PCUU$HflM2
`
`receiver from the satellites, related by the speed of
`light. Prior to correction for the clock bias C5,
`the
`apparent ranges of the satellites are all in error by a
`fixed amount and are called pseudoranges.
`Two positioning services are provided by the NAVSTAR
`The precise positioning service (PPS) which is
`GPS.
`reserved for military use provides accuracy to within
`twenty-one meters (2drms). The statistical term "zdrms"
`refers to a value that falls within two standard
`deviations (using the root-mean-squared method) of the
`sampled performance data mean. Therefore, a stated
`accuracy of twenty-one meters (zdrms) means that the
`position error has an error of less than twenty-one
`meters approximately ninety-five percent of the time.
`The standard positioning service (SPS) which is
`available for general use provides accuracy to within
`thirty meters (zdrms). However,
`the SPS signal accuracy
`is intentionally degraded to protect U.s. national
`security interests. This process, called selective
`availability, degrades the accuracy of SPS position fixes
`to within one hundred meters (zdrms).
`The SP5 may be
`degraded in a number of ways, for example, by providing
`slightly inaccurate satellite orbital data to the
`receivers or by dithering the ranging intormation.
`Certain applications require better accuracy than
`provided by degraded SPS, SPS, or even PPS.
`Differential GPS technology (DGPS) may provide
`Such
`location accuracies to within three meters (zdrms).
`accuracies allow, for example, accurate positioning of a
`delivery truck on a street map or precise locating for an
`in-vehicle navigation system.
`The precision of the GPS
`system is improved by broadcasting differential
`correction data to a GPS receiver.
`A typical DGPS
`positioning system, such as the one implemented by the
`U.S. Coast Guard, uses known position coordinates of a
`reference station to compute corrections to GPS
`
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`
`
`receiver from the satellites, related by the speed of
`light. Prior to correction for the clock bias C5,
`the
`apparent ranges of the satellites are all in error by a
`fixed amount and are called pseudoranges.
`Two positioning services are provided by the NAVSTAR
`The precise positioning service (PPS) which is
`GPS.
`reserved for military use provides accuracy to within
`twenty-one meters (2drms). The statistical term "zdrms"
`refers to a value that falls within two standard
`deviations (using the root-mean-squared method) of the
`sampled performance data mean. Therefore, a stated
`accuracy of twenty-one meters (zdrms) means that the
`position error has an error of less than twenty-one
`meters approximately ninety-five percent of the time.
`The standard positioning service (SPS) which is
`available for general use provides accuracy to within
`thirty meters (zdrms). However,
`the SPS signal accuracy
`is intentionally degraded to protect U.s. national
`security interests. This process, called selective
`availability, degrades the accuracy of SPS position fixes
`to within one hundred meters (zdrms).
`The SP5 may be
`degraded in a number of ways, for example, by providing
`slightly inaccurate satellite orbital data to the
`receivers or by dithering the ranging intormation.
`Certain applications require better accuracy than
`provided by degraded SPS, SPS, or even PPS.
`Differential GPS technology (DGPS) may provide
`Such
`location accuracies to within three meters (zdrms).
`accuracies allow, for example, accurate positioning of a
`delivery truck on a street map or precise locating for an
`in-vehicle navigation system.
`The precision of the GPS
`system is improved by broadcasting differential
`correction data to a GPS receiver.
`A typical DGPS
`positioning system, such as the one implemented by the
`U.S. Coast Guard, uses known position coordinates of a
`reference station to compute corrections to GPS
`
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`parameters, error sources, and resultant positions. This
`
`correction data is transmitted to GPS receivers to refine
`
`received position signals or computed position.
`
`Traditional DGPS positioning systems require the
`user to carry both a GPS receiver and an additional
`
`communications device to receive the correction data.
`
`the Coast Guard implementation requires a
`For example,
`maritime radio beacon receiver to obtain GPS correction
`
`data. This Coast Guard system is described in a document
`
`entitled "Implementation of the U.S. Coast Guard's
`
`Differential GPS Navigation Service," U.S.C.G.
