United States Patent (19)
`Vlcek et al.
`
`|||||IIII
`USOO54.93694A
`5,493,694
`11
`Patent Number:
`Feb. 20, 1996
`(45) Date of Patent:
`
`(54) FAST RESPONSE SYSTEM FOR A FLEET OF
`VEHICLES
`
`75 Inventors: Charles Vlcek, Greenland, N.H.; James
`Reynolds, San Jose, Calif.
`73 Assignee: Trimble Navigation Limited,
`Sunnyvale, Calif.
`
`21 Appl. No.: 148,998
`22 Filed:
`Nov. 8, 1993
`(51) Int. Cl. ......................... H04B 7/26
`52 U.S. Cl. ........................ 455/53.1; 455/541; 455/54.2
`58) Field of Search .................................. 455/12.1, 33.1,
`455/54.1, 54.2, 56.1, 53.1, 57.1, 58.1; 340/988,
`989
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,907,290 3/1990 Crompton .............................. 455/54.1
`5,029,234 7/1991 Kawai et al. ...
`... 455/54.1
`5,068,654 11/1991 Husher ................................... 455/54.1
`Primary Examiner-Edward F. Urban
`Assistant Examiner-Thanh Le
`Attorney, Agent, or Firm-John Schipper
`(57)
`ABSTRACT
`A method for directing a vehicle from a fleet of N vehicles,
`
`numbered n=1,2,..., N(N22), to respond to a call for
`assistance received at a central processing station that com
`municates with a communications unit on each of these
`vehicles. The central station receives information by radio
`wave signals on the present location and present status of
`each vehicle on patrol at a sequence of times. Optionally, the
`vehicles may be divided into mutually exclusive groups,
`with each group reporting its location and status at separate
`times with different frequencies of reporting. An information
`response interval for a group includes a number of time
`slots, with each time slot allocated to a vehicle in the group
`so that no slot is empty. When the central station receives a
`call for assistance, an assistance message is transmitted to
`the fleet, specifying the site at which assistance is needed
`and any available information on the nature of the assistance
`required. Each available vehicle receives the assistance
`message and determines its distance d(n) from the assistance
`site. Each available vehicle whose distance d(n) is no greater
`than a selected distance D replies to the assistance message
`by transmitting its status and distance d(n) to the central
`station with a time delay that is proportional to d(n). Vehicle
`selection may be based upon first-to-reply and/or upon any
`specialized equipment or personnel training needed to
`respond. Vehicle location may be determined by GPS,
`GLONASS, Loran, or any other suitable location determi
`nation system.
`
`34 Claims, 4 Drawing Sheets
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`central station
`received an
`incident call
`
`Central station transmits
`interrogation signal for
`group q of vehicles
`
`Vehicles in group q transmit,
`in specified order, present
`location and status and/or
`orher requested information
`for each such vehicle
`
`Each available vehicle receives
`incident message and determines
`its location, status, etc.
`
`5
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`Each vehicle n determines its
`distance d(n) fron incident site
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`55
`No
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`Vehicle m replies to
`central station and
`transmits d(n) information,
`using backoff time At(n)
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`Central station receives
`replies and selects
`vehicle(s) to respond
`to incident
`
`Central station notifies
`selected vehicle(s)
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`U.S. Patent
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`Feb. 20, 1996
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`Sheet 1 of 4
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`5,493,694
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`U.S. Patent
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`Feb. 20, 1996
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`Sheet 2 of 4
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`5,493,694
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`Transceiver
`
`Controller
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`FIG. 2
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`81
`83
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`FIG. 4
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`U.S. Patent
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`Feb. 20, 1996
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`Sheet 3 of 4
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`5,493,694
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`central station
`received an
`incident call
`
`Central station transmits
`interrogation signal for
`group q of vehicles
`
`45
`
`Central station broadcasts
`incident message
`
`49
`
`Vehicles in group q transmit,
`in specified order, present
`location and status and/or
`orher requested information
`for each such vehicle
`
`Each available vehicle receives
`incident message and determines
`its location, status, etc.
