United States Patent [19J
`Sheffer et al.
`
`I 1111111111111111 11111 111111111111111 111111111111111 IIIII IIIIII Ill lllll llll
`US005844522A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,844,522
`Dec. 1, 1998
`
`[54] MOBILE TELEPHONE LOCATION SYSTEM
`AND METHOD
`
`[75]
`
`Inventors: Eliezer A. Sheffer, San Diego; Paul J.
`Bouchard, Valley Center, both of Calif.
`
`5,394,158
`5,434,904
`5,515,419
`5,602,903
`
`2/1995 Chia ........................................ 342/457
`7/1995 Tsuzuki et al. ........................... 379/58
`5/1996 Sheffer ...................................... 379/58
`2/1997 LeBlanc et al.
`.......................... 379/60
`
`FOREIGN PATENT DOCUMENTS
`
`[73] Assignee: Trackmobile, Inc., San Diego, Calif.
`
`0417944
`
`3/1991 European Pat. Off ..
`
`[21]
`
`Appl. No.: 543,031
`
`[22]
`
`Filed:
`
`Oct. 13, 1995
`
`[51]
`[52]
`[58]
`
`[56]
`
`Int. Cl.6
`........................................................ G0lS 3/02
`U.S. Cl. ......................... 342/457; 455/32.1; 455/33.1
`Field of Search ........................... 342/457; 455/32.1,
`455/33.1, 54.1, 56.1
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,177,466 12/1979 Reagan .............................. 343/112 TC
`4,704,734 11/1987 Menich et al. .
`4,726,050
`2/1988 Menich et al. .
`4,728,959
`3/1988 Maloney et al. ........................ 342/457
`4,730,187
`3/1988 Menich et al. .
`4,742,357
`5/1988 Rackley .................................. 342/457
`4,891,650
`1/1990 Sheffer .................................... 342/457
`5,055,851 10/1991 Sheffer .................................... 342/457
`5,203,009
`4/1993 Bogusz et al. ......................... 455/33.1
`5,218,367
`6/1993 Sheffer et al. .......................... 342/457
`5,293,642
`3/1994 Lo .......................................... 455/33.1
`5,293,645
`3/1994 Sood ...................................... 455/54.1
`5,317,323
`5/1994 Kennedy et al.
`....................... 342/457
`5,327,144
`7/1994 Stilp et al. .............................. 342/387
`
`Primary Examiner-Theodore M. Blum
`Attorney, Agent, or Firm-Brown, Martin, Haller &
`McClain, LLP
`[57]
`
`ABSTRACT
`
`A wireless network based location system and method uses
`an existing wireless communication network to locate the
`position of any active phone or transceiver unit in the
`network. The system includes a plurality of agile vector
`sensor units, one each installed at each antenna site in the
`network, and a remote central monitoring station to which
`wireless network users can call for assistance in the event of
`an emergency. The system is designed to locate a portable
`phone transceiver unit using the reverse voice channel signal
`transmitted by the transceiver unit. Each agile vector sensor
`unit locks onto the reverse voice channel to determine
`azimuth and signal strength in the reverse voice channel, and
`also tracks any changes in voice channel by monitoring the
`forward voice channel. In the event of a voice channel
`change, the sensor unit tunes to the new voice channel. The
`azimuth, signal strength, and reverse voice channel infor(cid:173)
`mation collected is used to track the location of a transceiver
`unit in real time.
`
`25 Claims, 13 Drawing Sheets
`
`14
`
`TELEPHONE
`COMPANY CO
`
`/-26
`'
`,-------7'-7
`LOCAL
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`
`28~
`
`CSCO-1005
`Cisco v. TracBeam
`Page 1 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 1 of 13
`
`5,844,522
`
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`CSCO-1005
`Cisco v. TracBeam
`Page 2 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 2 of 13
`
`5,844,522
`
`METRO #1
`
`METRO #2
`
`41
`
`24
`
`PHONE
`CO.
