5,587,715
`[11] Patent Number:
`[19]
`Unlted States Patent
`
`Lewis
`[45] Date of Patent:
`Dec. 24, 1996
`
`llllllllllllllllllllllllllllllllllllllllllIllllllllllllllllllllllllllllllll
`USOOSSS7715A
`
`[54 METHOD AND APPARATUS FOR
`TRACKING A MOVING OBJECT
`
`
`
`[75
`
`Inventor: Peter T. Lewis, Washington, DC.
`
`[73. Assignee: GPS Mobile, Inc., Washington, DC-
`
`[21: Appl. No.2 36,533
`.
`.
`Mar. 19’ 1993
`Flled'
`[22]
`Int. Cl.6 ............................... HMB 7/185; G018 5/02
`[51
`[52, us. C1.
`.
`.........
`.. 342/357; 455/121
`[58
`Field of Search ............................. 342/357; 455/121
`
`[56
`
`References Cited
`
`.
`
`,
`
`_
`
`U.S. PATENT DOCUMENTS
`3/1971 Knickel .
`3,568,161
`4/1984 Taylor et a1.
`4,445,118
`6/1986 Wanka .
`4,596,988
`.
`5/1987 Gray et a1.
`4,651,157
`5/1987 Counselman, HI .
`4,667,203
`4,701,760 10/1987 Raoux .
`4,728,959
`3/1988 Maloney e, a].
`4,731,613
`3/1988 Endo et a1.
`_
`4,740,792
`4/1988 Sagey et 31,
`4,751,512
`6/1988 Longaker .
`4,754,283
`6/1988 Fowler.
`4791572 12/1988 Green, III et 31- -
`4,809,005
`2/1989 Counselman, III .
`4,812,991
`3/1989 Hatch.
`4,891,650
`1/1990 She/fer .
`.
`4,891,761
`1,1990 Gray et a1.
`4,894,662
`1,1990 Counselrnan, Ill .
`4,897,642
`1/1990 DiLullo et a1.
`,
`4,907,290
`3/1990 Crompton .
`4,912,756
`3/1990 Hop ,
`4,924,699
`5/1990 Kuroda 6t 81.
`4,928,107
`6/1990 KWOda 6t 31-
`4,953,198
`8/1990 Daly et a1.
`.
`4,983,980
`1/1991 Ando .
`.
`5,003,317
`3/1991 Gray et a1.
`5 014 066
`5/1991 Counselrnan III
`5’021’794
`6/1991 Lawrence .
`’
`5,043,736
`3,1991 Darnell et al.
`5,055,851
`10/1991 Sheifer.
`
`-
`-
`
`_
`
`'
`
`FOREIGN PATENT DOCUMENTS
`8706713
`5/1987 WIPO.
`
`OTHER PUBLICATIONS
`
`Communications Daily; vol 9, No. 61, p. 3; “Mobile Com-
`munications; Joint Venture Plans to Compete With Geostar
`and Qualcomm Satellite Systems”.
`Communicatins of the ACM; vol. 31, No 6, p. 638; “Personal
`computer in the year; Winning entry in 1987 Sponsored by
`Apple Computer Inc.”, By Mel; Bartlett W.; Gmohundro,
`Stephen M, Robinson. Arch D.; Skiena, Steven 5.; Thear—
`1mg, Km 11.; Young, Luke T; and Wolfram, Stephen.
`
`Primary Examiner—Theodore M. Blum
`Attorney, Agent, or Firm—Cowan, Liebowitz & Latman,
`RC
`
`ABSTRACT
`[57]
`An apparatus and a method are described for determining
`with a high degree of accuracy the location of an object,
`based upon signals transmitted from aplurality of satellites
`rotating about the earth in known orb1ts. First, the latitude
`and longitude of a fixed point is determined with accuracy.
`The object is capable of moving with respect to the fixed
`point. Second, the satellite signals are received at the fixed
`point and processed to determine the approximate latitude
`and longitude of the fixed point. A first difference between
`the accurate and approximate latitudes is taken to provide a
`diiferential latitude correction of a magnitude corresponding
`to the first difference and of a direction to the north or south.
`A second difi'erence between the accurate and approxrmate
`longitudes is also taken to provide a differential longitude
`correction of a magnitude corresponding to the second
`difference and of a direction to the west or east. The satellite
`signals are received at the object and processed to determine
`the approximate latitude and longitude of the object. The
`approximate object latitude and diiferential latitude correc-
`tion are then combined to provide a corrected object latitude
`of improved accuracy, and the approximate object longitude
`and differential longitude correction are combined to pro-
`vide a corrected object longitude of improved accuracy.