`
`Headquarters, Office of Navigation Safety and Waterway
`Services, Radio Navigation Division, June 28, 1993.
`
`Another system, described in U.S. Patent No. 5,311,194,
`entitled "GPS Precision Approach and Landing System for
`Aircraft" and issued to Brown, describes a differential
`
`GPS implementation for use in a precision approach and
`
`landing system for aircraft.
`
`In this system,
`
`the
`
`aircraft is required to carry a broadband GPS receiver
`
`with added functionality to receive pseudolite signals
`that contain the correction data.
`
`Differential positioning system 10 1h FIGURE 1
`
`implements the DGPS concept using posztionxnq system 12
`integrated with mobile communications netuorx 14 to
`
`accurately determine the location or vehicle 16.
`
`Differential positioning system 10 utilizes components of
`mobile communications network 14 as reference stations
`
`that provide correction data to vehicle 16 over an
`
`existing communications link, such as the control
`
`channel, overhead message stream, or paging channel of a
`
`cellular telephone network. Mobile communications
`
`network 14 may be a cellular telephone network,
`
`specialized mobile radio (SM), enhanced specialized
`
`mobile radio (ESMR), a personal communications service
`
`(PCS), a satellite—based or land-based paging system, a
`
`citizen's band (CB), a dedicated radio system, such as
`
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`those used by police and firefighters, or any other
`appropriate mobile communications technology.
`Differential positioning system 10 is described with
`reference to location of vehicle 16.
`The present
`invention contemplates location of all types of vehicles,
`including cars,
`trucks, airplanes, boats, barges, rail
`cars,
`truck trailers, or any other movable object that is
`desirable to locate or track. Furthermore, differential
`positioning system 10 can also be used to accurately
`locate a person carrying a portable or hand-held mobile
`unit 17. Potential applications of this technology may
`include delivery service dispatch,
`less-than-full-load
`(LTL)
`trucking applications,
`in—vehicle navigation
`systems, surveying applications, collision avoidance,
`emergency location using mobile 911 services, or any
`other application requiring accurate positioning
`information of a vehicle, object, or person.
`Differential positioning system 10 provides a more
`accurate position fix than currently available navigation
`services, and may provide these fixes near
`instantaneously or "on the fly."
`In some applications,
`low frequency and low accuracy updates are sufficient,
`but other applications may need better accuracy and
`higher frequency updates in near real—time.
`For example,
`a delivery truck may require accurate, high frequency
`position fixes for in-vehicle navigation to locate a
`specific delivery address or to provide real—time
`directions to the driver. Differential positioning
`system 10 may provide these high frequency updates
`without relying on off-vehicle computations prevalent in
`previous DGPS implementations.
`In addition,
`the same
`delivery truck may send lower frequency position reports
`to a remote location. These position reports may be sent
`at fixed time intervals, on-demand, or as a result of a
`predetermined reporting event. Differential positioning
`system 10 may provide both low and high frequency
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`position fixes and reports in such a hybrid navigation
`and position reporting system.
`
`Satellite—based positioning system 12 is a
`
`navigation system using NAVSTAR GPS, GLONASS, or other
`
`satellite-based or land-based radio navigation system to
`provide ranging data to mobile unit 17. Satellites 18,
`20, 22 maintain accurate and synchronized time and
`
`simultaneously transmit position signals that contain
`
`satellite specific and system information required by
`mobile unit 17 to generate position fixes.
`The position
`
`signals transmitted by satellites 18, 20, 22 may include
`
`high precision clock and ephemeris data for a particular
`
`satellite,
`
`low precision clock and ephemeris (called
`
`"almanac") data for every satellite in the constellation,
`
`health and configuration status for all satellites, user
`
`text messages, and parameters describing the offset
`between GPS system time and UTC.
`
`Mobile unit 17 receives position signals over
`
`message data streams 26, 28, 30 from satellites 18, 20,
`
`22, respectively. Additional satellites (not shown) may
`also communicate message data streams to mobile unit 17.
