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`preferred over those with lower average SNR; SCS/antennas with lower SNR variance are
`
`preferred to those with higher SNR variance; and SCS/antennas with a faster SNR rate of
`change.atthe on-set ofthe transmission are preferred to those with a slowerrate of change.
`The weighting applied to each of these criteria may be adjusted by the operatorofthe
`Wireless Location System to suit the particular design of each system.
`
`The candidate list of SCS’s 10 and antennas 10-1 are selected using a predeterminedset of
`criteria based, for example, upon knowledgeofthe typesofcell sites, types of antennas at
`the cell sites, geometry of the antennas, and a weighting factor that weights certain
`
`10
`
`antennas morethan other antennas. The weighting factor takes into account knowledge of
`
`the terrain in which the Wireless Location System is operating, past empirical data on the
`
`contribution of each antenna has made to goodlocation estimates, and other factors that
`
`maybe specific to each different WLSinstallation. In one embodiment, for example, the
`Wireless Location System mayselect the candidate list to include all SCS's 10 up to a
`maximum numberofsites (max_number_of_sites) that are closer than a predefined
`
`maximum radius from the primary site (max_radius_from_primary). For example, in an
`
`urban or suburban environment, where there may bea large numberof cell sites, the
`
`max_number_of_sites may be limited to nineteen. Nineteen sites would includethe
`primary,the first mng of six sites surrounding the primary (assuming a classic hexagonal
`distribution ofcell sites), and the next ring of twelve sites surroundingthefirst ring. This
`
`is depicted in Figure 9. In another embodiment, in a suburban or rural environment,
`
`max_radius_from_primary maybeset to 40 miles to ensure that the widest possible set of
`
`candidate SCS/antennas is available. The Wireless Location System is provided with
`
`meansto limit the total numberof candidate SCS’s 10 to a maximum number
`
`(max_number_candidates), although each candidate SCS maybe permitted to choose the
`
`best port from amongits available antennas. This limits the maximum time spent by the
`Wireless Location System processing a particular location. Max_number_candidates may
`be set to thirty-two, for example, which meansthatin a typical three sector wireless
`
`communications system with diversity, up to 32*6 = 192 total antennas could be
`
`considered for location processing for a particular transmission. In orderto limit the time
`spent processing a particular location, the Wireless Location System is provided with
`meansto limit the numberof antennas usedin the location processing to
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`max_number_antennas_processed. Max_number_antennas_processedis generally less
`
`than max_number_candidates, andis typically set to sixteen.
`
`While the Wireless Location System is provided with the ability to dynamically determine
`
`the candidate list of SCS’s 10 and antennas based uponthe predeterminedsetofcriteria
`described above,the WirelessLocation System can also store a fixed candidate list in a
`table. Thus, for each cell site and sector in the wireless communications system,the
`Wireless Location System has a separate table that defines the candidate list of SCS’s 10
`
`and antennas 10-1 to use whenever a wireless transmitter initiates a transmission in that
`
`cell site and sector. Rather than dynamically choose the candidate SCS/antennas each time
`
`a location requestis triggered, the Wireless Location System reads the candidatelist
`
`directly from the table when location processingis initiated.
`
`In general, a large number of candidate SCS’s 10 is chosen to provide the Wireless
`
`Location System with sufficient opportunity and ability to measure and mitigate multipath.
`On any given transmission, any one or moreparticular antennas at one or more SCS’s 10
`
`may receive signals that have been affected to varying degrees by multipath. Therefore,it
`
`is advantageousto provide this means within the Wireless Location System to
`
`dynamically select a set of antennas which may havereceived less multipath than other
`
`antennas. The Wireless Location System uses various techniques to mitigate as much
`
`multipath as possible from any received signal; howeverit is frequently prudent to choose
`
`a set of antennas that contain the least amount of multipath.