`
`(List continued on next page.)
`
`62 Claims, 15 Drawing Sheets
`
`/10
`
`
`
`4511:1255»W1]
`
`AW
`
`srA'noN
`A 35‘
`
`COMMAND
`
`“rum (”self
`(36‘ 36b
`CENTRAL
`TELEPFDNE
`CENTER
`
`
`
`CELL
`STATION
`NEFWDRK
`33
`
`6e: fig 3)
`36d
`1
`3
`
`
`
`
`'
`
`
`
`‘0‘\ Diifemnnal
`Station
`
`
`
`
`4219
`
`GH EXHIBIT 1009
`GH EXHIBIT 1009
`
`

`

`5,587,715
`
`Page 2
`
`Us. PATENT DOCUMENTS
`
`5,081,462
`5,093,669
`5,119,101
`5,119,102
`5,119,504
`5,132,695
`5,142,281
`5,148,179
`
`.......................... 342/352
`
`1/1992 Tachita et a1.
`3/1992 Kasiyama.
`6/1992 Barnard ................................... 342/357
`6/1992 Barnard.
`6/1992 Duboraw, III .
`7/1992 Dumas et a1.
`.
`8/1992 Park.
`9/1992 Allison.
`
`5,155,490
`5,155,491
`5,155,689
`5,223,844
`5,225,842
`5,323,164
`5,323,322
`5,365,447
`5,390,124
`5,438,517
`
`.
`
`10/1992 Spradley, Jr. et al.
`10/1992 Ando.
`10/1992 Worlham ,
`......................... 342/357
`6/1993 Mansell et a1.
`342/357
`.
`7/1993 Brown et a1.
`342/357
`6/1994 Endo .............
`364/449
`6/1994 Mueller et a1.
`364/449
`9/1991 Dennis ..........
`364/449
`2/1995 Kyrtsos ....................
`8/1995 Sennott et a1.
`.......................... 364/449
`
`
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 1 of 15
`
`5,587,715
`
`
`
`TELEPHONE " CENTER
`CENTRAL
`STATION
`NETWORK I 38
`
`(see fig 3)
`16
`3
`a
`:
`:
`36d
`; IDEDICATED:
`:
`“1TELEPHON .LJ
`I
`LINE
`1
`L__ ____________ J
`32_J
`
`
`
`From 12
`
`Differential
`Station
`
`
`
`FIG.
`
`1A
`
`

`

`7
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`
`7.,mmbw
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`:w.mm-ov
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`US. Patent
`
`m.
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`
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`vm-ovmm-o¢
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`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 3 of 15
`
`5,587,715
`
`Nationwide
`or Regional
`Page Link
`
` 720 V
`
`SM
`
`Splitter
`54
`
`Radio
`56
`
`53
`
`55
`
`57
`
`Switching
`
`
`
`Auto
`Dialer
`58
`
`Paging
`Receiver
`59
`
`
`
`Cellular
`Junction
`TransCeiver
`
`
`Port
`
`76
`
`71
`
`
`
`
`
`
`Distribution
`Switch
`54
`
`78
`
`Non-1
`-
`destructive
`0109—
`nostic "B Mer7n201'y
`System ~
`80
`
`F”
`82
`
`om ute
`P
`84
`
`Horn
`66
`
`Lights
`68
`
`Ignition
`70
`
`Dis la
`
`Ke board
`
`Mouse
`
`65
`
`626
`
`Alarm
`S
`tem
`
`ygo
`
`61 Doors
`62a
`
`Trunk
`62b
`
`H°°d
`62C
`
`Panic
`
`62d
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 4 of 15
`
`5,587,715
`
`
`
`FIG. 2B
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 5 of 15
`
`5,587,715
`
`48
`
`RF
`Amplifier
`
`1 8
`
`'
`
`1 7
`
`1 50
`
`RF
`Receiver
`
`152
`
`1 54
`
`1 56
`
`51 B
`
`)
`
`51A
`
`1 60
`
`
`
`
`
`Computer
`Processin
`Unit
`(CPU
`
`IO Gates
`8:
`Multi—
`Channel
`Signal
`
`Processor
`
`1 58
`
`49
`
`Reomme
`
`
`
`Clock
`
`FIG. 2C
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 6 of 15
`
`5,587,715
`
` FIG.2D
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 7 of 15
`
`5,587,715
`
`FIG.2F
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 8 of 15
`
`5,587,715
`
`From 12
`
`FIG. 8
`38 \ 120
`From 28 /
`From 12
`v
`From 40
`4
`56°
`132'
`V
`36c
`Telephone
`Network
`
`353
`
`46
`
`F
`
`ram
`
`122
`
`130
`
`GPS
`Receiver
`
`94 m Computer
`Q.