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`53
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`Each vehicle n determines its
`distance d(n) fron incident site
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`55
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`Vehicle in replies to
`central station and
`transmits d(n) information,
`using backoff time At(n)
`
`
`
`Central station receives
`replies and selects
`vehicle(s) to respond
`to incident
`
`Central station notifies
`selected vehicle(s)
`
`FIG. 3
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`U.S. Patent
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`Feb. 20, 1996
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`Sheet 4 of 4
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`5,493,694
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`5,493,694
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`1.
`FAST RESPONSE SYSTEM FOR A FLEET OF
`VEHICLES
`
`FIELD OF THE INVENTION
`This invention relates to a communication system for
`providing fast response to an incident by one of a fleet of
`vehicles operating within a region.
`
`2
`partly avoided by having each station listen to sense the
`presence of one or more carriers (indicating that a message
`is presently being transmitted by another station) before
`transmitting. Collisions are still a problem here, and another
`collision reduction algorithm is often imposed on the sta
`tions. One popular, algorithm, known as random backoff,
`requires two stations whose signals have collided (and any
`other station that senses the presence of this signal collision)
`to "back off” or refrain from further transmissions for a
`randomly determined time drawn from a time interval of
`selected maximum length, such as 16 seconds. Schwartz,
`ibid, shows that, without time slot allocation, a system will
`theoretically saturate, so that signal collisions allow sub
`stantially no transmissions, as soon as at least 19 percent of
`the stations are attempting to transmit. If time slot allocation
`is implemented, the threshold at which saturation will occur
`improves to about 37 percent. These results indicate that
`saturation can be continually present where a large fleet of
`vehicles communicates with a central station, even with
`modest throughput requirements, unless the communication
`protocol is carefully selected and implemented.
`Where system saturation is a concern, some workers have
`assigned general time slots for use in exchanging informa
`tion between stations. Fujiwara, in U.S. Pat. No. 4.513,416,
`discloses a TDMA satellite-communication system with a
`ground station that counts the number of idle time slots in
`each uplink or downlink signal. When this idle number
`exceeds a selected number, one of the idle time slots in an
`uplink and in a down link signal is assigned to time axis
`adjustment and is no longer available for its original use.
`However, at any time some of the time slots may be idle and
`unused.
`A radio communication system that adapts itself to the
`amount of signal traffic is disclosed in U.S. Pat. No. 5,103,
`445, issued to Ostlund. A receiving station determines
`whether a given time slot allocated for transmission is likely
`to be filled or empty, based on the signal traffic sensed by the
`station in a time slot used for an earlier invitation-to-transmit
`message. This patent disinguishes between three types of
`time slots (containing an understandable message, empty,
`and mutilated) as they appear in the system.
`In U.S. Pat. No. 5,168.271, Hoff discloses a packet-based
`paging and timekeeping system in which time slot identifi
`cation is used to transfer packets from a station on one
`network to a receiver on a second network. A sequence of
`time slots is allocated, and a packet is transmitted during a
`selected time slot that corresponds to and identifies the
`addressee network.
`Yamao, in U.S. Pat. No. 5,203,024, discloses an antenna
`selection system that selects a particular antenna for signal
`reception in a specified time slot, based upon comparison of
`a predicted signal quality parameter for each of the antennas.
`The parameter may be present error rate, receive level at the
`center of the assigned slot, minimum receive level required,
`or some other parameter.
`Another antenna selection system is disclosed in U.S. Pat.
`No. 5,203,026, issued to Ekelund. Antenna selection for the
`present time slot is based upon comparison of the signal
`quality in the immediately preceding time slot for each of a
`plurality of antennas.
`In U.S. Pat. No. 5,126,733, Sagers etal disclose a polling
`system for a plurality of location determination units, here
`Loran signal receivers. A polling station can transmit an
`interrogation signal, requesting location information from
`all receiving stations. Alternatively, a polling station can
`request location information from a specified station.
