Abstract:
A method and system for knowing positioning system standard time at a mobile unit with respect to a system, such as the Global Positioning System, when normal, direct measurement may be impracticable owing to low signal-to noise ratio, by calibrating the timing signal of an available communication network, such as a cellular telephone transmission network ( 610, 615 ). A time reference is set for the communication network with respect to the positioning system at a time when an adequate signal-to-noise ratio prevails and the offset of a timing event in the communication network control signal measured by means of the mobile unit&#39;s internal clock may be determined ( 620, 625 ). Subsequent times are measured with respect to this time reference by using the internal clock to measure time intervals therefrom.

Description:
This application claims benefit of 60/193,235, filed Mar. 30, 2000. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of positioning systems and, more particularly, to providing time synchronization to Earth satellite navigation system receivers. The invention is described in terms of the Global Positioning System (GPS) but is applicable to similar systems, such as Glonass. 
     The present invention makes use of a terrestrial, communications network, such as a cellular communications network. In particular, the use of the Global System for Mobile (GSM) network is instanced. 
     The operation of the present invention sill be better understood with reference to  FIGS. 3 and 4 , that respectively describe features of the GPS navigation data message and of the GSM signal control channel, and to  FIG. 1 , that shows the elements of the disclosed invention. 
     GPS Fundamentals 
     A GPS locator determines its position by measuring the transit time of signals transmitted simultaneously from a plurality of satellite-borne beacons in the Navstar satellite constellation, inferring therefrom the ranges from the respective beacons, and, from the known locations of the respective beacons at time of transmission, triangulating the ranges to fix a locator position. Normally, the beacons are satellite-borne, but the system may also include fixed, ground-based beacons at known locations, called “pseudolites”, that emulate the signal structure of the satellites to compensate for limited or no satellite visibility. References to satellites or beacons in the following may be taken also to include pseudolites. 
     The Navstar constellation consists of 24 satellites orbiting the earth every 12 hours. The positions of all satellites are known at all times. This constellation provides any user with 5–8 visible satellites at any time. Each satellite transmits a continuous, unique pseudo-random noise (PN) sequence, or C/A (coarse acquisition) code, at a chip rate of 1023 kHz and a repetition period of 1023 chips (=1 millisecond). The 1023 chips of the C/A code constitute a frame. The several PN sequences from the satellites are mutually orthogonal and have good correlation properties 
     A 50 Hz navigation or satellite data message (SDM) is superimposed on the PN sequence of each satellite. This data message includes satellite position and velocity data (Almanac—approximate orbital data parameters for all satellites—and Ephemeris—frequently updated parameters describing short sections of the satellite orbits), clock timing data, and transmit time of week (TOW) data, according to GPS system or standard time. The data is sufficient to enable a GPS receiver to compute the respective positions and velocities of the several satellites at any time. 
     The SDM consists of a sequence of data frames,  300  in  FIG. 3 , each having five sub-frames  310 . Each sub-frame  310  is, in turn, subdivided into ten words  320 , each word containing thirty bits  330 . The first two words of each sub-frame  310  are the telemetry word (TLM)  340  and the hand-over word (HOW)  350 . 
     A GPS receiver&#39;s processor locks onto a particular visible satellite&#39;s signal by correlation with a locally stored copy of that satellite&#39;s PN sequence. After lock-on has occurred, the processor demodulates the GPS signal to decode the positioning and time data (ephemeris and TOW) from the GPS carrier signal. 
     The correlation peaks obtained during continued tracking of the satellites provide times of arrival of the PN sequence frames, as measured by a local clock in the GPS receiver. The differences between an arbitrary reference time and measured times of arrival from each satellite, multiplied by the speed of light, are pseudo-ranges ρ from those satellites to the GPS receiver. Typically, the reference time is the (common) time at which the satellites commenced transmission of their respective PN sequences, as measured by the GPS receiver clock. This clock usually differs from the GPS system clock by an unknown time offset, T 0 . The pseudo-range ρ is related to the true range R of the respective satellite by ρ=R+c b , where the range offset, c b =cT 0 , c being the speed of light. 
     The pseudo-ranges are computed from the correlator output from at least four satellites. Using these computed pseudo-ranges and the known satellite positions at transmit time, a position is fixed by triangulation. Pseudo-ranges to at least four satellites are needed to solve at least four simultaneous equations of the form:
 
