United States Patent
`
`1191
`
`[11] Patent Number:
`
`5,687,196
`
`Nov. 11, 1997
`[45] Date of Patent:
`Proctor, Jr. et al.
`
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`l
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`USOO5687196A
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`[75]
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`[54] RANGE AND BEARING TRACKING SYSTEM
`WITH MULTIPATH REJECTIQN
`'
`Inventors: James Arthur Proctor, Jr.. Indialantic;
`:22‘: Egg gtgfindlan H”b°"’
`_
`.
`.
`[73] Assignee: Hams Corporation. Melbourne. Fla.
`
`21 A l.N .:315
`I
`1
`pp
`°
`[22] Filed:
`
`54
`Sep. 30, 1994
`
`5
`
`Int. CL6 ................................. I-104B 7/10; H04L 1/02
`[51]
`[52] U.S. Cl.
`.......................... 375/347; 342/457; 342/463;
`455/132
`[5 8] Field of Search ..................................... 375/200. 209.
`375/343. 346. 348. 349. 347. 367; 342/417,
`421, 423, 450_ 463; 455/55, 132
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,752,969
`5,2g(6),7él
`5,2
`,8 1
`
`6/1988 Rilling .................................... 455/278
`11/1993 Sterner .......
`..... 342/375
`
`3/1994 Knight ...............
`342/357
`31811110 .........
`.....
`3; fine“ ~-''---
`-----
`1/1997 Yokev et all
`........................... 342/337
`5,596,330
`Primary Examiner-—Tesfa.ldet Bocure
`Am-s,a,,, Exam,-,,e,_B1-yan Webster
`Attorney Agent, or Firm—Rogers & Killeen
`[57]
`ABSTRACT
`
`A system and method for tracking a remote RF transmitter
`with reduced susceptibility to the effects of multipath in
`which the distance and direction of an arriving RF chirp
`signal are determined with respect to the earliest arriving
`portion of the signal which is presumed to be the direct path
`signal. The received chirp signals. including the direct and
`mullipath signals, from a remote transmitter are correlated
`into plural path signals. and the direction and distance to the
`transmitter is determined from the earliest arriving path
`signal‘
`
`4,443,801
`
`4/1984 Klose et al.
`
`............................ 343/242
`
`32 Claims, 2 Drawing Sheets
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`PETITIONERS 1001-0001
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`U.S. Patent
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`Nov. 11, 1997
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`Sheet 1 of 2
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`5,687,196
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`26
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`FIG.
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`“ME
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`PETITIONERS 1001-0002
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`U.S. Patent
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`Nov. 11, 1997
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`Sheet 2 of 2
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`5,687,196
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`5,687,196
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`2
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`10
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`1
`RANGE AND BEARING TRACKING SYSTEM
`WITH MULTIPATH REJECTION
`
`BACKGROUND OF THE INVENTION
`
`The present invention is related generally to systems and
`methods for determining the range and bearing of the source
`of radiofrequency (“RF”) signal and. in particular. to sys-
`tems and methods for determining the range and bearing of
`such signals in the face of multipath and similar noise
`sources.
`
`Systems and method for determining the distance and
`bearing of an RF signal are well known. In some systems, a
`outbound signal having a known power is sent from a base
`station to a remote station at an unknown location. The
`remote station may “respond” to the signal from the base
`station by returning a signal upon receipt of the outbound
`signal. Such a system is presently produced by Cubic
`Defense Systems. Inc. as the ANIARS-6(V) PLS system or
`by Rockwell International Inc as the Target Locating System
`(TLS). The distance between the base station and the remote
`station may be determined by any of the known methods.
`For example. the distance can be computed by timing the
`total transit time between the transmission of the outbound
`signal and the receipt of the response signal. By subtracting
`the estimated time of the delay in the remote station from the
`total transit time. the time to traverse twice the distance
`between the base and the remote stations can obtained and
`the distance readily computed. By way of another example
`of prior art systems. the distance between the base and the
`remote stations can be estimated by knowing the power of
`the signal transmitted from the remote and measuring the
`power of the signal received at the base station. Using the
`inverse square law of signal strength over increasing
`distances. an estimate of the distance can be obtained from
`the difference between the transmitted power and the
`received power.