`
`Typically, mobile unit 17 receives at least four
`
`satellite message data streams to solve for position
`
`information independent of inherent clock bias (cg
`between positioning system 12 and mobile unit 17.
`
`Currently the NAVSTAR GPS system has twenty—one active
`
`satellites at 11,000 mile orbits of fifty-five degrees
`
`inclination with the equator.
`
`In normal conditions,
`
`mobile unit 17 may receive position signals from seven
`satellites.
`
`Using information from position signals 26, 28, 30
`
`and optionally additional message data streams, mobile
`
`unit 17 may determine its position using accurate
`
`satellite position information transmitted by satellites
`
`18, 20, 22 and pseudorange data represented by the time
`
`of arrival of message data streams 26, 28, 30 to mobile
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`unit 17. Using SPS this position fix may be accurate to
`within 30 meters (2drms) or 100 meters (zdrms) when
`selective availability degradation is activated.
`If
`mobile unit 17 is allowed to operate using PPS,
`then the
`position fix may be accurate to within 21 meters (2drms).
`To provide a more accurate position fix for mobile
`unit 17, satellites 18, 20, 22 also transmit message data
`streams 32, 34, 36, respectively,
`to a reference
`positioning receiver 38 on or in proximity to a
`transmitter site 40 of mobile communications network 14.
`Reference positioning receiver 38 performs similar
`calculations to determine a position fix from position
`signals received from satellites 18, 20, 22. Reference
`positioning receiver 38 compares the computed position
`fix to known position coordinates and generates
`correction data for transmission over correction data
`stream 44 to mobile unit 17 for further refinements of
`position fix provided by mobile positioning receiver 24
`(FIGURE 4) .
`The known position coordinates of transmitter site
`40 may be determined by traditional surveying techniques.
`In addition, reference positioning receiver 38 may
`perform position fixes over a statistically significant
`
`coordinates. Filtering or averaging position fixes by
`reference positioning receiver 38 over time removes or
`substantially reduces the effect of selective
`availability degradation and may provide a more accurate
`position determination than uncorrupted SPS or even PPS.
`One type of correction data generated by reference
`positioning receiver 38 is a position correction which is
`applied to the position fix of mobile positioning
`(FIGURE 4) of mobile unit 17 to achieve a
`receiver 24
`The position correction may
`more accurate position fix.
`be in latitude/longitude, compass direction and distance,
`or any other appropriate coordinate system. when using a
`
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`GPS positioning system 12, this technique provides
`
`accurate correction data when mobile unit 17 and
`
`reference positioning receiver 38 are located in a
`
`satellite common view area of approximately thirty square
`miles.
`In the satellite common view area all receivers
`
`operating in positioning system 12 receive approximately
`the same pseudorange errors assuming they are all
`
`listening to the same group of satellites 18, 20, 22.
`This correction method places less correction data in
`
`correction data stream 44 than other methods, but the
`
`validity of those correction terms decreases rapidly as
`the distance between mobile unit 17 and reference
`
`positioning receiver 38 increases.
`
`The usefulness of
`
`this correction method is impaired when mobile unit 17
`
`and reference positioning receiver 38 compute their
`
`position fixes using position signals from different
`
`satellites.
`
`Furthermore, this method requires that both
`
`mobile unit 17 and reference positioning receiver 38
`compute a navigation solution.
`
`In an alternative correction method, reference
`
`positioning receiver 38 computes pseudorange corrections
`
`(PRCs)
`
`to each satellite 18, 20, 22, which are then
`
`transmitted over correction data stream 44 to mobile unit
`
`17 to refine its navigation solution.
`
`The PRCs for
`
`satellites 18, 20, 22 in view of reference positioning
`
`receiver 38 are the difference between the pseudorange
`
`and the computed range to each satellite 18, 20, 22 based
`on the known position coordinates of reference
`
`positioning receiver 38.