`
`Switch Board
`
`124
`
`Differential
`
`A
`
`II
`
`940
`
`.
`
`136
`
`8c Billing
`Terminal
`
`Modern=-
`
`B
`
`90
`
`94
`
`I|
`
`118
`
`114
`
`Printer
`
`Plotter
`
`116
`
`— C
`
`D—l
`
`—
`To se Drive
`
` Tracking
`
`Computer
`
`1 10d
`
`II
`PC Cueing
`
`
`
`
`
`M 33rd
`CRT
`
`Patch
`Panel
`
`98b
`
`Multiplexer
`
`l
`
`D'ff re t'
`IMfmlerirnm
`
`
`
`126
`
`Emergency
`Power
`Supply
`
`
`
`
`
`100
`
`
`
`“5
`VAC
`
`To all Cn'ticol
`
`Command Center
`Components
`
`K ._
`
`104
`
`105
`
`108
`
`

`

`US. Patent
`
`m%
`
`5,587,715
`
`u,S..‘m..0:.N:
`
`
`
`
`
`.03om‘5:95xyzDboEozcoruBmlnwmEoco:$9323
`
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`BEE1A3.$32
`
`
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`av:Room.2:
`
`.00..
`
`w.UE
`
`
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 10 of 15
`
`5,587,715
`
`UNF
`
`in
`
`2.2323
`
`fofimz
`
`/
`
`o.‘
`
` LoSano
`
`mmo
`
`$2801
`
`Escfiota
`
`8:230”.
`
`+2
`
`
`
`

`

`U.S. Patent
`
`Dec. 24, 1996
`
`Sheet 11 of 15
`
`5,587,715
`
`
`GPS
`
`constellation
`
`180
`
`182
`
`SAon
`
`
`
`Differential
`Correction
`Activated
`o
`
`Yes
`
`Constant, Surveyed
`and stored DlFF
`station Lot/Long
`
`
`
`
`
`
`
`Full duplex
`connect with
`distant DlFF
`station and
`command center
`
`
`
`
`
`
`Tracking Dot
`display, up to
`a 100 meter
`error
`
`No
`
`186
`
`1 88
`
`190
`
`
`
`Add or subtract variable
`
`
`GPS—generated Lot/Long
`
`
`w/position error from
`
`
`constant surveyed
`position
`
`
`192
`
`
`
`
`Output correction A
`
`
`to command center
`
`
`194
`
`196
`
`
`Process
`
`
`correction to
`tracking PC.
`
`
`DlFF correction i
`vehicle Lat/Long
`
`
`
`
`No
`
`
`
`nd correction A
`to vehicle ?
`
`
`Yes
`
`196’
`
`Process
`correction at
`vehicle
`receiver.
`DlFF
`correction 3;
`vehicle
`location
`w/lnduced
`
`SA
`
`198
`
` Corrected
`
`position display
`dot at PC
`
`
`FIG. 6A
`
`9'
`18
`
`
`
` Corrected
`
`
`position display
`dot at PC
`
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 12 of 15
`
`5,587,715
`
`\
`
`224
`
`downlinks
`(3- )
`
`
`
`
`182'
`
`at DIFF station receiver l 86’
`
`Process GPS position
`
` GPS
`
`
`
`inducement
`Purposeful
`of SA causing
`positioning inaccuracy
`
`and / or
`Variable
`atmospheric delays
`6: minute clock
`error causing
`postioning
`inaccuracy
`
`
`Display
`
`
`
`
`No
`tracking dot
`with up to a
`
`
`
`100 meter error
`
`
`
`
`Differential
`correction
`activated ?