`
`Choosing Reference and Cooperating SCS/Antennas
`in choosingthe set of SCS/antennas to use in location processing, the Wireless Location
`
`System orders the candidate SCS/antennas using severalcriteria, including for example:
`average SNR over the transmission interval used for location processing, the variance in
`the SNR over the sameinterval, correlation of the beginning of the received transmission
`
`against a pure pre-cursor(i.e. for AMPS,the dotting and Barker code) and/or demodulated -
`
`data from the primary SCS/antenna, the time of the on-set of the transmissionrelative to
`
`the on-set reported at the SCS/antenna at which the transmission was demodulated, and
`
`the magnitude andrate of change of the SNR from just before the on-set of the
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`transmission to the on-set of the transmission, as well as other similar parameters. The
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`average SNR is typically determined at each SCS, and for each antennain the candidate
`
`list either over the entire length of the transmission to be used for location processing, or
`
`over a shorterinterval. The average SNR overthe shorter interval can be determined by
`
`performing a correlation with the dotting sequence and/or Barker code and/or sync word,
`
`depending on the particular air interface protocol, and over a short range oftime before,
`
`during, and after the timestamp reported by the primary SCS10. The time range may
`typically be +/- 200 microsecondscentered at the timestamp, for example. The Wireless
`
`Location System will generally order the candidate SCS/antennas using the following
`criteria, each of which may be weighted when combiningthecriteria to determine the final
`decision: average SNR for a given SCS/antenna mustbe greater than a predetermined
`
`threshold to be used in location processing; SCS/antennas with higher average SNR are
`preferred over those with lower average SNR; SCS/antennas with lower SNR variance are
`
`preferred to those with higher SNR variance; SCS/antennas with an on-set closerto the
`
`on-set reported by the demodulating SCS/antennaare preferred to those with an on-set
`
`moredistant in time; SCS/antennas with a faster SNR rate of change are preferred to those
`with a slowerrate of change; SCS/antennas with lower incremental weighted GDOPare
`preferred over those with higher incremental weighted GDOP,where the weighting is
`based upon estimated path loss from the primary SCS. The weighting applied to each of
`these preferences may be adjusted by the operator of the Wireless Location System to suit
`the particular design of each system. The numberofdifferent SCS’s 10 used in the
`jocation processing 1s maximized up to a predetermined limit; the number of antennas
`used at each SCS 10 in limited to a predeterminedlimit; and the total number of
`
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`SCS/antennas usedis limited to max_number_antennas_processed. The SCS/antenna with
`the highest ranking using the above described processis designated as the reference
`
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`SCS/antennafor location processing.
`
`Best Port Selection Within an SCS 10
`
`Frequently, the SCS/antennas in the candidate list or in thelist to use in location
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`30
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`processing will include only one or two antennas at a particular SCS 10. In these cases, the
`Wireless Location System may permit the SCS 10 to choosethe “best port” from all or
`someofthe antennas at the particular SCS 10. For example, ifthe Wireless Location
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`Systemchoosesto use only one antennaata first SCS 10,then thefirstSCS 10 mayselect
`thebest antenna port from the typical six antennaports that are connected to that SCS 10,
`or it may choosethe best antenna port from amongthe two antennaports ofjust one sector
`ofthe cell site. The best antenna port ischosen by using the same process and comparing
`the same parameters as described above for choosing the set of SCS/antennas to use in
`
`location processing, except that all of the antennas being considered for best port are all in
`
`the same SCS10. In comparing antennas.for best port, the SCS 10 mayalso optionally
`divide the received signal into segments, and then measure the SNR separately in each
`segmentofthe received signal. Then, the SCS 10 can optionally choose the best antenna
`port with highest SNR either by(i) using the antenna port with the most segments with the
`highest SNR, (ii) averaging the SNR in all segments and using the antenna port with the
`highest average SNR, or(iii) using the antenna port with the highest SNR in any one
`
`segment.
