WO1999030181A9 - Method and system for determining position of a mobile transmitter - Google Patents
Method and system for determining position of a mobile transmitterInfo
- Publication number
- WO1999030181A9 WO1999030181A9 PCT/US1998/024788 US9824788W WO9930181A9 WO 1999030181 A9 WO1999030181 A9 WO 1999030181A9 US 9824788 W US9824788 W US 9824788W WO 9930181 A9 WO9930181 A9 WO 9930181A9
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- WO
- WIPO (PCT)
- Prior art keywords
- frequency
- ofthe
- recited
- signal
- determining
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W64/00—Locating users or terminals or network equipment for network management purposes, e.g. mobility management
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/06—Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements
Definitions
- the present invention relates generally to position detection methods and systems, and more particularly, to a method and system for detecting a position of a mobile device or transmitter, such as a cellular telephone, based on phase differences of transmitted signals of two or more frequencies received at two or more receiving sites.
- Prior position detection, or navigation, systems may be generally divided into two categories: passive navigation systems and active navigation systems.
- a mobile device determines its position based on signals received from transmitters positioned at known locations.
- the mobile device transmits signals which are received by one or more receivers positioned at known locations. The position of the mobile device is then determined based on the received signals and the known position of the receivers.
- the GEOSTAR system In the GEOSTAR system, at least one orbiting satellite transmits signals to mobile transponders and receives replies from the mobile transponders. For calculation purposes, the GEOSTAR system assumes that the mobile transponder is on the earth's surface. By measuring the time a signal takes to travel from a satellite to a mobile transponder and back to the satellite (loop propagation delay), the mobile transponder can be determined to lie somewhere on a calculated sphere of appropriate radius. Since the intersection of the calculated sphere and the earth's surface is a circle, the GEOSTAR system thereby locates the mobile transmitter somewhere on the circle. If two satellites are employed, the mobile transponder can be deduced to also lie on a second circle. Since the intersection of these two circles is two points, the GEOSTAR system can therefore locate the mobile transponder at one of the two points.
- the GEOSTAR system unfortunately exhibits some significant deficiencies. Firstly, the GEOSTAR system suffers from the problem of congestion as experienced by other prior active navigation systems. Secondly, the GEOSTAR system relies upon loop delay measurements which are typically of questionable reliability or accuracy. For example, the accuracy of loop delay measurements is deleteriously affected by timing errors in the transponder hardware which may be caused by any number of known factors, such as temperature, imprecise manufacturing tolerances and the like. In addition, GEOSTAR systems typically use wideband signal transmissions to assure accurate loop delay measurements. Unfortunately, such wideband signal transmissions occupy a large portion of available bandwidth, thus significantly contributing to the aforedescribed problem of congestion.
- Capacity is an issue at the heart of cellular communications systems and satellite- based mobile communications systems. Enough capacity for all users is guaranteed by dividing the service area into a large number of small cells with the ability to re-use the limited number of available radio frequencies again in different cells which are adequately sparated.
- U.S. Patent No. 5,619,503 issued to Dent on April 8, 1997 describes improvements to multi-cell or multi-beam communications systems that permit higher capacity by denser frequency re-use - ultimately permitting every frequency channel to be used for a different purpose in every cell or beam.
- the disclosure of U.S. Patent No. 5,619,503 is hereby incorporated by reference herein and provides the capacity improvements necessary to allow an active navigation system to succeed. Methods to obtain an initial coarse position estimate are also described and may be used in the current invention.
- a method and system in accordance with the present invention wherein a first signal at a first frequency and a second signal at a second frequency are received from a transmitter at first and second receiving stations. Based on phase shifts of the signals received at the first and second receiving stations, a range difference is calculated which locates the transmitter on a first hyperbola having the stations as foci.
- a method for determining position of a mobile transmitter, such as a cellular telephone, relative to first and second receiving sites.
- the first and second receiving sites may be a land-based cellular base station or a satellite relay station.
- a first signal at a first frequency is transmitted by the mobile transmitter and received at the first and second receiving sites, as respective first and second received signals.
- a second signal at a second frequency is transmitted by the mobile transmitter and received at first and second receiving sites, as respective third and fourth received signals.
- a first phase difference measurement is made based on the first and second received signals.
- a second phase difference measurement is made based on the third and fourth received signals.
- a position of the mobile transmitter is determined based on the first and second phase difference measurements, the first and second frequencies and the first and second known locations.