`
`
`184
`
`Yes
`
`222
`
`188
`
`
`
`
`DIFF PC on; survey
`marker in place
`directly under
`
`receiver antenna
`
`
`
`226
`
`Enter accurate
`posrtion of reference
`
`marker to
`
`DIFF PC
`
`
`
`
`————————————
`
`Full duplex
`connect
`between
`DIFF station
`and
`command
`center
`
`190’
`
`Save accurate
`position
`and
`
` 230
` Enter accurate
`
`altitude data
`
`altitude of reference
`
`
`marker to
`DIFF PC
`
` 228 J
`
`FIG. 6B
`
`TTO step 232, Fig. 6C
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 13 of 15
`
`5,587,715
`
`J’ From step 228, fig. SB
`
`FIG. 6C
`
`
`
`
`
` input real—time 232
`inaccurate GPS
`
`
`POS/ALT data
`to PC com port
`
`1
`
`
`
`
`
`
`Process
`Process
`Process
`latitudinal
`longitudinal
`altitude
`
`
`correction
`correction
`correction
`
`
`
`
`
`
`Convert Latitude
`Convert Longitude
`
`decimal degrees to
`decimal degrees to
`
`
`
`decimal minutes
`decimal minutes
`
`
`
`236
`or vice verso
`or vice versa
`
`
`
`
`
`
`238'
`238
`
`
`If cps Lat
`If cps Long
`If GPSthLong
`if cps Lat
`
`Less than
`313110” On
`more than
`less than
`
`
`
`
`
`
`Ref Lat
`Ref Long
`Ref Lat
`Latitude
`g
`Longitude
`
`
`
`correction
`correction
`
`
`
`
`
`?
`
`234'
`
`236’
`
`240'
`
`DIFF modem
`processing
`
`
`
`{To step 262, fig. 6D
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 14 of 15
`
`5,587,715
`
`260
`
`\ J/From step 250, fig. 6C
`
`
`Command
`Center
`
`modem
`Processing
`
`252
`
`Patch
`P0"?!
`routine
`
`
`
`? Mobile
`differential
`
`
`capability
`
`
`Yes
`
`No
`
`Apply
`correction A
`at Command
`Center
`
`Mobile GPS
`receiver data
`via modern
`
`276
`
`I
`
`1 '
`
`L
`
`Full duplex
`cellular ac
`
`PSTN circuit
`
`272
`
`
`
`
`Command
`
`
`
`Center
`PC
`
`270
`
`
`
`Display
`
`accurately
`positioned
`
`track dot
`
`
`FIG. 6D
`
`

`

`US. Patent
`
`Dec. 24, 1996
`
`Sheet 15 of 15
`
`5,587,715
`
`
`
`Remove main power
`from GPS receiver
`or enter signal blocking
`
`
`
`
`
`Environment
`
`
`
`
`298
`
`304
`
`Restart cold
`
`2—15 minutes
`
`support capability
`in GPS receiver
`
`306
`
`Old
`
`Ephemeris
`
`-Old
`
`Almanac
`
`Yes
`
`I
`Old
`
`date/time group
`
`300
`
`FIG.
`
`'7
`
`312
`
`Old
`
`position
`
`
`
`530
`
`
`
`
`
`_estart_—15minutescold
`
`
`
`
`.£33231:dauge
`
`
`
`
`
`line
`environment;
`old and new
`
`
`of sight path
`position
`
`
`
`enabled
`
`
`
`
`Re—apply main poweror exit signal blocking
`
`
`
`
`
` Compare old
`
`
`GPS time with
`
`Real
`time
`
`clock
`
`
`
`332
`
`
` < =
`
`150 km
`difference
`
`
`
`
`3—8 second
`
`
`Read
`reaquisition if all
`
`Ephemeral
`variables are yes
`
`
`data
`
`326
`
`
`
`
`
`328
`
`

`

`1
`METHOD AND APPARATUS FOR
`TRACKING A MOVING OBJECT
`
`FIELD OF THE INVENTION
`
`This invention relates to a method and apparatus for
`tracking the location in terms of latitude and longitude of an
`object with a high degree of accuracy. This invention is
`designed for use with a wide range of vehicles including
`those adapted to be used on land, on water and in the air and,
`in particular, with automobiles and trucks,
`to track their
`location. In the event of unauthorized entry and/or theft, this
`invention provides a warning signal along with the present
`location of that vehicle to a command center. This invention
`in its preferred, illustrative embodiment combines in a new
`and nonobvious manner well known technologies such as
`the available Global Positioning Systems (GPS) and the
`cellular telephone systems.