`
`Detection and Recovery From Collisions
`Because the Wireless Location System will use data from many SCS/antenna ports in
`location processing, there is a chance that the received signal at one or more particular
`SCS/antenna ports contains energy that is co-channelinterference from another wireless
`transmitter (i.e. a partial or full collision between two separate wireless transmissions has
`
`occurred). There is also a reasonable probability that the co-channel interference has a
`
`much higher SNR than the signal from the target wireless transmitter, and if not detected
`
`by the Wireless Location System, the co-channel interference may cause an incorrect
`
`choice of best antenna port at an SCS 10, reference SCS/antenna, candidate SCS/antenna,
`
`or SCS/antennato be used in location processing. The co-channel interference mayalso
`
`cause poor TDOAand FDOAresults, leading to a failed or poor location estimate. The
`
`probability of collision increases with the density of cell sites in the host wireless
`
`communications system, especially in dense suburban or rural environments where the
`
`frequencies are re-used often and wireless usage by subscribersis high.
`
`Therefore, the Wireless Location System includes meansto detect and recover from the
`
`types of collisions described above. For example, in the process of selecting a bestport,
`
`reference SCS/antenna, or candidate SCS/antenna, the Wireless Location System
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`determines the average SNR ofthe received signal and the variance of the SNR overthe
`
`interval of the transmission; when the variance of the SNR is above a predetermined
`
`threshold, the Wireless Location System assigns a probability that a collision has occurred.
`
`If the signal received at an SCS/antenna has increased or decreased its SNR in a single
`step, and by an amount greater than a predetermined threshold, the Wireless Location
`System assigns a probability that a collision has occurred.-Further, if the average SNR of
`
`the signal received at a remote SCSis greater than the average SNR that would be
`
`predicted by a propagation model, given the cell site at which the wireless transmitter
`
`initiated its transmission and the known transmit powerlevels and antenna patterns of the
`
`transmitter and receive antennas, the Wireless Location System assigns a probability that a
`
`collision has occurred. If the probability that a collision has occurred is above a
`
`predetermined threshold, then the Wireless Location System performsthe further
`
`processing described below to verify whether and to what extent a collision may have
`impaired the received signal at an SCS/antenna. The advantage of assigning probabilities
`is to reduce or eliminate extra processing for the majority of transmissions for which
`
`collisions have not occurred. It should be noted that the threshold levels,. assigned
`
`probabilities, and other details of the collision detection and recovery processes described
`
`herein are configurable, i.e., selected based on the particular application, environment,
`system variables,etc., that wouldaffect their selection.
`
`For received transmissions at an SCS/antenna for which the probability of a collision is
`above the predetermined threshold and before using RF data from a particular antenna port
`in a reference SCS/antenna determination, best port determination or in location
`
`processing, the Wireless Location System preferably verifies that the RF data from each
`antennaport is from the correct wireless transmitter. This is determined, for example, by
`demodulating segmentsof the received signal to verify, for example, that the MIN, MSID,
`or otheridentifying informationis corrector that the dialed digits or other message
`characteristics match those received by the SCS/antennathat initially demodulated the
`transmission. The Wireless Location System mayalso correlate a short segmentofthe
`received signalat an antennaport with the signal received at the primary SCS 10 to verify
`that the correlation result is above a predetermined threshold. If the Wireless Location
`System detects that the variance in the SNR overthe entire length of the transmission is _
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`above a pre-determined threshold, the Wireless Location System may divide the
`
`transmission into segments and test each segment as described herein to determine
`whether the energy in that segmentis primarily from the signal from the wireless
`transmitter for which location processing has been selected or from an interfering
`transmitter.
