(12) United States Patent
`(10) Patent NO.:
`US 6,373,429 B1
`
`Eschenbach
`(45) Date of Patent:
`*Apr. 16, 2002
`
`USOO6373429B1
`
`(54) DIFFERENTIAL GLOBAL POSITIONING
`SYSTEM USING ALMANAC DATA FOR A
`FAST TIME TO FIRST FIX
`
`6,211,817 B1 *
`6,225,945 B1 *
`
`4/2001 Eschenbach ........... 342357.03
`5/2001 Loonlis ................. 342/357.12
`
`* cited by examiner
`
`<75)
`
`<73)
`
`Inventor: Ralph F. Eschenbach, Woodside, CA
`(US)
`
`Primary Examiner—Thomas H. Tarcza
`Assistant Examiner—Fred H. Mull
`
`Assignee: Trimble Navigation Limited,
`Sunnyvale, CA (US)
`
`(74) Attorney, Agent, or Firm—David R. Gildea
`
`(57)
`
`ABSTRACT
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`This patent is subject tO a terminal dis-
`claimer.
`
`<21)
`
`<22)
`
`<63)
`
`<51)
`<52)
`<58)
`
`(56)
`
`Appl. NO.: 09/777,425
`
`Filed:
`
`Feb. 6, 2001
`
`Related US. Application Data
`
`Continuation of application No. 09/301,91(), filed on Jul. 27,
`1999, now Pat. No. 6,211,817.
`
`Int. Cl.7 ................................................ H04B 7/185
`.......... 342/357.03
`
`Field of Search ............................ 342/35703, 358,
`342,857.02; 701/215
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`A DGPS system using GPS almanac data for determining
`the locations-in-space and the DGPS corrections for GPS
`satellites for providing a differentially corrected location of
`a remote GPS user receiver. A GPS reference receiver
`determines almanac-based DGPS corrections from the dif-
`
`ferences between ranges that are measured to GPS satellites
`and ranges that are calculated to the GPS almanac-based
`locations-in-space of the GPS satellites from the known
`location of the GPS reference receiver. The GPS user
`
`receiver measures pseudoranges to the GPS satellites. Then,
`in a first embodiment, the GPS reference receiver radios the
`DGPS corrections to the GPS user receiver. The GPS user
`
`receiver uses almanac-based locations-in-space for the GPS
`satellites and the almanac-based DGPS corrections for dif-
`ferentially correcting the measured user pseudoranges for
`providing a differentially corrected user location. In a second
`embodiment, the GPS user receiver radios the measured user
`pseudoranges to the GPS reference receiver. The GPS ref-
`erence receiver uses the measured user pseudoranges, the
`almanac-based locations-in-space for the GPS satellites, and
`the almanac-based DGPS corrections for providing the
`differentially corrected user location.
`
`5,764,184 A
`
`6/1998 Hatch et a1.
`
`................ 342/357
`
`27 Claims, 7 Drawing Sheets
`
`
`
`
`
`
`/
`‘
`ALMANAC
`
`1
`/
`‘ /
`
`GPS SOURCE
`LOCATION
`4.4
`CODE
`
`REFERENCE RANGE
`CALCULATION
`45
`CODE
`
`32
`Aw
`
`
`
`ALMANAC-
`5 BASED
`DGPS
`CORRECTIONS
`
`USER
`PSEUDO-
`RANGES
`
`REFERENCE
`52
`DIFFERENTIAL
`CORRECTION CODE
`
`ALMANAC-BASED
`——>1DGPS
`LOCATION CODE
`\
`
`5L4
`
`ALMANAC»
`BASED
`DGPS
`CORRECTED
`USER
`LOCATION
`
`
`
`TRANSCEIVER
`
`1%
`
`17
`
`‘
`
`\
`ALMANACEBASED
`DGPS CORRECTIONS
`
`
`
`
`1
`
`REFERENCE RANGE
`MEASURMENT
`fl
`7 CODE
`
`GPS SIGNALS
`15
`
`,
`
`21
`
`,
`RADIO
`FREQUENCY 22
`gIRCUITRY
`
`DIGITAL
`_
`SIGNAL
`EROCESSOR
`.
