Title of Invention

A METHOD FOR SYNCHRONIZING TIME OF ARRIVAL MEASUREMENTS

Abstract

A method for synchronizing time of arrival measurements made with a wireless communication device to a global positioning system (GPS) time, receiving time of I arrival measurements made at a wireless communication system and signals from an external source synchronized to GPS time, the signals received from the external source including an indication from which the amount of delay encountered by a signal transmitted between the external source and the wireless communication device can be determined and adjusting the received time of arrival measurements by an amount equal to the delay encountered by a signal transmitted between the external source and the wireless communication device.

Full Text

BACKGROUND OF THE INVENTION L Field of the Invention; The present invention relates to communicaaons systems. More specifically, the present invention relates to systems and techniques for locating the position of a wireless communication device in a code di\Tsion muiriple access system. E. Description of the Related Art Deployment of location technologies in wireless networks is being driven by regulatory forces and carriers" desires to enhance revenues by differentiating the services offered by one carrier from the services offered by others. In addition, in June 1996, ttie Federal Communications Commission (FCC) mandated support for enhanced emergency 911 CE-911) service. Phase I of the Order requires that sector and cell information be set bade to the PSAP (Public Safety Answering Point) agency. Phase 11 of the Order requires that Ihe location of the cellular transceiver be sent back to the PSAP. To comply with the FCC mandate, 77,000 total sites are to be equipped with automatic location technologies by the year 2005. Many techniques axe being considered to provide automatic location capability. One technique being considered involves measuring the time difference of arrival of signals from, a number of cell sites. These signals are triangulated to extract location information. Unfortunately, this technique requires a high concentration of cell sites and/or an increase in the transmission power of the sites to be effective. This is due to the fact that in a typical CDMA system, each telephone transmits with only enough signal power to reach the closest cell site. As triangulation requires communication with at least three sites, tiie concentration of cell sites would have to be increased or the signal power of each wireless communication device would have to be increased. In any event, each alternative has significant drawbacks. An increase in the number of cell sites would be too cosdy. Irureases in signal power would add to the wei^t and cost of each wireless communication device and increase the likelihood of interference between wireless users. In addition, the network triangulation approach does not appear to meet the FCC mandate requirements. Another approach being considered involves the addition of GPS (Global Positioning System) hmctionality to the cellular telephone. Although this approach would add signi5cant cost and weight to the wireless communication device, require a line-of-sight to four satellites, and would be somewrtiat slow, nevertheless, it is the most accurate approach to support location services. To speed the process, a third apprt^dv sends aiding infoimation to the wireless communication device indicating where the wireless communication device should look in frequency for GPS carriers. Most GPS recavers use what is known as a GPS satellite almanac to minimize a seardi performed by the receiver in the frequency domain for a signal from a visible satellite. The almanac is a 15,000 bit block of coarse ephemeris and time model data fcr the entire constellation. The information in tfie almanac regarding the position of the satellite and the current time of day is approximate only. Without an almanac, tlie GPS recsiver must conduct the widest possibleTrequehcy seardt to acquire a satellite signaL Additional processing is required to.attain additional information that will aid in acquiring otiier satellites. T^e signal acquisition process can take several minutes due to the large number of frequency bins tinat need to be searched. Each frequency bin has a center frequency and predefined width. The availability of the almanac reduces ti\e uncertainty in satellite Doppler and therefore the number of bins that must be searched. The satellite almanac can be extracted from the GPS ruvigation message or sent on the down (forward) link as a data or signaling message to the receiver. Upon receipt of this information, the receiver performs GPS signal processing to determine its location. While this approach may be somewhat faster, it suffers from the requirement of a line-of-sl^t to at least four satellites. This may be probVematic in urban environments. Hence, a need remains in the art for a fast, accurate and inexpensive system or technique for locating a cellular. SUMMARY OF THE INVENTION The rttcd in the art is addressed by the ^rstem and method presently disQosed for determining the position of a wireless transceiver. In the most goieral sense, the inventive method is a hybrid approad\ for detgnnining position uang ranging information from a terrestrial system, timing information from a wireless communication device, and ranging information from GPS satellites. This information is combined to allow the position of a wireless communication device toj"rapidly and reliably determined. The disclosed method includes the steps of receiving at a wireless commimication device, a first signal transmitted from a first GPS satellite", a second signal transmitted from a second GPS satellite, and a third agnal form a third satellite. The wireless communication device k adapted to receive these GPS agnals and transmit a fotirth signal to the base station in response thereto. The base station receives the foiu^ signal, corrects for the dock bias imposed on the fourth signal by the roiind trip delay between the base station and the wireless communication device and uses the unbiased fourth signal to calculate the position of the wireless communication devire. In a specific implementation, the base station sends aiding information to die wireless communication device. The aiding information is used by the wireless communication device to quickly acquire the signals transmitted by the first, second and third satellites. The aiding signals are derived from information collected at the base station transceiver subsystem (BTS) serving the wireless communication device. Base Station Controller (BSC), or some other entity and includes; (1) satellite identification inionnation; (2) Doppier shift or related information; (3) values indicating the distance between the base station and each satellite; and (4) a search window size associated with each satellite, the search window size being calculated based on ihe round trip delay between ihe wireless communication device and fee base station and tiie elevation angle of each satellite. Upon acquisition by the wireless communication de\"ice of the agnals transmitted by the first, second and third satellites^ the wireks conummJcation device -aV-nlatP*; fee range pml, between fee wireless communication device and the fiist satellite, range ptrH between ti^ wireless communication device and the second satellite, and range pm3 between the wireless communication device and the third satellite. This range information is trangnttted bade to the base station along wife information as to the time at i^feid\ the measurement was made. In a CDMA implementation, the time it takes fee signal to propagate between the base station antenna and the wireless conununiratiQn device antenna is half fee round trip delay and is known by the base station. A measure of fee round trip delay between the wireless oimmunication device and the base station indicates the distance between the wireless communication device and fee base station. In addition^ this delay provide a means for correcting the wireless communication device absolute time. A device external to the wireless comir.unication device, such as tiie baae station controller or some ofeer entity associated wife the cellular infrastructure, utilizes information known to fee serving base station to calculate the position of fee wireless commtmication device. Sudi information may include the position of the first, second, and third satellites relative to the wireless communication device and the distance between fee wireless commtmication device and the base station. Determining the position of fee wireless communication device is achieved by finding: (1) an intersection of a first sphere of radius cpl around a first satellite, (2) a second sphere of radii cp2 around the second satellite, (3) a third sphere of radii cp3 around fee third satellite, and (4) a fourfe ^here of radius cpb around the base station, "c" is the speed of li^t, "pi" is fee pseudo-range associated wife fee first satellite and fee wireless communication device, "p2" is the pseudo-range a^odated wife fee second satellite arvd the wireless communication device, "p3" is fee pseudo- range associated with the third satellite and the wireless communication device, and "cpb" is the pseudo-range associated with the base station and the wireless communicatiOTi device. Note that if a line-of-sight (no multipadi) exists between the wireless cominunication device and the base station, then the prc^xjsed approach requires measurements from only two satellites and one base statical- In the case o; a communication system that is synchronized to GPS time, such as a d^AA cominunication system, the pseudorange measurement taken from, the signais transmitted by the base station will be used both to remove the bias from d« satellite pseudorange measurements and as an additicmal ranging measurement Additional infom^tion from anottier base station, if available, can be used to further reduce ^e number of satellites required to determine the position of the wireless communication device. Also in situations, where only two-dionensional locations are needed, only one satellite and one base station are needed. One key advantage of this approach over other known GPS approaches is the speed with which the wireless communication device can determine the pseudo-range. Since the serving base station transcaver, base station controller, or other entity coupled to the base station has its own GPS receiver, and also knows d\e pseudo-rai^es of all satellites being tracked with respect to the serving base station location, it is possible to determine a seardi window center and search window size for each satellite being tracked. The information is sent to the wireless communication device to increase the speed of the search process. That is, a clock onboard each GPS satellite controls the timing of the broadcast of the ranging signal by the satellite. Each such clock is syitchroruzed to GPS system time. The base station also contains a dock fttat is synchronized to GPS system time. The wireless communication.device synchronizes its clock