`
`Each PRC message includes an
`
`identification of the satellite 18, 20, 22 and a linear
`
`measure of the PRC. Although this method may include
`
`more transmission of data, it may result in a more
`
`accurate position fix. Furthermore, such a scheme
`
`provides additional flexibility to allow mobile unit 17
`
`to use navigation data from any of the satellites that
`
`reference positioning receiver 38 has furnished PRCs.
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`An additional correction method generates position
`corrections based on possible combinations of satellites
`18, 20, 22 currently in view of reference positioning
`receiver 38. This approach may be computationally
`intensive at reference positioning receiver 38, but would
`allow for a simple adjustment of the solution computed by
`mobile unit 17.
`The number of position corrections (Pcs)
`may be computed using the following formula:
`
`Ab.qfPCs=
`
`m
`
`r! (n-r)!
`
`where n is the number of satellites in the common view
`area and r is the number of satellites used in the
`position correction calculation.
`For example, for a
`position fix using four satellites and with six
`satellites in the satellite common view area, reference
`positioning receiver 38 would have to generate fifteen
`Pcs corresponding to fifteen combinations of four
`satellites each.
`Each satellite 18, 20, 22 sends an identifier in its
`respective message data stream. Both mobile unit 17 and
`reference positioning receiver 38 may use these
`that
`identifiers to generate satellite group IDs (SGIDS)
`identify the specific combination of satellites used for
`a position fix. Reference receiver 38 may generate the
`position correction for fifteen combinations (four
`satellites chosen from a total of six), and tag the
`position corrections with the appropriate sGIDs. Mobile
`unit 17, having determined an SGID for its position fix,
`may then choose the proper position correction identified
`by the same SGID to ensure that mobile unit 17 and
`reference positioning receiver 38 use the same
`combination of satellites. Using this scheme with the
`NAVSTAR GPS,
`there would be 10,626 unique SGIDs for
`
`Page 000839
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`satellite combinations of four out of twenty—four
`satellites in the planned constellation.
`
`The size and structure of a correction data message
`generated by reference positioning receiver 38 and
`
`transmitter over correction data stream 44 depends on the
`correction method employed and the precision required.
`A
`single pseudorange correction (PRC) message for a
`
`satellite in the satellite common view area may include a
`satellite ID,
`the range correction in a selected
`
`precision, and other associated portions of the message,
`such as a header, delimiter, and checksum.
`A typical PRC
`message for six satellites described in the Motorola GPS
`
`is fifty—two
`(October 1993)
`Technical Reference Manual
`bytes long,
`including the header, delimiter, and
`checksum.
`
`The size and structure of a single position
`
`correction message also depends on the precision required
`and the transmission protocol.
`A typical position
`
`correction message may include a four byte SGID (1
`
`through 10,626), a one byte latitude correction, and a
`
`one byte longitude correction.
`
`A multiple position
`
`correction message for fifteen satellite combinations
`
`(four satellites chosen from a total of six) may total 90
`
`bytes of correction data. Appropriate header, delimiter
`
`and checksum bytes consistent with the communication
`
`protocol of mobile communications network 14 may be
`added.
`
`The precision of pseudorange or position corrections
`
`depends on the anticipated range of error and the number
`
`For example,
`of bytes allocated to the correction data.
`one byte of eight bits may provide correction in the
`
`range of +/- 127 meters with one meter bit resolution.
`
`One byte may also provide correction in 0.25 meter bit
`
`resolution over a range of approximately +/- 32 meters.
`
`The precision, correction range, and byte allocation is a
`
`design choice that considers various factors, such as the
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`the
`available bandwidth in correction data stream 44,
`accuracy of the unrefined position fix at mobile unit 17,
`the correction method employed, and the inherent
`
`inaccuracies of positioning system 12.
`Correction data stream 44 allows correction data to
`be transmitted from reference positioning receiver 38 to
`mobile unit 17.
