(12) United States Patent
`Wortham
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,748,226 B1
`Jun. 8, 2004
`
`US006748226B1
`
`(54) SYSTEM AND METHOD FOR LOCATINGA
`MOBILE UNIT WITHIN THE SERVICE
`AREA OF A MOBILE COMMUNICATIONS
`
`NETWORK
`_
`(75) Inventor‘ Larry C'W°I1ham>Gar1and’TX(US)
`
`(73) Assignee: Minorplanet Systems USA, Inc.,
`Richardson, TX (US)
`
`4,382,178 A
`4,428,052 A
`4,428,057 A
`4,435,711 A
`
`5/1983 Mori ......................... .. 377/17
`1/1984 Robinson et al. ......... .. 364/436
`1/1984 Setliff et al. .............. .. 364/521
`3/1984 Ho et al. . . . . . . . .
`. . . .. 343/389
`
`343/357
`4/1984 Taylor et al.
`4,445,118 A
`343/456
`4,547,778 A 10/1985 Hinkle et al.
`4,590,569 A
`5/1986 Rogoff et al. ............. .. 364/452
`
`(List continued on next page.)
`
`FOREIGN PATENT DOCUMENTS
`
`( * ) Notice:
`
`Subject‘ to any disclaimer, the term of this
`Pawnt 1S mended 0r adlllsted under 35
`U-S-C- 154(b) by 0 days-
`
`AU
`EP
`EP
`
`133767589
`0242099
`0290725
`
`4/1993
`10/1987
`11/1988
`
`___________ __ 6015/5/02
`........... .. G01S/5/14
`.......... .. H04Q/7/04
`
`(21) AppL NO‘: 09/219,113
`(22) Filed:
`Dec. 23, 1998
`
`Related US. Application Data
`
`(63) Continuation of application No. 08/340,755, ?led on Nov.
`16, 1994.
`
`(51) Im. c1.7 ................................................ .. H04Q 7/34
`(52) US. Cl. .............................. .. 455/456.6 455/456.1-
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`(58) Field of Search ............................... .. 455/456, 457,
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`451, 457, 463, 464, 389
`
`(56)
`
`References Cited
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`Date
`
`(L151 Continued 0H IleXt page.)
`Primary Examiner—Nay Maung
`Assistant ExaminerA’h?ip J- Sobutka
`(74) Attorney) Agent» 0’ Fi””—L°Cke Lidden 8‘ SaPP LLP
`(57)
`ABSTRACT
`
`A differential positioning system (10) includes components
`of a satellite-based or land-based positioning system (12)
`and components of a mobile communications network (14).
`The differential positioning system (10) provides accurate
`and immediate position information to a mobile unit (17). A
`transmitter site (40) of a mobile communications network
`(14) is associated With a reference positioning receiver (38).
`The reference positioning receiver (38) generates correction
`data for transmission to the mobile unit (17). The mobile unit
`(17) includes a mobile communications device (42) for
`receiving the correction data generated by the reference
`positioning receiver (38) and a mobile positioning receiver
`(24) for generating a position ?x. The mobile unit (17)
`re?nes the position ?x generated by the mobile positioning
`receiver (24) using correction data received by the mobile
`communications device (42).
`
`16 Claims, 3 Drawing Sheets
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 1
`
`

`
`US 6,748,226 B1
`Page 2
`
`US. PATENT DOCUMENTS
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`. . . .. 455/56
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`- 342/457
`2
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`golp ~~~~ ~~~~~~~~~~~~~~~~~~~~ ~~
`/
`/
`a y et a ' """"""""" "
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`342/457
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`.. 340/825.06
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`_ _ _ _ _ __ 342/457
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`342/357
`9/1991 Duffett-Smith ~~~~~~~~~~~ ~~ 342/457
`ghelffei
`~~~~~~~~~~~~~~~~~~ ~~
`/
`ut er an '
`'
`/
`’
`’
`2/1992 Heffernan .................. .. 379/60
`5,090,050 A
`6/1992 Barnard .................... .. 342/357
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`. 342/419
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`7/1992 Sheffer et al.