to GPS time with a delay corresponding to the one-way delay between the base station and the wireless communication device. Timing information is embedded within the satellite ranging signal that enables the wireless communication device to calculate when the signal was transmitted from a specific satellite. By recording tiie time when the signal was received, the distaiKfi (range) from the satellite to the wireless commiinication device can be computed. As a resiilt, the locus of the location of the wireless communicatjon de\Tce is a sphere with center at the satellite location and radius equal to the calculated range. If a measurement is simiiltaneously made using the ranging of two other satellites, the wireless communication device would be somiew4«re on the surface of three ^heres. The ti^ee ^heres intersects in two points, however, only one of the points is &e correct wireles user positioa The candidate locations are mirror images of one another with respect to the plane containir-g the three satellites. In one embodiment of the disclosed method and apparatus, the GPS satellites for locating the position of the wireless communication device at a given point in time are identified by the base station. This information is forwarded to the wireless communication device to facilitate die search operation performed by the wireless commimication device. In addition to the above, when the wireless oammimication device is a Code Division Multiple Access (CDMA) receiver, the presently disclosed method and apparatus takes advantage of the fact that CDMA is a syndircmous sj^tem. Being synchronous, the time of arrival of a refErencc pilot ai the wireless conununication device can be used as a time reference. Accoitiingly, die Vkoreiess communication device can measure the time difference of arrival between the reference pilot, GPS signals, and other pilot signals. Accordingly, tiie problem of detennining the location of the wireless commimication device becomes a time difference of arrival (TDOA) problem, resulting in a furtJier reduction in the number of satellites required to determine the location of the wireless oDmmunication device. In one embodiment, the wireless communication device can have several modes of operation: (1) Hybrid mode using information from both the wireless system infrastructure and ihe GPS satellites; (2) Stand-alone (standard or conventional) GPS mode; (3) Aided stand-alone CPS mode; (4) Inverted differential GI*S mode; and (5) Aided and inverted differential GPS mode. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an illustrative implementation of a base statioT: and wireless communication device of a wireless (CE^IA.) communication system. Hg- 2a is a block diagram of an exemplary CDMA cellular telephone system-Fig. 2b is a simplified representation of a first, secOTvd, and tiiird base staticKv and a wireless communication device. Hg. 3 is an illustrative simplified representation of a base staticm constructed in accordance with the teaching of the present invention. Fig. 4 is a block diagram of the wireless communication device of the system, for determining die position of a wireless CDMA transceiver of the present invention. Fig. 5 is a block diagram of an illustrative implementation of a portion of the receiver, control signal interface, digital IF, and wireless demodulator drcuits of the wireless commimication device of the present invention. Hg. 6 is an illixstration of a functional model for determining the location of a wireless communication device. Fig. 7 shows the calculations of the search window size and center in the time domain. Fig. 8 is a diagram which illustrates correction of the local clock bias. Fig. 9 illustrates the relationship between the pilot FN sequences that are transmitted from three base stations. Fig. 10 illustrates &e relationship between the pilot FN sequences that are transmitted from three base stations. Fig. 11 shows a simplified block diagram of si,ich a remote synchronizing station located a known distance away from a plurality of base stations. Fig, 12 is an illustration of the meihod used to determine the amount of delay introduced between a base station control processor and center of radiation of the transmit antenna using a remote synchronizing station at a known distance or to which signals can be transmitted with a known transmission delay. Rg, 13 illustrates the method by which live TDOA foi base statimis is calculated in accordance with the synd\ionous GPS / forward link mode- DETAILED DESCRIFnON OF THE INVENTION Ohisjative embodiments will now be described with reference to the accoiiir>an\"ing drawings. Vvhiie the present method and apparatus is described herein with reference to illustrative embodiments for particular applications, it should be understXKxi Ihat tiie invention is not limited thereto. Those having ordinary skill in the art and access to tiie teachings provided herein will recognize additional modificatkms, applications, and embodin^ts within the scope of the prKent invention and additional fields in which the present invention would be of significant utility. Hg, 1 is a diagram showing an illustrative implementation of a wireless communication device 20 and an external signal source, such as a base station 10 or satellite 60, 70, 80,90, of a wireless code division multiple access (CDMA) communication system. The communication system is surrounded by buildings 40 and ground based obstacles 50- The base station 10 and wireless communication device 20 are disposed in a GPS. (Global Positioning System) environment having several GPS satellites, of whidi four are shown 60, 70, SO and 90. Such GPS environments are well known. See for example Hofmann-Wellenhof, B., et al., GPS Theory and Practice, Second Edition, New York, NY: Springer-Verlag Wien, 1993. Those of ordinary skill in the art will appreciate that the present teachings may be applied to other communication systems, such as advanced mobile phone system (AMPS), Global system for mobile commurucations (GSM), etc. without departing from, the scope of the present invention. In a typical GPS application, at least four satellites are required in order for a GPS receiver to detennine its position. In contrast, tiie presently disdosed method and apparatus is for determining the three-dimeisional position of a vtireless rom:nunication device 20 using only three GPS satellites, the round Crip deiav from the wireless communication device to an external signal source, sudi as rK serving base station 10, and the known locatksi of ^ serving base station i;. In nispt where there is a direct line-of-sigJ\t available, only two GPS satellites, round trip delay, and tiie known location of the serving base station 10 are required to locate a wireless communication device 20. "nus number can be reduced even further by using time difference of arrival information from the forward link of a CDMA cellular commimication system or from any other synchronous cellular commimication system. For the purpose of this disclosure, a cellular commimication system, is defined as a communication system in which multiple cells are used to allow a wireless communication device to receive signals from the communication system bora at least one of the piuraBty of cells as the wireless communication device moves about within thesystenu Fig. 2a is a block diagram of a CDMA cellular telephone system 30. The system 30 includes a mobile svritching center (MSC) 12 having a base station controller (BSC) 14. A public switched telephone network (PSTN) 16 routes f-allt; from telephone lines and other networks (not shown) to and from the MSC 12- The MSC 12 routes calls from the PSTN 16 to and from a source base station 10 associated with a first ceE 19 and a target base station 11 associated witii a second cell 21. In addition, the MSC 12 routes calls between the base stations 10, 11. The source base station 10 directs calls to the first wireless communication device 20 within the first cell 19 via a first communications path 28. The commimications path 28 is a two-way link having a forward link 31 and a reverse link 32. Typically, when the base station IC has established voice communications with the wireless communication device 20, the link 28 includes a traffic channel. Although each base station 10,11 is associated with only one cell, a base station controller often governs oj is associated with base stations in several cells. Whffi the wireless commuiucation device 20 moves from the first cell 19 to the second cell 21, tiw wireless comntimication device 20 begins communicating with the base station associated with the second cell. Ibis is commonhr referred to as a *"hand-off" to the target base station 11. In a "soft" hando^, the wireless commimication device 20 establishes a seccavi communics£ions link 34 wid\ the target base station 11 in addition to the first communicatxjns link 28 with the source base station 10. After the wireles communication device 20 crosses into ^e second cell 21 and the link with the second ceD has been established, the wireless comffiunication device may drc^ tiie first communications link 28. In a hard handoff, &\e operation of the source and tar^ base stations typically are different enough that the commimications link 34 between the source base station must be dropped before the link to the target base station can be established. For example, when a source base station is wifliin a CDMA system using a first frequency band and target base statiOTi is in a second CDMA system, using a second frequency band, the wireless commimication device will not be ah\e to maintain links to berth base stations concurientiy, since mcst wireless communication devices do T«>t have ttie ability to tune to two different freqtttsicy bands corKniirently (one transmission frequerwzy band and one receive frequency band). When ihe first wireless comm.unicaticm device 20 moves from d\e first cell 19 to the second cell 21, the link 28 to &e source base station 10 is dropped and a new link is formed witii the target base station 11. Fig. 2b is a simplified representation of a first, second, and durd base station 10a, lOb, 10c, and a wireless communication device 20. As shown in Hg. 2b, each base station comprises: a GPS transceiver/time unit 203; processing circuitry, such as a control processor 62; a GPS antenna 76, communication circuits 207; and communication antenrtas 201. It will be imderstood by those skilled in the art that the control processing circuitry may be a general purpose computer, a microprocessor, micro-computer, dedicated state machine. dedicated discrete hardware circuitry, application spedSc integrated drcuit (ASIC), or any other circuitry that allows the functions described as being performed by the control processor to be performed. Fig. 3 is more detailed represen^tion of a base station 10 constructed in accordance with the teachings of the presently disclosed method and apparatus and will now be discussed. In accordaiKE with the embodiment shown in Figs. 3 and D, the base station 10 is essentia L"y conventionaL In an alternative embodiment, the base station 10 includes additional functionality