`In one embodiment, correction data
`stream 44 may be the control channel, paging channel, or
`overhead message stream currently implemented in cellular
`telephone technology. Currently,
`the control channel
`provides paging of incoming calls, hand-off instructions,
`and other features of the cellular telephone network, but
`may be modified by one skilled in the art to include
`transmission of correction data. Correction data stream
`44 may also be implemented using any other communication
`link between transmitter site 40 and mobile
`communications device 42 (FIGURE 4)
`in mobile unit 17,
`whether or not the communication link requires seizing of
`a voice or data channel.
`There are several developing technologies that may
`provide a convenient implementation of correction data
`stream 44.
`For example, cellular digital packet data
`(CDPD)
`technology allows integration of data and voice
`using the existing cellular telephone infrastructure.
`In a CDPD system, digital packets of data and analog
`
`code division multiple access (CDMA
`multiple access (TDMA), allow digital data and digital
`voice signals to be interspersed on a communications
`channel. These technologies integrate digital data
`transmission in a mobile communications network 14, and
`therefore provide a convenient implementation scheme for
`correction data stream 44.
`Using the technologies mentioned above or other
`appropriate digital communications link, transmitter site
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`40 may either continuously broadcast correction data over
`
`correction data stream 44, such as in the control channel
`
`of the cellular telephone network, or only send
`
`correction data to mobile unit 17 when requested by a
`feature code request or by any other appropriate manner.
`
`Transmitter site 40 may send correction data to mobile
`
`unit 17 in one large packet or in several smaller packets
`interspersed with other data used for mobile
`
`The correction data may be packaged in
`communications.
`existing, but unused, bytes of the control channel or in
`
`a dedicated protocol.
`
`one possible implementation would
`
`place correction data in the extended protocol described
`
`in the EIA/TIA—533 mobile communications standard, which
`provides for bidirectional communication between
`
`transmitter site 40 and mobile unit 17.
`
`Reference positioning receiver 38 may continuously
`receive position updates and continuously compute
`correction data for transmission to mobile unit 17 over
`
`correction data stream 44. Alternatively, reference
`
`positioning receiver 38 may send correction data over
`
`correction data stream 44 at predetermined time
`
`intervals, at designated times when correction data
`
`stream 44 can accommodate the additional traffic, or when
`requested by mobile unit 17.
`
`Reference positioning receiver 38 may include an
`additional capability to ensure that correction data
`
`transmitted to mobile unit 17 by transmitter site 40 is
`
`current. This may be accomplished by including a time
`
`stamp in the correction data message to account for
`
`latency in the system. Using GPS technology as an
`
`example, satellites 18, 20, 22 in positioning system 12
`provide position navigation data each second.
`Reference
`
`positioning receiver 38 may include an additional byte
`that indicates the delay in seconds of the correction
`
`data.
`
`The mobile unit 17 may save time-stamped position
`
`signals and later synchronize and correct the position
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`signals with the time-stamped correction data received
`from transmitter site 40.
`The post—processing to refine
`past position fixes may be performed by mobile
`positioning receiver 24 (FIGURE 4) or other separate
`processor in mobile unit 17.
`correction data stream 44 may be part of the control
`
`channel, part of a seized voice or data channel, or a
`separate channel requiring mobile unit 17 to re-tune to
`the correction data stream channel to receive valid
`
`corrections for the area. Mobile unit 17 may
`
`continuously monitor correction data stream 44
`transmitted from transmitter site 40. Furthermore,
`mobile unit 17 may alternately tune between several
`correction data streams 44 from several transmitter sites
`40 to determine the strongest signal, usually relating to
`the nearest transmitter site 40. This strongest channel
`select feature of mobile unit 17 assures that reference
`positioning receiver 38 and mobile unit 17 will be in
`close proximity and receive position signals from the
`same group or nearly the same group of satellites 18, 20,
`22.
`For a typical transmitter site spacing in a cellular
`telephone network,
`the distance between BOblXe unit 17
`and reference positioning receiver 38 may be less than
`five miles, well within the satellite common view area of
`the GPS system.