`
`14
`
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`
`44
`
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`SITE
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`
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`
`CSCO-1005
`Cisco v. TracBeam
`Page 3 of 28
`
`

`

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`37
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`36
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`COMPUTER
`ALGORITHM
`
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`. 3
`
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`.. TO ~OUTER
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`39
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`-88
`111 1 001 .. 11 1 011 0
`TIME STAMP/
`
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`2
`1
`
`CSCO-1005
`Cisco v. TracBeam
`Page 4 of 28
`
`

`

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`SECTOR #
`:CTIVE CELL ID
`
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`
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`
`CSCO-1005
`Cisco v. TracBeam
`Page 5 of 28
`
`

`

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`
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`64
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`
`CSCO-1005
`Cisco v. TracBeam
`Page 6 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 6 of 13
`
`5,844,522
`
`Fig. 7
`
`IDLE STATE
`
`\82
`
`/
`
`SCAN FCC FOR:
`MIN 1, MIN2,
`INIT. VC
`CDCj DIALS
`AVSi
`i - - - ~ SCAN FVC FOR:
`REASSIGNED VC
`
`\
`SCAN RVC FOR:
`RSSI, AZIMUTH. TEMP.
`SCAN FCC FOR:
`MIN1, MIN2,
`INIT. VC
`SCAN FVC FOR:
`REASSIGNED VC
`
`/
`
`TRANSMIT
`SCANNED DATA
`TO CDCj:
`ALSO APPEND
`AVSi
`ID
`
`86\
`
`YES
`
`"-
`"SHUT -oFr'
`NO
`i - - - - - - - - - - - - - -1 COMMAND
`FROM
`CDCj ?
`
`CSCO-1005
`Cisco v. TracBeam
`Page 7 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 7 of 13
`
`5,844,522
`
`SUBSCRIBER
`CALL
`
`88
`
`CELL NET
`ASSIGNS CELL SITE
`TO SUBS. PHONE
`
`89
`
`MTSO ROUTES
`CALL THROUGH
`PSTN TO CDC
`
`CDC OPERATOR
`QUERIES CALLER
`AS TO HIS/HER
`NAME
`
`90
`
`91
`
`CDC OPERATOR
`QUERIES SUBS.
`DATA BASE
`FOR ESN
`/ NAME
`
`CDC OPERATOR
`REQUESTS A CALL
`TRACE FROM MTSO
`BASED ON SUBS.
`NAME
`
`CALL TRACE RECORD
`INCLUDES ACTIVE CELL
`SECTOR
`ID
`VOICE CHANNEL
`
`NEIGHBOR CELLS
`TO ACTIVE CELL
`ARE SELECTED
`FROM CELL
`SITE DATABASE
`
`92
`
`93
`
`94
`
`95
`
`Fig.BA
`
`CSCO-1005
`Cisco v. TracBeam
`Page 8 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 8 of 13
`
`5,844,522
`
`Fig.BB
`
`96
`
`97
`
`98
`
`REQUEST SENT FOR
`SELECTED CELL SITES
`TO ACTIVATE THERE
`AVS
`
`AVS MONITORS
`CHANGES
`IN FCC,
`FVC AND RVC
`
`ALL AVS'S SEND
`DATA COLLECTED
`BACK TO REQUESTING
`CDS,
`INCLUDING NAME,
`FCC, RSSi AZIMUTH,
`VOICE CHANNEL, PREV.
`VOICE CHANNEL
`
`CDC RECEIVES
`DATA PACKETS
`
`99
`
`DETERMINE
`CURRENT VOICE
`CHANNEL ASSIGNED
`TO CALLING PHONE
`
`COMPUTE
`POSITION
`USING RSSi
`AND AZIMUTH
`
`129
`NO
`
`100
`
`101
`
`HAVE
`>3 UNITS
`RESPONDED
`?