`Likewise. it is known in the prior art to determine the
`direction of the response signal by one of many techniques.
`For example. in one of the most simple methods. a loop
`antenna may be rotated and the strength of the response
`signal measured. The transmitting station is estimated to be
`along the line corresponding to the axis of the loop when the
`loop is position to maximize response signal power. In
`another example in the prior art. a base station may use
`plural antennas having a known geometric relationship to
`one another. The angle of arrival of the response signal may
`be determined by evaluating the phase of the response signal
`simultaneously at each of the antennas. The simultaneous
`phase relationships at the antennas. the geometric relation-
`ship of the antennas and the frequency of the response signal
`can be used to estimate the angle of arrival of the response
`signal with respect to the antennas.
`All of the above-noted systems and methods for deter-
`mining range and bearing in the prior art experience some
`ditficulty in multipath and other noisy environments typical
`of where many such tracking and ranging systems are used.
`For example. with reference to FIG. 1, an RF signal source
`10 may be located at location remote and unknown to a base
`station 12. Plural blocking and/or reflecting elements 14.
`such as buildings.
`towers. mountains may exist in the
`proximity of and in the direct path between the RF signal
`source 10 and the base station 12. The blocldng and/or
`reflecting elements cause RF signals impinging upon such
`elements to be blocked. absorbed, reflected. and often a
`combination of all three. Generally. such elements cause RF
`signals to be diminished in strength and to change direction.
`
`When a source of RF signals such as the remote source 10
`radiates RF signals, such signals are blocked andlor reflected
`by the elements 14 such that instead of a single signal
`arriving at the base station 12. multiple versions of the same
`5 or slightly altered signal arrive at the base station 12. The
`different versions of the signals arrive at difierent times
`because they have travelled different paths of diflerent
`distances than either the direct version or other indirect
`versions. The signals may also be altered from one another
`because each of the signals has experienced a diflerent
`environment and may have been subject to dilferent noise
`and interference sources along the different paths.
`With continued reference to FIG. 1.
`in a multipath
`environment. the signal which arrives directly from the RF
`signal source 10 at the base station 12 may not be the
`strongest signal. For example. in the system of FIG. 1 three
`difierent paths 20. 22. and 24 between the RF signal source
`10 and the base station 12 are shown. (It being understood
`that generally communications are conducted across an arc
`and not just at selected lines from the RF signal source.) The
`first signal path 20 proceeds directly from the RF signal
`source 10 to the base station 12. Because the first signal path
`intersects two of the elements. and each element tends to
`diminish the strength of the signal. the signal arriving at the
`base station 12 is lower in amplitude or power than a signal
`arriving without being partially absorbed. Note that when
`the signal on the first signal path impinged on the elements.
`it is likely that some portion of the signal was reflected and
`some portion was refiacted and never reached the base
`station 12 but such reflection and refraction are not shown
`
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`with respect to the first signal path 20 for simplicity of
`illustration.
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`The second signal path 22 in the illustration of FIG. 1 is
`reflected off two of the elements 14 before reaching the base
`station 12 (refraction and absorption not being shown). Ifthe
`reflecting surfaces of these two elements are relatively
`eflicient. a relatively strong signal will reach the base station
`12 along the second signal path 22. Because the signal
`travelling the second signal path 22 travelled a longer
`distance than the signal travelling the first signal path 20, the
`signal on the second path will arrive at the base station 12
`after the signal on the first signal path 20. Similarly. the third
`signal path 24 is reflected ofl an element 14 to reach the base
`station 12.
`
`Note that in the system of FIG. 1. the various signals
`arrive at the base station from entirely ditferent angles. In
`some systems in the prior art. the locating system will
`operate on the signal having the strongest signal power. As
`can be seen from the illustration in FIG. 1. such a procedure
`will lead to an erroneous result as the signal with the
`strongest power arrives along the second signal path 22.
`from almost the very opposite of the actual angle to the RF
`signal source. Note also that if ranging is done on the basis
`of the strongest signal. the ranging determination will be in
`error because the strongest (second signal path 22) travels
`more distance than the distance between the RF signal
`source 10 and the base station 12.