`
`24
`
`25
`
`TIMER
`_
`7
`MICRO
`PROCESSOR 25
`_
`
`
`
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`REAL TIME
`CLOCK
`_
`A
`INTERFACE
`A
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`34
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`
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`MEMORY
`2§
`12
`
`LYFT 1010
`
`LYFT 1010
`
`1
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 1 0f 7
`
`US 6,373,429 BI
`
`1
`
`5
`
`$6)1/
`
`14
`
`17
`
`1217
`
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`
`Fig.1
`
`V15
`
`19
`
`16
`
`2
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 2 0f 7
`
`US 6,373,429 B1
`
`ITRANSCEIVER
`‘
`
`33
`
`17
`
`E
`ALMANAC-BASED
`DGPS CORRECTIONS
`
`
` REFERENCE RANGE
`
`MEASURMENT
`i8
`
`CODE
`
`
`
` GPS SOURCE
`
`LOCATION
`
`CODE
`
` REFERENCE RANGE
`
`CALCULATION
`4.6
`CODE
`
`
`
`
`
`
` REFERENCE
`
`DIFFERENTIAL
`52
`
`
`CORRECTION CODE
`ALMANAC-
`
`
`BASED
`
`DGPS
`
`CORRECTIONS
`ALMANAC-BASED
`
`DGPS
`5A
`USER
`
`
`LOCATION CODE
`PSEUDO-
`
`
`\
`RANGES
`
`
`
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`ALMANAC-
`BASED
`DGPS
`CORRECTED
`USER
`LOCATION
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`MEMORY
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`GPS SIGNALS
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`
`35
`
`12
`
`Fig. 2A
`
`
`
`
`
`3
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 3 0f 7
`
`US 6,373,429 B1
`
`TRANSCEVER
`133
`
`17
`
`\
`———————————-————————>
`MEASUREDUSER
`PSEUDORANGES
`
`GPSSKSNALS
`15
`
`121 “"
`
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`
`FREQUENCY 122
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`
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`ClOCK
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`
`INTERFACE
`
`135
`
`
`
`
`USER
`
`
`RANGE
`LIES
`
`
`CODE
`
`
`
` ALMANAC
`
`
`GPSSOURCE
`
`LOCAHON
` MA
`CODE
`
`
`
`
`
` ALMANAC—BASED
`
`
`
`
`
`ALMANAC-
`BASED
`DGPS
`CORRECNONS
`
`
`
`
`
`154
`DGPS
`LOCATION CODE
`
`
`
`ALMANAC-
`BASED
`DGPS
`CORRECTED
`USER
`LOCAHON
`
`MEMORY
`
`128
`
`
`4
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 4 0f7
`
`US 6,373,429 B1
`
`300
`
`START
`
`301
`
`GPS REFERENCE RECEIVER
`
`RECEIVES GPS SIGNALS
`
`GPS REFERENCE RECEIVER
`
`MEASURES TIMES-OF-
`
`
`
`
`
`TRANSMISSION
`
`OF GPS SIGNALS
`
` GPS REFERENCE RECEIVER
`
`COMPUTES SV LOCATIONS
`
`
`BASED UPON GPS ALMANAC
`
`AND TlMES-OF-TRANSMISSION
`
`
`
`
`306
`
`GPS REFERENCE RECEIVER
`
`MEASURES RANGES TO SVS
`
`302
`
`304
`
`308
`
`310
`
`
`
`
`GPS REFERENCE RECEIVER
`CALCULATES RANGES FROM
`
`
`REFERENCE LOCATION
`
`TO ALMANAC-BASED
`
`
`
`SV LOCATIONS
`
`
`
`CALCULATES ALMANAC—BASED
`
`
`GPS REFERENCE RECEIVER
`
`DGPS CORRECTIONS FROM
`
`DIFFERENCE OF CALCULATED
`
`AND MEASURED RANGES
`
`Fig. 3A
`
`5
`
`

`

`US. Patent
`
`Apr. 16,2002
`
`Sheet 5 0f7
`
`US 6,373,429 B1
`
`324
`
`
`GPS USER RECEIVER
`RECEIVES GPS
`ALMANAC DATA
`
`
`
`328
`
`GPS USER RECEIVER
`
`RECEIVES ALMANAC-
`BASED DGPS
`CORRECTIONS FROM
`
`GPS REFERENCE
`RECEIVER
`
` GPS USER RECEIVER
`320
`
`RECEIVES GPS SIGNALS
`
`
`
`322 \[GPS USER RECEIVER
`
`MEASURES
`PSEUDORANGES TO SVs
`
`326
`
`GPS USER RECEIVER
`DETERMINES ALMANAC-BASED
`SV LOCATIONS
`
`330
`
`GPS USER RECEIVER
`
`DIFFERENTIALLY CORRECTS
`PSEUDORANGES WITH
`ALMANAC-BASED
`
`DGPS CORRECTIONS
`
`332
`
`GPS USER RECEIVER
`
`DETERMINES USER LOCATION
`FROM ALMANAC-BASED DGPS
`CORRECTED PSEUDORANGES
`