`
`The Wireless Location System may chooseto use the RF data from a particular
`- §CS/antennain location processing even if the Wireless Location System has detected that
`
`a partial collision has occurred at that SCS/antenna.In these cases, the SCS 10 uses the
`means described abovetoidentify that portion of the received transmission which
`represents a signal from the wireless transmitter for which location processing has been
`
`selected, and that portion of the received transmission which contains co-channel
`
`interference. The Wireless Location System may commandthe SCS 10 to send or use only
`
`selected segments of the received transmission that do not contain the co-channel
`
`interference. When determining the TDOA and FDOAfora baseline using only selected
`
`segments from an SCS/antenna, the Wireless Location System uses only the
`
`corresponding segmentsof the transmission as received at the reference SCS/antenna. The
`
`Wireless Location System may continueto use all segments for baselines in which no .
`
`collisions were detected. In many cases, the Wireless Location System is able to complete
`
`location processing and achieve an acceptable location error using only a portion ofthe
`
`transmission. This inventive ability to select the appropriate subset of the received
`transmission and perform location processing on a segment by segment basis enables the
`Wireless Location System to successfully complete location processing in cases that might
`
`have failed using previous techniques.
`
`Multiple Pass Location Processing
`Certain applications may require a very fast estimate of the general location of a wireless
`transmitter, followed by a more accurate estimate of the location that can be sent
`
`subsequently. This can be valuable, for example, for E9-1-1 systems that handle wireless
`
`calls and must makea call routing decision very quickly, but can wait a little longer for a
`more exactlocation to be displayed upon the E9-1-1 call-taker’s electronic map terminal.
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`The Wireless Location System supports these applications with an inventive multiple pass
`
`location processing mode.
`
`In manycases, location accuracy is enhanced by using longer segments ofthe
`
`transmission and increasing the processing gain through longerintegration intervals. But
`
`longer segments of the transmission require longer processing periods in the SCS 10 and
`
`TLP 12, as well as longer time periods for transmitting the RF data across the
`
`communications interface from the SCS 10 to the TLP 12. Therefore, the Wireless
`
`10
`
`Location System includes meansto identify those transmissionsthat require a fast but
`rough estimate of the location followed by more complete location processing that
`producesa better location estimate. The Signal of Interest Table includesaflag for each
`SignalofInterest that requires a multiple pass location approach. This flag specifies the
`maximum amountof time permitted by the requesting location application for the first
`estimate to be sent, as well as the maximum amountof time permitted by the requesting
`location application forthe final location estimate to be sent. The Wireless Location
`
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`System performsthe rough location estimate by selecting a subset of the transmission for
`
`whichto perform location processing. The Wireless Location System may choose, for
`example, the segmentthat was identified at the primary SCS/antenna with the highest
`average SNR. After the rough location estimate has been determined, using the methods
`described earlier, but with only a subset ofthe transmission, the TLP 12 forwardsthe
`location estimate to the AP 14, which then forwards the rough estimate to the requesting
`application with a flag indicating that the estimate is only rough. The Wireless Location
`
`System then performsits standard location processing using all of the aforementioned
`
`methods, and forwardsthis location estimate with a flag indicatingthefinal statusofthis
`location estimate. The Wireless Location System may perform the roughlocation estimate
`andthe final location estimate sequentially on the same DSP in a TLP 12, or may perform |
`the location processing in parallel on different DSP’s. Parallel processing may be
`necessary to meet the maximum time requirements of the requesting location applications.
`The Wireless Location System supports different maximum time requirements from
`different location applications for the same wireless transmission.
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`Very Short Baseline TDOA
`The Wireless Location System is designed to operate inurban, suburban,andruralareas.