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`O
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`BACKGROUND OF THE INVENTION
`When a central station that controls a fleet of vehicles,
`such as police cars, taxicabs or ambulances, receives a call
`for assistance, one question of immediate concern is which
`vehicle can respond most quickly to the call. A 911 or
`taxicab dispatcher will normally question the caller and
`determine the type of assistance needed and the location of
`the caller, using a Geographic Information System (GIS) or
`similar database to identify the caller's location on a grid or
`other coordinate system. The dispatcher must now deter
`mine which vehicle or vehicles is in the best position to
`respond to the call, based upon vehicle availability, vehicle
`proximity to the caller's location, special equipment and/or
`personnel training needed for response to this call, and
`possibly other factors. Vehicle availability and proximity to
`the caller's location require current information on each of
`the fleet vehicles.
`If the number of vehicles in the fleet is small (e.g., five)
`and not too widely dispersed, it may be feasible to contact
`each vehicle by radiowave and learn the vehicle's present
`location and availability every 15-30 seconds. However, if
`the fleet is a large one (e.g., with 200 or more vehicles), this
`frequent interrogation/response mode cannot be imple
`mented using a single channel or a reasonably small number
`of channels. For example, the New York City Police Depart
`ment has a fleet of about 4600 vehicles, and about 5 police
`vehicles per second can report their present location and
`availability or status. If 16 channels are dedicated for such
`reporting, assuming 5 reports per second in a round-robin
`system, each vehicle can report at most once in a time
`interval of length 58 seconds. Further, some police vehicles
`that are currently in the "pursuit or deployment” mode
`should report their present location and status more often,
`perhaps once every ten seconds, and a round-robin system
`that assigns uniform priorities to each vehicle is not appro
`priate here. Further, some vehicles may not be on patrol nor
`operational at any given time, and interrogation of the
`present availability and/or location of such vehicles (which
`will not respond) is a waste of air time.
`Local area networks (LANs) whose stations communicate
`by cable signals or by radio waves often face a problem of
`collision of two or more signals sent by one station to
`another station, where the content of both signals may be
`lost. In one of the original LANs, the ALOHA network
`connecting five of the Hawaiian islands, the signal collisions
`were severe enough that the ALOHA system experimented
`with an allocation of specific time slots for each station,
`which was of some help in the small ALOHA system. This
`is discussed in some detail by Mischa Schwartz in Com
`puter-Communication Network Design and Analysis, Pren
`60
`tice Hall, 1977, pp. 288-320. A fixed allocation of time slots
`is wasteful if many of the responding stations are not
`operative at a given time.
`Another popular approach for LANs is CSMA/CD, or
`carrier sense, multiple access with collision detection, in
`which any station may contend for "possession' of the air
`waves for a selected maximum time. Signal collisions are
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`5,493,694
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`3
`Other workers have used a known signal transmission
`backoff algorithm in the presence of signal collisions, to
`reduce the likelihood of subsequent signal collisions. Hoch
`sprung et al disclose a local area network with carrier sense
`collision avoidance, using a signal backoff, in U.S. Pat. No.
`4,661,902. If a first station, wishing to transmit, senses the
`presence of a carrier or other indicia of a second station's
`signal, the first station executes a signal backoff for a time
`RAt, where At is a selected number (100 usec) and R is a
`positive integer that is randomly chosen based upon recent
`network experience with signal collisions.
`In U.S. Pat. No. 5,018,138, Twitty et al disclose a signal
`backoff algorithm for a network of communicating stations.
`A first station, wishing to transmit, that senses the presence
`of a signal already transmitted by a second station, waits for
`a backoff period of length specified by the I.E.E.E. 802.3
`truncated binary exponential backoff standard. The backoff
`time is a selected time. At multiplied by a random integer R
`that is uniformly distributed over an integer interval defined
`by 0<R<expmin(IO, n)), where n is a statistically deter
`mined number of signal collisions in a selected time interval
`for the network, based upon recent experience and I0 is a
`selected integer.
`The I.E.E.E. 802.3 truncated binary exponential backoff
`standard is also adopted for signals in response to collision
`detection in U.S. Pat. No. 5,164,942, issued to Kamelman et
`al. None of these patents uses a deterministic, as opposed to
`statistically determined, backoff time based upon some
`physically measurable quantity that is distinct for each
`station.