- the first and second phase difference measurement are scaled based on the first and second frequencies and a range difference is determined.
- a frequency difference may be determined between the first and second frequencies and the difference between the first and second phase difference measurements is scaled by the frequency difference to obtain a range difference.
- the first phase difference measurement is preferably performed by determining a firsit phase shift for the first received signal; determining a second phase shift for the second received signal; and making the first phase difference measurement based on the first and second phase shifts.
- the second phase difference measurement is preferably performed by determining a third phase shift for the third received signal; determining a fourth phase shift for the fourth received signal; and making the second phase difference measurement based on the third and fourth phase shifts.
- the calculated range difference places the transmitter on a first hyperbola with foci at the first and second receiving stations.
- the method may further comprise the steps of calculating third or fourth phase difference measurements for a third receiving site and one ofthe first or second receiving sites; localizing the transmitter on a second hyperbola related to the third receiving site and the one ofthe first and second receiving sites; and determining the position ofthe mobile transmitter based on the intersection ofthe first and second hyperbolas.
- the first frequency may be a random access channel and the second frequency may be a traffic channel.
- at least one ofthe first and second frequencies is contained in set of frequencies allocated to the mobile transmitter for frequency hopping communication.
- the signal on at least one ofthe first and second frequencies may be comprised of signal bursts formatted in accordance with a time division multiple access format.
- the step of making a first phase difference measurement and the step of making a second phase difference measurement may comprise the steps of converting the first, second, third and fourth received signals into a digital stream of numerical values representative of instantaneous phases ofthe first, second, third and fourth received signals; and determining the position of the transmitter based on the stream of numerical values and the first and second frequencies.
- the step of converting may comprise the steps of downconverting the first and third received signals to a first intermediate frequency; downconverting the second and fourth received signals to a second intermediate frequency; and sampling and analog-to-digital converting the downconverted first, second, third and fourth received signals.
- the first and second intermediate frequencies may be generated by local oscillators which may be synchronized to a common reference frequency.
- the common reference frequency could be derived from satellite navigation signals.
- the step of sampling and analog-to-digital converting may comprise the step of quadrature converting the downconverted first, second, third and fourth received signals to yield the stream of numerical values containing complex values.
- the step of sampling and analog-to-digital converting may comprise the step of logpolar converting the downconverted first, second, third and fourth received signals to yield the stream of numerical values containing numerical value pairs representative of instantaneous signal phase and amplitude.
- the step of determining the position may include the step of determining first, second, third and fourth phase difference measurements for the first, second, third and fourth received signals.
- the stream of numerical values can then be correlated to determine an integral number of samples delay between arrival ofthe first and second received signals and the third, and fourth received signals and to determine respective first and second correlation phases.
- the correlation step may comprise determining the correlation phases at the first frequency and the second frequency and using the correlation phases to determine the first and second phase difference measurements.
- the present invention further provides for translation in time ofthe phase measurements.
- the step of making a first phase difference measurement comprises the step of performing multiple measurements ofthe first phase difference measurement at different instants of time and interpolating or extrapolating the multiple measurements to determine a value for the first phase difference measurement at an instant of time at which the second phase difference measurement was made.
- a frequency difference between the first and second frequencies is determined.
- the frequency difference is then compared to a predetermined frequency value. If the determined frequency difference is less than the predetermined frequency value, then a new value for at least one of the first and second frequencies is selected such that the frequency difference is larger.
- the first and second phase difference measurements are then repeated using the new value for the at least one ofthe first and second frequencies.
- a system for determining position of a mobile transmitter is provided.
- the mobile transmitter is capable of transmitting first and second signals at respective first and second frequencies.
- the system comprises a first and second receiving station for receiving the first signal and a network control processor for computing a first phase difference based on the first signal received at the first and second receiving stations.
- the first and second receiving stations also receive the second signal and the network control processor coputes a second phase difference based on the second signal received at the two stations.
- the network control processor determines a range difference measurement ofthe mobile transmitter based on the first and second phase difference measurements and the first and second frequencies.
- the first receiving station comprises a first phase measuring circuit for determining a first and third phase value for the first and second signals respectively as received at the first receiving station.
- the second receiving station comprises a second phase measuring circuit for determining a second and fourth phase value for the first and second signals respectively as received at the second receiving station.