`In rural areas, when there are not sufficient cell sites available from a single wireless
`carrier, the Wireless LocationSystem can be deployed with SCS’s 10 located at the cell
`
`sites of other wireless carriers or at other types of towers, including AM or FM radio
`station, paging, and two-way wireless towers. In these cases, rather than sharing the
`existing antennas ofthe wireless carrier, the Wireless Location System mayrequire the
`installation of appropriate antennas,filters, and low noise amplifiers to match the
`
`frequency bandof the wireless transmitters of interest to be located. For example, an AM
`
`10
`
`radio station tower may require the addition of 800 MHz antennas to locate cellular band
`
`transmitters. There may be cases, however, where no additional towers of any type are
`
`available at reasonable cost and the Wireless Location System must be deployed onjust a
`
`few towersof the wireless carrier. In these cases, the Wireless Location System supports
`
`an antenna mode known as very short baseline TDOA.This antenna mode becomesactive
`
`whenadditional antennas are installed on a single cell site tower, whereby the antennas are
`
`placed at a distance of less than one wavelength apart. This may require the addition of
`
`just one antennapercell site sector such that the Wireless Location System uses one
`
`existing receive antennain a sector and one additional antenna that has been placed next to
`
`the existing receive antenna. Typically, the two antennas in the sector are oriented such
`that the primary axes,orline ofdirection, of the main beamsareparallel and the spacing
`betweenthe two antenna elements is known with precision.In addition, the two RF paths
`
`from the antenna elements to the receivers in the SCS 10 are calibrated.
`
`In its normal mode,the Wireless Location System determines the TDOA and FDOAfor
`
`pairs of antennathat are separated by many wavelengths. For a TDOAona baseline using
`
`antennas from two difference cell sites, the pairs of antennas are separated by thousands of
`
`wavelengths. Fora TDOAona baseline using antennas at the samecellsite, the pairs of
`
`antennas are separated by tens of wavelengths. In either case, the TDOA determination
`
`effectively results in a hyperbolic line bisecting the baseline and passing through the
`location of the wirelesstransmitter. When antennas are separated by multiple wavelengths,
`the received signal has taken independent paths from the wireless transmitter to each
`
`antenna, including experiencing different multipath and Doppler shifts. However, when
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`two antennas are closer than one wavelength, the two received signals have taken
`
`essentially the same path and experienced the same fading, multipath, and Dopplershift.
`Therefore, the TDOA and FDOAprocessing of the Wireless Location System typically
`produces a Doppler shift of zero (or near-zero) hertz, and a time difference on the order of
`zero to one nanosecond.A time difference that short is equivalent to an unambiguous
`
`phase difference between the signals received at the two antennas on the very short
`
`baseline. For example, at 834 MHz, the wavelength of an AMPSreverse control channel
`
`transmission is about 1.18 feet. A time difference of 0.1 nanosecondsis equivalent to a
`
`received phase difference of about 30 degrees. In this case, the TDOA measurement
`
`10
`
`producesa hyperbolathat is essentially a straightline, still passing through the location of
`
`the wireless transmitter, and in a direction that is rotated 30 degrees from the direction of
`
`the parallel lines formed by the two antennas on the very short baseline. Whenthe results
`
`of this very short baseline TDOAatthe single cell site are combined with a TDOA
`measurementon a baseline between twocell sites, the Wireless Location System can —
`determinea location estimate using only twocell sites.
`
`Bandwidth Monitoring Method For Improving Location Accuracy
`
`AMPScellular transmitters presently comprise the large majority of the wireless
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`transmitters used in the U.S. and AMPSreverse voice channel transmissions are generally
`
`FM signals modulated by both voice and a supervisory audio tone (SAT). The voice
`
`modulation is standard FM,andis directly proportional to the speaking voice of the person
`using the wireless transmitter. In a typical conversation, each person speaksless that 35%
`of the time, which meansthat mostofthe time the reverse voice channelis not being
`modulated due to voice. With or without voice, the reverse channel is continuously
`modulated by SAT,which is used by the wireless communications system to monitor
`channel status. The SAT modulationrate is only about 6 KHz. The voice channels support
`in-band messagesthat are used for hand-off control and for other reasons, such as for
`
`establishing a 3-waycall, for answering a second incomingcali while already ona first
`
`call, or for responding to an ‘audit’ message from the wireless communications system.