which allows the base station to determirtf the positjon or a wireless commimicadon device 20, as will become dear from the description provided below. The communication antennas 201 include a receive CDMA anterma 42 for receiving CDMA signals and a transmit Cn*4A anteima for transmitting CDMA signals. Signals received by ihe antenna 42 are routed to the commimication circuits 207. The comm.unication circuits 207 include: a commtinicatioris reaver 44, a rate detector 61, a switch 63, a vocoder 64, an digital to arialog (D/A) converter 65, a transmitter 69, a vocoder 68, and an analog to digital (A/D) convertor 66. The receiver 44 receives signals directly from the antenna 42. In practice, the recover 44 includes demodulator, de-interleavers, decoders and other circuits as will be appreciated by tiiose skilled in the art. The received signal is allocated to an appropriate channel for which a rate detector 60 is associated. The control processor 62 uses the rate of the detected sigr\ai to detect speech. If speech is detected in a received frame, the control processor 52 switdies ^e received frame to the vocoder 64 via a switch 63. The vocoder 61 decodes tiie variable rate enoDded signal and provides a digitized output signal in response ttiereto. The digitized de-vocoded signal is converted to speech by the D/A converter 65 and an output device such as a speaker (not shown). Input ^>eech from a microphone or other input device (not shown) is digitized by the A/D converter 66 and vocoded by the vocoder encoder 68. The vocoded speech is input to the transmitter 69. In practice, the transmitter 69 includes modulators, interleavers and encoders as will be appreciated by those skilled in the art. The output of the transmitter 69 is fed to the transmit antenna 43. As shown in fig3, the GPS transceiver/time unit 203 includes a recsiver 74, and a timing and frequency unit 72. The timing aiui frequency imit 72 accepts signals from the GPS engine of the GPS receiver 74 and uses the signals to generate timing and frequency references for the proper operation of die CDMA sv-stenv Accordingly, in many such CDMA systems, each cdl site is syndironized to GPS time (Le-, uses a GPS time base reference from whidi time critical CDMA transmissions (including pilot PN sequences, frames and Walsh funrtkjns) are derived). Such conventional timing and frequency units and GPS engines arc cianmon in CDMA systems and are well Imown in the art. ConventianaJ timing and frequency imits provide frequency pulses and timing information. In contrast, ihe timing and frequency imit 72 of the presentiy disclosed method and apparatus preferably also outputs the elevation angie, pseudo range, satellite identification (i.e., pseudo noise (PN) offset associated with each satellite) and information related to die Doppler shift associated with each satellite in order to assist the wireless communication device 20 in acqiiiiing the satellites (i.e., decrease the amount of time required to acquire a satdlite). This information is typically available within conventional timing and frequCTicy units, but is typically neither needed nor provided to external devices. The additional information provided by the timing and frequency "oriii 72 is preferably communicated to tiie BSC 14 in the same manner as is conventionally done witii regard to frequency and timing information in a conventional base station. Fig. 4 is a block diagram of Ihe wireless communication device 7Q in acojrdaiKe with one embodiment of the disclosed method and apparatus. The wireless communication device 20 preferably includes a bi-directional antenna 92 adapted to receive CDMA transmissions well as GPS signals. In an alternative embodiment of the disclosed metiiod and apparatus, separate antennas may be used for receiving and transmitting GPS signals, CDMA signals, and other agnals, such as alternative system signals. The antenna 92 preferably feeds a duplexer 94. The duplexer 94 preferably feeds a receiver 100 and is preferably fed by a transmitter 200. A time frequency subsystem 102 provides analog and digital reference signals for the receiver 100, a control signal interface 300, and the transmitter 200, as will be appreciated by those skilled in the art. CDMA power control is provided by a gain control circuit 104. In one embodiment of the disclosed method and apparatus, the control signal interface 300 is a digital signal processor (DSP). Altaiatively, the control signal inErfare may be anotiier circuit capable of performing gain control functions The control signal interface 300 provides control signals for the wireless cDinmunicatk)n device 20. The receiver 100 provides for radio frequenc} ■."RF) down conversion and a first stage of intermediate frequency (IF) down corr^-ersion, A digital IF application specific integrated circuit (ASIQ 400 provides tor a second stage of IF to baseband down conversion, samplir^ and analog to digital (A/D) conversion. A mobile demodulator ASIC 500 seardies and correlates digital baseband data from the digital IF ASIC 400 to ascertain pseudo-ranges as discussed more fully below. The pseudo-ranges, along with any voice or data, is passed by the mobile demodulator 500 to the digital IF modulator 400. The digital IF modulator 400 provides a first stage IF up conversion of the data received from the mobile demodulator 500. A second stage of IF up conversion and RF up conversion of these signals is provided by the transmitter circuit 200. These signals are then transmitted to the base station 10 and processed in accordance with the method of the invention discussed below. It should be rusted that location information to be communicated between the wireless communication device 20 and the BSC 14, such as pseudo-ranges received by tiie wireless communication device 2D, are preferably communicated by the wireless communication device 20 to the base station 10 via a data burst type message, such as short message service (SMS) defined by industry standard TIA/EIA/IS-167, published fay the Telecomm-unications Industry Assodation/Electronics Industry Association (TIA/EIA). Such messages are transmitted through the base station 10 to the BSC 14. Alternatively, a newly defined burst type message could be transmitted by the wireless commimication device 20 to the base station 10. Fig. 5 is a block diagram of an illustrative iniplementation of a portion of the receiver, control signal interface, digital IF, and mobile demodulator circuits of the wireless communication device 20 of the disclosed method and apparatus. The transmitter portion of the wireless communication device 20 is eKentially idaitical to the transmitter portion of a conventional wireless corrununiration device and therefore is not discussed herein for the saie of brevity, in the preferred embodiment, tiie receiver ITO is implemcnled wiih first and second paths 103 and 105, respectively, which are connected to the antenna 92 ^^a the duplexer 94 and a first switch 106. It will be understood b)-those skilied in tiw art that more integration between the two-way communication device and the GPS receiver could taie place. Alternatively, two sepaiatt receivers with an appropriate interfece could achieve the objective of the disclosed method and apparatus. The first path 103 dowiuxmverts received ODMA signals and provides conventianai CDMA RP downconverted output signals. The first path 103 includes a kjw noise amplifier 108, a first bandpass filter 112, a first mixer 118 and a second bandpass filter 126. The second path 105 downconverts GPS signals from the GPS satellites 60, 70, 80 or 90 of Fig. 1. The second path 105 includes a second low noise amplifier 110 which feeds a third bandpass filter 114. The output of the bandpass filter 114 is input to a second mixer 120. The output of the second mixer is fed to a fourth bandpass filter 128. The first and second mixers are fed by fir^t and second local osdUators 122 and 124, respectively. The first and second local osdBators 122 and 124 operate at different frequencies under control of a dual phase locked loop (PLL) 116. The dual PLL insures that each local oscillator 122 and 124 maintains a reference frequency effiective to down convert either a received CDMA signal, in the case of the first mixer 118, or a received GPS signal, in the case of the second mixer 120. The outputs of the second and fourth bandpass filters 126 and 128 are coupled to a first IF section 130 of conventional design. The output of ttve IF demodulator 130 is input to a second switch 402 in the digital IF ASIC 400. The first and second switches 106 and 402 operate under control of the control signal interface 300 to divert a received signal for voice or data output processing in a conventional CDMA manner or GPS processing by a third mixer 404, fifth barwipass filter 4Q6, an automatic gain control circuit 408 and an A/D converter 410. The second input to the third mixer 404 is a local oscillator output. The mixer 404 converts the applied signal to baseband. The filtered, gain controlled, signal is fed to tiie A/D converter 410. Tne output of the A/D converter 410 includes a first digital stream of in-phase (!"■ components and a second digital stream of quadrature components (Q)- These digitized signals are fed to a digital signal processor 520, which processes the GPS signal and outputs the pseudo-range information required for posioor detenninaticffL In ar. alternative embodiment of the disclosed method and apparatus, the outp".is from the two bandpass filters 126, 128 are fed to a basriand applicaticn spednc integrated circuit (ASIC) vi^uch digitally converts the IF irequEncy s^r\als output from the baseband filters 126, 128 to baseband and outputs a stream of digital values that represent the quadrature and irt-phase baseband signals. These signals are then applied to a searcher. The searcher is essentially identica] to conventional searches used in CDMA demodulators. However, the searcher that is preferably used is programmable to allow the searcher to search for either a PN code associated with the CDMA signals transmitted from, the base station or the PN code associated with ttie GPS satellites. Tlie searcher discriminates between CDMA channels when receiving CDMA sign?!