- Fig. 1 is a diagram of a system in accordance with the present invention comprising a mobile transmitter and three receiving stations;
- Fig. 2 is graphical representation of successive phase difference measurements taken while the transmitter shown in Fig. 1 is frequency hopping;
- Fig. 3 is a graphical representation of phase difference measurements made on sequentially different frequencies
- Fig. 4 is a diagram showing non-simultaneous arrival of a signal at two ofthe receiving stations shown in Fig. 1;
- Fig. 5 is a diagram showing phase difference measurements between signals not arriving at the same time at the two receiving stations
- Figs. 6a and 6b are graphical representations of an exemplary autocorrelation functions of random digitally-modulated signals
- Fig. 7 is a schematic of a system in accordance with the present invention wherein at least one ofthe receiving stations is a satellite; and
- Fig. 8 is a schematic diagram of an exemplary dual channel receiving system in accordance with one aspect ofthe present invention.
- a system 100 for determining position of a mobile transmitter 102 in accordance with the present invention is shown in Figure 1.
- the mobile transmitter 102 may be a cellular telephone, satellite communicator or any other mobile device which transmits signals.
- First, second and third receiving stations, or sites, 104, 106 and 108 located at known positions receive signals transmitted by the mobile transmitter 102.
- the receiving sites 104, 106 and 108 may be land-based cellular base 'Stations belonging to a mobile telephone network, may be orbiting satellite relay stations in communication with at least one land-based station, or combinations thereof.
- a network control processor 110 may control the receiving stations 104, 106 and 108, receives information from the receiving stations 104, 106 and 108, and, based on this information, calculates the position ofthe mobile transmitter 102.
- the network control processor 110 may also provide a phase or frequency reference signal S R to the receiving stations 104, 106 and 108. It is to be understood that the receiving stations 104, 106 and 108 and the network control processor 110 may be widely separated, thus signals are transmitted between the devices over communications links, such as radio links.
- the network control processor 110 consists of various circuits and processors for performing calculations. As those skilled in the art will readily comprehend, these circuits and processors may alternatively be contained in one ofthe receiving stations 104, 106 and 108 or may be divided among a plurality of sites.
- the structure, control and arrangement ofthe conventional components and circuits have, for the most part, been illustrated in the drawings by readily understandable block representations and schematic diagrams. Only those specific details are shown that are pertinent to the present invention in order not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art having the benefit ofthe description herein.
- the present invention may be advantageously implemented using two or more receiving stations.
- the mobile transmitter 102 transmits a first signal S, at a first frequency w ⁇ which is received by the first and second receiving stations 104 and 106, as respective first and second received signals.
- the first frequency w may be a Random Access Channel (RACH) which is used when the transmitter 102 first initiates contact with the stations 104, 106 and 108 and the processor 110.
- RACH Random Access Channel
- the first signal S will experience a first phase shift ⁇ A1 during transmission to the first receiving station and a second phase shift ⁇ B1 during transmission to the second receiving station 106.
- phase shifts ⁇ A1 and ⁇ B depend upon the distance of travel of the first signal S t to the respective stations 104 and 106. Consequently, the first and second phase shifts ⁇ A1 and ⁇ B1 will not be equal unless the mobile transmitter 102 is equidistant from the first and second receiving stations 104 and 106.
- First and second receiving stations 104 and 106 receive a frequency and phase reference signal from a common source, such as the GPS satellite system ofthe network control processor 110.
- the stations 104 and 106 measure the received phases ⁇ A1 and ⁇ B1 ofthe first signal S t using the common phase reference signal, and pass the phase measurements to the network control processor.
- the network control processor 110 determines a first phase difference measurement ⁇ , in accordance with the following equation:
- the mobile transmitter 102 next transmits a second signal S 2 at a second frequency w 2 which is similarly received by the first and second receiving stations 104 and 106, as respective third and fourth received signals.
- the second frequency w 2 may be a traffic channel, for example, assigned to the transmitter 102 for passing communications traffic after the initial contact on the RACH channel.
- the mobile transmitter 102 may change between, or transmit at, the first and second frequencies w x and w 2 in a predetermined frequency change schedule (frequency hopping schedule), on command from the network control processor 110 or other control mechanism.
- the second signal S 2 will experience a third phase shift ⁇ A2 during transmission to the first receiving station 104 and a fourth phase shift ⁇ B2 during transmission to the second receiving station 106.