`Some of these systems use signal analysis in a given time
`slot to determine whether a signal should be transmitted, or
`received, during that time slot or a subsequent time slot but
`do not provide an approach that reduces the required number
`35
`of time slots to a minimum. Other systems use well known
`transmission backoff algorithms that do not take account of
`the special needs of a fast response system for a fleet of
`vehicles that continuously communicates with a central
`station.
`40
`What is needed is a system for communication between a
`central station and each vehicle in a fleet that: (1) allows the
`central station to poll the present status and location and
`other necessary information for each vehicle with a fre
`quency that need not be uniform for all vehicles in the fleet;
`(2) allows the central station to advise each fleet vehicle of
`the location and other necessary information for each call for
`assistance received by the central station; (3) allows each
`fleet vehicle to determine its present distance from the
`location of a call for assistance and to advise the central
`station if that vehicle is within a selected distance of the
`caller; (4) allows each fleet vehicle to communicate with the
`central station, using a protocol that minimizes the likeli
`hood of signal collision; (5) allows the central station to
`adjust the criteria to be used, including but not limited to a
`vehicle's proximity to the caller's location, in determining
`which vehicle(s) will respond to a call for assistance; and (6)
`performs these tasks with a minimum of radio channels and
`does not saturate as 100 percent utilization is approached.
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`SUMMARY OF THE INVENTION
`These needs are met by the invention, which provides a
`system for communication between each vehicle in a fleet
`and a central station, where the present status and location
`of each vehicle is to be reported to the central station with
`a reporting frequency that may vary with each vehicle and
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`may vary with time as well.
`The Invention provides information on the present loca
`tion and status of each of a fleet of vehicles and of any
`"incident” to be responded to, and can direct a vehicle that
`is "closest to' the site of the incident to report to that site.
`The system works with a small or large number N of
`vehicles, numbered n=1, 2, . . . , N, each of which has a
`location determination (LD) unit including an LD signal
`antenna and LD signal receiver/processor (providing a loca
`tion fix every 0.5-5 seconds), a radiowave transceiver, and
`an LD receiver/processor-transceiver interface connecting
`the indicated devices. The radiowave transceivers all com
`municate with a central processing station that has a similar
`transceiver but need not have an LD antenna or LD receiver/
`processor. At any given time: (1) a first portion N1 of these
`vehicles are "parked", unavailable or otherwise not in opera
`tion; (2) a second portion N2 of these vehicles are already
`assigned or responding to other incidents, and thus am not
`available to respond to a call; and (3) a third portion N3 of
`these vehicles are on patrol, and not in pursuit or otherwise
`deployed, and are thus available to respond to a call for
`assistance or service, where N1+N2+N3=N. The LD signals
`used for location determination may be produced and ana
`lyzed by a Global Positioning System (GPS), a Global
`Orbiting Navigational System (GLONASS), any other sat
`ellite-based system, a Loran or similar system, an FM
`subcarrier system, or any other system that uses electromag
`netic waves to determine location.
`At a selected time, the central station broadcasts an
`interrogation signal in a time slot of length T(s1 sec),
`requesting that vehicles number n=n,n-, ... in a selected
`group of k(q) vehicles respond with the present location and
`status of each vehicle. Using a protocol known by the central
`station and by each of the vehicle transceivers, the central
`station then ceases its broadcast and waits a certain time
`interval of length T for the vehicle responses. This time
`interval is divided into k(q) sub-intervals or time slots, each
`of approximately equal length At-T/k(q) (s50-500 msec),
`and vehicle number n=n, replies with the requested infor
`mation during the rth consecutive time slot. Transceiver n,
`(in vehicle number n) has listened to the central station's
`interrogation signal and knows (1) whether it is one of the
`transceivers queried and (2) if it has been queried, what is its
`numerical position in the queue for responding to this query.