| s−r|=ρ−c   b 
 
where
         s is the position vector of a satellite and   r=(x,y,z) is the position vector of the GPS receiver, for the three unknown Cartesian coordinates x, y, z, and for c b . The satellites are sufficiently far from the GPS receiver that these equations can be linearized in x, y, and z with no loss of accuracy.       

     Thus a standard GPS locator calculates position by solving the equation:
 
| s   j ( t )− r ( t )|= R   j ( t )=ρ j ( t )−cT b 
 
where:
         s j (t) and r(t) are the respective position vectors of the j th  satellite  150  (see  FIG. 1 ) of the GPS constellation and mobile unit  140  at transmission time t;   R j (t) is the absolute distance of mobile unit  140  from the j th  satellite  150  at transmission time t;   ρ j (t) is the pseudo-range measured to the j th  satellite  150  at transmission time t;   c is the velocity of light; and   T b  is the clock bias of local time (mobile clock time) with respect to absolute (GPS system) time, hereafter referred to as standard time.       

     ρ j (t) is measured by correlating the incoming GPS signal with a locally stored copy of the signal transmitted from the j th  satellite  150 , thereby deducing the signal transit time from the j th  satellite  150 , and converting that transit time to distance. 
     s j (t) is calculated from the demodulated position and timing data superimposed on the PN signal. 
     A comprehensive account of the GPS system may be found in  Understanding GPS: Principles and Applications , Elliott D. Kaplan, ed., Artech House Publications, 1996, which is incorporated by reference for all purposes as if filly set forth herein. 
     GSM Data Structure 
     GSM operates on a plurality of carrier frequencies within a reserved frequency band. These frequencies, or a sub-set thereof, are allocated to a base station  110  in each cell of the network. Each carrier frequency is partitioned in time, using a TDMA (Time-Division Multiple-Access) scheme which is illustrated in  FIG. 4 , into slots or burst periods  440  of 15/26 ms (≈0.577 ms) each containing 156.25 bits. Eight slots, or burst periods, are grouped into a TDMA frame  430  ( 120/26 ms, ≈4.615 ms). Frames  430  are grouped to form multi-frames  420 . The number of TDMA frames per multi-frame depends upon Whether the channels are used as traffic channels (26 frames), which are transiently assigned to individual mobile units to carry speech and data, or as network control channels (51 frames). The latter is illustrated in  FIG. 4 . Super-frames  410  consist of 51 multi-frames  420  (for traffic), or 26 (for control). Hyper-frames  400  contain 2048 super-frames  410  and span 3.4816 hr. A logical channel corresponds to a specific time slot  440  within successive TDMA frames  430 . 
     In the case of signal control channels, lime-slot zero of each TDMA frame forms a logical channel, as described above, and is utilized, inter alia, as a frequency-correction and a synchronization channel (SCH)  450 . This serves to synchronize a GSM mobile unit to the time-slot structure of a network cell by defining the boundaries of burst periods and time-slot numbering. Synchronization channel  450 , in distinction to traffic channels, is one of a group of common channels accessible to all mobile units. Common channels are defined within a 51-frame multi-frame  420 . The present invention, as disclosed, exploits the SCH but other embodiments might utilize other features of the GSM signal structure to the same end. 
     GSM employs four types of burst, distinguished by internal structure and, in one case, length. The S burst, used on the synchronization channel, consists of 156.25 bits transmitted in a slot length of 0.577 ms. 
     Because the GSM signal has a known and precisely regulated structure, it is possible to know the time interval between two identified events in that signal. 
     GPS Operation 
     The standard GPS process outlined above has severe drawbacks in urban areas, where signal blockage may result in a signal-to-noise ratio too low to allow demodulation of the data message bits (20 ms bit duration) in order to decode the satellite navigation data message and therefrom compute a location, even though the receiver can acquire and track the signals. In this event, the PR may still be measured with sufficient accuracy by operating coherently on longer signal strings (˜100–1000 ms), thus achieving an enhanced signal-to-noise ratio, as taught in co-pending U.S. patent application Ser. No. 09/585,619. 
     Krasner, in U.S. Pat. No. 5,945,944, Method and apparatus for determining time for GPS receivers, discloses a method for providing GPS standard time to a wireless GPS locator operating within the coverage area of a cell-based communication network The GPS receiver includes an auxiliary receiver suitable for the cell-based network and acquires GPS standard time by communicating measurements of pseudo-ranges and received GPS time indicators to a GPS base station where a position is calculated and transmitted back to the mobile unit, all via the cell-based network. But this method also suffers a drawback in that it requires the addition of a GPS base station connected to the cell-based network. 
     There is thus a need for, and it would be highly advantageous to have, an alternative method and system of providing GPS standard time to a wireless GPS receiver. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method for calibrating a network time of a communications network that has at least one cell and that transmits a control channel signal including a plurality of known bit sequences according to the network time from at least one base transceiver station into at least one cell of the communications network, to provide an absolute time reference for a local clock in a mobile unit located in one of the at least one cell including the steps of: (a) providing: (i) a source of standard time that transmits at least one synchronization signal; and (ii) a transceiver operative to communicate with the communications network and with the mobile unit; (b) synchronizing the local clock with the standard time; (c) synchronizing the network time of the communications network with the local clock; (d) calculating a time offset, according to the local clock, of the network time of the communications network from standard time; and (e) calculating an absolute time reference as a sum of the time offset and the standard time. 
     According to the present invention, the method includes the further step of providing at least one location server communicating with at least one base transceiver station for storing the absolute time reference. 
     According to the present invention, the method includes the further step of: (h) providing the absolute time reference to the location server. 