`
`All of these messages, though carried on the voice channel, have characteristics similar to
`
`the control channel messages. These messagesare transmitted infrequently, and location
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`systems have ignored these messages and focused on the more prevalent SAT
`
`transmissions as the signalof interest.
`
`In view of the above-described difficulties presented by the limited bandwidth of the FM
`
`voice and SAT reverse voice channel signals, an object of the present inventionis to
`
`provide an improved method by which reverse voice channel (RVC)signals may be
`
`utilized to locate a wireless transmitter, particularly in an emergency situation. Another
`
`object of the invention is to provide alocation methodthat allows the location system to
`
`avoid makinglocation estimates using RVC signals in situations in whichit is likely that
`
`the measurementwill not meet prescribed accuracy andreliability requirements. This
`
`saves system resources and improvesthe location system’s overall efficiency. The
`
`improved method is based upon two techniques. Figure 10A is a flowchart ofa first
`
`method in accordance with the present invention for measuring location using reverse
`
`voice channel signals. The method comprises the followingsteps:
`
`(i) It is first assumed that a user with a wireless transmitter wishes to be located, or
`
`wishesto have his location updated or improved upon. This maybethe case, for
`
`example, if the wireless user has dialed “911” and is seeking emergencyassistance.
`
`It is therefore also assumed that the user is coherent and in communication with a
`
`centrally located dispatcher.
`
`(ii) Whenthe dispatcherdesires a location update for a particular wireless transmitter,
`
`the dispatcher sends a location update command with the identity of the wireless
`
`transmitter to the Wireless Location System over an application interface.
`
`(iii) The Wireless Location System respondsto the dispatcher with a confirmation that
`
`the Wireless Location System has queried the wireless communications system and
`has obtained the voice channel assignmentfor the wireless transmitter.
`
`(iv) The dispatcherinstructs the wireless user to dial a 9 or more digit number and then
`
`the “SEND”button. This sequence may be somethinglike “123456789”or
`
`“911911911”. Two functions happen to the reverse voice channel whenthe
`
`wireless user dial a sequenceofat least 9 digits and then the “SEND”button.First,
`especially for an AMPScellular voice channel, the dialing of digits causes the
`
`sending of dual tone multi-frequency (DTMF) tones over the voice channel. The
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`modulation index of DTMF tonesis very high and during the sending of each digit
`
`in the DTMF sequencewill typically push the bandwidth ofthe transmitted signal
`
`beyond +/- 10 KHz. The second function occurs at the pressing of the “SEND”
`
`button. Whether or not the wireless user subscribes to 3-way calling or other
`
`special features, the wireless transmitter will send a message overthe voice using a
`
`“blank and burst” mode where the transmitter briefly stops sending the FM voice
`
`and SAT,and instead sends a bursty message modulated in the same manner as the
`
`control channel (10 Kbits Manchester). If the wireless user dials less than 9 digits,
`
`the messagewill be comprised of approximately 544 bits. If the wireless user dials
`
`9 or moredigits, the message is comprised of approximately 987 bits.
`
`(v) After notification by the dispatcher, the Wireless Location System monitors the
`
`bandwidth of the transmitted signal in the voice channel. As discussed earlier,
`
`when only the SAT is being transmitted, and even if voice and SAT are being
`transmitted, there maynotbe sufficient bandwidth in the transmitted signal to
`calculate a high quality location estimate. Therefore, the Wireless Location System
`conserves location processing resources and waits until the transmitted signal
`
`exceeds a predetermined bandwidth. This may be, for example, set somewhere in
`
`the range of 8 KHz to 12 KHz. When the DTMF dialed digits are sent or when the
`
`bursty messageis sent, the bandwidth would typically exceed the predetermined
`
`bandwidth.In fact, if the wireless transmitter does transmit the DTMF tones during
`_ dialing, the bandwidth would be expected to exceed the predetermined bandwidth
`multiple times. This would provide multiple opportunities to perform a location
`
`estimate. If the DTMF tonesare not sent during dialing, the bursty messageisstill
`sent at the time of pressing “SEND”, and the bandwidth would typically exceed the
`predetermined threshold.