^ from the base station and determines the GPS satellite from which received GPS signals are being transmitted when in the GPS mode. In addition, once the GPS signals are acquired, d\e searcher indicates the time offset associated with the PN code essentially in a conventional manner in order to detErmine the pseudo range associated with satellites from \^u£h signals are being received, as will be understood by those skilled in the art. It will be understood by those skilled in the art that a double conversion process, such as is shown in Fig. 5, or alternatively, a single conversion and IF sampling technique, or a direct conversion could be used to produce the required I and Q samples. Furthermore, the structure of the embodiment shown in Fig. 5 may be altered in many ways that would not affect the operation of the disclosed method and apparatus. For exam^ple, a conventional programmable processor may be used in place of the DSP that is shown in Fig. 5. The memory 510 may not be required if the rate at which data flows through the system is such that no buffers are required. The bandpass filter 406 and automatic gain control circuit 408 may be omitted under certain conditions, implemented using digital tedmiques or analog techniques, or other wise altered. Many other such variations to tiie structure that is shown in Fig. 5 may be made %^iihout altering the inventioru Furthermore, it should be noted that an altematr^"e embodiment may have greater or lesser sharing of hardw^are and software resaurcES between the GPS and wireless receiver. Fig. 6 is a hig^ level block diagram of the compOTients of a communication system whidi indudes die disdosed method and apparatus, b operatiiMi, in accordance with the disdosed method, the BSC 14 requests GPS inionnation from the control processor 62 (Fig 3) within the base station 10. This information indudes, but is not limited to, all of the satellites cuiienlly being viewed by the GPS transceiver 74 (Fig. 3), their elevation angle, Dt^pler shift, and pseudo ranges at a spedfu: time. Note that the GPS receiver at &e base station 10 has up-to-date information cm the location, frequency, and FN offset of each satellite in view, because it is always traclong all satellites that are in view. Alternatively, the base station 10 could send date corresponding to a subset of only those satellites diat can be viewed by fee wireless communication device 20, a^amiing that the base station 10 has stored information regarding the street widtti and height of the surrounding buildings. That is, if the base station 10 has the ability to determine that the wirdess communication device will have an obstructed view of one or more satellites, then the base stetion 10 will not send information regarding those satellites -amt are obstructed. It should be noted that a conventional GPS receiver notes the time at which satellite signals are received with respect to the receiver"s internal GPS dock. However, the receiver"s internal GPS dock is not accurately synchronized to "true" GPS time. Therefore, the receiver cannot know fee exact point in "true" GPS time at which the satellite signals are received. Later, a .navigation algorithm corrects this error by using a fomth satellite. That is, if fee dock within the receiver were accurately synchronized to fee dock in each satellite, feen a conventional GPS receiver would only require three satellites to accurately determine the position of fee receiver. However, since the receiver clock is not accurately synchronized to the satellite"s dock, additional information is required. This additional information is provided by notiiig tJie time at whidi a fourdi satellite"s signal is received by the recpiver. This can be understood by noting liiat there are four equations (i.e., one equation assodated with each a the four satellites) and four unknowns which must be solved (i^., the X, y, anc z coordinates of the receiver, and the error in ti>e receiver ciock;. Therefore, ior three-dimensional solutions, at least four measurements from four differera satellites are required in a conventional GPS receiver. In contrast, die present system utilizes an earth based station which is synchronized to true GPS time. In one embodiment, this station is a CDMA base station- It will be understood by Ihose skilled in the art that CDMA base stations are synchronized to GPS time. In cases wdiere the base is not perfectly synchronized, the time offset can be calibrated out In addition, all wireless communication devices that communicate througji such CDMA base stations using tiie CDMA protocol are also synchronized to an offset GPS time that is unique to each wireless communication device 20. The offset in time is equal to the "actual delay" in the commimication of the signal (i.e., the "transmission delay" due to the one-way delay caused by ^ propagation of the radio ngnal frotn the base station antenna to the wireless comm.ui\ication device antenna, plus the internal delay caused by hardware delays in the transmission chain of the base station). This is due to the fact that the wireless commimication device synchronizes its dock (within the time/frequency subsystem 102) by receiving an indicatiOTi from the base station of the GPS time. However, by the time the indication arrives at the wireless commuiucation device, ti\e indication is in error by an amoimt equal to die actual delay encountered while the signal travels from the base station to die wireless communication device. This actual delay can be determined by measuring how long it takes a signal to make a round-trip between the base station and the wireless comm.ui\ication device. The one way delay will be equal to approximately half the round trip delay. Many ways for measuring the round trip delay are available to those skilled in the art. Irv accordance witi\ the disclosed method and apparatus, processing circuitry within the time/frequency subsystem 1"02 corrects the interna] clock within the time/frequency subsystem to more accurately ^mdironize the wireless communication device to GPS time by accounting for the delav encountered by the signal transmitted between the base station 10 and the wireless amucunication device 20. It shDu^ be noted that Ihe processing tbat is performed -widiin ti* wireless communication device 20 is shown to be divided among functional blocks in Hg. 4. However, the particular structure that is used to perform the processing functions may be a single processing circuit, or may be individual processing circuits that are functions that are groiqwd differently &om liw grotq:ing diown in the present disclosure. Such alternative grouping of the functions within *e hardware can be done without affectis^ the operation of the disclosed method and apparatus. That is, as will be understood by those skilled in the art, processing functions can be split or combined within various processing circuits throughout the wireless communication device 20 without significantly affecting the operation of the disclosed method and apparatus. In addition, the distance between the base station 10 and ihe wireless communication device 20 can be used to assist ir, ^fetermming the location of die wireless communicatfon device 20. Hence, in the case of direct line-of-sight (LOS) between the base station 10 and the wireless communication devia 20, one needs only two satellite range measurements and one base station range measurement In cases where there is no direct LOS between the serving base station and the wireless communication device, three satellite measurements and one round trip delay measurement are required to calculate a three-dimensional location. The extra satellite measurement is required to correct for the additional distance introduced by the additional delay caused by the mtdtipatix The round trip delay is used to correct the clock error in the wireless communication device. The system described herein allows the position of a valid CDMA wireless communication device to be determined at.any time utilizing a Wireless Positioning Pimction (WPF) 18 (Fig. 6), as long as the wireless commurucation device 20 is within the radio coverage area of the CDMA network and as long as there is sufficient quality of service on the CDMA network. The WPF comprises an input and output port and processing circuitrv". I: will be understood by those skilled in the art that the processing dmiitn." rr^y be a general purpose computer, a microprocessor, mkro-computer, dedicated state machine, dedicated discrete hardware draotry, applicatjor. speafk integrated circuit (ASIQ, or any other circuitiy that allows the functions described as being performed by the WFF to be performed. The process of determining the position of a wireless communicatian device may be initiated by the wireless communication device 20, the netwodt, or an extan^al entity, such as an internal location application (ILA) 17, an exterT\al location application (ELA) 15, or an emergency service application (ESA) 13. Each of these components 13, 15, 17 may be either hardware or software Which is capable of requesting and/or receiving location informatioiL In one embodiment, the ILA 17 is a terminal coupled to the BSC 14 which allows an operator to directly request and receive location information regarding a wireless communication device 20. Alternatively, the ILA 17 is a software application executed by a processor within the MSC 12. The WPF 18 is preferably a conventional programmable processor capable of accepting the raw data that is received from the wireless commimication device and from the satellites (Le., the pseudo ranges from two satellites, the distance from the wireless communication device to the base station and the time correction factor) and calculating the position of the wireless oDmmujucation device. However, any device that is capable of reaving the information required to calcuUte the location of ihe wireless commimication device 20 based on such received information and output this location determination may be used. For example, the WPF 18 may be implemented as an ASIC, a discrete logic circuit, a state machine, or a software applicataon within another network device (such as the BSC 14). Furthermore, it should be understood ^lat ihe WPF 18 may be located within die base station 10, the BSM 14, oi elsewhere in the MSC 12. Preferably, the WPF 18 is a software application that is executed by a dedicated processor that is in communication witji the BSC 14. Accordingly, the base station 10, the BSC 14, and the MSC 12 need not be significantly modified in order to implement the disclosed method and apparatus with conventional components. Alternatively, tiie WFF IS is a software application that is executed by a processor within the BSC 14. The WPF 18 preferably communicates with the BSC 14 via 3 communication port similar to that used by osnventimiai billing function, managesnerr functions, home location register/visitor kxation register functions, mn olher ancillaiy functions that are perfoimed by processors that are coupled tc conventional BSCs. The aigorithm used to calculate the position is provided in Parkinson, B.W., and SpSkei, J.J.,. Editors, Global Positioning System: Theory and Applications, Volume. I, American Institute of Aeronautics and Astronautics Inc., Washington DC, 1996. Additionally, it should be rKrted that Volume U teadies how O) perform, differential GPS correction. It will be understood by those skilled in His art that such correction may have to be performed by the WFF 18 in order to calculate the position of the wireless communication device accurately. In accordance vn1h one embodimCTit of the disclosed metiwxi and apparatus, a service provider can restrict posiUoiiing services based on several conditions, such as capability, security, service profiles, etc. Location services may si^port each, or some subset, of the following services: (1} Wireless mmmunication device originated request for positioning (WFF). (2) Network originated request for positioning (NRP). (3) Petitioning allowed on a per service instance (FSI): The wireless cotrununication device gjves