`AND ALMANAC-BASED
`SV LOCATIONS
`
`STOP
`
`Fig. SB
`
`6
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 6 0f7
`
`US 6,373,429 B1
`
`342
`
`GPS USER RECEIVER
`
`RECEIVES GPS SIGNALS
`
`346
`
`GPS USER RECEIVER
`
`MEASURES
`
`PSEUDORANGES TO SVS
`
`348
`
`GPS USER RECEIVER
`
`TRANSMITS PSEUDORANGES
`
`TO GPS REFERENCE RECEIVER
`
`350
`
`352
`
`GPS REFERENCE RECEIVER
`
`USER PSEUDORANGES
`
`WITH ALMANAC-BASED
`
`
`
`
`
`
`DIFFERENTIALLY CORRECTS
`
`
`
`
`DGPS CORRECTIONS
`
`
`GPS REFERENCE RECEIVER
`
`
`
`DETERMINES USER LOCATION
`
`
`FROM DGPS CORRECTED
`
`USER PSEUDORANGES AND
`
`
`
`
`
`ALMANAC-BASED
`
`DGPS CORRECTIONS
`
`Fig. 3C
`
`7
`
`

`

`US. Patent
`
`Apr. 16, 2002
`
`Sheet 7 0f 7
`
`US 6,373,429 B1
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`

`US 6,373,429 B1
`
`1
`DIFFERENTIAL GLOBAL POSITIONING
`SYSTEM USING ALMANAC DATA FOR A
`FAST TIME TO FIRST FIX
`
`This application is a continuation of a pending applica-
`tion filed Jul. 27, 1999 having a Ser. No. 09/361,916 now
`U.S. Pat. No. 6,211,817.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The invention relates generally to a differential global
`positioning system (DGPS) and more particularly to a DGPS
`system using GPS almanac data for determining the
`locations-in-space of GPS signal sources and the DGPS
`corrections for providing a differentially corrected location.
`2. Description of the Prior Art
`The global positioning system (GPS) is a satellite based
`location and time transfer system developed by the United
`States government and available free of charge to all users.
`A GPS user location is based upon one-way ranging between
`the user and GPS satellites. The GPS satellites transmit
`
`signals having the times-of—transmission and orbital param-
`eters for their respective time variable locations-in-space. A
`GPS receiver measures the ranges to typically four satellites
`simultaneously in-view by correlating the incoming GPS
`signals to internal GPS rcplica signals and measuring the
`received phases against an internal clock. These ranges are
`generally called pseudoranges because they include a term
`for the error of the internal clock. The pseudoranges are then
`used in location equations having calculated quantities for
`the locations-in-space for several satellites and the direc-
`tional cosines from the user location to the satellites. With
`
`four equations for four GPS satellites, respectively, the GPS
`receiver can resolve the four unknowns of a three dimen-
`sional geographical user location and a correction to the
`internal clock. Fewer than four satellites are required if other
`location information is available. However, many GPS
`receivers today use up to twelve GPS satellites in an
`overdetermined solution in order to improve location
`accurary.
`Two types of orbital parameters are transmitted for deter—
`mining locations-in-space for the satellites, almanac data
`and ephemeris data. The almanac data includes relatively
`few parameters and is generally sufficient for determining
`locations-in-space to a few kilometers. Each GPS satellite
`broadcasts the almanac data for all the GPS satellites on a
`twelve and one-half minute cycle. Almanac data is updated
`every few days and is useful for several weeks. Because of
`its relatively long lifetime, the almanac data is typically used
`by GPS receivers that have been off for more than a few
`hours for determining which GPS satellites are in-view.