     According to the present invention, providing the absolute time reference to the location server is effected by steps including: (i) transmitting the absolute time reference by the associated transceiver to the base transceiver station; and (ii) communicating the absolute time reference by the base transceiver station to the location server. 
     According to the present invention, the source of the standard time includes an Earth satellite navigation system. 
     According to the present invention, synchronizing the local clock with the standard time is effected by steps including: (i) receiving at least one synchronization signal by the mobile unit; and (ii) noting a time according to the local clock when a known bit sequence of the synchronization signal is received. 
     According to the present invention, synchronizing communications network time is effected by steps including: (i) receiving the control channel signal of the communications network from at least one base transceiver station by the transceiver; and (ii) noting a time according to the local clock when a known bit sequence is received. 
     According to the present invention, the synchronizing of communications network time includes the further step of: (iii) updating the absolute time reference to provide an updated time reference to the mobile unit. 
     According to the present invention, updating the absolute time reference is effected by steps including: (A) receiving the control channel signal of the communications network from at least one base transceiver station by the transceiver; (B) demodulating the frame number of a subsequent known bit sequence in the control channel signal; (C) noting the time, according to the local clock, when the subsequent known bit sequence is received; (D) calculating from known network signal characteristics the time difference, according to the communications network, between the first known bit sequence and the subsequent known bit sequence; and (E) correcting the local clock time according to the time difference. 
     According to the present invention, updating the absolute time reference is performed at least twice per day. 
     According to the present invention, the time offset is the time period, according to the local clock, between the reception of a known bit sequence within the synchronization signal and reception of a known bit sequence within the control channel signal. 
     According to the present invention the method includes the further steps of: (f) determining, according to the local clock, the time offset of a subsequent event from the most recent time reference; and (g) adding the time offset thereto. 
     According to the present invention, the most recent time reference is received from the location server via the base transceiver station serving the cell wherein the mobile unit is located. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  depicts the system elements; 
         FIG. 2  is a block diagram representation of the wireless locator mobile unit; 
         FIG. 3  is a chart describing the GPS navigation message structure; 
         FIG. 4  is a chart describing, by a way of example, the frame stricture of the GSM control channel including synchronization bursts; 
         FIG. 5  is a diagram showing timing information related to time synchronization and time update; and 
         FIG. 6  is a flow chart showing two modes of operation of the process described in the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a device and method for calibrating network time of a terrestrial wireless communications network against standard time of an Earth-satellite navigation system and subsequently using calibrated network time in place of directly obtained standard time in circumstances when the standard time is difficult to get. The invention is described in terms of the Global Positioning System (GPS) but is applicable to similar systems, such as Glonass. The calibrated network time is obtained and updated in a mobile GPS unit through signals transmitted by the wireless communications network (for example, a cellular network) which covers the operation area of the GPS device. 
     GPS standard timing information from an external source may be essential for enabling a location fix in urban areas where demodulation of the 50 Hertz SDM information may otherwise not be possible because of blockage of signals by urban structures. 
     Fundamentals of the Present Invention 
     The principles and operation of a GPS wireless locator or Mobile Unit (MU), according to the present invention, may be better understood with reference to the drawings and the accompanying description. 
       FIG. 1  illustrates a wireless locator mobile unit (MU)  140 , capable of receiving both the signals  130  transmitted by GPS satellites  150  (only two satellites are shown, distinguished by subscripts j and j′), and the control channel  120  transmitted by a Base Transceiver Station (BTS)  110  of a cell-based network within a cell  100  thereof wherein unit  140  operates. Optionally, a location server  160  may communicate with mobile unit  140  via the communications network. Location server  160  may include reference data passed thereto and may also calculate locations for MU  140  and for other MUs in cell  100 , if required. 
     The method disclosed in the present invention relates to a situation where the position, s j (t), of satellite  150   j  at transmission time, t, cannot be generated in the GPS device because of low signal-to-noise ratio that prevents extraction of the needed timing data in that signal. 
     To overcome this difficulty, mobile unit  140  uses communication control signals  120  transmitted by a communication network having a coverage area wherein mobile unit  140  is located. For illustrative purposes, the invention is described in relation to a GSM telephony network, although any suitable communications network may be used. 
     Wireless locator  140  (depicted in block-diagram form in  FIG. 2 ) includes:
         A GPS engine  200  including: an antenna  250  capable of receiving signals  130  from GPS satellites  150 , a GPS receiver  210 , a processor  220  for performing correlation and location calculations, and a local clock  230 ; and   A communications transceiver  240 , which may be part of a standard cellular transceiver, connected to a communication antenna  260 , able to receive a control channel of communication network signal  120 , and to communicate with processor  220 , the output of transceiver  240  being a digital signal which is further processed by GPS engine processor  220 .
           Optionally, transceiver  240  also transmits timing information from processor  220  to location server  160  via BTS  110  and receives timing information therefrom.   
               