an external application a temporary allowance ba position the unit for the purpose of delivering a specific service. (4) Positkanir^ vrith/without v»nreless comm^mication device identification (PWI/PWO): will position all wireless communicatian devices in a defined geographical area. PWI will give die identity and the location of these uruts while FWO will only give their location. (5) Positioning within a dosed group (PCG): Allows for the creation of groups ■within which special rights for positioning can be detennined (fleet management). In accordance with one embodiment of the disclosed method and apparatus in which a wireless communication device 20 originates a request for the position of that wireless communication device 20 to be determined, the wireless communication device 20 sends a position request to the MSC 12. The MSC 12 validates the request to ensure that the wireless communication device 20 has s^Ibscribed to the type of service requested. The MSC 12 then sends a request to the serving BSC 14 to find tiie position of the wireless communication device 2D. The BSC 14 asks the serving base station 10 for position aiding information. The serving base station 20 responds to the request by sending: (1) a list of satellites in view, (2) their Doppler shift, (3) their rate of Doppler change, (4) their pseudo-ranges, (5) their elevation angles, (6) their Signal-to-Noise ratio (SNR), and (7) an indication from which the amount of delay encountered by a sigr^ transmitted between the base station and the wireless communication device can be detennined (e.g., Roimd Trip Delay (RTD) between the wireless communication device and the serving base station). It should be noted that the indication that is used to determine the amount of delay encountered by a signal transmitted between the base station and the v."trekss communication device may be the round trip delay encotmtered in a round trip from the base station to the wireless communicatian device and back or from the wireless communication device to the base statian and back. This calculation may be done by noting tfie time at which a signal is transmitted from the point of origin of the rotmd trip, knowing the aniount of time required to retransmit the signal at the far end erf the trip, and noting the time at w^ch the retransmitted signal is received. If the base station onginates ^\e signal ai\d does the measurement of the round trip delay, then the base station can; (1) trarsmit infoimaticm from whidi the wirriess commimicaticin device can compute the one way delay between the base station and the wireless comrmmication device (such as the roiand trip delay), or (2) calculate the amount of delay in a one way trip from the base station to the wireless communication device (assuming that the either the turn around time is negligible between receipt of the signal at the wireless commimicatinn device or receiving information on the turn around time) and trar\smit the cme vray delay to fee wireless CQinnriunira.tiQn device. Likewise, if tfie wireless communication device originates the signal and does the measurement of round trip delay, then the wireless commimication device can" (1) calculate the one way delay directly from die measured roimd trip delay, assuming that the amoimt of time between receipt and retransmission at the base staticm is negligible; (2) receive an indication from the base station as to how much time elapses between the receipt and retransmission of a signal received from the wireless communication device, from which the wireless communication system can calculate the one way delay; or (3) transmit the recorded round trip delay back to the base station, which then calculates the one way delay and transmits a value indicating the one way delay to the wireless communication device. In accordance with one embodiment of the disclosed method and appartatiis, round trip delay between the base station and the wireless commuricarion device 20 is determined as ti\e difference between the beginning of a frame on the signal timt is transmitted from the base station, and the beginning of a frame on the signal dnat is received by the base station from the wireless communication device 20. This is known as the finger offset delay of the first arri\*ing finger measured by the CSM (Cell site Modem). It should be rioted th*: the round trip delay is the sum of the followng: (1) hardware delay of the forward link (base station transmission d\ain}: (2) one way propagation between the base station antenna and fiie antenna of the wireless commtmication device; (3) hardware delay wilhin the wireless conununication device (receive and transmit chains); and (4) one way delay between the anterma of the wireless communication device and the base station antenna. According to the telecommunications industry standard, IS-95, promulgated by the Telecommunication Industry Association/Electronics Industry Association (TIA/EIA), the wireless communication device should adjust its transmit timing to compensate for its own hardware delay siidi that the begirming of a frame of the signal transmitted by the wireless communkation device 20 lines up with the beginning of the frame received by the wireless communication device 20. Accordingly, the hardware delay in item (3) is automatically removed to within an acceptable tolerance. The delay in item (1) can be calibrated widi accuracy of approximately 50 nano-seconds. Hence under line of sight conditions, an RTD measurement can be used to determine die distance between the wireless communication device 20 and the base station 10. Note that the GPS receiver 74 witiiin the base station 10 is continuously tracking the satellites in view and hence can have up-to-date information on parameters related to liie satellite. The BSC 14 will use the RTD, pseudo-range, satellite elevation angle, Doppler shift and rate of change of Doppler for each satellite to calculate the search window center and search window size in both time aiui frequency as follows (see also Fig. 7); In the time domain the center of the search window for the i* space vehicle ("S\"0 is equal to the pseudo-range, pb between the serving base station 10 and the S%* in Fig. 7. The search window size for SVj is equal to dcos^^), wberc d is tqaai lo one half the round trip delay between the base station BS and ftie ^-irekss cocununication device (noted as MS in Fig. 7) and, where cos (*) is the cosine of aw ang^ of &e elevation of the satellite vnnii reelect to the radius of the earth n-tadi originates at the center of the earth and passes ferou^ the receiver. One sadUed in the art will understand this relationship by noting that the distance between ftie base station aixd dve satellite is much, ntnch greater than the distance between the base station and Ihe wireless communication device-According^, when the satellite is essentially overhead, the distances pml, pb, and pm2 will all be essentially equal As the elevation angle of the satellite approaches 90 degrees, the difference between pnil and pm2 will approadi 7d, and the search window size will approach d. In accordance with one embodiment of the .disclosed method and apparatus, the search window center and size can be further refined by information regarding: (1) any information regarding the recent location of the wireless communication device, (2) information regarding from which, if any, other base stations the wireless communication device can receive signals, (3) the relative strength of signals recaved from other base stations, (4) the relative locations of other base stations from which additional signals can be received by the wireless commimicalion device, (5) whe^er signals received by tiie wireless conunurdcatron device are transmitted from a base station titat is sectorized, and if so from which sector the signals are being transmitted, and (6) any attempt to triangulate the location of the v\"ireless cominunication device using signals transmitted from any source, including any base stations, by either time of arrival or time difference of arrival information regarding such transmitted signals. In the frequency domain, tt^ search window center for SV, is equal to i, ■^ f^- where f, is equal to the carrier frequency of the GPS signal and f^ is equal to the Doppler shift of the signal transmitted by SV.. The search window size for 5V^ is equal tc i« uncertainty in frequency due to receiver frequency error and Doppler raK of d^ar^. Tlie BSC 14 sends the information induding satellites in \-iew, searcher window centers, sizes, in both time and frequency, and the minimum number of satellites needed to determine the posilian of fttt wireless commimicatiofi device 20. In accordance with one embodiment, a message to the wireless communicatkjn device 20 will trigger a re-tuning signal at the wireless communication device 20. The message also could have an "action time" (a particular time in the future when the receiver will retune to a GPS recover frequency). In response, the wireless communication device 20 will activate the first and second switches 106 and 402 at the acticm time (Fig. 5) and tfiereiiy retune itself to the GPS frequency. The digital IF ASIC 400 dwi^es its FN generator (not shown) to GFS mode and starts to search all specified satellites. Once the wireless communication device 20 acquires the minimum number of the required satellites, it computes the pseudo-ranges based on the GPS ck»ck wiftun the wireless commxmication device 20, re-tunes to the conununication system frequerxcy, and sends Ae pseudo-range results along with the measured signal-to-noise ratio of the first ^iree satellites and a mo^ recent CDMA pilot search result to the BSC 14. The pilots search results are needed if the unit cannot acqioire three satellites and there is no direct line of sight path between the serving base station and the wireless communication device 20. Nonetheless, less than three satellite can be used, as long die roxmd trip delay from another device, such as another base station, can be computed using available information, such as pilots search information. Techniques for detennjiurtg round trip dday based on pilot search iniormation are well known intheart- The BSC 14 sends the pseudo-range measurements made by the wireless communication device 20, together with the position of the serving base station 10, the corresponding round trip delay measurements, the positicm (in space) of the satelHtES under consideration (with reference to a fixed, predetermined reference origin), and differaitial GFS correction to the WPF 18 where the position of the wireless ajmmimication device 20 is calculated. The pseudo-ranges received from the wireless communication device M by the BSC 14 and passed to ir« WPF 18 are relative to the dock within the wirrfess communication, device 20. Therefore, they are erroneous (Le., biased by the round trip delay between the serving BTS 10 and the wireless communication device 20). Hg. 8 is a diagram that illustrates how the WPF 18 corrects for die local dock bias. In Hg. 8, Si represents the pseudo-range (half die round trip delay) in the receipt of signals transmitted from the base station 