, Quasi Zenith Satellite System (QZSS), and so on.

It will be appreciated that the invention finds further advantage when used with a method of synchronising a receiver which waits for the time data to be transmitted. Thus in accordance with a second aspect of the invention we provide 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 performing each of the following methods:- i) performing a method according to the first aspect of the present invention; ii) waiting for the time data signal to be received by the receiver; and, establishing synchronisation using which of the methods (i) or (ii) allows the synchronisation to be achieved first. It will be appreciated that 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.

Brief Description of the Drawings

An example of a method according to the invention will now be described with reference to the accompanying drawings, in which :-

Figure 1 is a flow diagram of the example method; Figure 2 shows the content of the reference signal and predicted signal; and, Figure 3 illustrates the bit comparison of the method.

Description of Examples

We now describe an example implementation of the invention in which a global positioning system (GPS) receiver, such as a handheld or vehicle mounted receiver acquires synchronisation with a signal from a remote transmitter onboard a satellite.

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) is used for this purpose. In this GPS example 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. There are two primary components of the remainder of the data within each frame, these being ephemeris data (which is high accuracy orbital information of the particular satellite) and almanac data (which is part of a larger sequence of data giving the orbits of each of the satellites in the GPS constellation). 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").

In order to determine the position of a receiver on Earth, it is essential to know the time at the receiver and the time of transmission of signals from a number of satellites. The difference between the time of transmission and the time of receipt allows the calculation of the distance to the satellite in each case. This time difference requires a synchronisation to be performed by the receiver. This involves the receiver determining which part of a frame is being decoded at any particular time.

The normal method for establishing synchronisation between a GPS receiver and a satellite transmission, known as a "Time of Week Sync" 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. This means that a normal time of week synch for the worst case (under good signal conditions) 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. It should be noted that the known method of establishing a receiver position involves performing this method for each of the 4 satellites that are required for positioning.

The present applicants have realised that 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. This might originate from 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.

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. By performing a bit comparison in the time range, between the data received from the satellite in the transmitted signal and the reference data, it is possible to establish the exact position within the transmitted signal that it presently being received and decoded. Thus synchronisation is achieved. Note that 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.

The method is now described in further detail in association with Figures 1 to 3. Figure 1 is a flow diagram setting out the main steps of the method.

At step 100 in Figure 1 , the receiver is initialised, involving a power up from a dormant state. At step 101 , 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.

At step 102 the receiver acquires a signal from a particular satellite and begins decoding and sampling the signal.

Then, at step 103, 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.

At 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. Thus 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

TOW.

As shown in Figure 2, 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. In principle 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.

At step 105 of Figure 1 , 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). Thus, with the predicted inaccuracy of the GPS time based upon the old GPS time, 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

106. A more accurate GPS time would give a smaller range.

As a specific example, let it be assumed that 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.

If then a unique match between the satellite signal bits and the bits of the reference data can be made, then the exact TOW can be calculated for the received data from the satellite. So if it then turns out that a match of the start of the decoded data match the start of word 8 of subframe 3 as shown in Figure 3, then the time of week of the start of the decoded data would be:

300006 + 750 ^* 6 (elapsed super frames) + 30 ^* 6 (elapsed frames) + 6 ^* 1 (elapsed subframes) + 7 ^* 0.6 (elapsed words) + 0^*0.02 (elapsed bits into words) giving a GPS time of week of the first compared decoded bit to 304696.2 seconds. This is performed at step 107 where 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 synchronisation between the received data and its time of transmission is therefore finally resolved at step 108.

In this method it should be noted that 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. Thus 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). Note that 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.

Using this scheme, 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. Thus 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.

Claims

1. 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.

2. A method according to claim 1 , wherein the transmitted signal comprises data transmitted at a predetermined transmission rate.

3. A method according to claim 1 or claim 2, wherein the time data comprises Time Of Week data.

4. A method according to any of the preceding claims, wherein the orbit data comprises position data from which the position of the transmitter at a given time may be determined.

5. A method according to claim 4, wherein the position data comprises ephemehs data defining the orbit of the satellite.

6. A method according to claim 5, wherein a number of additional transmitters are provided on-board further satellites and wherein the orbit data comprises almanac data defining the position of the transmitter and each of the additional transmitters as a function of time.

7. A method according to any of the preceding claims, wherein the reference data comprises bit true time data and/or orbit data transmitted previously by the transmitter or, when a number of additional transmitters are provided, by one of the additional transmitters.

8. A method according to any of claims 1 to 7, wherein the additional data comprises time data and/or orbit data provided by an assistance server.

9. A method according to claim 8, wherein the assistance server provides an estimate of the receipt time to the receiver.

10. A method according to any of the preceding claims, wherein the receiver comprises an on-board clock for use in estimating the receipt time.

11. A method according to any of the preceding claims, wherein the transmitted signal comprises time divisions as superframes, each superframe comprising a number of frames and each frame comprising a number of subframes.

12. A method according to claim 8, wherein each subframe comprises time data as Time Of Week data acting as a time stamp.

13. A method according to claim 9, wherein the estimate of the receipt time is sufficient to give a time of transmission which is accurate to the nearest frame.

14. A method according to any of the preceding claims, further comprising estimating the accuracy of the receipt time and using the accuracy estimate in selecting the predicted signal.

15. A method according to any of the preceding claims, wherein the predicted signal is selected based upon the reference data.

16. A method according to any of the preceding claims, wherein the comparing step comprises a data bit comparison between the transmitted data and the predicted data.

17. A method according to claim 16, wherein the number of bits compared is 25 bits or more.

18. A method according to any of the preceding claims, wherein the transmitted signal is provided as a CDMA signal having a chip rate of 1.023 MHz wherein a

1023 bit code repeats every 1 millisecond.

19. A method according to any of the preceding claims, wherein the method is performed using global positioning system (GPS) transmitters and a global positioning system receiver.

20. 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 performing each of the following methods:- i) performing a method according to any of the preceding claims; ii) establishing synchronisation by waiting for the time data signal to be received by the receiver; and, establishing synchronisation using which of the methods (i) or (ii) allows the synchronisation to be achieved first.

21. A computer program product comprising computer program code for performing some or all of the method steps of any of the preceding claims when such code is executed upon a computer.

22. A satellite receiver adapted to receive a signal from remote transmitters onboard a satellite and further adapted to perform a method according to any of the claims 1 to 20 when in use.