`
`~~~ - - - - -+ - - . ~~~
`
`ASSIGN
`LOWEST
`CONFIDENCE
`LEVEL TO
`POSITION
`
`132
`COMPUTE
`POSITION
`A FROM
`AZIMUTH
`DATA
`
`YES
`COMPUTE POSITIONS
`USING AZIMUTH FOR
`DIFFERENT TRIPLETS
`
`102
`
`124
`CALCULATE
`POSITION B
`FROM RSSi
`
`CSCO-1005
`Cisco v. TracBeam
`Page 9 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 9 of 13
`
`5,844,522
`
`AVERAGE
`POSITIONS TO
`POSITION A
`
`104
`
`POSITION C
`FROM CELL
`SITE AND SECTOR
`
`125
`
`CALCULATE
`POSITION B
`FROM RSSi
`
`105
`
`110
`
`CALCULATE
`POSITION C
`FROM CELL
`SITE AND SECTOR
`
`128
`USE BAS
`POSITION
`AND ASSIGN
`CONFIDENCE
`LEVEL 6
`
`ASSIGN
`CONFIDENCE
`LEVEL 5 TO
`POSITION
`
`122
`USE RESULT
`A AND
`ASSIGN
`CONFIDENCE
`LEVEL 4
`
`120
`
`ASSIGN
`CONFIDENCE
`LEVEL
`3 TO
`POSITION
`
`ASSIGN
`CONFIDENCE ASSIGN HIGHEST
`POSSIBLE
`LEVEL 2
`TO POSITION CONFIDENCE
`..__ ___ _, LEVEL 1 TO
`POSITION
`
`DISPLAY POSITION ON MAP
`COMPUTER WITH SIZE OF SPOT
`CORRESPONDING TO CONFIDENCE i---------'
`LEVEL
`
`133
`
`SEND POSITION
`CONFIDENCE
`LEVEL AND VOICE
`CHANNEL TO FRY
`
`134
`
`GO TO
`99
`
`136
`SEND MESSAGES
`AVS UNITS TO
`DISCONTINUE
`TRANSMISSION
`
`CSCO-1005
`Cisco v. TracBeam
`Page 10 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 10 of 13
`
`5,844,522
`
`AVS #3
`
`Fig.9
`
`PROBABLE PHONE
`LOCATION
`
`~
`AZIMUTH TOLERANCE
`
`AVS #1
`
`#2
`
`AVS #3
`
`SECTOR #1
`
`Fig. 10
`
`CSCO-1005
`Cisco v. TracBeam
`Page 11 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 11 of 13
`
`5,844,522
`
`Fig. 11
`
`AVS #1
`
`/-------.. .......
`
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`/
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`CSCO-1005
`Cisco v. TracBeam
`Page 12 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 12 of 13
`
`5,844,522
`
`Fig. 1 3
`CHANNEL NUMBER
`CONTROL CH.
`VOICE CH # 1 VOICE CH #2 3 3
`..
`1 -354 ..
`
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`VOICE CH. #1 VOICE CH. #2 .. 313-354 ..
`7
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`CSCO-1005
`Cisco v. TracBeam
`Page 13 of 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 13 of 13
`
`5,844,522
`
`Fig. 1 5 CHANNEL NUMBER
`
`CONTROL CH.
`VOICE CH. #1 VOICE CH. #2 .. 313-354 ..
`
`7
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`RSSI AZIMUTH RSSI AZIMUTH RSSI AZIMUTH
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`
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`Fig. 16 CHANNEL NUMBER
`CONTROL CH.
`VOICE CH. #1 VOICE CH. #2 .. 313-354 ..
`
`7
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`RSSI AZIMUTH RSSI AZIMUTH RSSI AZIMUTH
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`CSCO-1005
`Cisco v. TracBeam
`Page 14 of 28
`
`

`

`5,844,522
`
`1
`MOBILE TELEPHONE LOCATION SYSTEM
`AND METHOD
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to a location
`system and method for locating the position of a mobile
`telephone, and is particularly concerned with a system using
`an existing wireless telephone or communications network,
`such as cellular, PCS, or other types of wireless communi(cid:173)
`cation networks, in locating a target telephone within the
`network in the event of an emergency situation, or for other
`reasons.