`379/39
`5,131,019 A
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`.. 379/59
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`. . . . .. 379/59
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`.
`5,159,625 A 10/1992 Zicker ....................... .. 379/59
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`. 342/357
`
`,
`
`,
`
`rei ......................... ..
`
`EP
`EP
`EP
`
`GB
`
`WO
`WO
`
`* 6/1989
`320913
`0 320 913 A2
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`
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`
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`
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`
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`
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`
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`_ "
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`”
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`,
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`Board System Document <DESIGN~TXT:_DeSign Process
`-
`,
`-
`-
`-
`for the United States Coast Guard s Differential GPS Nav1
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`”
`gat1on Servece, US. Coast Guard, US. Coast Guard
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`ior lgtegrgteld Dlffirenngl S132’ zDl?eremml Corrections
`"6'
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`> Pages
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`Author Unknown, “Westinghouse Sens
`Transit Flee
`B .
`_ E S t
`F
`F.
`,, I .a, IVHS J
`16
`lglgsgne? _ ys ems Orms 1m’ "S1 e
`a
`an‘
`’
`
`a,
`
`-
`
`-
`
`-
`
`~
`
`-
`
`342/357
`2/1995 Eberwine
`5,392,052 A
`455/54_1
`2/1995 Sasuta er a1, __
`5,392,458 A
`. . . .. 379/59
`3/1995 Gooch . . . . . . .
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`3/1995 Jones et al. .
`5,400,020 A
`342/457 X
`4/1995 Luna ~~~~ "
`574067491 A
`455/56'1 X
`6/1995 Schuchman et a '
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`6/1995 Sprague et al. ........... .. 364/449
`5,422,816 A
`.
`7/1995 Rrrner ....................... .. 379/59
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`8/1995 Sennott et al. ....... .. 342/357.03
`5,438,517 A
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`5,463,554 A 10/1995 Araki et al
`364/444
`455/12-1
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`2/1997 Dunn et al. ............. .. 379/60 X
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`
`_
`_
`_
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`.
`.
`.
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`.
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`_
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`
`* cited by examiner
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 2
`
`

`
`U.S. Patent
`
`Jun. 8,2004
`
`Sheet 1 of3
`
`US 6,748,226 B1
`
`1
`
`FIG.
`
`2
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 3
`
`

`
`U.S. Patent
`
`Jun. 8,2004
`
`Sheet 2 of3
`
`US 6,748,226 B1
`
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`CENTRAL
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 4
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 5
`
`

`
`US 6,748,226 B1
`
`1
`SYSTEM AND METHOD FOR LOCATING A
`MOBILE UNIT WITHIN THE SERVICE
`AREA OF A MOBILE COMMUNICATIONS
`NETWORK
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This application is a continuation of pending U.S. appli
`cation Ser. No. 08/340,755, ?led Nov. 16, 1994, by Larry C.
`Wortham and entitled “Locating System and Method Using
`a Mobile Communications Network.”
`
`10
`
`TECHNICAL FIELD OF THE INVENTION
`
`This invention relates to locating systems, and more
`particularly to a locating system and method using a mobile
`communications netWork.
`
`15
`
`BACKGROUND OF THE INVENTION
`Mobile communications technology has enjoyed substan
`tial groWth over the past decade. Many cars, trucks,
`airplanes, boats, and other vehicles are equipped With
`devices that alloW convenient and reliable mobile commu
`nication through a netWork of satellite-based or land-based
`transceivers. Advances in this technology have also led to
`Widespread use of hand-held, portable mobile communica
`tions devices.