`
`BACKGROUND OF THE INVENTION
`
`GPS Systems have been adapted in the prior art to track
`objects and vehicles. All-weather GPS systems, which have
`been designed and implemented by the United States
`Department of Defense, are intended to be comprised of 24
`satellites (21 of which shall be active and 3 of which will be
`on standby), ground control stations, and individual GPS
`receiver units throughout the world. The satellites are placed
`in elliptical orbits and are evenly distributed in 6 spheres of
`four satellites each. The satellites are disposed approxi-
`mately 10,900 nautical miles above the earth and maintain
`orbit longitudinal spacing angles of about 60 degrees from
`each other. The GPS satellites orbit about the centrally
`disposed earth. The semimajor axis of each satellite is
`controlled to maintain equal spacing from the earth so that
`the satellites pass over a given location on earth at predict—
`able, periodic pass-by times, e.g., regularly in 12 hour
`intervals. Thus, each GPS satellite concludes a complete
`orbit twice daily. Thus, assuming a complete constellation of
`GPS satellites, an average of 4.8 satellites would be in view
`at any given time from any given location on earth, not-
`withstanding signal obscuration by trees, mountains, build-
`ings and other natural and manmade obstacles.
`The former Soviet Union had been launching similar
`positioning satellites dubbed “GLONASS.” There is a
`strong possibility that the GPS and GLONASS systems may
`be combined into one mega-constellation of positioning
`satellites. Accordingly, the invention herein contemplates
`the usage of GLONASS and other like systems.
`The position of each GPS satellite in its orbit may be
`precisely determined. Each satellite includes an atomic
`clock, whereby the time at which a signal is transmitted from
`that satellite is precisely known. The object, whose latitude
`and longitude on earth is to be tracked, includes a ground
`GPS receiver for receiving and processing these satellite
`signals. A ground GPS receiver also includes a clock and a
`computer processing unit (CPU), which together are capable
`of determining the propagation time, i.e., the time required
`for signals to be propagated from the satellite to the ground
`GPS receiver, and therefore is capable of calculating the
`distance between each of at least three satellites and the
`ground GPS receiver to thereby accurately determine by
`well known triangulation techniques its position in terms of
`latitude and longitude on earth. In particular, the distance
`between a particular satellite and the ground GPS receiver is
`the product of the velocity of light, i.e., 186,000 miles per
`second, and the determined propagation time. To calculate
`
`5,587,715
`
`2
`
`object location it is also necessary to know accurately the
`positions of the satellites. The ground GPS receivers store
`therein data indicative of the continuously changing posi-
`tions of all of the active satellites in the GPS system. Such
`data is transmitted by each satellite to the ground GPS
`receivers to use in these object location calculations. When
`signals from three satellites are received by a ground GPS
`receiver, a 2-dimensional position, i.e., latitude and longi-
`tude, may be determined. When signals from four satellites
`are received by the GPS receiver, a 3-dimensional position,
`i.e., latitude, longitude and altitude, may be determined.
`The Department of Defense operates its GPS system to
`provide two distinct services. The first or Precise Positioning
`Service (PPS) is reserved for military use and is believed to
`be capable of determining object location to an accuracy of
`at least one meter. A second, less precise system known as
`the Standard Positioning Service (SPS) is available for
`general civilian use.
`The accuracy of the propagation time determination and
`therefore the calculations of the distances between the
`ground GPS receiver and each of the overhead satellites, is
`dependent directly on the accuracy of the clock included in
`the ground GPS receiver. The accuracy of the receiver clock
`is maintained by synchronizing it with the operation of the
`satellite’s atomic clock by transmitting a binary pseudo—
`random code from each satellite to the ground GPS receiver.
`As will be explained, the Precise Positioning Service and the
`Standard Positioning Service use different methods and
`pseudo-random codes for synchronizing the receiver clocks.
`The accuracy of the object location calculations is thus
`dependent upon the accuracy of the clock of the ground GPS
`receiver. To calculate position location to an accuracy of one
`meter, the ground receiver clock and therefore the calcula—
`tion of the propagation times require an accuracy of better
`than 100 ns. To maintain receiver clock accuracy,
`the
`satellites transmit timing marks at approximately one micro—
`second intervals. The ground receivers’ clocks difier from
`the satellite clocks by an error or clock bias CB. Dependent
`upon the error or bias C,3 of the ground receiver clock, the
`object location calculations performed by the ground receiv-
`ers are all in error by a fixed amount, which is called a
`pseudo-range “”.n
`Relative uncertainties in the calculations of object loca-
`tion by the ground GPS receiver occur because of several
`factors such as ionospheric delays, ambient temperature
`fluctuations and Doppler
`shift. Such uncertainties are
`expressed collectively as the dilution of precision. The
`Department of Defense increases the dilution of precision
`when it implements a policy of unscheduled Selective
`Availability in its Standard Positioning Service, which
`causes the calculated object location to appear off by the
`pseudo-range n, where n is whatever the Department of
`Defense selects, but generally, not in excess of such a value
`where n would cause an inaccuracy over 100 meters. The
`Department of Defense uses Selective Availability to pre-
`vent potential aggressors against the United States to employ
`the GPS system in a potential attack. However, Selective
`Availability, especially when combined with those elements
`contributing to normal dilution of precision, could prove to
`be detrimental to civilian uses of the Standard Positioning
`Service, inducing varying errors into the calculations of
`object location by ground GPS receivers.