10 to die wireless asmmunication device 20 and vice versa, rml, rm2 and nnS are the pseudo-ranges from the wireless communication device to the first, second and third selected GPS satdlit» 60, 70 and 80, iKpectively. These measuiMnents are taken with respect to the local clock in the wireless CGmmurocatiori device 20. But since the local clock is offset from die true GFS time by 51, the corrected pseudo-ranges are then: pi =pml +51 p2 = pEo2 + 81 p3 = pm3 + 51 The WPF 18 uses the above three equation, position (in space) of die three satellites, position of the severing base station, and corresponding RTD measurements to calculate die position of the wireless communication device 20. Note that knowing the RTD is equivalent to exactly Jmowing the local clock bias of the wireless communication device relative to the true GFS time. That is, it is sufficient to solve the diree range equations from the three satellites. Furthermore, if there is a direct propagation path between the base station and the wireless commLmication device, then three satellites provide an over-determined solution, since the RTD can be used to determine both the dock offset and a pseudorange measurement to the base station. Note also that the minimxim number of satellites required can be reduced to two if there is a direct line of sight connection between the i^ireless communicaacc de\"ice 20 and a base station 10, such that the distance between the Vk-irekffl commimication device 20 and tiie base station 10 can be determined dnKify from the RTD between tiie wireless commtmication de\"ice 20 and the base station 10. This number can be further reduced if information about other pilots (sites) is available. For example, if the wireless commimicatkri device 20 is in communication with two or more base statimis (e.g., soft handoff), neither of which have a direct line of site to the wireless communicaticm device 20, more than one round trip delay may be calculated, and hence two satellites are all tiiat is needed to determine the position of the wireless commimication device 20. That is, the calculations can be made based on the five equations (two equations related to the two pseudo range measurements associated with the two satellites, two equations related to the two base station RTD measurements, and one equation related to the RTD fco &e serving base station that allows the local dock within die wireless communication device 20 to be synchronized to true GPS time). This is very useful in scenarios where GPS satellites are blocked or diadowed by buildings of trees. In additiorv it reduces the time to search for GPS satellites. The WPF18 sends the calculated position to BSC 14 and the BSC 14 forwards the calculated position to MSC 12 or sends it directly to the wireless communication device 20. In addition to using the RTD from another base station, the disdosed mettiod and apparatus can use the time difference of arrival (TDOA) between pilots from different base stations or between a base station and a satellite to assist in determining the position of a wireless commimication device. Such TDOA is used in addition to the TEXDA of satellite signals from GPS satellites. Such use assists in determining the location of a wireless communication device when at least one synchronized base station is available (i.e., base station synchronized to GPS time) or at least two unsynchronized base stations (i.e.. base stations synchronized to one another, but not to GPS time) and less than the desired number of satellites are available. Use of TDOA measurements from forward link signals can reduce the number of satellites even in the absence of KTD infonnation. The folicrwing is a description of several embodiments of the presently disdcsed method and apparatus for using forward link information to assist in dcterrrJrJn£ re position of a wireless commumcation device. It should be undersMod th*; the apparatus disclosed is essentially a general purpose processor, diecal signal processor, dedicated drcuit, state machine, ASIC, or other such drcurrv that can perform the function disclosed, as is well known in theart- In oroer to determine the position of Ihe wireless communication device, the number of imknowns (e.g., x, y, and z coordinates of the device and exact time) must match the number of equations that include dwDse unknowns. The following equation can be written for each pair of signals from which a TDOA measurement can be made (i.e., each pair comprising one signal transmitted by a first base station BSl and one signal from traiismitted by a second base station BS2): TDOA wherein TDOA^_ b^ woi is the TDOA between signals received by the wireless communication device (wed), from a first base station (bsl), and a second base station (bs2); At is the offset between the dock used to generate the signals transmitted by each signal source and any difference between the internal BS delay of base stations bsl and bs2 Xj^j is the X coordinate determining the location of base station bsl; x^ is the X coordinate determining the location of base station bs2; }\^ IS ij\e y coordinate determining the location of base station bsl; y^ is d^ y coordinate determining the location of base station bs2; Z;_. is the 2 coordinate determining die location of base station bsl; 2^ is the z coordinate determining the location of base station bs2; x_^ 15 die X coordinate determining the location of wireless cosimurica TOT dev-ice; y,^ is the y coordinate determining the location of vMTf"tess camcrjnicatton device; and z^ s 3ie z cooniinate determining the location of wireless ."TTTT"TTnTniraTinr. device . For the above equation, the unknowns are the x, y, z of the wireless communicaaoffi device and At. Similar equations can be written for the satellite TDOA measurements. Since there are four unknowns, thei« must be four such equations, requiring at least four satellites or base stations in any combination, assuming the At is constant for both the satellites and the base stations. If this assun^tion is rwt true (Le., the base stations are not synchronized to GPS time), then one additional unknown will be added, and so orxe additional signal source is required- In addition, if the base stations and GPS system are not syiKhronized, tten there must be at least two satellites and at least two base stations in order to use both satellites and base stations. If communication base stations operate synchronously with respect to one another, the TDOA of each pilot with respect to each other pilot can be determined using the above equation. If the base stations are also synchronized to GFS time, as is the case with CDMA commimication base stations, the method of using forward link pilot TDOA and satellite TDOA which are both synchronized to GPS time is referred to herein as a "synchronous GPS/forward link mode", In the case in which the communication base stations are not synchronized to GPS time, the method of using both forward link TDOA and satellite TDOA is referred to herein as an "asynchronous GPS/forward link mode". In this case, the term "asynchronous" refers to the fact that there is an unknown offset between the time reference used to make time of arrival measurements on the forward link and the time reference used to make time of arrivai measurements on the GPS satellite signals. It should be understood tiiat the term "asiF-nchronous" is not intended to convey that the base stations are not s\-nchroni2ed with one another, nor that the GPS satellites are not sj-nchronoiis v.-iih one another. In feet, each base station is preferably synchronized ■v^irh eadi other base station to allow the time difference of arrival of each signal received from a base station to be determined with respect to each other base staaon. Likewise, each satellite is synchronized with each other sateliia in the GPS canstellation. Seferrin^ to Figs. A-D, ftie timing between pilot PN sequences that are generated and transmitted by tl:^ tiiuee base statwjns 10a, lOb, 10c are discussed. It should be rested that while the presently disclosed method and apparatus is described essentially with, respect to a CDMA system, the asyndttonous GPS/forward iink mode is most useful when used with synchnDnous communications systems that are unrelated to GPS time, such as time division multiple access (TDMA) communication systems. Synchronous GPS/forward Link Mode Fig. 9 iliustrates the relationEhip bet%vccn the pilQt FN sequences that are transmitted from three base stations. Fig. 9 shows three pilot PN sequences 901, 903,905, each pilot FN sequence having nine chips numbered one tiuough nine. A "ch^" is defined herein as tiie smallest unit of information within a pseudo¬random noise (FN) sequence. Each chip will typically have a binary (or logical) value (Le., either one or zero). A pilot FN sequence is defined for the purposes of ftiis disckwure as a sequence having a length N, which repeats every N chips (in the illustrated exasnple N = 9 chips), but which appears to be random in any sequence of N consecutive dups. It dusuld be understood by those skilled in the art that the pilot PN sequences that are transmitted by QDMA base stations are typically on the order of 2" diips in length. However, for the sake of simplicity and clarity in describing the presort method and apparatus, the pilot FN sequences shown in Fig. 9 are shown to be only nine chips in length. Fig. 9 shows the timing of three pilot PN sequences 901, 903, 905 generated "uy the control processors 62 of three base stations 10a, 10b, lOc. Each base station iC generates one such pilot PN sequence. Each base station 10 Vtithx. a CDVL^ communication system transmits the same pilot PN sequence. However, the beginnii^ of the pilot PN sequence generated by each base station ICfc, 13c a intentionally offset a predetermined amount of time by the cooJol processor 6Z For the purposes of this discussion, ttie control processor 62 vv-ithin the base station 10a transmits a "zero FN sequence". The zero PN sequence is the refereni^ from which each other pilot PN sequence is offeet. Accordingh", rw control processor 62 that generates the zero PN sequence introduces an afiset of zero (hence the name "zero PN sequence"). It should also be noted triat the presently disclosed method and apparatus does not need the zero FN sequence to be generated by any particular base station. The zero PN sequence is shown merely for illustrative purposes. It will be understood by those skilled in the art that in conventional CDMA communication systems there is a one to one correspondence between each pilot PN sequence offset and each base station. That is, each base station is assigned a unique offset and generates pilot FN sequences with that one offset only. The offeet intentionally introduced by the control processor 62 of each base station ICb, 10c preferably has a duration that is equal to tt« amount of time required to transmit an integer number of chips. The offeet is unique to each base station in the communication system. In the example shown in Fig. 9, the zero FN sequence 901 is generated wittiin the control processor 62 of the base station 10a beginning at a time T, and repeating beginning