`Numerous location systems have been proposed in the
`past for locating the position of a target emitting a radio or
`cellular signal. All previous systems have been subject to
`various disadvantages. Some proposed methods are satellite(cid:173)
`based, for example using the Global Positioning System, or
`GPS, with receivers receiving signals from orbiting satellites
`in order to determine approximate location. However, such
`a system has limited operation in heavily built urban areas
`and cannot readily provide voice or data communications.
`Such systems also require specialized end-user equipment.
`Additionally, these systems can locate a target to an approxi(cid:173)
`mate area of 100 yards. In practice, this may not be sufficient
`to pinpoint a phone which may be within a building, for
`example.
`Other systems involve use specialized radio positioning
`methods. However, this also requires installation of heavy
`infrastructure, and requires specialized end-user telephone 30
`equipment. A number of location systems have also been
`proposed which use existing cellular telephone network
`infrastructure. For example, U.S. Pat. Nos. 5,218,367, 5,055,
`851 and 4,891,650 of Sheffer all describe locating systems
`which utilize cellular technology. In U.S. Pat. No. 5,218, 35
`367, the cellular telephone itself is modified to receive signal
`strength and cell identifying information from all adjacent
`cell sites, and to transmit this information to a monitoring
`station which includes a computer programmed to calculate
`an approximate vehicle location. One disadvantage of this 40
`system is that it can only be used to locate specialized, or
`modified, cellular telephones and cannot locate
`conventional, unmodified phones.
`In U.S. Pat. No. 4,728,959 of Maloney et al. a location
`system is described in which detector units are used to detect 45
`reverse control channel signals emitted from mobile phones,
`and determine azimuth angles for the phone to determine the
`direction from which the signals are emitted. Phase angle
`measurements of the direction of the phone from a plurality
`of land stations are processed to produce a probability 50
`density function, producing an x-y coordinate area repre(cid:173)
`senting the most likely location of the phone.
`This system is subject to a number of disadvantages,
`along with the other known system which locate only a
`rough, x-y coordinate position. An x-y coordinate position is
`unlikely to be sufficient to pinpoint the location of the
`mobile phone with sufficient accuracy, particularly in a
`heavily built-up, urban area including multi-story buildings
`and underground parking lots. Even if the correct building
`can be located, which is not certain given the accuracy of
`these systems, the location of the individual calling for help
`within the building will be completely unknown, and a
`significant amount of extra time will be necessary in search(cid:173)
`ing the building from floor to floor. In an emergency
`situation, the caller must be found as quickly as possible, and
`this is not feasible with systems of the type described by
`Maloney.
`
`5
`
`2
`Another disadvantage of Maloney is the use of a reverse
`control channel signal to locate a mobile phone. Each cell
`site in a cellular system has a fixed control channel fre(cid:173)
`quency over which forward control channel signals are
`transmitted to mobile phones in the vicinity. All mobile
`phones assigned to that cell site will use the same frequency
`to transmit reverse control channel, or RCC, signals back to
`the cell site. The identification code or NAM for each phone
`will be embedded in the emitted RCC signal packet. RCC
`10 signals are not emitted continuously, but only in an irregular
`fashion depending on reorder conditions, such as hand-off.
`For each mobile phone, the RCC signal is emitted in short
`bursts at irregular time intervals. Thus, there may be rela(cid:173)
`tively long periods of time when no RCC signal is emitted.
`15 Another problem is that up to 60 phones may be assigned to
`the same cell site at any time, and the only way they can be
`distinguished from one another using the RCC signal is by
`deciphering each RCC received to determine the NAM of
`the desired target, which may or may not be emitting an
`20 RCC signal at the time. Thus, this location system is
`relatively unreliable and does not allow a target phone
`location to be pinpointed with sufficient accuracy to enable
`emergency assistance to be deployed quickly to the target.