`Differential positioning system 10, as illustrated
`in FIGURE 1, contemplates placing reference positioning
`receiver 38 on each transmitter site 40 within mobile
`communications network 14. When using GPS technology as
`positioning system 12 and a cellular telephone network as
`mobile communications network 14,
`the satellite common
`view area may be much larger than the coverage area of a
`single transmitter site 40,
`thereby obviating the need to
`have reference positioning receivers 38 on each
`transmitter site 40.
`For example, differential
`positioning system 10 may include reference positioning
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`receivers 38 on selected transmitter sites 40 of mobile
`
`comunications network 14.
`
`In this configuration, mobile
`
`unit 17, which may be capable of simultaneously
`
`monitoring correction data streams 44 from multiple
`transmitter sites 40, may still receive correction data
`
`from a transmitter site 40 that is currently not
`
`providing communication service to mobile unit 17.
`
`Selected transmitter sites 40 equipped with reference
`
`positioning receivers 38 may be spaced so that mobile
`
`unit 17 located anywhere in mobile communications network
`
`14 can receive correction data of sufficient signal
`strength from one of the selected transmitter sites 40
`
`equipped with reference positioning receivers 38.
`FIGURE 2 shows an alternative embodiment of
`
`differential positioning system 10 that places reference
`receivers 38 on selected transmitter sites 40 in mobile
`
`communications network 14. As in FIGURE 1,
`
`transmitter
`
`site 40 is associated with reference positioning receiver
`
`38, which receives position signals in message data
`
`streams 32, 34, 36 from satellites 18, 20, 22,
`
`respectively. However, mobile unit 17 is located in an
`
`area serviced by transmitter site 46, which is not
`
`equipped with reference positioning receiver 38.
`
`Furthermore, mobile unit 17 is unable to receive
`
`correction data directly from transmitter site 40 due to
`
`the inability to monitor communications from transmitter
`
`sites 40 and 46,
`
`the distance from transmitter site 40,
`
`or other reasons. However, mobile unit 17 is close
`
`enough to reference positioning receiver 38 to receive
`
`navigation data from at least a subset of satellites 18,
`
`20, 22 serving reference positioning receiver 38. Using
`
`any of the correction methods described above with
`
`reference to FIGURE 1, reference positioning receiver 38
`
`generates correction data and transmits this correction
`
`data through link 48 to transmitter site 46. Transmitter
`
`site 46 transmits correction data generated by reference
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`positioning receiver 38 over correction data stream 44 to
`mobile unit 17. Mobile unit 17 uses the correction data
`
`to refine a position fix derived from position signals
`
`received from satellites 18, 20, 22 over message data
`
`streams 26, 28, 30.
`
`Differential positioning system 10, illustrated in
`
`FIGURE 2, reduces the number of reference positioning
`
`receivers 38 required by networking correction data
`
`through link 48 between transmitter sites 40, 46. Link
`48 between transmitter sites 40, 46 may include microwave
`
`communications, bidirectional paging or control channels,
`
`direct land-line connections, switching stations such as
`
`MTSOS, or any other appropriate communications device to
`
`send correction data from transmitter site 40 to
`
`transmitter site 46.
`
`FIGURE 3 is a schematic representation of
`
`transmitter site 40 associated with reference positioning
`
`receiver 38. Reference positioning receiver 38 may be
`
`mounted directly on transmitter site 40 or on a separate
`structure or mounting. Reference positioning receiver 38
`
`includes an antenna 50, receiver 51, controller 52, and
`
`The following description relates to the
`memory 54.
`operation of reference positioning receiver 38 with a GPS
`positioning system, however,
`the same concepts apply to
`other land—based and satellite-based positioning systems.
`Reference positioning receiver 38 receives position
`
`signals in message data streams 32, 34, 36 from
`satellites 18, 20, 22, respectively.
`The position
`signals include navigation data, such as ephemeris,
`almanac, and clock correction data. Ephemeris data
`includes detailed information about the specific
`
`the almanac
`satellite course over the next two hours,
`data includes less detailed information about the
`complete satellite constellation for a longer period, and
`the clock correction data includes information to correct
`for clock errors.