- the magnitudes ofthe phase shifts ⁇ and ⁇ B2 will depend upon the distance of travel ofthe second signal S 2 to the respective stations 104 and 106.
- the first and second stations 104 and 106 determine the phases ⁇ A2 and ⁇ B2 using the common reference signal #nd pass the measurements to the network control processor 110.
- the network control processor 110 then computes a second phase difference ⁇ 2 using:
- a time difference dT is next determined using the following equation:
- the time difference dT is a measure ofthe difference in propagation delay ofthe first and second signals S, and S 2 to the respective first and second receiving stations 104 and 106. Since this difference in the propagation delay is dependent upon the difference in the distances between each ofthe stations 104 and 106 and the mobile transmitter 102, the time difference dT is also a measure ofthe range difference between each station 104 and 106 and the mobile transmitter 102.
- a range difference dR is calculated, preferably by the network control processor 110, using the following equation:
- c is the speed of light (3 x 10 8 m/s).
- the value (range difference measurement) calculated for the range difference dR places the location ofthe transmitter 102 on a first hyperbolic curve contained in a family of hyperbolic curves having the first and second receiving stations 104 and 106 as foci.
- the network control processor 110 can receive other measurements that locates the transmitter 102 on a second hyperbolic curve. The intersection ofthe first and second hyperbolic curves defining the position ofthe transmitter 102.
- loop delay measurements if available, can be used to determine the distances ofthe transmitter 102 from the first station 104, the second station 106 or both, thereby identifying a segment ofthe first hyperbolic curve on which the transmitter 102 is located.
- the invention is envisaged to be most useful when loop delay measurements are not available or not as accurate as the delta-range measurements made in the above described manner.
- loop delay measurements may be used only to resolve ambiguities, although a position uncertainty will still be present in the dimension lying along the first hyperbolic curve.
- the position accuracy is much better, however, in the dimension perpendicular to the first hyperbolic curve, as this is determined by the more accurate delta-range measurement.
- the transmitter 102 does not transmit at the first and second frequencies w l and w 2 concomitantly, one source of error may arise if the transmitter 102 or any ofthe receiving stations 104, 106 or 108 are moving. In such a scenario, it is possible for the transmitter 102 to transmit at the first frequency w , in one location and then to have moved to another location before transmitting at the second frequency w 2 .
- the signal at the second frequency w 2 would therefore have phase shifting characteristics based, in part, on the distance traveled from the second location.
- One solution to this problem, and most likely the optimum solution, is to use a Kalman filter algorithm to track changes in all parameters, as discussed more fully below.
- phase difference measurements ⁇ , and ⁇ 2 A hypothetical progression of phase difference measurements taken at times t 0 , t l5 1 2 , t,.... is shown in Fig. 2.
- the phase difference measurements have been labeled ⁇ lx and ⁇ 2x where the subscripts "1" and "2" represent the first frequency w, and the second frequency w 2 , respectively, and the subscript "x" represents a variable integer indicating a time the measurement was taken.
- a phase difference measurement ⁇ 16 is a measurement of a signal having the first frequency w ⁇ taken at the time t 6 .
- the transmitter 102 alternates, or "hops", between transmitting the first signal S, at the first frequency w l during even times to, t 2 , t 4 ... and transmitting the second signal S 2 at the second frequency w 2 during odd times t l5 1 3 , t 5 ...
- the graph in Fig. 2 has further been drawn in a modulo-2 ⁇ format in that the phase difference measurements begin again at zero for every 2 ⁇ or 360°.
- phase difference measurements ⁇ leven-x for the first frequency w and phase difference measurements ⁇ 2odd , x f° r tne second frequency w 2 move progressively up sloped lines.
- the slope ofthe lines is not due to making the measurements at different frequencies, as all measurements on a line are made at either the first or second frequency w, or w 2 . Consequently, the slope must be due to the sources of frequency error mentioned above.
- the sloped lines' representing the phase difference measurements ⁇ 2odd . x for the second frequency w 2 may have a slightly different slope than the sloped lines representing the phase difference measurements ⁇ ev ⁇ .n - f° r tne first frequency w,.
- the slight difference in slope is due to the fact that the rate of change of phase caused by a fixed frequency error is constant, but the rate of change of phase due to a constant percentage of frequency error is proportional to the frequency,
- the difference in slope between the two lines will most likely be practically negligible, so that an average slope can be determined using the slope of both lines.