`Each satellite transmits at a rate of 50 bps a tri-group of
`data in a direct sequence spread spectrum (DSSS) form,
`containing therein information concerning the almanac,
`ephemeris, and clock correction. The almanac, which is
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`generally reliable for a period of at least 30 days, contains
`general information regarding the position of the entire GPS
`constellation. The ephemeris is satellite-specific progression
`and path information, which is generally reliable for up to
`120 minutes (the duration of time during which the geo-
`graphical footprint generated by reliable signals made on
`earth from a satellite vehicle is of suflicient strength to
`reliably participate in a positioning fix). The clock correction
`parameters are necessary because even atomic clocks are not
`perfect and such timing oifsets, while greatly compensated
`for with ground relayed referencing to the National Obser—
`vatory time standardization in the District of Columbia, may
`be further corrected with user-corrected referencing. The
`satellites transmit
`their signals in both the Precise and
`Standard Positioning Services on a common carrier fre-
`quency within the L-band’s upper limit at 1575.42 MHz
`(L1), carrying with this frequency two distinct, binary
`pseudo—random codes emitted at two chip rates correspond-
`ing respectively to the Precise Positioning Service and the
`Standard Positioning Service. The chip rate for the Precise
`Positioning Service is 10.23 MHz, which is associated with
`a Precise or P-code. In the case of the Standard Positioning
`Service, a pseudo-random noise signal (PRN), which has a
`chip rate of 1.023 MHz and is unique to each satellite, is
`used to spread the spectrum of the transmitted information
`about the center frequency. The pseudo-random noise signal
`is known as a coarse/acquisition (CIA) code since it provides
`the timing marks required for fast acquisition of GPS signals
`and coarse navigation. Each satellite has a dilferent spread
`spectrum access code for both a clear acquisition (CIA) and
`a precision (P) code. The CIA code is a pseudo-random
`string of ones and zeros applied to a device which controls
`the carrier phase in 180 degree increments. This technique is
`known as bi-phase direct sequence spread spectrum at the
`1.023 MHz chip rate. The P code is much longer in length
`and is applied at the 10.23 MHz chip rate. Details of the GPS
`are given in NAVIGATION: Journal of the institution of
`Navigation, Vol. 25, No. 2, December 1978. The satellites
`repeatedly transmit at l—millisecond intervals their pseudo-
`random codes to the ground GPS receivers. The signals
`received at a ground receiver have a bandwidth of approxi—
`mately 2 MHz and a signal-to-noise ratio (SIN) of approxi-
`mately —20 db.
`Since the satellites are each moving at a speed in excess
`of 3 krn/s, the GPS signals are received with a Doppler
`frequency offset from the GPS center frequency. As a result,
`a stationary ground GPS receiver has to be capable of
`receiving signals with frequencies of up to + or—4 KHz from
`the GPS center frequency, and a mobile receiver (as is
`usually the case) has to be able to receive signals over an
`even greater frequency range. To recover the data and
`measure the propagation time of the satellite signals, the
`ground GPS receiver must compensate for the Doppler
`frequency offset and also synchronize its clock with the
`atomic clock of a satellite by generating the CIA code
`corresponding to each satellite. In particular, the ground
`GPS receiver must generate a replica of the pseudo code
`transmitted from the satellite for control of an internal phase
`switch and synchronize the code in time with the code
`received at
`its antenna in order to recover the carrier
`frequency. The code time with respect to the receiver’s clock
`is measured for four satellites and used for determining the
`position of the GPS receiver on the earth. See, for example,
`U.S. Pat. Nos. 4,457,006 and 4,114,155. Initially, at least,
`this synchronizing can be very time consuming since to
`despread the DSSS signals, the incoming and locally gen-
`erated PRN code delay,
`the ground GPS receiver must
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`compare the locally generated code and the incoming code
`at a number of diiferent positions until the point of synchro-
`nism or correlation is found. With a code length of 1023
`chips this comparison can be a lengthy procedure. However,
`once the frequency offset and the PRN code delay for each
`satellite are known, tracking them is relatively easy.