`However, as the inaccuracy of the location-in-space of a
`GPS satellite transfers to an inaccuracy in the user location,
`almanac data is not used by existing GPS receivers for
`ranging. The ephemeris data provides relatively more
`parameters and is much more accurate. Typically, current
`ephemeris data is sufficient for determining locations—in—
`space to a few meters or a few tens of meters at current levels
`of selective availability. Each GPS satellite broadcasts its
`own ephemeris data on a thirty second cycle. Ephemeris data
`is updated each hour. However, after about two hours the
`accuracy of the ephemeris data begins degrading. Typically,
`ephemeris data that is more than about two to four hours old
`is not used for ranging.
`A stand alone accuracy of existing commercial GPS
`receivers is a typically within about twenty meters or within
`
`10
`
`15
`
`.
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`
`about one-hundred meters with selective availability (SA).
`In order to achieve these accuracies, existing GPS receivers
`use the current ephemeris data for determining the locations—
`in-space of the GPS satellites in location equations. In order
`to improve the stand alone accuracy with or without SA,
`differential GPS (DGPS) systems use a GPS reference
`station having an accurately known reference location for
`providing DGPS corrections. The DGPS corrections are
`computed from the differences between the ranges that the
`GPS reference station measures in a conventional manner
`and ranges that are calculated based upon the known loca-
`tion. A remote GPS user receiver receives the DGPS cor-
`
`rcctions with radio signals for correcting the raw pseudor-
`anges that it measures to the same GPS satellites at the same
`times as the GPS reference station. Using such DGPS
`corrections, GPS receivers can obtain a location accuracy
`within a meter or even better. Alternatively, the raw pseu-
`doranges can be transmitted or put onto a disk and carried to
`another site for differential correction.
`
`Several applications for GPS receivers require or make
`desirable a fast time to a first location fix after having been
`off or in a standby mode for more than a few hours. One of
`the problems in getting a fast location fix is that the GPS
`receiver needs to collect new ephemeris data before the
`location can be computed. Typically, the ephemeris data is
`obtained directly from the GPS satellites in the GPS signals.
`However, up to about thirty seconds is required to acquire
`ephemeris data in this manner. It has been proposed that this
`thirty seconds can be eliminated in one of two ways. First,
`if location is not needed at the remote, the solution can be
`computed at a network base station where current ephemeris
`data is available. In this case, the raw pseudoranges are sent
`to the base station along with the satellite identifications and
`times. Second, if the location is needed at the remote, the
`base station sends the current ephemeris data including the
`satellite identifications and times to the remote for location
`determination. These schemes are attractive for real time
`
`DGPS systems where radio equipment is already required.
`Unfortunately, these proposals require the transmission and
`reception of a relatively long data string for the ephemeris
`data of approximately 1500 bits per satellite or 15000 bits
`for ten in-view satellites. Further,
`the transmission and
`reception must be accomplished every few hours in order
`that thc cphcmcris data be up-to-datc.
`Hatch et al. in US. Pat. No. 5,764,184 discloses a method
`and system for post—processing DGPS system satellite posi—
`tional data. Hatch recognizes that almanac data is suffi-
`ciently accurate for computation of directional cosines
`because the high altitude of the GPS satellites renders the
`directional cosines relatively insensitive to errors in satellite
`locations-in-space. The almanac-based directional cosines
`are then used in post-processing to map reference station
`corrections to user receiver stand-alone position information
`for correcting the user position. An advantage asserted by
`Hatch is that substantially less information is required to be
`saved by the user for post-processing. However, IIatch does
`not address the issue of the acquisition time for the user
`receiver for receiving ephemeris data for ranging in order to
`obtain the stand-alone position.
`There is a need for a differential GPS system where a
`remote GPS user receiver has a fast time to first fix without
`
`a requirement of receiving ephemeris data.
`
`SUMMARY OF THE INVENTION
`
`It is therefore an object of the present invention to provide
`a differential global positioning system (DGPS) using the
`
`9
`
`

`

`US 6,373,429 B1
`
`3
`GPS almanac data for measuring ranges to GPS signal
`sources and for determining almanac-based DGPS correc-
`tions.
`
`in a preferred embodiment, a system of the
`Briefly,
`present invention includes a GPS reference receiver and a
`remote GPS user receiver. The GPS reference receiver
`determines almanac-based DGPS corrections from the dif-
`ferences between ranges that are measured to GPS signal
`sources and ranges that are calculated to GPS almanac-based
`locations-in-space of the GPS signal sources from a known
`location of the GPS reference receiver. The GPS user
`
`receiver measures pseudoranges to the GPS signal sources.