at a time T^. In a CDMA commimication system, times Tj and T^ are predetermined times with respect to GPS time. Since, the pilot FN sequence is a fixed length and repeats, the begimung of the pilot PN sequence will occur on regular predictable intervals with respect to GPS time. The first such pilot PN sequence 901 is the zero FN sequence that is used as a timing reference. The second such pilot PN sequence 903 begins at time T, and is offset from the zero PN sequence by ei^t chips. Accordingly, when the base station 10a generating the zero PN sequence 901 is generating the first chip 907 of the pikit PN sequence 903. base station 10b generating the pilot FN sequence 903 is generating the ninth chip 909. The pilot PN sequence 905 begins at time T. and is offset from the zero PN sequence by three chips. Therefore, wher. the zero FN sequence is generating the first chip 907, the base stadar. generating the pilot PN sequence 905 is generating the fourth chip 911. Each base stabor. is synchronized to base station time. Therefore, each base station m ^ sysKm can generate the offset pilot PM sequences synchronously with respect to each other base station in ttie system. It can be seen that, even wirwct kncwirtE :he time at which the zero sequence begins, the offset between the pikrt PN seq-jence 903 and the pilot PN sequence 905 at the time each signal was generated czn be determined to be four chips. It win be understood by those skilled in the art that true GPS time can be attained within any base station having a GPS receiver and processor by calculating a coir^lete solution for the location of ^ base station from GPS signals received from four satellites. Sudi a complete solution will yield the coordinates x, y, and z, of the receiving base station 10a and GPS time t, as is well known in the art. More generally, true GPS time can be determined accurately by calculating an N dimensional solution for location using N+1 satrflitea. Acrnrdingly, ihs base station ICa will cause tiK -znszi FN sequence 901 to begin at a particular time widi respect to true GPS time. It should be noted that the absolute time at whidi the zero PN sequence begins need not be known, since the time difference of arrival between two signals received at tf\e wireless communication device is determined as a relative measurement Once a wireless commimication device determines that a pilot signal that is being transmitted from a base station can be received, the recover can thei demodulate information transmitted on a "sync" signal transmitted from the same base station. The information demodulated on the sync signal includes the PN offset (with respect to the zero PN sequence) that was applied to the pilot PN sequence. Therefore, tiie zero FN sequence serves as a reference for determining the relative timing of eadi of the pilot PN sequences transmitted by each base station within a CDMA communications system. However, even though the control processor 62 vrithin each CDMA base station 10 is synchronized to GPS tdme, there is a propagation delay associated vi-ith iransrrjrtznz each pilot FN sequence 901 to the center of radiation of the transirut antenr^ -43 within the commuiucation antermas 201 of the transmitting base station iC. Tnis hardware delay dirough the transmission chain (hereafter reierred ra as "cirernal BS delay") introduces an offeet between the time at •.4-hidi the pikK ?N" sequery:e began with respect to true GPS time (i.e., the begmxung of iie pilot FN sequence at tiie control processor 62) and the begimrig of the pilot FN sequence at the time of transmission from the center of radiation of the antenna 43. The internal BS delay may be different for each base station IXl Accordingly, CDMA base stations are not accurately synchronized to GPS time, or to each other, at the CKiter of radiation of the base station transmit antennas 43. This inaccuracy is not significant erusugh to be problematic for commimications, but does present a problem when attenipting to very accurately determine the time difference of arrival for the purpose of determining position location. Fig. 10 ilhistrates the effect on the relative timing of the pilot FN sequences 901, 903, 905, caused by the internal BS delay. The first pilot PN sequence 901 is delayed from time T, to time Tf Likewise, the pilot FN sequence 903 is delayed from time T. to time T^. Pilot PN sequence 905 is delayed from time T, to time T^. It can be seen in Fig. 10 that the pilot PN sequences 903 and 905 have been shifted by an amoimt of time S, = T^ - T^ and Sj = T„ - T^ re^)ectrvely, with respect to the pilot PN sequence 901, due to the differences in the internal BS delays through each base station 10. The relative offsets S^ S, must be known in order to accurately determine the TDOA of the three pilot sigr\als 209, 211, 213 received from the three base station 10 at the wireless communication device 20. Neither the internal BS delay of each base station 10, nor the relative offiset S will typically vary greatly over time. Accordingly, in one embodiment of the presently disclosed method and apparatus, either die internal BS delay of each base station 10 or the relative offeet S can be measured at the time a base station is commissioned. Alternatively, the internal BS delay oi relative offset S can l?e detErmined at regular intervals and commimicated to the wireless communication device. In yet another embodiment, the internal BS delay or relative ofeet S can be continuously determined and communicated to the v.-irekss osmmunication device, either upon demand, at regular intervals, or in re^xinse to a change in &e value. One metnod for measuring the internal BS delay and the relative offset amor^g CDMA base stations (or base stations from any other system l±\flt is s^TvJroaized t GPS time) is to \ise a remote syrvchronizing station located a kno**Ti distance away {Le., having a known propagation delay from tiie antenna of the base stataon 10 to fee remote synchronizing station) to receive a pilot PN sequence from tiis base station 10. Fig. 11 shows a simplified block diagram of such a renwjte s^iidironizing station 1101 located a known distance away from a plurality of base stations 10a, 10b, 10c. The remote synchronizing station 1101 comprises a dock 1103, a receiver 1105, a transmitter 1107, and processing circuitry 1109. Since the distances between base stations 10a, 10b, 10c and the remote synchronizing station 1101 are fixed, such a remote synduonizing station capable of receiving signals from more tiian one base station and operating at a known location can be used to determine the oHset between base statiorts. Since the time differsncs of arrival foi signals coming from sources at krwwn locations can be easily determined for a remote synchronizing station 1101 at a known location, the remote synchronizing station 1101 can determine the difierence between the escpected time difference of arrival and the measured time difference of arrival. Alternatively, if &e remote synchronizirtg station 1101 has a GPS receiver, it can determine the amount of delay introduced as follows. The base station can determine true GPS time from the GPS receiver. The base station can also determine the time at which the signal was generated at the contnil processor 62 within the base station 10, since the base station will generate the pilot PN sequence at a predetermined time with respect to true GPS time. Fig. 12 is an iliustxation of the method used to determine the amount of delay introduced between a base station control processor 62 and center of radiation of the transmit anterma 43 using a remote synchronizing station 1101 at z known distance or to which signals can be transmitted with a known transmission aeUy. As shown, a pilot PN sequence is generated at a first time t. Tne pilot FN" sequence is commimicated through the base station to the cET-ter of radiaaor. of \h& transmit antenna 43 and begins transmission at time t,. The pilot PN sequence is ihen transmitted over the air to the remote syi^d^rorizing sation, where it is received at time t^. Accoidingly, since both dme t snd tiiae ", are known with respect to true GPS time, the amount of time rKr.i:;ec for the pilot FN sequence to propagate from lJ\e control processor 62 svachrtffiizing station 1101 is known. In addition, the amount of ±ne required cr the signal to propagate from the tranKnit antenna 62 to the remote syncfartsuzing station 1101 (i.e., the differen^^ between time t, and tj is know, sirvce the distance and/or path traversed by the signal can be measured and determined. One way to determine the difference between time tj and t, is to measure the round trip delay- For example, a wireless communication device that receives the signal may be a conventional wireless mobile phone adapted to provide the round trip delay between the base station 10 and the phone. Alternatively, the base station 10 may be a conventional base station having the ability to detennine the round trip delay between the base station 10 and the wireless aammunication device 20. It will be understood by tiiose skilled in the art that these are several ways to determine the propagation delay that occurs between the base station 10 and the wireless communication device. Several other well know techniques muld be used to perform this measurement. It will also be imderstood by those skilled in the art tiiat the remote synchronizing station 1101 may be a conventional wireless telephone, a piece of test equipment specially designed to perform the synchronization function, or any other receiver, such as another base station, capable of being adapted to perform the functions described herein. Once these values are know, the difference in time from tj to t, can be calculated by subtracting the anxount of time between time t^ and t^ from the amount of time between t, and tj resulting in the time between tj and t,. alculated in accordance "with liie synchronoiis GPS/forward link mode. As ho^Ti in Fig. 13, each of three base stations 10a, 10b, 10c generates a pilot PN equence at the same time T^. However, each base station 10a, 10b, 10c has a iitfejer.t internal B5 delay. Accordingly, the first base station 10a transmits the rilot PS sequence generated in that base station 10a at time Tj, the second base tation 10b transrrJts the pilot PN sequence generated by that base station 10b t time Ty ar^i the third base station 10c transmits the pilot PN sequetice ;enera»l by that base station 10c at time T,. The internal BS delays within each ase station 10 on be determined as described above. Once determined, the nteiT^al BS ttelays can &en be eiti\er stored in, or communicated to, the wireless Once the three pilot PN sequences are transmitted, each by an associated >ne of the base stations 10, each pilot PN sequence will encounter a different >ropagation delay through the air from the center of radiation of the transmit mterma 43 of the transmitting base station 10 to the receiving wireless rommunication device 20. It can be seen that the TDOA between die base itation 10a and 10b, for example, appears to be the difference between times T^ end T,. However, since the time difference of arrival should measure only ilie iifference in the amount of time required for the signal to propagate through he air, the error introduced by the differences in the internal BS delays mcomitered in asrmnuiucating Ihe pilot FN sequences from the control processor 62 to the center of radiation of the transmit antenna 43 must b? Xaksn nto aorovmL This can be done by simply subtracting the known internal BS ielays from the times T^ T^ and T„ resulting in the times T,^ T^ and T,(,. Accordingly, the corrected TDOA is the difference between each pair of d\ese hree times T^ T», and T„. In addition, subtracting the internal BS delays jyndTTonizes each of the base stations with GPS time- Therefore, since each :;n^ satellite is synchronized with GPS time, the time of arrival of the base itation signals and die time of arrival of the satellite signals can be used :ogether to form, a TDOA value that can be used in a least mean square TDOA equation, as is well known in the art. It will be understood by those skilled in the art since the satellite signals and the base station signals are synchronized, the relationship between the pilot PN sequence that is transmitted from the base stations ani the satellites is known. Accordingly, without knowing the exact time at ^*udi the signals were transmitted from either the base stations or the satellites, the tiine at which the base station signals were transmitted with .