`
`The influences of multipath signals on distance and angle
`location has been recognized in the prior art. Some prior art
`systems ignore the influence of multipath by utilizing a
`composite signal based on the strengths of the various
`multipath signals identified by the base station 12 The
`systems of FIG. 1 illustrate how the composite signal may
`be erroneous as signal which is the composite of the arriving
`signals may yield a signal which is misaligned such as the
`composite signal 26. As can be seen from the illustration. the
`direct signal path (first signal path 20) will yield the best
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`the “composite” signal 26
`“direction" information but
`received by the base station is a combination of signals from
`different angles of arrival.
`It is accordingly an object of the present invention to
`provide a novel system and method of tracking a remote RF
`transmitter which obviates these and other known problems
`in the prior art.
`It is a further object of the present invention to provide a
`novel system and method of tracking a remote RF transmit-
`ter which has a reduced susceptibility to the eflects of
`multipath.
`It is another object of the present invention to provide a
`novel system and method of tracking a remote RF transmit-
`ter by determining the range and direction of an arriving
`signal with respect to the portion of the signal arriving
`directly from the RF transmitter.
`These and many other objects and advantages of the
`present invention will be readily apparent to one skilled in
`the art to which the invention pertains from a perusal of the
`claims. the appended drawings. and the following detailed
`description of the preferred embodiments.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a pictorial representation of a typical environ-
`ment in which RF tracking is accomplished. showing the
`various signal paths.
`FIG. 2 is a signal power level graph showing the corre-
`lations of signal power in a typical multipath signal.
`FIG. 3 is a simplified block diagram of a system in which
`the present invention may be used.
`FIG. 4 is a simplified block diagra.rn of another embodi-
`ment of a device in which the method of the present
`invention can be implemented.
`
`DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`The principles of operation of the present invention can be
`described with reference to the correlated signal power level
`graph of FIG. 2. The graph of FIG. 2 depicts the correlated
`power level of the signal received plotted against time
`starting from the time of transmission of the signal. In a
`typical multipath environment. such as shown in the illus-
`tration of FIG. 1. the signal power level may have several
`peaks. each peak corresponding to the arrival of a signal
`which has taken a difierent route. For example, in FIG. 2. the
`first peak could correspond to the signal traveling along the
`first signal path 29 of FIG. 1.
`the second peak could
`correspond to the signal travelling along the second signal
`path 22 of FIG. 1. et cetera. Note that in the graph of FIG.
`2. the “strongest” signal corresponds to the signal travelling
`on the second signal path 22 and that this is a multipath
`signal. not the signal arriving directly from the transmitter.
`Rather, the signal arriving directly from the transmitter is the
`first signal having a significant peak. the signal travelling
`along the first signal path 20. The first arriving signal in this
`example is weaker than the later arriving signal because the
`signal was attenuated by passing through the blocking
`elements 14 illustrated in FIG. 1. Because the “shortest
`distance between two points is a straight line" and the speed
`of transmission of the signal
`through various media is
`approximately equal. the signal travelling directly from the
`transmitter to the base station will always be the first signal
`to arrive (assuming that the signal is not wholly blocked.)
`The present invention takes advantage of the fact that the
`first arriving signal is the signal which has travelled the
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`shortest distance and is most likely the signal corresponding
`to the direct path to the transmitter by selecting this signal
`from which to determine the range to and angle of arrival
`from the remote transmitter.
`
`With reference to FIG. 3. the present invention may be
`embodied in a tracking and locating system in which a base
`station 10 transmits a signal to a remote unit 12 which relays
`the signal back to the base station 10. The signal may be a
`spread spectrum signal, such as a chirp signal. By determin-
`ing the round trip time and subtracting the known delay
`within the remote unit 12 and within the base station's
`detection system. the propagation time is determined and the
`distance may be calculated. In a preferred embodiment. the
`base station’s receiver is a conventional correlation receiver
`in which the power level of the arriving signal is correlated
`in time and the signal arriving first in lime is used to
`determine the total propagation time and angle of arrival.
`With reference to FIG. 4. the present invention may be
`embodied in a direction finding unit having multiple chan-
`nels 50. each channel being associated with a ditferent
`antenna 52 (or dement in a plural element antenna array).