`Then,
`in a first embodiment,
`the GPS reference receiver
`radios the almanac-based DGPS corrections to the GPS user
`receiver. The GPS user
`receiver uses almanac—based
`
`10
`
`15
`
`locations-in-space for the GPS signal sources and the
`almanac-based DGPS corrections for differentially correct-
`ing the measured user pseudoranges for providing a differ-
`entially corrected user location. In a second embodiment, the
`GPS user receiver radios the measured user pseudoranges to '
`the GPS reference receiver. The GPS reference receiver uses
`
`the almanac-based
`the measured user pseudoranges,
`locations-in-space for the GPS signal sources, and the
`almanac-based DGPS corrections for providing the differ-
`entially corrected user location.
`An advantage of the present invention is that a remote
`GPS user receiver in a DGPS system does not require GPS
`ephemeris data, thereby providing a fast time to first fix for
`a differentially corrected GPS location.
`These and other objects and advantages of the present
`invention will no doubt become obvious to those of ordinary
`skill
`in the art after having read the following detailed
`description of the preferred embodiments which are illus-
`trated in the various figures.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a differential global posi-
`tioning system of the present invention including a global
`positioning system (GPS) reference receiver and a GPS user
`receiver;
`FIG. 2a is a block diagram of the GPS reference receiver
`of FIG. 1;
`FIG. 2b is a block diagram of the GPS user receiver of
`FIG. 1;
`FIGS. 3a—c are flow charts of a method of operation of the
`system of FIG. 1.
`FIG. 4 is a table of GPS almanac parameters used in the
`system of FIG. 1.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`FIG. 1 is a block diagram of a differential global posi-
`tioning system of the present invention referred to by the
`general reference number 10. The system 10 includes a GPS
`reference receiver 12 and a GPS user receiver 14. The GPS
`reference receiver 12 and the GPS user receiver 14 both
`receive GPS signals 15 from GPS signal sources 16.
`Typically,
`the GPS signal sources 16 are GPS satellites.
`However, other sources of GPS signals 15 such as GPS
`pseudolites can be used. The GPS signal 15 from each of the
`GPS signal sources 16 has an L-band carrier signal modu-
`lated by GPS data bits of twenty milliseconds that are spread
`by a pseudorandom (PRN) code that repeats every one
`millisecond. The GPS data bits and the PRN codes of all the
`GPS signal sources 16 are synchronized to transmit at the
`
`25
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`30
`
`35
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`40
`
`45
`
`50
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`55
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`60
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`65
`
`4
`same times beginning with 00 hours, 00 minutes, 00.000
`seconds of each GPS week and continuing throughout the
`week. The PRN code from each GPS signal source 16 is
`distinct, thereby allowing a GPS receiver to distinguish the
`GPS signal 15 from one of the GPS signal sources 16 from
`the GPS signal 15 from another of the GPS signal sources
`16.
`
`The GPS data bits are segmented into 1500 bit frames,
`also called pages, of thirty seconds. The frame in each GPS
`signal 15 includes the ephemeris orbital parameters for the
`GPS signal source 16 transmitting that GPS signal 15 and a
`portion of the almanac orbital parameters for all the GPS
`signal sources 16. The frames are segmented into five 300 bit
`subframes of six seconds each. The subframes are seg—
`mented into thirty 10 bit words. Each subframe begins with
`a known preamble and includes a Z-count. The Z-count
`gives GPS—based time—of—transmission for the preamble.
`Approximately two subframes are used for the ephemeris
`and approximately one subframe is used for the portion of
`the almanac within each frame. The complete almanac is
`transmitted by each GPS signal source 16 in twenty-five
`frames (pages). The ephemeris orbital parameters are highly
`accurate and are updated each hour. The almanac orbital
`parameter are about 100 times less accurate and are updated
`every few days.
`The GPS reference receiver 12 and the GPS user receiver
`
`14 communicate with radio signals 17 either directly or
`through a network control base station 18. The base station
`18 may be located with or separate from the GPS reference
`receiver 12. The system 10 may include more than one GPS
`reference receiver 12 communicating through the base sta-
`tion 18 and usually includes several independent units of the
`GPS user receiver 14. The GPS reference receiver 12, the
`GPS user receiver 14, and the base station 18 are shown on
`the surface of the earth 19. However, there is no reason that
`the GPS user receiver 14 and/or the base station 18 could not
`be air or space borne.