- the average slope can be determined by averaging as set forth below:
- phase differences are computed modulo-2 ⁇ (i.e. 2 ⁇ is added if the difference becomes negative).
- slope dW can now be determined by the following equation:
- values of phase differences at the first frequency v. , and the second frequency w 2 may be computed at substantially the same instant in time by interpolation.
- phase measurement ⁇ 2I at the second frequency w 2 referred to instant of time t may be determined by:
- any prior art method of interpolation such as least squares curve fitting, may alternatively be used to obtain a best estimated for the phase measurement ⁇ 21 .
- the value of the phase measurement at the second frequency w 2 may now be subtracted from the phase measurement made at the first frequency w at the same instant of time t ⁇ to obtain a difference value which is corrected for the slope error.
- This may repeated at other instants of time t 2 , t 3 ... by interpolating either the line for phase measurements at the first frequency w or the line for phase measurements at the second frequency w 2 , as appropriate, to obtain a succession of phase differences that depend only on the frequency change.
- the transmitter 102 and the receiving stations 104, 106 and 108 are known to be stationary, these phase measurements must all correspond to the same range difference dR and the computed range differences at times ⁇ ⁇ .... may be averaged to obtain higher accuracy.
- the successive range differences dR computed at times t, , t 2 , t 3 .... may show a systematic drift, revealing a possible movement ofthe transmitter 102 or the receiving stations 104, 106 or 108 that was not previously modeled.
- the principle of a Kalman filter is to model all such movement in order to predict what the range difference should be at a given instant of time.
- motion ofthe transmitter 102 may be modeled by assuming a constant velocity which has to be estimated.
- motion of two receiving stations in earth orbit may be modeled by use of sets of orbital parameters, such as altitude, inclination, eccentricity and the like.
- the Kalman procedure updates the model parameters such that, on average, the error between the prediction and the measurement is minimized in a least squares sense.
- the system ofthe present invention is used to determine the positions of large numbers of mobile transmitters, the parameters modeling the motion ofthe receiving stations are common, thus resulting in a very accurate determination of the motion of a receiving station (e.g. satellite orbits) based on receiving signals from many mobile transmitters. This in turn aids accurate position determination for each individual transmitter.
- Figure 3 illustrates an alternative measurement sequence when frequency hopping between two. or more frequencies is not employed, but other programmed frequency changes are made.
- the transmitter starts out transmitting at the first frequency w x and a signal phase difference at two receiving stations, such as the first and second receiving stations 104 and 106, is determined at times t 0 , t, and t 2 to be phase differencemeasurements ⁇ 1(J , ⁇ u , and ⁇ 12 respectively.
- the first frequency w can, for example, be the RACH channel frequency as explained above.
- the transmitter then changes to the second frequency w 2 which can be a first traffic channel frequency having a small frequency difference from the RACH channel frequency, for example 50KHz.
- Phase difference measurements are then made at the second frequency w 2 at times t 3 , t 4 and t 5 to obtain phase difference measurements ⁇ 23 , ⁇ 24 and ⁇ 2J .
- the transmitter may optionally then switch to another traffic channel frequency, or third frequency w 2 , at times t 6 , t 7 and t 8 at which phase difference measurements ⁇ 36 , ⁇ 37 and ⁇ 38 are made.
- phase difference measurements at the first frequency w l and the second frequency w 2 at the same instant in time may be obtained by backward extrapolation ofthe slope line for the second frequency w 2 measurements or by forward extrapolation ofthe slope line for the first frequency w , measurements, or both.
- a first estimate for a first range difference dR is obtained.
- the slope line at the third frequency w 3 may be backwards extrapolated and the slope line at the second frequency v. 2 may be forward extrapolated to obtain a second range difference which must be ofthe same order as the first range difference dR,.
- the transmitter 102 transmits signals that are received at two separated receiving stations, the receivers then being able in some way to convey the received signals to a common point for phase comparison.
- Making a phase comparison implies that a signal from one station is available at the same time as a signal from another station.
- a preferred waveform to be transmitted by the transmitter for communications purposes is a short signal burst occupying a timeslot of a repetitive TDMA frame period.
- the differential delay between to paths exceeds the duration of a timeslot, so that the signal is not received at the first receiving station 104 and the second receiving station 106 in overlapping time periods.