`The satellite transmissions received
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`by antenna 50 consist of a direct sequence spread
`
`spectrum signal containing the ephemeris, almanac, and
`
`clock correction data at a rate of fifty bits per second.
`
`In the case of the SPS, a pseudorandom noise signal with
`
`a chip rate of 1.023 MHZ that is unique to each satellite
`
`is used to spread the spectrum of the information which
`
`is then transmitted on a center frequency of 1575.42 Mz.
`
`Receiver 51 receives satellite position signals
`
`having a bandwidth of approximately 2 MHz and a signal-
`
`to-noise ratio of approximately -20 dB.
`
`The relative
`
`movement between satellites 18, 20, 22 and reference
`
`positioning receiver 38 causes an additional Doppler
`
`frequency offset from the GPS center frequency.
`
`To
`
`recover the navigation data and measure the propagation
`
`time of the satellite position signals, receiver 51 must
`
`cancel or allow for the Doppler frequency offset and
`
`generate the proper coarse/acquisition code associated
`
`with each satellite 18, 20, 22 to despread the signal.
`
`once synchronization with the pseudorandom noise signal
`
`is achieved, receiver 51 may extract the ephemeris,
`
`almanac, and clock correction data and pass this
`information to controller 52.
`
`Controller 52 receives navigation data from at least
`three satellites and uses this information to determine a
`
`navigation solution based on well—known triangulation
`
`techniques.
`
`In a four satellite fix, with each satellite
`
`position represented by coordinates (Xn, Yn, Zn) with the
`
`indice n equal to one through four,
`
`the position
`
`coordinates (X, Y, Z) of reference positioning receiver
`
`38 may be determined by solving the following equations:
`
`(x,
`
`(x2
`
`(x.
`
`(x.
`
`+ (Y. - W + (2.
`
`z)2 = (R.
`
`+ (Y. - W + (22
`
`(Y. - if
`
`(2.
`
`(Y. - W (2.
`
`z)2
`
`z)2
`
`z)2
`
`(R.
`
`(R.
`
`(R.
`
`Page 000846
`
`
`
`WO 96115636
`
`PCTlUS95Il4862
`
`where R1, R2, R3, R. are pseudorange measurements from the
`satellites and C5 is a common clock bias. Controller 52
`
`may use certain data stored in memory 54 to arrive at a
`navigation solution. Controller 52 may then compare the
`instantaneous navigation solution (X, Y, Z)
`to known
`
`position coordinates (X0, Y9, Z0) stored in memory 54 to
`generate position correction data in latitude/longitude,
`compass direction and distance, or other appropriate
`coordinate system.
`
`In an alternative embodiment, controller 52 may
`
`receive ephemeris, almanac, and clock correction data
`from satellites 18, 20, 22 and compute a pseudorange (Rm
`
`for each satellite.
`
`Since the satellite signal contains
`
`information on the precise satellite orbits and
`
`controller 52 has known position coordinates (X0, Y0, ZQ
`
`stored in memory 54,
`
`the true range to each satellite 18,
`
`By comparing the true range
`20, 22 can be calculated.
`and the measured pseudorange, a pseudorange correction
`(PRC) for each satellite 18, 20, 22 may be computed and
`sent as correction data. As described above with
`
`reference to FIGURE 1, controller 52 may also provide
`position correction data based on navigation solutions
`using all possible combinations of satellites 18, 20, 22
`currently in view of reference positioning receiver 38.
`Correction data in any of the various forms
`
`described above is sent by controller 52 to channel
`controller 56 of transmitter site 40 over communication
`link 58. Communication link 58 may be a direct wire
`connection, a radio communication link, a connection
`through a switched telephone system, or other appropriate
`communication link. Depending on the configuration of
`differential positioning system 10, channel controller 56
`may send correction data to radio duplexer 60 for
`transmission over transmitter site antenna 62 to mobile
`unit 17. Alternatively, channel controller 56 may pass
`
`Page 000847
`
`
`
`PCUU$HnM2
`
`correction data through link 48 to transmitter site 46
`
`currently serving mobile unit 17.