`If a particular transceiver is not among those queried, that
`transceiver ignores the interrogation signal and waits for the
`next interrogation signal. With the interrogation/response
`time intervals thus allocated, only the k(q) transceivers
`whose numbers are broadcast or otherwise identified
`respond to the central station, and each such transceiver
`responds only in its allocated time slot. Each interrogation
`signal sent by the central station can be directed to a different
`group of vehicle transceivers, and the number the k(q) in
`each such group can vary from one interrogation group to
`the next. Further, if one group of vehicles needs to be
`interrogated more frequently than other groups, this is easily
`accomplished. Using this protocol, a time interval allocated
`to interrogation of, and responses from, k(q) vehicle trans
`ceivers is "collapsed” to a time interval of length 2-3
`seconds for a group of k(q)s 10 vehicles. Undiscriminating
`interrogation of a group of, say, 200 vehicle transceivers
`could require 40-120 seconds by conventional approaches.
`Interrogation of non-parked vehicles can be done in different
`size groupings, and different vehicle groups can be interro
`gated at different frequencies.
`When the central station receives a call requiring assis
`tance, such as response to a break-in or robbery in progress
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`5,493,694
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`5
`(an "incident'), the central station broadcasts an incident
`message that includes the location of the incident and
`whatever is known about the incident. Each vehicle message
`unit (n) receives the incident message, determines its present
`location and the distance d(n) from that vehicle to the site of
`the incident, determines if that vehicle can respond to the
`incident (i.e., whether the vehicle is presently in an available
`mode), and replies to the incident message by transmitting
`its present location and other pertinent information. The
`vehicle message unit (n) transmits this reply if and only if
`the distance d(n) is less than a predetermined distance D,
`which might be in the range 0.5-3 miles, depending upon the
`estimated density of vehicles near the incident site. Any
`vehicle LD unit whose distance d(n) is greater than the
`predetermined distance D does not respond. Each patrolling
`vehicle whose distance d(n)sD does not reply immediately
`but waits a certain backoff time that is equal to Md(n)(T-
`T)/D where M is a selected constant, T is a selected trans
`mission cut-off time, and t is a selected vehicle communi
`cation unit response time. Thus, the vehicle LD units reply
`in order of their distance d(n) from the site of the incident,
`with the closest LD unit replying first. The radio system used
`for this reply may provide one channel or many channels for
`replies to incident messages.
`If certain vehicles have special equipment, or if the
`occupants of certain vehicles have specialized training, and
`such equipment or training is needed for the response to such
`incident, the central station need not select the first-to
`respond vehicle to respond to the incident. The central
`station may select the first vehicle (and vehicle occupants)
`that replies and has the needed equipment or personnel
`training.
`An optional reply transmission cut-off time, or length of
`a time-out interval, T, which begins as soon as a transceiver
`receives the incident message, defines the maximum time
`interval for the vehicle transceivers to reply, after which no
`further replies are transmitted. Use of this time-out limit
`(optional) limits the number of vehicle transceivers that can
`reply, a useful feature if the number of vehicles near the
`incident site is large.
`The invention will provide the greatest benefits where a
`large fleet of vehicles is used, probably in an urban envi
`ronment. Although the invention has been discussed with
`reference to a fleet of police vehicles, the invention will
`work equally well with a fleet of taxicabs or a fleet of
`radio-dispatched service vehicles, such as ambulances,
`vehicle towing trucks or other emergency or general service
`responders.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates the invention in use in a large region R.
`FIG. 2 is a schematic view of a location determination
`unit carried on a fleet vehicle according to the invention.
`FIG. 3 illustrates a procedure for determining which fleet
`vehicle will respond to a call for assistance, according to the
`invention.
`FIGS. 4-6 illustrate particular location determination
`systems that can be used to practice the invention.
`
`DESCRIPTION OF BEST MODE OF THE
`INVENTION
`FIG. 1 illustrates use of the invention in one embodiment.