`The geometric relationship of the plural antennas 52 is
`known to the DF unit. Each of the channels 50 may include
`a low noise amplifier 54 and a bandpass filter 56 which
`provides the received signal to a mixer 58 which mixes the
`received signal with a locally generated signal 60. The
`mixed signal may then be amplified by an intermediate
`frequency arnpli.fier 62, finther filtered by a bandpass filter
`64 and adjusted by a gain control circuit 66. The mixed
`signal may then be applied to a quadrature downconverter
`which down converts the mixed signal to baseband. The
`baseband signal may be converted by an analog-to-digital
`(“A/D”) converter 70 to a digital signal which is supplied to
`a digital signal processor (“DSP”) 72. The DSP 72 may be
`under the control of a small logic device. such as a personal
`computer 74. which determines the angle and direction of
`the remote transmitter from the DSP and provides and
`appropriate display or announcement to a user (not shown).
`In the preferred embodiment. it has been found advanta-
`geous for the remote t:ransmit:ter to transmit a signal having
`two portions. the first portion being a preamble which alerts
`the base station that the signal is arriving and indicates to the
`receiver that it should begin receiving. The second portion of
`the transmitted signal may be a chirp waveform. i.e.. a
`waveform in which the frequency is varied. usually at a
`linear rate. for a period of time. The transmitted signal may
`be generated remotely at the transmitter or may be a replay
`of a signal originally sent by the base station.
`Use of a chirp waveform has several advantages in the
`present invention because of a useful property of mixed
`chirp signals. It is known that if two identical chirp signals.
`one time delayed from the other. are mixed. the resulting
`signal will be a sinusoidal signal with a frequency which is
`directly proportional to the amount of delay between the two
`signals.
`In operation. when the transmitted signal is received at the
`base station. it is detected by the receiver which recognizes
`the preamble and starts a chirp generator 76 which modu-
`lates a quadrature modulator 78 to provide an RF chirp
`signal as the locally generated signal 60 on the mixer input.
`When the RF chirp signal is mixed with the chi.rp signals
`arriving from the remote transmitter (i.e., several
`time
`delayed versions of the transmitted chirp signal). a set of
`sinusoidal signals is generated whose frequencies are pro-
`portional to the time they arrived. Thus. after the mixing. the
`signals in each channel represent the multipath profile of the
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`received signal in the frequency domain. In the present
`invention. the mixing is accomplished at an intermediate
`frequency and then the frequency spectrum of signals is
`further downconverted by the quadrature downconverter 68
`to baseband. The quadrature modulator 68 may be provided
`with a modulating/downconverting signal from the Timing
`and Frequency Synthesizer 40 via a line 42 in a conventional
`fashion. The signal may then be digitized by the A/D
`converter and the digitized form of the signal applied to a
`multichannel DSP 72 which may use conventional tech-
`niques (such as a fast Fourier Transform) to determine the
`spectrum of frequencies at which signal energy is signifi-
`cantly present so that a power level profile may be devel-
`oped The PC 74 may review the power level profiles
`developed by the DSP for all the channels 50 to determine
`which signal arrived first. Because the relative phase rela-
`tionships among the signals have been preserved and the PC
`74 has been provided with the geometric relationships
`between the various channels. the PC 74 may conventionally
`determine the angle of arrival of the first arriving signal.
`The filtering and amplifying elements of the DF unit of
`FIG. 4 may be conventional. The DSP 72 may be a com-
`mercially available device such as the TMS 320 C 30 device
`sold by Texas Instruments.
`While not critical to the invention. in one embodiment. an
`acceptable signal from the transmitter had a center fre-
`quency of 915 MHz with a chirp of +/-l0 MHz for a
`duration of approximately 10 millisec. The locally generated
`(reference) RF chirp used to downconvert to an intermediate
`frequency may be a signal having a center frequency of 880
`MHz.
`
`In the foregoing description. for ease of understanding.