     According to the present invention, wireless locator  140  operates in three modes, a synchronization mode, a time update mode, and a roaming mode. The relationship between time synchronization and time update is shown in  FIG. 5 , and the sequence of operations in the first two modes is depicted in the flow chart of  FIG. 6 . 
     Synchronization 
     Operation in this mode sets an absolute reference time (ATR) for Wireless locator  140 . In a synchronization event, wireless locator  140  ascertains GPS standard time by locking onto at least one sufficiently strong GPS satellite signal  130  that is in view during idle time, by using correlation techniques (block  600  in  FIG. 6 ). 
     As described in the background, signal  130  includes information about GPS standard time and identifiable markers, such as the start of a PN sequence, that enable signal  130  to be used to provide standard time to a suitably equipped receiver. Signal  130  will be referred to as a synchronization signal. 
     This procedure is standard in GPS devices. Once lock-on is achieved, the TOW data is demodulated and the GPS standard time, T GPS , of the GPS epoch (1 ms GPS frame) is determined (block  605  and see also  FIG. 5 ). According to local clock  230  this time is T. 
     Concurrently, communication receiver  240  detects a known event in the communication control channel of the local cell (block  610 ); for the purposes of discussion here, a synchronization burst (SB)  510  is assumed, although other suitable events might be used. After detection, processor  220  demodulates the frame number of this burst (block  615 ). According to local clock  230 , the time difference between the GPS epoch (T) and the leading edge of SB  510  (T BTS  by local clock  230 ) is (block  620 ):
 
Δ T=T   BTS   −T. 
 