`A number N of vehicles, numbered n=1,2,..., N, operate
`in a region R and individually communicate with a central
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`processing station C by means of electromagnetic wave
`signals (radiowaves, infrared, optical, etc.), referred to as
`"radiowaves' for convenience herein. The central process
`ing station C periodically interrogates one or a group of
`vehicles to determine: (1) the vehicle status ("parked” or
`inoperative, on patrol and available, on patrol but not
`available because it is responding to an incident or call for
`assistance, etc.); (2) the present location of each vehicle; and
`(3) other necessary information concerning the vehicle and/
`or its occupants.
`As illustrated in FIG. 2, each vehicle carries a commu
`nication unit CU that includes the following components: (1)
`a location determination unit 21 including an LD signal
`antenna 23 and LD signal receiver/processor 25 to receive
`and process LD signals and to determine the present location
`of the LD signal antenna; (2) a transceiver 27 and transceiver
`controller 29 that allows exchange of radiowave communi
`cations with the central station C (FIG. 1 ), using selected
`protocols that are stored for use in the transceiver controller,
`(3) an interface 31 between the LD signal receiver/processor
`25 and the transceiver 27 and transceiver controller 29, to
`allow exchange of information and requests for information;
`and (4) a power supply 33 that supplies electrical power for
`one or more of the other components.
`FIG. 3 illustrates a procedure according to the invention.
`The vehicles are divided into Kgroups of vehicles, which
`may be mutually exclusive, numbered q=1,2,..., K, where
`each group may have the same or a different number of
`vehicles. Each vehicle in a group has approximately the
`same reporting interval length so that the central station can
`interrogate each group of vehicles together. At each of a
`Selected sequence of times, the central station determines if
`it has recently received an incident call, in step 43.
`If the central station has not recently received an assis
`tance or incident call, the central station broadcasts an
`interrogation signal, in step 45, in a time slot of length T.
`(s0.2-1 sec), requesting that the communication units for
`vehicles number n=n-1, n2,..., n in a selected group q
`respond with the present location and status of the vehicle
`and/or other requested information.
`Using a protocol known by the central station and by each
`of the vehicle communication units, the central station then
`ceases its broadcast and waits a certain time interval of
`length T for the responses of the vehicles number n, n, .
`.., n. This time interval is divided into k(q) sub-intervals
`or time slots, each of length approximately At-T/k(q)
`(as50-500 msec, preferably as200 msec), and vehicle number
`n=n, responds with the requested information during the rth
`consecutive time slot, in step 47. Optionally, the vehicles in
`group q can reply to the interrogation in numerical order or
`in reverse numerical order, according to their respective
`vehicle numbers in that group. The system then recycles to
`step 43.
`Optionally, the central station can determine which
`vehicles are currently operating and include only operating
`vehicles among an interrogated subset {n, n, ..., n} of
`vehicles, or can interrogate all vehicles in the various
`groups. Optionally, the central station can separately inter
`rogate the non-operating vehicles in one or more separate
`subsets and determine when a vehicle transfers from non
`operational to operational status, or inversely, so that such a
`vehicle can be assigned to an appropriate subset of interro
`gated vehicles.
`Transceiver n (in vehicle number n) has listened to the
`central station's interrogation signal and knows (1) whether
`it is one of the transceivers queried and (2) if it has been
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`7
`queried, what is its numerical position in the queue for
`responding to this query. If a particular transceiver is not
`among those queried, that transceiver ignores the interroga
`tion signal until receipt of the next interrogation signal from
`the central station. With the interrogation/response time
`intervals thus allocated, only these k(q) transceivers whose
`numbers are specified will transmit a response to the central
`station interrogation signal, and each such transceiver trans
`mits a response only in its allocated time slot, transmitting
`its present status, location and any other requested informa
`tion. Although the central station may not have received the
`present status and location of a given vehicle within, say, the
`last 15 seconds, a vehicle LD unit determines its own present
`status and present location at prescribed time intervals, for
`example second-by-second or more often.
`Each interrogation signal sent by the central station can be
`directed to a different group q of vehicle communication
`units, and the number k(q) of vehicles in each such group
`can vary from one interrogation group to the next. Further,
`if one group of vehicles needs to be interrogated more
`frequently than other groups, this is easily accomplished.