`U.S. Pat. No. 4,983,980 contemplates the mounting of a
`GPS receiver on a vehicle, for determining the location of
`that vehicle as it moves from place to place. This patent
`contemplates that such a vehicle may pass through a tunnel,
`whereby the GPS receiver may lose the transmission of the
`GPS signals from the satellites. Even after the vehicle
`emerges from the tunnel, it takes time for the vehicle‘s GPS
`receiver to reestablish reception of the satellite signal. In
`particular, GPS satellites continuously rotate about the earth,
`whereby the center frequency of the satellite signal is shifted
`due to the Doppler eifect when received by the ground GPS
`receiver disposed at a relatively stationary position on the
`earth. The ground GPS receiver initiates receiving of the
`spread-spectrum signal from the satellite by locking aphase-
`locked loop (PLL) circuit of the GPS receiver to the center
`frequency of the GPS signal which may be shifted by the
`Doppler eifect. Upon locking of the PLL circuit, the spread-
`spectrum signal is despread and demodulated to receive the
`GPS signal. Thus even after the vehicle emerges from the
`tunnel and its GPS receiver again has a line of sight contact
`with an overhead signal, the GPS receiver of the vehicle
`requires some delay before the satellite signal is received
`and demodulated and may again start calculating the vehicle
`position. This patent discloses a ground GPS receiver, which
`comprise a clock and a random access memory for storing
`the latitude and longitude of a last-known location, e.g., the
`latitude and longitude of Tokyo when the vehicle is driven
`in Japan, and for using the almanac information of each GPS
`satellite to determine the position of the satellites, when the
`vehicle reemerges into direct line of sight with the satellites.
`In particular, the GPS receiver identifies the strongest sat-
`ellites at the highest mask angle (reference to the horizonal
`plane) at the time when the vehicle reappears from the tunnel
`and has a direct line of sight with the satellites.
`In those applications where a GPS receiver is mounted on
`a vehicle, the receiver may be used for security application.
`For example, the GPS receiver may continue to calculate the
`vehicle location and to transmit that location to a distant
`point, where location data may be used by the police to track
`the vehicle. For example, if the vehicle is stolen, the vehicle
`owner or, preferably, the police could use the vehicle loca-
`tion to retrieve the vehicle, apprehend the thief and to
`discourage the theft of the vehicle, in the first instance. In
`potential security applications as well as in everyday track-
`ing of the vehicle,
`the vehicle may be taken to places,
`wherein its GPS receiver may no longer receive satellite
`signals. For example, the vehicle may be taken into an
`underground garage. Vehicles may be kept in such places for
`hours or even days and then emerge so that its GPS receiver
`may again reacquire transmission of the satellite signals and
`to again calculate the vehicle’s location.
`U.S. Pat. Nos. 5,043,736 and 5,119,102 disclose the
`combination of a GPS receiver and a transmitter for trans—
`mitting GPS system data from the receiver to a remote base
`station. The ’736 patent suggests that the transmitter be
`implemented by cellular system technology.
`U.S. Pat. No. 4,751,512 suggests improving the accuracy
`provided by a GPS system operated in the Standard Posi—
`tioning Service by operating such a system in a so called
`“differential mode”. Generally, operation in differential
`mode involves combining navigational information received
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`5,587,715
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`5
`at two difierent receivers, where the location of one of the
`receivers is known. By combining the data, the location the
`other receiver can be determined with greater accuracy than
`would be possible through using the data received by that
`other receiver alone. In particular, a GPS receiver may be
`disposed at a known location to determine the difference
`between its known location and its location predicted based
`upon receiving the satellite signals and calculating therefrom
`the approximate location. This diiTerence reflects errors in
`the information received including those deliberately
`induced by the Department of Defense in its Standard
`Positioning Service. This differential data must be commu-
`nicated from the reference receiver to a user, who is typically
`displaced from the reference station. The ’512 patent par-
`ticularly suggests that the associated transmitting unit trans:
`mit the diiierential data via a commercial geosynchronous
`earth satellite relay to a user located no more than 500 miles
`from the reference receiver.