- a different method of determining phase difference which does not rely on the signals from the stations 104 and 106 being present at both inputs of a phase comparator simultaneously is required.
- FIG. 5 illustrates the principle of such a solution.
- the transmitter 102 transmits a signal burst which is received at first and second antermas 112 and 114 ofthe respective first and second receiving stations 104 and 106.
- the respective received signals are downconverted in first and second receivers RX-A 116 and RX-B 118 to a suitable frequency for analog-to digital (AtoD) conversion and sampling.
- AtoD analog-to digital
- a suitable frequency spectrum for AtoD conversion and sampling is the complex baseband.
- a signal is mixed with Cosine and Sine oscillator signals to produce In-Phase and Quadrature (I, Q) signals that each extend in the spectral domain from zero (DC) to half the receiver bandwidth.
- I, Q signals are then sampled and digitized
- first and second AtoD converters 120 and 122 each comprises two conversion channels, one for the I signal component and one for the Q signal component.
- the resulting first and second signal sample streams Acute A 2 , A 3 ... and B
- B 2 , B 3 ... are complex samples having both a real (I) part and an imaginary (Q) part.
- oscillator synchronizing means 124 In order to preserve the relative phase-of-arrival information, it is necessary to employ oscillator synchronizing means 124 to synchronize all local oscillators used in the
- first and second receivers RX-A 116 and RX-B 118 so that the effects on signal phase of the downconversion processes are at least known.
- convertor synchronizing means 126 it is necessary to employ convertor synchronizing means 126 to synchronize the sampling ofthe first and second AtoD convertors 120 and 122 such that their sampling instants are the same or at least have a known relationship.
- _5 126 can, for example, comprise atomic clocks located at the first and second receiving stations 104 and 106 having stabilities of one part in 10 to the power 13, or can comprise GPS satellite navigation receivers which provide accurate time and frequency references.
- the GPS receivers may also be used to correct slow drifts ofthe atomic clocks.
- the GPS receivers are in any case desirable in order to accurately determine the locations ofthe
- the signal burst from the transmitter 102 affects sample numbers A_...A (n+L) generated from the first receiver RX-A 116 but, due to delay difference, affects different sample numbers B m ...B (m+L) generated from the second receiver RX-B 118.
- a conventional crosscorrelation between the first signal sample stream A and the second signal sample stream B x is defined by:
- the correlation value C(k) represents the similarity ofthe first sample stream A starting at sample A(j+1) to the second sample stream B delayed by c k' samples, i.e. starting at sample B(j+k+l). The similarity is determined over an interval of L consecutive signal samples.
- the correlation value C(k) is a complex number that has a correlation magnitude and a correlation phase.
- the correlation magnitude plotted as a function of 'k' , exhibits a characteristic curve, termed the autocorrelation function, which is determined by the modulation pattern imposed on the signal. If the signal is a random digital modulation pattern of sufficient length, or is deliberately chosen to be one of certain sequences that produce desirable autocorrelation functions, then the autocorrelation function is as depicted in Figure 6.
- Figure 6a shows the classical form ofthe autocorrelation function for a digital signal that is not restricted in bandwidth.
- the magnitude peaks at unity when the signals at the inputs ofthe correlator are perfectly aligned, and falls to zero at plus and minus one digital symbol period away from the peak.
- a digital symbol period is equal to a bit period if the modulation is binary phase shift keying (BPSK) or equal to two bit periods if the modulation is Quadrature Phase Shift Keying (QPSK).
- BPSK binary phase shift keying
- QPSK Quadrature Phase Shift Keying
- modulation signal bandwidfhs and receiver bandwidths are restricted to prevent adjacent channel interference.
- the Wiener-Khintchine theorem states that the autocorrelation function is the inverse Fourier transform ofthe signal spectrum, which is affected by transmitter and receiver filtering.
- a typical, practical autocorrelation function is more rounded than the function shown in Figure 6a and oscillates with decreasing amplitude away from the peak.
- the width ofthe main lobe between the first.nulls is approximately equal to the reciprocal ofthe signal's main spectral lobe. For example, if the signal spectral lobe is ofthe order of 40KHz wide, then the autocorrelation peak will be of the order of 25 microseconds wide.
- a first coarse delay estimate ofthe time difference of arrival may be obtained by correlating the first complex signal stream A with the second complex signal stream B and determining for which value ofthe sample delay 'k' the correlation magnitude is maximum. The accuracy of this determination will be some fraction of 25 microseconds, for example, one tenth of 25 microseconds or +2.5uS.