`Many customers of mobile communications systems also
`require an accurate determination of their position, and
`perhaps reporting of this position to a remote location. For
`example, a cellular telephone in a vehicle or carried by a
`person offers a convenient communication link to report
`position information. The position information may be gen
`erated by traditional positioning systems, including a
`satellite-based positioning system such as the global posi
`tioning system (GPS), or a land-based positioning system,
`such as LORAN-C. These approaches, hoWever, may not be
`suitable for particular applications that require great position
`accuracy.
`
`25
`
`35
`
`SUMMARY OF THE INVENTION
`
`45
`
`In accordance With the present invention, the disadvan
`tages and problems associated With previous techniques
`used to locate and report the position of a vehicle, person, or
`object equipped With a mobile communications device have
`been substantially reduced or eliminated. One aspect of the
`present invention provides a differential positioning system
`that integrates positioning technology With an existing
`mobile communications infrastructure.
`According to an embodiment of the present invention, a
`locating system using a cellular telephone netWork and a
`positioning system includes a reference positioning receiver
`having knoWn position coordinates. The reference position
`ing receiver receives ?rst position signals from the position
`ing system and generates correction data in response to the
`55
`?rst position signals and the knoWn position coordinates. A
`transmitter site of the cellular telephone netWork is coupled
`to the reference positioning receiver and transmits the cor
`rection data generated by the reference positioning receiver.
`A mobile unit in communication With the cellular telephone
`netWork and the positioning system receives correction data
`transmitted by the transmitter site. The mobile unit also
`receives second position signals from the positioning system
`and determines the location of the mobile unit in response to
`the second position signals and the correction data.
`According to another embodiment of the present
`invention, a system for locating a mobile unit Within the
`
`65
`
`2
`service area of a mobile communications netWork includes
`a plurality of transmitter sites having knoWn position
`coordinates, each transmitter site broadcasting time-of
`arrival (TOA) data. Amobile communications device on the
`mobile unit receives the TOA data transmitted by at least
`three transmitter sites. A memory on the mobile unit stores
`knoWn position coordinates of the transmitter sites. A pro
`cessor receives the TOA data from the mobile communica
`tions device and determines the position of the mobile unit
`in response to the TOA data received from the transmitter
`sites and the knoWn position coordinates of the transmitter
`sites stored in the memory.
`Important technical advantages of the present invention
`include improving the accuracy of existing positioning sys
`tems using a mobile communications system. In particular,
`existing transmitter sites of a mobile communications net
`Work may be used as reference points to transmit position
`correction data to mobile units Within the mobile commu
`nications netWork service area. Other important technical
`advantages include integration of communicating, locating,
`and reporting functions for an overall reduction in the cost
`and complexity of the system. For example, a differential
`GPS (DGPS) positioning system may use an existing com
`munications link, such as the overhead message stream of a
`cellular telephone netWork, to send correction data from the
`transmitter site to the mobile unit. Important technical
`advantages may also include accurate and immediate posi
`tion ?xes Without relying on calculations performed at a
`remote location. Other important technical advantages may
`also include implementation of a time-of-arrival (TOA)
`positioning system Within the mobile communications net
`Work Without land-based or satellite-based positioning tech
`nology. Other technical advantages are readily apparent to
`one skilled in the art from the folloWing ?gures, description,
`and claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present inven
`tion and for further features and advantages, reference is
`noW made to the folloWing description taken in conjunction
`With the accompanying draWings, Wherein like reference
`numerals represent like parts, in Which:
`FIG. 1 illustrates a differential positioning system;
`FIG. 2 illustrates an alternative embodiment of the dif
`ferential positioning system of FIG. 1;
`FIG. 3 is a schematic representation of a transmitter site
`associated With a reference positioning receiver;
`FIG. 4 is a schematic representation of a mobile unit;
`FIG. 5 is a schematic representation of a central host; and
`FIG. 6 illustrates an alternative positioning system.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 illustrates several components used in a differential
`positioning system 10. The system includes components of
`a satellite-based or land-based positioning system 12 and
`components of a mobile communications netWork 14. Dif
`ferential positioning system 10 provides accurate and imme
`diate position information to vehicle 16 equipped With a
`mobile unit 17.