-aspect T3 the tme at which the satellite signals were transmitted is known. Tnsrefore, the =ae difference of arrival can be accurately determined by sufcsacting the »rn\-al times of any signal from either a base station or satellite jom any olher a^r^ received from eitiier a base station or satellite. >.svnchronous GPS /forward Link Mode m *i — " — As is the case in the synchronous GPS/forward link mode, the asynchronous GPS/forwaid Imk mode assunws Ihat all base stations are synchronised to or\e another. However, in the asynchronous mode, the time of arrival of signals received over the forward link cannot be combined with the time of arriva] of the GPS signals to generate meaningful TEX)A measurements, as can be done in the synchronous GPS/ forward link mode. Rather, the time of arrival of signals received from base stations can be combined widi only the time of arrival of signals received from other base stations to form TDOA measurements, likewise, the time of arrival of signals received from GPS satellites can be combined with only the time of arrival of signals received from other satellites to form TDOA measuremeits. Nonetheless, if a wireless communication device can receive signals from N base stations (where N is equal to at least two), then all that is needed are signals from 4 - N - 1 satellites. Any additional satellites that can be received would provide an over-determined solution, thus leading to a more reliable and accurate solutioru It should also be clear from the present disclosure to those skilled in the art that two or more non-synchronous systems may be combined in order to reduce the number of satellites needed to 4 - (N, - Nj ... -NJ - X, where there are X systems in addition to the satellite system, each such system having at least one base station that can be received by the wireless communication device, and N base stations can be received foi ^st^n x. In addition, the procedure for determining the differences between iniema! SS delay"s in ^ asynchronous GFS/forward link mode differs frorci the procedure used ir. the synchronous GPS/forward link mode due to the fact that tr« base stations are not synchronized to GPS time. In the case of asynchronotis GPS/forward link =wde, a recaver receives signals from at least two of the base sations wtfftr: the communication system^ The receiver is placed in a knou-n iocatkm so iat the TEKDA between liie signals transmitted from each pair at zwo base sation is known. Accordingly, the difference between the knowTi value of tr« TDOA and liie measured value of TDOA is equal to the differaice in the Jntaaial BS delays (and any clock offsets between the base stations). For each, pair of base stations, such a measxurement is made. These dirferences are then either stored in, or communicated to, the wireless communication device 20 to be taken into account when calculating the TEOA values- Alternatively, these differences can be eidier stored in, or communicated to the WFF 18 to which the TEXDA measurements from the wireless communication device 20 will be sent In that case, the WFF IB corrects the TDOA to account for errors in the synchronJEatiori between the base stations. If the calculated values are communicated, they may be communicated at the tinve a new base station is commissioned, at regular intervals, on demand, or upon a change in the values. The GPS/forward link modes drecribed herein treat each base station as a "pscudo satellite". A pseudo satellite is defined as a device that transmits a sigral synchronous with the satellites and which can be used in a TDOA measurement with a satellite. The WPF 18 stores base station almanac, including base station location, antenna high, anterma characteristics (antenna pattern and gain), base station configuration, such as number of sectors, orientation of sectors, dock error for each sector. In .this way, die location system will consider all received signals by the wirieless communication device to have one common time reference (i.e., GPS time as the reference for all received signals). It should be noted that the presently disclosed method and apparatus may use both forward and reverse link measurements if they are available. That is, similar measurements made at a plurality of base stations based on signals transmitted by the wireless communication device could be used instead of, or in addition to, the signals received by the wireless communication device The same technique would apply. However, the time o: arrival infonnaaon would have to be transmitted to a common location so that differences s. trie relative arrival tinies at each base station of the signal transmitred from the wireless communication device could be determined. The disciosed method and apparatus has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additianal modincations, applications, and embodimerts within the scope of the present invention- It is therefore intended by the appended claims to cover any and all such applications, modifications, and embodiments within the scope of the present invention. We claim: 1. A method for synchronizing time of arrival measurements made with a wireless communication device to a global positioning system (GPS) time, comprising the step of: receiving time of arrival measurements made at a wireless communication system and signals from an external source synchronized to GPS time, the signals received from the external source including an indication from which the amount of delay encountered by a signal transmitted between the external source and the wireless communication device can be determined; and adjusting the received time of arrival measurements by an amount equal to the delay encountered by a signal transmitted between the external source and the wireless communication device. 2. The method as claimed in claim 1, wherein the indication from which the amount of delay can be determined is an indication of the amount of delay encountered by a signal making a round trip between a base station and the wireless communication device. 3. The method as claimed in claim 1, wherein the indication from which the amount of delay can be determined is a direct indication of the amount of delay encountered by a signal transmitted from a base station to the wireless communication device. 4. A method for synchronizing a wireless communication device to a global positioning system (GPS) time, comprising the steps of: receiving signals from an external source synchronized to GPS time, the signals received from the external source including an indication from which the amount of delay encountered by a signal transmitted between the external source and the receiver can be determined, and receiving a GPS time that includes an indication of when GPS time is transmitted from a base station; and synchronizing a clock to the received GPS time after adjusting the received GPS time for the delay encountered by a signal transmitted between the external source and the wireless communication device. 5. The method as claimed in claim 4, comprising determining a time of arrival (TOA) of the signals received from any source with respect to the synchronized clock. 6. The method as claimed in claim 5, comprising determining an n-dimensional position location of a wireless communication device from n time of arrival measurements, wherein the time of arrival measurements indicate the time of arrival of signals received from n satellites. 7. The method as claimed m claim 6, wherein determining an n-position location comprises determining an n-position location with a control processor residing within a base station transceiver subsystem. 8. The method as claimed in claim 6, wherein determining an n-position location comprises determining an n-position location with a control processor residing within a base station controller. 9. The method as claimed in claim 6, wherein determining an n-position location comprises determining an n-position location with a control processor residing within a dedicated position determination device. 10. A method for synchronizing time of arrival measurements made with a wireless communication device to a global positioning system (GPS) time, comprising: receiving signals from an external source synchronized to GPS time, the signals received from the external source including an indication from which the amount of delay encountered by a signal transmitted between the external source and a receiver can be determined, and receiving a GPS time that includes an indication of when GPS time is transmitted from a base station; determining the time of arrival of signals received by the receiver with respect to a clock; and adjusting the time of arrival by an amount equal to delay encountered by a signal transmitted between the external source and the wireless communication device. 11. A system for synchronizing time of arrival measurements made with a wireless communication device to a global positioning system (GPS) time by the method claimed in any one of the preceding claims.

Documents:

in-pct-2002-0628-che description (complete)-duplicate.pdf

in-pct-2002-0628-che description (complete).pdf

in-pct-2002-0628-che abstract-duplicate.pdf

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in-pct-2002-0628-che abstract.pdf

in-pct-2002-0628-che claims-duplicate.pdf

in-pct-2002-0628-che claims.pdf

in-pct-2002-0628-che correspondences-others.pdf

in-pct-2002-0628-che correspondences-po.pdf

in-pct-2002-0628-che drawings.pdf

in-pct-2002-0628-che form-1.pdf

in-pct-2002-0628-che form-19.pdf

in-pct-2002-0628-che form-26.pdf

in-pct-2002-0628-che form-3.pdf

in-pct-2002-0628-che form-4.pdf

in-pct-2002-0628-che form-5.pdf

in-pct-2002-0628-che others.pdf

in-pct-2002-0628-che pct.pdf

in-pct-2002-0628-che petition.pdf