`FIG. 2a is a block diagram of the GPS reference receiver
`12 of the present invention. The GPS rcfcrcncc rcccivcr 12
`includes a GPS antenna 21, radio frequency circuitry 22, a
`digital signal processor 24, a timer 25, a microprocessor 26,
`and a memory 28. The GPS antenna 21 receives radio
`frequency (RF) GPS 5 signals 15 from in-view GPS signal
`sources 16 and passes a representative conducted RF GPS
`signal to the radio frequency circuitry 22. The radio fre—
`quency circuitry 22 includes local oscillators, frequency
`downconverters, and samplers for receiving and issuing a
`sampled GPS signal at a lower frequency to the digital signal
`processor 24. The sampled GPS signal simultaneously
`includes carriers modulated by the GPS data bits spread by
`the pseudorandom (PRN) code from each of the in-view
`GPS signal sources 16.
`The timer 25 provides a reference clocking signal to the
`digital signal processor 24. The digital signal processor 24
`include numerically controlled signal generators using the
`reference clocking signal as a time base for generating
`internal GPS replica signals and correlators for providing
`correlation data for the correlation between the internal GPS
`replica signals and the sampled GPS signal. The replica
`signals replicate the PRN codes for each of the GPS signal
`sources 16 that the GPS reference receiver 12 is tracking or
`attempting to acquire. The microprocessor 26 receives the
`correlation data over a signal bus 32 and executes coded
`directions in the memory 28 for issuing rcsponsivc fccdback
`adjustments over the bus 32 to the digital signal processor
`24. The feedback adjustments offset the respective phases of
`the replica PRN codes with respect to the reference clocking
`
`10
`
`10
`
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`

`US 6,373,429 B1
`
`5
`signal to drive the replica PRN codes to correlate with the
`PRN codes in the sampled GPS signal. The microprocessor
`26 may include several chips interconnected directly or over
`the bus 32 in order to perform in a conventional manner for
`reading and writing data in the memory 28, reading execut-
`able code in the memory 28, and controlling and receiving
`information from the elements of the GPS reference receiver
`12. The memory 28 may include several chips or other
`storage devices.
`A real time clock 34 provides an approximate absolute
`time to the processor 26 over the bus 32. An interface 36
`coupled to the microprocessor 26 over the bus 32 enables a
`human user and electronic devices to communicate with the
`GPS reference receiver 12. A transceiver 38 transmits and
`receives the radio signals 17 as directed by the micropro-
`cessor 26.
`
`10
`
`15
`
`The memory 28 includes data that may be modified by the
`microprocessor 26 over the bus 32 and executable code that
`is read by the microprocessor 26 over the bus 32 for
`directing the operation of the microprocessor 26. The
`executable code in the memory 28 includes programs for a '
`GPS source location code 44, a reference range calculation
`code 46, a reference range measurement range code 48, a
`reference differential correction code 52, and an optional
`almanac-based DGPS location code 54 that are used for
`providing almanac-based differential global positioning sys-
`tem (DGPS) corrections to measured user pseudoranges to
`the GPS signal sources 16 and optionally for providing an
`almanac-based DGPS corrected user
`location. Detailed
`operation of the program codes is illustrated in the flow
`charts of FIGS. Safe and explained in the accompanying
`detailed descriptions. Although in the preferred embodiment
`certain elements are implemented with stored program code
`in the memory 28 that is executed by the microprocessor 26,
`it is to be understood that alternative embodiments can at
`least partially implement one or more of these elements in
`circuit hardware.
`
`25
`
`30
`
`35
`
`FIG. 2b is a block diagram of the GPS user receiver 14 of
`the present invention. The GPS user receiver 14 includes a
`GPS antenna 121, radio frequency circuitry 122, a digital
`signal processor 124, a timer 125, a microprocessor 126, and
`a memory 128 all operating in a similar manner to the GPS
`antenna 21, radio frequency circuitry 22, digital signal
`processor 24, timer 25, microprocessor 26, and memory 28
`in the GPS reference receiver 12. The memory 128 includes
`data that may be modified by the microprocessor 126 over
`a bus 132 and executable code that is read by the micro-
`processor 126 over the bus 132 for directing the operation of
`the microprocessor 126. A real time clock 134 provides an
`approximate absolute time to the processor 126 over the bus
`132. An interface 136 coupled to the microprocessor 126
`over the bus 132 enables a human user and electronic
`devices to communicate with the GPS reference receiver 14.