`Using this protocol, a time interval allocated to interrogation
`of, and responses from, k(q) vehicle transceivers is "col
`lapsed” to a time interval of length T+k(q)T, which can be
`of the order of 2-3 seconds for a group of k(q)=10 vehicles.
`Undiscriminating interrogation of a group of, say, 200
`vehicle transceivers could require 40-120 seconds by con
`ventional approaches. Interrogation of operational vehicles
`can be done in different size groupings, and different vehicle
`groups can be interrogated at different frequencies, if
`desired.
`Assume that the central station has recently received an
`assistance or incident call, requiring assistance or service,
`such as a break-in or robbery in progress (an "incident'), in
`step 43. The central station preferably stops its periodic
`transmission of interrogation signals temporarily when the
`central station receives an incident call. In step 49, the
`central station broadcasts an incident message containing the
`location of the incident and whatever is known about the
`incident, possibly in a coded message that is understood by
`the vehicle communication units. Each vehicle communica
`tion unit n receives the incident message, in step 51, and
`determines its present location and status and determines the
`distance d(n) from that vehicle's LD unit to the site of the
`incident, in step 53. In step 55, the vehicle communication
`unit in determines if that vehicle can respond to the incident;
`that is, whether the vehicle distance d(n) from the incident
`site is no greater than a predetermined distance D, which
`might be in the range 0.5-3 miles, depending upon the
`estimated density of vehicles near the incident site. Any
`vehicle LD unit whose distance d(n) is greater than the
`predetermined distance D does not reply to the incident
`message transmitted by the central station, and this part of
`the system recycles to step 43.
`The transceiver in each available vehicle whose distance
`is d(n)sD does not respondimmediately, because this might
`55
`produce signal collisions. In step 57, each transceiver num
`ber n waits at least a certain backoff time At(n)=Md(n)(T-
`t)/D and then transmits a reply, where T is a selected reply
`transmission cut-off time, t is a vehicle communication
`response time, and Mis a selected positive constant that may
`depend upon the selected distance D, the radio system used,
`the number of vehicles in the fleet that are likely to be within
`a distance D of the incident site, and other parameters. The
`constant M may be selected independently for each incident
`message, if desired. Typically, the cut-off time T might be
`2-20 sec and the response time t might be 50-500 m.sec.
`Preferably, Ts2-10 sec, te50-200 msec and Mas 1.
`
`8
`Table 1 presents a representative set of reply backoff times
`At(n) for the choices M=1, T-1 sec, t=0.2 sec and D=1600
`meters. Note that the difference in backoff times for each
`increase by 100 meters in the distance d(n) is 613 msec,
`which is probably adequate to avoid collision of reply
`signals transmitted by two vehicles whose distances d(n)
`from the incident site differ by 100 meters or more. More
`generally, the reply signal backoff time may be defined as
`At(n)=Md(n)(T-t)/D+Ato, where Ato is an arbitrary small
`time value that may be positive or negative or zero.
`
`TABLE 1.
`
`Reply Signal Backoff Time Versus Incident Site Distance
`M = 1, T is 10 sec, t = 0.2 sec, Dr. 1600 meters
`
`d(n)
`
`100 M
`200
`300
`400
`500
`600
`700
`800
`900
`1000
`1100
`1200
`1300
`1400
`1500
`1600
`
`Backoff Time
`
`0.613 sec
`1225
`1838
`2.450
`3.063
`3.675
`4.288
`4900
`5.513
`6.125
`6.738
`7.350
`7.963
`8.575
`9.188
`9.800
`
`Because each replying transceiver waits a backoff time
`that is proportional to its distance d(n), the LD unit with the
`shortest distance d(n) will reply first; the LD unit with the
`second shortest distance d(n) will reply second; and so on.
`The central station receives the replies to the incident
`message and selects one or more vehicles to respond to the
`incident call, in step 59. The distance D in the requirement
`d(n)sD can be chosen so that no more than 3-12 vehicle
`transceivers are likely to reply to an incident message. The
`central station