`
`25
`
`SUMMARY OF THE INVENTION
`It is an object of this invention to provide a new and
`improved location system and method using cellular or other
`wireless telephone technology.
`According to one aspect of the present invention, a
`portable phone tracking system is provided, which utilizes
`parts of an existing wireless telephone communication sys(cid:173)
`tem such as a cellular phone network. Each cellular phone
`network includes a plurality of cell or antenna sites for
`receiving and transmitting wireless voice and control chan-
`nel signals when linked with a wireless or cellular phone in
`the vicinity of the antenna site, and for linking the cellular
`phone to another phone via a mobile telephone switching
`office, or MTSO, and the public telephone network. Each
`antenna or cell site covers a predetermined area and will be
`the active cell site for any portable phones making or
`receiving calls within that area. Special hand-off procedures
`are used for transferring calls from an active cell site to a
`neighboring cell site when the phone moves from one cell
`area to another. The cellular phone system will operate in a
`certain frequency range divided into separate channels,
`some of which will be assigned as voice or communication
`channels for forward and reverse communication and others
`of which are assigned as control and/or signaling channels
`for sending forward and reverse control signals. Each caller
`in the network will be assigned an initial communication
`channel, for voice or other types of communications, via the
`forward control channel (FCC) of the active cell site. The
`initially assigned reverse voice channel may be changed
`quite rapidly and several times during a call. The tracking
`55 system of this invention is designed to track the initial
`reverse voice channel and all reassigned reverse voice
`channels for a particular call on a real-time basis during the
`entire call duration, and to use the determined voice chan(cid:173)
`nels as a beacon to direction finding equipment in order to
`60 locate the exact position of the cellular phone.
`According to the present invention, a wireless phone
`location system is provided, which includes a plurality of
`agile vector sensor units, each agile vector sensor unit being
`mounted at a respective cell site in a cellular telephone
`65 network such that an agile vector sensor unit is located at
`every cell site in the network, and each agile vector sensor
`unit comprising a scanning assembly for scanning over the
`
`CSCO-1005
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`Page 15 of 28
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`

`5,844,522
`
`3
`frequency range of the telephone network and detecting any
`calls within the range of the scanning assembly, a direction
`finder assembly for determining azimuth and signal strength
`of the particular voice channel, a receiver assembly for
`receiving control and voice channel signals of the cell site
`and the target phone and determining the identification code
`of all phones in the area connected to that cell site, and the
`initially assigned voice channel for each connected phone,
`an information storage system for storing the identification
`code and initial voice channel of connected calls, and the 10
`azimuth and signal strength for each detected voice channel
`signal, and a receiver assembly for detecting and storing new
`voice channel assignments, and tuning the direction finder
`assembly automatically and rapidly to the new voice chan(cid:173)
`nels to determine azimuth and signal strength in each new 15
`voice channel on a real-time basis. The system also includes
`a monitoring system at a remote facility for determining the
`currently active cell site for a selected phone in the network
`and the initially assigned voice channel for that phone,
`determining neighboring cell sites to the currently active 20
`cell, and sending control signals to the sensor units at the
`active and neighboring cell sites to begin transmitting col(cid:173)
`lected information for at least a selected phone to the
`monitoring station, and an analyzer for determining a prob(cid:173)
`able location area for the selected phone from the informa- 25
`tion received from the sensor units.
`The monitoring system may command the sensor units to
`transmit all stored information for all calls in the vicinity,
`and may be programmed to filter or extract information for
`the selected phone from the incoming data. Preferably, all
`sensor units continuously scan and collect information on at
`least the voice channel assignments, and write over the
`previously collected information unless activated by a con(cid:173)
`trol signal from the monitoring system. When activated,
`sensor units periodically transmit collected information to
`the monitoring system.