`
`(vi) Only when the transmitted bandwidth of the signal exceeds the predetermined
`
`bandwidth, the Wireless Location System initiates location processing.
`
`Figure 10B is a flowchart of another method in accordancewith the present invention for
`
`measuring location using reverse voice channel signals. The method comprises the
`following steps:
`
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`PCT/US99/29507
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`(i) It is first assumed that a user with a wireless transmitter wishes to be located, or
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`wishesto have their location updated or improved upon. This maybe thecase, for
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`example, if the wireless user has dialed “911” and is seeking emergencyassistance.
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`It is assumed that the user may not wish to dial digits or may notbe able to dial any
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`5
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`digits in accordance with the previous method.
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`(ii) When the dispatcher desires a location update for a particular wireless transmitter
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`user, the dispatcher sends a location update commandto the Wireless Location
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`System over an application interface with the identity of the wireless transmitter.
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`(iil) The Wireless Location System responds to the dispatcher with a confirmation.
`(iv) The Wireless Location System commands the wireless communications system to
`makethe wireless transmitter transmit by sending an “audit”or similar message to
`the wireless transmitter. The audit message is a mechanism by whichthe wireless
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`communications system can obtain a response from the wireless transmitter
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`without requiring an action by the end-user and without causing the wireless
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`transmitter to ring or otherwisealert. The receipt of an audit message causes the
`wireless transmitter to respond with an “audit response” message on the voice
`channel.
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`(v) After notification by the dispatcher, the Wireless Location System monitors the
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`bandwidth of the transmitted signal in the voice channel. As discussedearlier,
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`whenonly the SATis being transmitted, and even if voice and SATare being
`transmitted, there may notbe sufficient bandwidth in the transmitted signal to
`calculate a high quality location estimate. Therefore, the radio location conserves
`location processing resources and waits until the transmitted signal exceeds a
`predetermined bandwidth. This may be, for example, set somewherein the range
`of 8 KHz to 12 KHz. Whenthe audit response messageis sent, the bandwidth
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`would typically exceed the predetermined bandwidth.
`(vi) Only when the transmitted bandwidth ofthe signal exceeds the predetermined
`bandwidth, the Wireless Location System initiates location processing.
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`Estimate Combination Method For Improving Location Accuracy
`The accuracy ofthe location estimate provided by the Wireless Location System may be
`improved by combining multiple statistically-independent location estimates made while
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`Apple Exhibit 1002
`Page 1219 of 2414
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`Apple Exhibit 1002
`Page 1219 of 2414
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`WO 00/40992
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`PCT/US99/29507
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`the wireless transmitter is maintaining its position. Even when a wireless transmitteris
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`perfectly stationary, the physical and RF environment around a wireless transmitteris
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`constantly changing. For example, vehicles may changetheir position or another wireless
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`transmitter which had caused a collision during one location estimate may have stopped
`transmitting or changedits position so as to no longercollide during subsequent location
`estimates. The location estimate provided by the Wireless Location System will therefore
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`changefor each transmission, even if consecutive transmissions are made within a very
`short period of time, and each location estimateis statistically independentofthe other
`estimates, particularly with respect to the errors caused by the changing environment.
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`Whenseveral consecutive statistically independent location estimates are made for a
`wireless transmitter that has not changedits position, the location estimates will tend to
`cluster aboutthe true position. The Wireless Location System combinesthe location
`estimates using a weighted averageor other similar mathematical construct to determine
`the improved estimate. The use of a weighted averageis aided by the assignmentof a
`quality factor to each independentlocation estimate. This quality factor may be based
`upon, for example, the correlation values, confidenceinterval, or other similar
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`measurements derived from the location processing for e