`
`Also shown in FIGURE 3 as part of transmitter site
`
`40 are time—of—arrival
`
`(TOA) data generator 64 and clock
`
`66 that may be used in an alternative positioning system
`200 described with reference to FIGURE 6.
`TDA data
`
`generator 64 generates a TOA data message and sends this
`
`message to channel controller 56 for transmission to
`
`mobile unit 17 over transmitter site antenna 62.
`
`The TOA
`
`data message may include a precise time of transmission
`
`based on information maintained by clock 66. Clock 66
`
`and TOA data generator 64 are shown as elements of
`
`transmitter site 40, but it should be understood that
`
`their functions may also be implemented in a central or
`
`distributed device accessible by transmitter sites 40, 46
`of mobile communications network 14.
`
`FIGURE 4 is a schematic representation of a mobile
`
`unit 17 that includes mobile positioning receiver 24,
`
`mobile communications device 42, and other associated
`
`hardware and software, described below. Mobile
`
`positioning receiver 24 is similar in construction and
`
`function to reference positioning receiver 38 and
`
`includes an antenna 82, receiver 84, controller 86, and
`
`memory 88.
`
`In operation, mobile positioning receiver 24
`
`receives position signals from satellites 18, 20, 22 over
`
`message data streams 26, 28, 30 at antenna 82. Receiver
`
`84 processes these signals to extract ephemeris, almanac,
`and clock correction data. Controller 86 receives this
`
`information and computes a navigation solution or
`
`pseudorange measurements. These calculations performed
`
`by controller 86 may use data stored in memory 88.
`
`Mobile communications device 42 includes an antenna
`
`In operation,
`transceiver 92, and hand set 94.
`90,
`mobile communications device 42 receives correction data
`
`at antenna 90 over correction data stream 44.
`
`The
`
`correction data may be transmitted directly from
`
`Page 000848
`
`
`
`W0 96/15636
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`PCTIUS95/14862
`
`transmitter site 40 equipped with reference positioning
`
`receiver 38 as described with reference to FIGURE 1, or
`indirectly through link 48 and transmitter site 46 as
`described with reference to FIGURE 2. As described
`above, the correction data may be in a variety of forms,
`including single or multiple position corrections, or
`pseudorange corrections to each satellite. Correction
`data is then stripped from correction data stream 44 by
`
`transceiver 92. Correction data may be passed to
`processor 100 over link 95 or over any other appropriate
`path, such as through bus drivers 112 and modem or dual
`tone multifrequency (DTHT) coder/decoder 110. Hand set
`94 provides traditional voice or data communication using
`mobile communications device 42.
`locating,
`Processor 100 manages the communicating,
`and reporting features of mobile unit 17. Processor 100
`receives a navigation solution or pseudorange
`measurements from controller 86 and correction data from
`transceiver 92. Coupled to processor 100 is memory 102
`which may contain programs, databases, and other
`information required by processor 100 to perform its
`functions.
`For example, memory 102 may contain a table
`of known position coordinates of transmitter sites 40 for
`use in computing the position of mobile unit 17 in the
`alternative positioning system 200 described with
`reference to FIGURE 6. Memory 102 may be random access
`memory (RAM), read-only memory (ROM), CD—RoM,
`removable
`memory devices, or any other device that allows storage
`or retrieval of data.
`Processor 100 and controller 86, as well as memory
`102 and memory 88, may be separate or integral components
`of mobile unit 17.
`For example, controller 86 may
`include a port that directly receives correction data and
`allows mobile positioning receiver 24 to output a refined
`position fix. Mobile unit 17 contemplates any
`
`Page 000849
`
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`
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`PCT/US95/14862
`
`arrangement, processing capability, or task assignment
`between controller 86 and processor 100.
`
`In operation, processor 100 generates a refined
`
`position fix for mobile unit 17 based on the navigation
`solution or pseudorange measurements
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