`Preferably, a plurality of field response units or vehicles
`are provided, each having an extra, stand alone field direc(cid:173)
`tion finder unit including a receiver for linking the unit to the
`monitoring system at the remote site, and an automatic tuner
`for automatically tuning the unit to a selected voice channel
`received from the monitoring system. When the monitoring
`system determines an approximate location for a selected
`phone, one or more field response units are dispatched to the
`area, with the direction finder unit in each vehicle linked to
`the monitoring system. The monitoring system will transmit
`all voice channel changes in real time for that caller to the
`field direction finder units to tune them automatically to the
`current voice channel for the selected call, so that they can
`hone in on the selected call and locate the precise position 50
`of the portable phone. Thus, unlike previous phone location
`systems which locate call position to an accuracy of 100
`yards or so only, and provide only a two-dimensional or x-y
`position for the target, this system enables a direction finder
`to lock on to the uniquely assigned reverse voice channel of 55
`a calling phone, and to track and lock onto changed voice
`channels on a real-time basis, so that the caller can be
`located with pinpoint accuracy in x, y and z coordinates.
`This is extremely important in an emergency medical or
`crime situation, for example, where an accuracy of 100 60
`yards will not necessarily enable the caller to be found
`quickly. Also, if the caller is in an underground or above
`ground parking lot or in a multi-story building, the radio
`direction finder unit will still be able to pinpoint z coordinate
`position within the building.
`Previous location systems, such as Maloney, compute x-y
`coordinate location only, and use signals which are not on at
`
`4
`all times. Since these signals are not on at all times, they
`cannot be used as a beacon for direction finding equipment.
`The system of this invention, in contrast, uses the reverse
`voice channel signal, which is always on, as a tracking
`5 beacon for a field direction finder unit to track the x, y and
`z coordinate position quickly and accurately.
`The remote monitoring station will be able to determine
`the currently assigned voice channel for a selected call, and
`will control sensor units at neighboring cell site to scan this
`voice channel and collect azimuth and signal strength infor(cid:173)
`mation for the current voice channel, and to transmit that
`information to the monitoring station. This information can
`then be used in the location determination. The system
`preferably includes at least one local computer which is
`connected to each MTSO in the network, and which is
`programmed to trap all calls to the monitoring center,
`including the active cell identification, sector number and
`initial voice channel assignment from the cellular carrier
`MTSO (mobile telephone switching office) which is cur(cid:173)
`rently handling the selected phone, and to transmit that
`information to the central monitoring station. The local
`computer may be physically located at one of the MTSO
`sites in the system and include a modem for communicating
`with all the other MTSO sites, or it may be located at the
`central monitoring station and linked to all MTSO sites via
`modem. The system includes a cell site data base of the
`geographical location of each cell site, and a neighboring
`cell site data base of the nearest neighbors of each cell site.
`These data bases may be located at the local computer or at
`30 the central monitoring station.
`Preferably, the central monitoring station includes a plu(cid:173)
`rality of work stations each including a first, phone location
`computer for determining an approximate phone location
`and a second, map computer for displaying a map including
`35 the calculated position. Portable phones in the cellular
`network may make emergency calls to the central monitor(cid:173)
`ing station, and all such calls are assigned to a particular
`workstation based on current work load. Receipt of an
`emergency call will initiate a phone location procedure, in
`40 which the identification code for that phone is determined,
`the network is searched for that identification code and the
`currently active cell site for that phone, and the sensor units
`at the active cell site and neighboring sites are activated to
`begin transmitting data on at least that call to the monitoring
`45 station.