`Positioning system 12 is illustrated as a satellite-based
`radio navigation system, such as the NAVSTAR positioning
`system (GPS). The description uses the NAVSTAR GPS as
`a representative positioning system 12, but any land-based
`or satellite-based system may be used. For example, posi
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 6
`
`

`
`US 6,748,226 B1
`
`3
`tioning system 12 may be a land-based LORAN-C, a space
`based GLONASS, or any other appropriate positioning
`technology. In general, positioning system 12 comprises a
`plurality of space-based or land-based transmitters that emit
`position signals.
`The NAVSTAR GPS consists of a number of satellites in
`approximately tWelve hour, inclined orbits of the earth, each
`satellite transmitting position signals. The GPS concept of
`operation is based upon satellite ranging. With position
`signals from three satellites, a GPS receiver can make an
`accurate calculation of its position in three dimensions. To
`make a valid position ?x, the GPS receiver measures the
`propagation times of position signals from the satellites to a
`very high accuracy. This is accomplished by synchroniZing
`the transmission of position signals to an atomic clock.
`HoWever, to reduce costs and complexity, the GPS receiver
`may not maintain such an accurate clock, Which introduces
`a clock bias (CB) betWeen the satellite clock and the GPS
`receiver clock. By measuring the apparent satellite signal
`propagation times from four satellites rather than three, the
`redundancy can be used to solve CE. The signal propagation
`times correspond to ranges of the GPS receiver from the
`satellites, related by the speed of light. Prior to correction for
`the clock bias CB, the apparent ranges of the satellites are all
`in error by a ?xed amount and are called pseudoranges.
`TWo positioning services are provided by the NAVSTAR
`GPS. The precise positioning service (PPS) Which is
`reserved for military use provides accuracy to Within
`tWenty-one meters (2 drms). The statistical term “2 drms”
`refers to a value that falls Within tWo standard deviations
`(using the root-mean-squared method) of the sampled per
`formance data mean. Therefore, a stated accuracy of tWenty
`one meters (2 drms) means that the position error has an
`error of less than tWenty-one meters approximately ninety
`?ve percent of the time.
`The standard positioning service (SPS) Which is available
`for general use provides accuracy to Within thirty meters (2
`drms). HoWever, the SPS signal accuracy is intentionally
`degraded to protect US. national security interests. This
`process, called selective availability, degrades the accuracy
`of SPS position ?xes to Within one hundred meters (2 drms).
`The SPS 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 information. Certain
`applications require better accuracy than provided by
`degraded SPS, SPS, or even PPS.
`Differential GPS technology (DGPS) may provide loca
`tion accuracies to Within three meters (2 drms). Such accu
`racies 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 US. Coast Guard, uses knoWn
`position coordinates of a reference station to compute cor
`rections to GPS parameters, error sources, and resultant
`positions. This correction data is transmitted to GPS receiv
`ers to re?ne 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. For example, the Coast
`Guard implementation requires a maritime radio beacon
`receiver to obtain GPS correction data. This Coast Guard
`system is described in a document entitled “Implementation
`of the US. Coast Guard’s Differential GPS Navigation
`Service,” U.S.C.G. Headquarters, Of?ce of Navigation
`
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`Safety and WaterWay Services, Radio Navigation Division,
`Jun. 28, 1993. Another system, described in US. Pat. 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 in FIG. 1 implements
`the DGPS concept using positioning system 12 integrated
`With mobile communications netWork 14 to accurately
`determine the location of 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 communi
`cations netWork 14 may be a cellular telephone netWork,
`specialiZed mobile radio (SMR), 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 those
`used by police and ?re?ghters, or any other appropriate
`mobile communications technology.