`the elements of the system have been referred to as “base"
`and “remote". However. there is nothing critical to the
`present invention that requires one of the stations be fixed
`and the other mobile. Additionally. the detailed description
`may suggest that certain components may be utilized to
`construct a system of the present invention. However. that
`suggestion is not to be taken as limiting as it is known that
`many other components could be utilized to accomplish the
`same results. For example. the FET 74 could be replaced by
`a bank of bandpass filters and appropriate detectors. each
`measuring the power level of the signal at different fre-
`quency ranges. Similarly. while the above description uti-
`lizes plural channels 50.
`the invention could be readily
`implemented using a single channel which is appropriately
`multiplexed to the various antennas.
`While preferred embodiments of the present invention
`have been described. it is to be understood that the embodi-
`ments described are illustrative only and the scope of the
`invention is to be defined solely by the appended claims
`when accorded a full range of equivalence. many variations
`and modifications naturally occurring to those of skill in the
`art from a perusal hereof.
`What is claimed is:
`
`1. A method for determining the direction with respect to
`a receiver of a source of a radiated radiofrequency (RF)
`signal. comprising the steps of:
`(a) receiving a multipath signal which has been transmit-
`ted by a remote transmitter;
`(b) correlating the received multipath signal into plural
`path signals;
`(c) determining the time of arrival of each of the plural
`path signals;
`(d) determining the direction of the remote transmitter
`from the path signal having the earliest determined time
`of arrival.
`
`2. The method of claim 1 wherein said step of correlating
`comprises the step of determining the signal power level of
`each path signal.
`3. The method of claim 1 wherein said step of receiving
`is accomplished by plural antennas.
`4. The method of claim 3 wherein said step of receiving
`is accomplished by four antennas equally spaced along the
`comers of a square.
`5. The method of claim 3 wherein said step of determining
`the direction comprises the step of comparing the phase
`relationship of the earliest arriving signal at each of the
`plural antennas for correspondence with predetermined
`angles of arrival.
`6. The method of claim 1 wherein said radiated RF signal
`is a chirp signal.
`7. The method of claim 1 wherein said radiated RF signal
`is derived from an RF signal received at the remote trans-
`mitter.
`
`8. The method of claim 1 wherein step of determining the
`time of arrival further comprises the steps of:
`(c)(l) mixing the plural path signals with a predetermined
`mixing signal to produced plural mixed signals; and,
`(c)(2) calculating the time of arrival of the plural mixed
`signals.
`9. The method of claim 8 wherein said calculated time of
`arrival for each mixed signal is related to the frequency of
`each mixed signal.
`10. The method of claim 8 wherein said mixing signal is
`a chirp signal.
`11. A system for determining the direction from which a
`received multipath signal was transmitted. comprising:
`plural receiving channels. each channel comprising:
`RF signal receiving means for receiving a RF signal
`and converting said RF signal to a received signal;
`bandpass filtering means for filtering said received
`signal;
`a chirp mixer to mix the filtered electrical signal with
`a chirp signal to produce an intermediate frequency
`(“lF") signal;
`IF filtering means for filtering said IF signal;
`a quadrature downconverter to downconvert said fil-
`tered IF signal to baseband quadrature signals; and.
`an analog-to-digital converter to convert the baseband
`quadrature signals to digital signals;
`digital signal processing means operatively connected to
`receive and determine the power spectrum of each of
`said digital signals from said plural receiving channels;
`control means for determining the time of arrival of each
`of said digital signals and for determining the angle of
`arrival of the digital signal having the earliest time of
`arrival.
`
`12. A method for determining the distance between a
`receiver of a radiated radiofrequency (RF) signal and a
`transmitter of said signal. comprising the steps of:
`(a) receiving a multipath signal which has been transmit-
`ted by a remote transmitter;
`(b) correlating the received multipath signal into plural
`path signals;
`(c) determining the time of arrival of each of the plural
`path signals;
`(d) determining the distance of the remote transmitter
`from the path signal having the earliest determined time
`of arrival.
`
`13. The method of claim 12 wherein said step of corre-
`lating comprises the step of determining the signal power
`level of each path signal.
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`14. The method of claim 12 wherein said distance deter-
`mining step comprises a measurement of the time period for
`the signal to travel from the transmitter to the receiver.
`15. The method of claim 14 wherein said receiving step
`comprises the steps of:
`(a) transmitting an outward signal to the remote transmit-
`ter; and.