`
`SUMMARY OF THE INVENTION
`
`It is an object of this invention to improve the accuracy of
`determining the location of a object based upon signals
`received from a subset of a plurality of satellites, each of
`which is deposed in a known orbit about the earth.
`It is another object of this invention to compensate for
`errors, which are deliberately introduced or caused by envi-
`ronmental conditions in the determination of objects based
`on the reception and processing of signals from satellites.
`it is a further object of this invention to permit differential
`correction of object determinations based on the reception
`and processing of satellite signals over a large geographic
`area.
`
`It is a still further object of this invention to efliciently
`recapture lost satellite signals and to begin again to process
`the recaptured satellite signals to determine object location
`with a minimum of delay.
`It is another object of this invention to quickly establish
`communication with a person, object and/or vehicle whose
`location within a large geographic area is not known.
`It is a further object of this invention to prompt a person,
`object and/or vehicle to call via a cellular telephone to a
`known address or telephone number.
`It is another object of this invention to disguise an antenna
`for receiving satellite signals for mounting on a vehicle.
`It is a still further object of this invention to protect an
`object, e. g., an automobile, by detecting its unauthorized use
`to automatically transmit an alarm message carrying the
`current location of the object to a command center.
`In accordance with these and other objects of the inven-
`tion,
`there is described an apparatus and a method of
`determining with a high degree of accuracy the location of
`an object based upon signals transmitted from a plurality of
`satellites rotating about the earth in known orbits. First, the
`latitude and longitude of a fixed point is determined with
`accuracy. The object is capable of moving with respect to the
`fixed point. Second, the satellite signals are received at the
`fixed point and processed to determine the approximate
`latitude and longitude of the fixed point. A first difierence
`between the accurate and approximate latitudes is taken to
`provide a differential latitude correction of a magnitude
`corresponding to the first diiference and of a direction to the
`north or south. A second difference between the accurate and
`approximate longitudes is also taken to provide a differential
`longitude correction of a magnitude corresponding to the
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`second difi'erence and of a direction to the east or west. The
`satellite signals are received at the object and processed to
`determine the approximate latitude and longitude of the
`object. The approximate object
`latitude and differential
`latitude correction are then combined to provide a corrected
`object latitude of improved accuracy, and the approximate
`object longitude and differential longitude correction are
`combined to provide a corrected object
`longitude of
`improved accuracy.
`In a further aspect of this invention, there is disclosed an
`object location system, which comprises a command center,
`and a plurality of diiferential stations distributed throughout
`a geographical area at a plurality of corresponding fixed
`points. The object is capable of moving throughout this
`geographical area. Each fixed point is disposed at a known
`location. Each of the plurality of differential stations oper—
`ates to receive and process signals from a corresponding
`subset of the plurality of satellites presently in sight of the
`differential station to provide a signal
`indicative of the
`approximate location of the corresponding fixed point and
`diiferential station. Signals indicative of the approximate
`and accurate locations of each of the fixed points are
`compared to provide difierential data. A unit mounted on the
`object and movable with the object throughout the geo-
`graphical area, receives and processes signals from a given
`subset of the plurality of satellites presently in sight of the
`object to provide a signal indicative of the approximate
`location of its object. The unit includes a first actuable
`transmitter for transmitting a message indicative of the
`location of its object to the command center. The command
`center has a first receiver for receiving the differential data
`and the approximate object location signal from the unit’s
`transmitter, and a processor for determining based upon the
`approximate object location and the known locations of the
`fixed points a determined one of the corresponding differ-
`ential stations which is presently in sight of the same subset
`of satellites as the object. The system includes a processor
`for combining the approximate object location signal and the
`differential data from the determined one difl’erential station
`to provide an indication of the object’s location with greater
`accuracy.
`
`In another aspect of this invention, there is disclosed an
`apparatus and method for determining the location of an
`object based on satellite signals, which comprise satellite
`orbit data. A memory is provided for storing the satellite
`orbit data, and a clock provides the current time. Upon
`detecting the cessation of receiving the satellite signals, the
`last received satellite orbit data is stored in the memory. The
`recapture of the satellite signals is detected to determine a
`length of time that the apparatus was not receiving satellite
`signals. That length of time is compared with a given period
`and, if less, the stored satellite orbit data is accessed from the
`memory, and the current time is taken from the clock to
`determine the present subset of the plurality of satellites that
`is in direct view of the object, before restarting the location
`determining apparatus to receive again the signals from the
`determined subset of satellites. If the determined length of
`time is greater than the