`
`Atransceiver 138 transmits and receives the radio signals 17
`as directed by the microprocessor 126.
`The executable code in the memory 128 includes pro-
`grams for a GPS source location code 144, a user range code
`148, and an optional almanac-based DGPS location code
`154 that are used for measuring user pseudoranges to the
`GPS signal sources 16 and optionally for providing a
`differentially corrected user location. Detailed operation of
`the program codes is illustrated in the flow charts of FIGS.
`351—6 and explained in the accompanying detailed descrip-
`tions. Although in the preferred embodiment certain ele-
`ments are implemented with stored program code in the
`memory 128 that is executed by the microprocessor 126, it
`is to be understood that alternative embodiments can at least
`partially implement one or more of these elements in circuit
`hardware.
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`11
`
`6
`FIGS. 3a—c are flow charts of a method for a fast time to
`first fix of a differentially corrected location of the GPS user
`receiver 14. In FIG. 3a, at the start 300 the GPS reference
`receiver 12 stores data in the memory 28 for a pre-
`determined geographical reference location of the antenna
`21 and the GPS almanac data for the GPS signal sources 16.
`Alternatively, the GPS almanac data can be obtained from
`the GPS signals 15 from any one of the GPS signal sources
`16 over a period of twelve and one-half minutes. Typically,
`the reference location is precisely determined by a land
`survey. In a step 301 the GPS antenna 21 receives the RF
`GPS signal 15 from the in-view GPS signal sources 16. The
`radio frequency circuitry 22 downconverts the RF GPS
`signal 15, samples the downconverted GPS signal, and
`passes the sampled GPS signal to the digital signal processor
`24.
`
`The digital signal processor 24 in a step 302 provides
`correlation data for the correlation of the sampled GPS
`signal and an internal replica code. The microprocessor 26
`follows directions in the executable code in the memory 28
`for adjusting the phases of the internal replica PRN codes
`with respect to the internal reference clocking signal and
`determining the phase offsets of the replica codes that
`provide the best correlations to the PRN codes in the
`sampled GPS signal. The GPS source location code 44
`estimates the times-of—transmission for the GPS signals 15
`from the phase offsets and thc Z-count of one or more
`subframes. Then, in a step 304 the GPS source location code
`44 uses the times-of—transmission together with the GPS
`almanac data for computing almanac—based locations—in—
`space of the GPS signal sources 16. In the flow charts
`illustrated in FIGS. 3a—c,
`the GPS signal sources 16 are
`abbreviated as “SVs”.
`
`GPS almanac data and a satellite (SV) health word are
`carried in pages one through twenty-four of subframe five,
`as well as pages two through five and seven through ten of
`subframe four, for up to thirty-two GPS signal sources 16.
`The almanac data is a reduced-precision subset of the clock
`and ephemeris orbital parameters. The data occupies all bits
`of words three through ten of each page except the eight
`MSBs of word three (data ID and SV ID), bits seventeen
`through twenty-four of word five (SV health), and the fifty
`bits devoted to parity. The number of bits, scale factor
`(LSB), range, and units of the almanac parameters are given
`in FIG. 4. In FIG. 4 the asterisk (*) denotes that the number
`of bits is in two’s complement, with the sign bit (+ or —)
`occupying the MSB. A more complete description of the
`GPS almanac data is given in a “GPS Interface Control
`Document ICD-GPS-200” for the “NAVSTAR GPS Space
`Segment and Navigation User Interfaces” published by
`NaVTech Seminars & NaVTech Book and Software Store,
`Arlington, Va., reprinted February, 1995.
`The reference range measurement code 48 in a step 306
`corrects the bias error in the timer 25 and the transit time
`
`estimate of step the 302 and measures ranges to several GPS
`signal sources 16 from the phase offsets and the almanac
`based locations-in-space for preferably about ten ofthe GPS
`signal sources 16 using equations and methodology that are
`conventional except for the use of almanac-based locations-
`in-space in place of locations-in-space based upon the GPS
`ephemeris parameters.
`In a step 308 the reference range calculation code 46
`calculates ranges to the almanac-based locations-in-space
`from the known rcfcrcncc location of the GPS antenna 21.