`According to another aspect of the present invention, a
`method of locating a portable phone is provided, which
`comprises the steps of:
`providing an agile vector sensor unit at each cell site in a
`cellular network;
`determining the active cell site and neighboring cell sites
`for a selected phone which is currently in use;
`activating the sensor unit at the active cell site to scan the
`forward voice channel of the active cell site to detect
`any voice channel assignment changes for connected
`calls;
`determining the new voice channel of any connected call;
`tuning the sensor unit to all currently assigned reverse
`voice channels in turn and determining the azimuth and
`signal strength for each reverse voice channel signal;
`transmitting new voice channel assignments, azimuth and
`signal strength information to a central monitoring
`station;
`at the central monitoring station, determining the cur(cid:173)
`rently assigned voice channel for a selected phone and
`
`65
`
`CSCO-1005
`Cisco v. TracBeam
`Page 16 of 28
`
`

`

`5,844,522
`
`5
`controlling sensor units at neighboring cell sites to scan
`that voice channel and collect and transmit azimuth and
`signal strength information to the central monitoring
`station; and
`determining an approximate phone location from the 5
`azimuth and signal strength information received from
`the active and neighboring cell sensor units.
`During the location process, the central monitoring station
`will receive information on new voice channel assignments
`on an ongoing basis, so that the voice channel for the 10
`selected phone can be updated as necessary, and the correct
`voice channel information can be used to re-tune the sensor
`units as necessary. Field response units can then be
`dispatched, with their own direction finder units being
`automatically tuned and re-tuned with each change in voice
`channel.
`This system and method uses the reverse voice channel of
`a portable phone as a signal to lock onto. The reverse voice
`channel is the only uniquely identifiable signal emitted by a
`portable phone which is emitted, or "on," continuously over
`the entire duration of a call. Thus, it is the best signal to use 20
`for locking onto, and tracking, a portable phone. However,
`the reverse voice channel assignment may be changed
`several times during a call, and this system enables such
`changes to be detected quickly, so that the system can tune
`to a newly assigned voice channel on an ongoing basis, and 25
`receive and analyze azimuth and signal strength information
`for the current voice channel at all times and in real time.
`The system of this invention does not require any modi(cid:173)
`fication whatsoever to the hardware or software of any
`portable phone, cell site, or MTSO of an existing cellular 30
`system. It simply adds on to an existing system or network,
`in the form of an agile vector sensor unit mounted at each
`cell site but not connected, in any way, to the cell site
`electronics/network, a separate central monitoring station
`for controlling the sensor units and receiving data from the 35
`sensor units, and one or more local computers for connection
`to each MTSO either locally or remotely, as well as auto(cid:173)
`matically tunable direction finder units in each of a plurality
`of field response units for pinpointing a call location once an
`approximate location has been determined. Only one central 40
`monitoring station is needed for locating any mobile phone,
`nationwide or even worldwide.
`This system is intended to be used primarily for locating
`a portable phone in the event of an emergency. However, it
`may also be used to detect cellular phone clones, since it can 45
`locate any currently active phone by identification number
`or NAM, and can detect if more than one phone having the
`same NAM is in use simultaneously, or if a NAM is being
`used in the wrong city for a particular subscriber. The central
`monitoring station may broadcast to all agile vector sensor 50
`units, nationwide, to determine the location of any phone
`having a selected NAM, for anti-fraud purposes. The system
`may also be used for network management purposes.
`The system of this invention enables a portable phone
`making a call to be located much more accurately and 55
`exactly than was previously possible, so that field response
`units can hone in on the originating voice channel signals
`with great precision. This can enable a caller in an emer(cid:173)
`gency situation to be located much faster than was previ(cid:173)
`ously possible, which can be critical.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention will be better understood from the
`following detailed description of a preferred embodiment of
`the invention, taken in conjunction with the accompanying
`drawings, in which like reference numerals refer to like
`parts, and in which:
`
`6
`FIG. 1 is a schematic block diagram of a cellular phone
`system incorporating a location system according to a pre(cid:173)
`ferred embodiment of the present invention;
`FIG. 2 is more detailed block diagram of the location
`system;
`FIG. 3 is a schematic block diagram of workstation at the
`central monitoring station or communication and dispatch
`center of the location sy