`Differential positioning system 10 is described With ref
`erence 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, emer
`gency 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 accu
`rate position ?x than currently available navigation services,
`and may provide these ?xes near instantaneously or “on the
`?y.” In some applications, loW frequency and loW accuracy
`updates are suf?cient, but other applications may need better
`accuracy and higher frequency updates in near real-time. For
`example, a delivery truck may require accurate, high fre
`quency position ?xes for in-vehicle navigation to locate a
`speci?c 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 fre
`quency position reports to a remote location. These position
`reports may be sent at ?xed time intervals, on-demand, or as
`a result of a predetermined reporting event. Differential
`positioning system 10 may provide both loW and high
`frequency position ?xes and reports in such a hybrid navi
`gation 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 simulta
`neously transmit position signals that contain satellite spe
`ci?c and system information required by mobile unit 17 to
`generate position ?xes. The position signals transmitted by
`satellites 18, 20, 22 may include high precision clock and
`
`T-Mobile / TCS / Ericsson EXHIBIT 1009
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 7
`
`

`
`US 6,748,226 B1
`
`5
`ephemeris data for a particular satellite, loW precision clock
`and ephemeris (called “almanac”) data for every satellite in
`the constellation, health and con?guration 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, respec
`tively. Additional satellites (not shoWn) may also commu
`nicate 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 (CB) betWeen positioning system 12 and
`mobile unit 17. Currently the NAVSTAR GPS system has
`tWenty-one active satellites at 11,000 mile orbits of ?fty-?ve
`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 pseu
`dorange data represented by the time of arrival of message
`data streams 26, 28, 30 to mobile unit 17. Using SPS this
`position ?x may be accurate to Within 30 meters (2 drms) or
`100 meters (2 drms) When selective availability degradation
`is activated. If mobile unit 17 is alloWed to operate using
`PPS, then the position ?x may be accurate to Within 21
`meters (2 drms).
`To provide a more accurate position ?x 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 commu
`nications netWork 14. Reference positioning receiver 38
`performs similar calculations to determine a position ?x
`from position signals received from satellites 18, 20, 22.
`Reference positioning receiver 38 compares the computed
`position ?x to knoWn position coordinates and generates
`correction data for transmission over correction data stream
`44 to mobile unit 17 for further re?nements of position ?x
`provided by mobile positioning receiver 24 (FIG. 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 ?xes over a statistically signi?cant period of time to
`determine the knoWn position coordinates. Filtering or aver
`aging position ?xes 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 SP5 or
`even PPS.
`One type of correction data generated by reference posi
`tioning receiver 38 is a position correction Which is applied
`to the position ?x of mobile positioning receiver 24 (FIG. 4)
`of mobile unit 17 to achieve a more accurate position ?x.
`The position correction may be in latitude/longitude, com
`pass direction and distance, or any other appropriate coor
`dinate system. When using a 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 cor
`rection data stream 44 than other methods, but the validity
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`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 ?xes 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 re?ne 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 identi?cation 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 ?x. Furthermore, such a scheme provides
`additional ?exibility to alloW mobile unit 17 to use naviga
`tion data from any of the satellites that reference positioning
`receiver 38 has furnished PRCs.
`An additional correction method generates position cor
`rections based on possible combinations of satellites 18, 20,
`22 currently in vieW of reference positioning receiver 38.
`This approach may be computationally intensive at refer
`ence 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:
`
`No. of PCs :
`
`n!
`
`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 ?x using
`four satellites and With six satellites in the satellite common
`vieW area, reference positioning receiver 38 Would have to
`generate ?fteen PCs corresponding to ?fteen combinations
`of four satellites each.
`Each satellite 18, 20, 22 sends an identi?er in its respec
`tive message data stream. Both mobile unit 17 and reference
`positioning receiver 38 may use these identi?ers to generate
`satellite group IDs (SGIDs) that identify the speci?c com
`bination of satellites used for a position ?x. Reference
`receiver 38 may generate the position correction for ?fteen
`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
`?x, may then choose the proper position correction identi
`?ed 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 satellite combina
`tions 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 trans
`m