`
`The reference differential correction code 52 in a step 310
`calculates DGPS corrections from the differences between
`the measured ranges that were determined in the step 306
`
`11
`
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`

`US 6,373,429 B1
`
`7
`and the calculated ranges that were determined in the step
`308. An equation 1 shows the DGPS correction A‘ for the “i”
`th one of the GPS signal sources 16, termed the ith source
`in the discussion below.
`i
`i
`i
`A=r,..—r,.
`
`1)
`
`The equation 1 shows that the DGPS correction N for the
`range to the ith sources equals the measured range rim,
`between the GPS reference receiver 12 and the ith source
`
`minus the calculated range ri,C between the GPS reference
`receiver 12 and the ith source. In a first embodiment the flow
`chart of FIG. 3a continues through “A” to the flow chart of
`FIG. 3b. In a second cmbodimcnt thc flow chart of FIG. 3a
`
`10
`
`8
`to use and the GPS reference receiver 12 provides DGPS
`corrections based upon that particular update. The GPS
`almanac data, a wakeup call, and/or the DGPS corrections
`may be transmitted in a single transmission of the radio
`signal 17 or multiple transmissions.
`The almanac-based DGPS location code 154 in a step 330
`differentially corrects the pseudoranges with the almanac-
`based DGPS corrections using measurements by the GPS
`reference receiver 12 and the GPS user receiver 14 taken at
`
`the same times for the same GPS signal sources 16. Equation
`2 shows a relationship between the measured user pseudo-
`range pi of the ith one of the GPS signal sources 16 and an
`almanac-bascd rangc bctwccn thc location (Xu, y“, z“) of thc
`GPS user receiver 14 and the almanac-based location-in-
`
`space (Xls, yiszis) of the ith source.
`
`Pi-Ai=[(XL.-X';)2+(yu-Yis)2+(Zu-Z’;)2]1/z+bu
`
`2)
`
`The almanac-based range to the ith source is the square root
`of the sum of the squares of the range ('Xu—X‘IS) in an “X”
`dimension, the range (yu—yis) in a “y” dimension, and the
`range (zu—zis) in a “z” dimension, where (Xu, y“, z“) is the
`location of the GPS antenna 121 of the GPS user receiver 14
`
`that is yet to be determined and (X2, yis, zis) is the almanac-
`bascd location-in-spacc of thc ith sourcc. The equation 2 is
`a location equation showing the user pseudorange pi mea-
`sured by the GPS user receiver 14 to the ith source minus the
`almanac—based DGPS correction for the ith source equals the
`almanac-based range to the ith source plus the time error bu
`in the internal reference clock signal
`that
`is yet
`to be
`determined. The almanac-based DGPS location code 154
`
`resolves the differentially corrected user location (Xu, y“, z“)
`by a simultaneous solution of four of the equation 2 for four
`GPS signal sources 16. In order to linearize the mathematics,
`a location of the GPS antenna 121 of the GPS user receiver
`14 is assumed and the almanac-based DGPS location code
`
`154 calculates a user range riuC between the almanac-based
`location-in-space of the ith source (Xis, yis, zis) and the
`assumed user location. An incremental pseudorange, termed
`an almanac-based linearized pseudorange y‘, is defined as
`cqual to thc diffcrcncc bctwccn thc mcasurcd u uscr pscu-
`dorange pi and the almanac-based calculated user range riuC
`as shown in equation 3.
`i_ii
`yep rue
`
`3)
`
`The equations 2 and 3 may be manipulated and reformatted
`as shown in equation 4.
`
`yi—Nfli-Xfin.
`
`4)
`
`In the equation 4, the Q is the unit vector, often termed the
`directional cosines, in X,y,z dimensions to the ith source that
`are calculated based upon the GPS almanac data. The X is
`the vector location difference in X,y,z dimensions that are yet
`to be determined between the assumed user location and the
`
`actual user location. The equation 4 is a location equation
`showing that the almanac-based linearized pseudorange yi
`minus the almanac-based DGPS correction Ai equals the dot
`product of the directional cosines or unit vector Qi times the
`vector location diiference X; plus the internal time error b,,.
`In a step 332 the almanac-based DGPS location code 154
`uses the known almanac-based linearized pseudorange yi
`and the known almanac-based DGPS correction N in the
`equation 4 for four GPS signal sources 16 for resolving four
`unknowns of thc vcctor location diffcrcncc X in X,y,z
`dimensions and the i