`(b) transmitting a signal to the receiver responsively to the
`receipt at the transmitter of the outward signal.
`16. The method of claim 12 wherein said multipath signal
`is a spread spectrum signal.
`17. A system for determining the direction to a remote
`transmitter from which a chirp signal was transmitted.
`comprising:
`a receiver for receiving a first chirp signal and any
`multipath chirp signals related thereto;
`a chirp generator for providing a locally generated chirp
`signal;
`a mixer for combining the locally generated chirp signal
`with each of the received chirp signals to generate
`plural sinusoidal signals whose frequencies are related
`to the arrival times of the received chirp signals;
`a signal processor operatively connected to receive the
`sinusoidal signals and for determining a power spec-
`trum of the sinusoidal signals; and
`a control means for determining from the power spectrum
`the relative times of arrival of the received chirp signals
`and for determini.ng the angle of arrival of one of the
`received chirp signals having the earliest
`time of
`arrival.
`
`18. The system of claim 17 further comprising a trans-
`mitter for transmitting an outgoing triggering signal to the
`remote transmitter causing the remote transmitter to transmit
`the first chirp signal.
`19. The system of claim 18 wherein the outgoing trigger-
`ing signal is a further chirp signal and the first chirp signal
`transmitted by the remote transmitter is a replay of the
`further chirp signal.
`20. The system of claim 18 further comprising a timer for
`determining elapsed time between transmission of the trig-
`gering signal and reception of the earliest one of the received
`chirp signals.
`21. A method of determining the direction to a remote
`transmitter of an RF chirp signal. comprising the steps of:
`(a) receiving plural RF chirp signals resulting from a
`remotely transrnitted RF chirp signal;
`(b) generating a local RF chirp signal;
`(c) mixing the local RF chirp signal with each of the
`received RF chirp signals to generate a set of sinusoidal
`signals whose frequencies are related to the arrival
`times of the received chirp signals;
`
`8
`(d) correlating the sinusoidal signals into plural path
`signals;
`(e) determining which one of the plural path signals
`arrives first; and
`
`(f) determining the direction to the remote transmitter
`using the earliest arriving one of the plural path signals.
`22. ‘The method of claim 21 wherein the step of correlat-
`ing comprises the steps of determining a spectrum of fre-
`quencies with signal energy present, and developing a power
`level profile.
`23. The method of claim 21 wherein the step of deter-
`mining the earliest arriving one of the path signals comprises
`the step of comparing the frequencies of the sinusoidal
`signals.
`24. The method of claim 21 wherein the transmitted RF
`chirp signal is transmitted in response to a trigger signal
`received at the remote transmitter.
`25. The method of claim 24 wherein the trigger signal
`received at the remote transmitter is an RF chirp signal.
`26. A method of determining the distance to a remote
`transmitter of an RF chirp signal. comprising the steps of:
`(a) receiving a multipath RF chirp signal resulting from
`the remotely transmitted signal;
`(b) correlating the multipath RF chirp signal into plural
`path signals;
`(c) determining the time of arrival of the first arrived one
`of the path signals; and
`(d) determining the distance of the remote transmitter
`using the first arrived one of the path signals.
`27. The system of claim 11 further comprising a trans-
`mitta for transmitting a trigger signal to a remote transmit-
`ter to initiate transmission of the signal received at said
`plural receiving channels.
`28. The system of claim 27 wherein said trigger signal is
`a chirp signal.
`29. The system of claim 11 wherein the received RF signal
`comprises a preamble portion and a following chirp wave-
`form portion. and wherein said RF signal receiving means is
`activated upon receipt of the preamble portion of the
`received RF signal.
`30. The system of claim 17 wherein the received first
`chirp signal comprises a preamble portion for activating said
`receiver and a following chirp waveform portion.
`31. The method of claim 21 wherein each of the received
`plural RF chirp signals comprises a preamble portion for
`activating a receiver and a following chirp waveform por-
`tion.
`32. The method of claim 26 wherein the received RF chirp
`signal comprises a preamble portion for activating a receiver
`and a following chirp waveform portion.
`*
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`PETITIONERS 1001-0007