     The Absolute Time Reference (ATR) of the leading edge of SB  510  according to GPS standard time is (block  625 ):
 
 ATR=T   GPS   +ΔT 
 
     ATR time tags generated from time to time by mobile units  140  within cell  100  are optionally transmitted via cellular network  100  to location server  160  and stored therein as absolute cell-timing data (block  680 ) for local cell  100 , and are available, on request (block  685 ), to other mobile units. This helps other mobile units that have missed a synchronization opportunity or that are roaming into cell  100 . 
     In a cellular network where base stations are not synchronized (as in GSM and some other networks), the synchronization of the present invention is valid only within the cell where synchronization was performed. In networks, like CDMA, wherein base stations are synchronized, a time tag is valid for the whole network. 
     Time Update 
     In time update mode, previously set absolute time ATR is updated by communication transceiver  240  locking on to a succeeding incoming communication channel timing burst (block  640 ), herein referred to as an update burst (UB)  520 , and demodulating a signal frame number thereof (block  645 ), thereby deducing the time difference ΔT BTS  between SB  510  and UB  520  (block  650 ) according to cellular network time, which is more accurate than local clock  230  time. ΔT BTS  is added to ATR, thereby establishing an Updated Time Reference (UTR) (block  655 ). As before, the update information is optionally provided (block  680 ) to location server  160 . UTR may be extrapolated, using local clock  230  time, for any future time event required by GPS engine  200 . 
     For example, if GPS engine  200  measures a pseudo-range (PR) to a visible satellite, the standard time, T PR , of a received signal event, such as a PN frame start, can be calculated as:
 
 T   PR   =UTR+δT 
 
where δT is the time lapse from UTR to T PR , as measured by local clock  230 .
 
     Since in most networks, the cellular network BTS network clock is stable (1 sec in 2×10 8  sec for GSM), a synchronization event taken only twice a day will suffice to maintain adequate accuracy. As an example, for a GSM network five hours after synchronization, the accumulated time offset is:
 
Δ UTR ≈5×3600×2×10 −8 sec=0.36 ms
 
     Thus, the UTR drifts less than 1 ms after five hours. Since the required accuracy for the PR for a navigation solution is ≦10 ms, such an accuracy in the absolute time of reception at the GPS locator is higher than required for precise location. 
     Roaming 
     When roaming from one network cell to a second cell, the time difference, according to clock  230 , between communication bursts transmitted by the respective cell base transceiver stations  110  (the BTS of one cell and the BTS of a second cell) is measured by clock  230 . This observed time difference (OTD) is added to the first cell ATR, consequently establishing a new ATR that is valid in the second cell. This process is identical to the previously described time-update process, whereby OTD replaces UTR. 
     If optional location server  160  has been provided, then a roaming mobile unit  140  can call for a current UTR of the network cell just entered. 
     In the present invention, unlike Krasner (U.S. Pat. No. 5,945,944), attaching GPS standard time tags to the communication signals is not required, nor is there any need for a GPS base station. Absolute time is generated and updated in wireless locator MU  140  by using known communication signal  120 . 
     Application to Event Timing 
     The GPS standard time of any event subsequent to any of the actions described above is obtained by adding a time lapse, according to local clock  230 , to the most recent of ATR and UTR. 
     While the present invention has been described with respect to a limited clamber of embodiments and with reference to the GPS and GSM systems, it will be appreciated that variations, modifications, and other applications of the invention may be made.