- the coarse delay- estimate was obtained from the magnitude ofthe correlation 0 value C(k) ignoring the phase.
- the phase of the correlation value C(k) allows a much more accurate determination ofthe delay however.
- a more mathematical formulation will illustrate the value of phase.
- Z(t) is a complex function describing both phase and amplitude modulation (if not a pure phase modulation or a pure amplitude modulation) ofthe signal's carrier frequency
- EXPQwt represents the unmodulated carrier frequency.
- the transmitted signal S(t) is received with a first delay TI at the first receiving station 104 and with a second delay T2 at the second receiving station 106.
- Signals A(t) received at the first receiving station 104 and signals B(t) received at the second receiving station 106 are thus given by:
- dT is the interval between samples assuming synchronization at both stations 104 and 106.
- B(i) - B(i.dT) Z(i.dT-Tl)EXPGw(i-dT-T2))
- This ambiguity is resolved according to the invention by repeating the correlation when the transmitter is using a different frequency.
- the about example was performed with a carrier frequency w equal to the first frequency w
- the different frequency is the second frequency w 2 .
- the accuracy has thus been improved by almost an order of magnitude from 2.5 ⁇ S to 30 0.27 ⁇ S accuracy.
- the measurements can now be repeated with a greater value ofthe frequency difference W,- > 2 .
- the accuracy may be improved by a further factor of 20 to +0.0135 ⁇ S which represents a range difference error of +4 meters.
- the rate of change can be determined by the method illustrated graphically in Figures 2 and 3, namely by determining the difference between successive measurements on the same frequency and then interpolating or extrapolating to determine the difference between measurements on different frequencies translated to the same instant of time.
- Figure 7 shows the use of first and second orbiting satellites 128, 130 as the at least two separated receiving stations needed by the invention.
- a signal from the mobile transmitter 102 is received at the satellites 128 and 130 and, after translation ofthe signal from the transmitter's original frequency to a feederlink frequency, the satellites 128 and 130 relay the signal to respective first and second ground antennas 132 and 134.
- the antennas 132 and 134 are located at approximately the same site on the ground, thus facilitating connection to a common, dual-channel receiving system 136.
- Figure 8 shows more detail of an exemplary dual-channel receiving system having first and second receiving channels.
- the signal from the first antenna 132 is fed to a first downconvertor 201 and the signal from the second antenna 134 is fed to a second downconvertor 202.
- the first downconvertor 201 comprises a low noise amplifier 201a, a filter 201b, a mixer 201c and a local oscillator 201d.
- the second downconvertor 202 is identically constructed with a low noise amplifier 202a, a filter 202b, a mixer 202c and a local oscillator 202d.
- the downconvertors 201 and 202 preferably reside at the focus of their respective antennas since it is easier to transmit an intermediate frequency output to further processing circuits using flexible coaxial cable than to transmit the original microwave feederlink signal using a waveguide.
- the intermediate frequency outputs from the downconvertors 201 and 202 are further amplified and filtered in IF filter amplifiers 203 and 204.
- the outputs ofthe IF filter amplifiers 203 and 204 are fed to respective quadrature downconvertors.
- Quadrature downconvertors 205 and 207 are supplied the signals received from the first satellite 128 and quadrature downconvertors 206 and 208 are supplied the signals received from the second satellite 130.
- the quadrature downconvertors 205, 206, 207 and 208 mix their input signals with a cosine and a sine wave signal from a third local oscillator 210 which is common to both receiver channels.
- the quadrature downconvertors 205 and 206 output I-waveforms to respective demultiplexer/digitizers 211 and 212.
- the quadrature downconvertors 207 and 208 output Q- waveforms to the respective demultiplexer/digitizers 211 and 212.
- the operation of a time-multiplexed feederlink is described in more detail in U.S. Patent Nos. 5,596,961, 5,555,271, 5,619,503, 5,619,210, 5,574,967, 5,568,088 and 5,642,358, the disclosures of which are hereby incorporated by reference herein.
- the feederlink signal comprises a time multiplex of signals from the satellite's receiving antenna elements, together with some known test or calibration signals.
- the calibration signal may for example be set on board the satellite to 1+jO so that, when its sample occurs in the time-multiplexed feederlink stream, it is known that the I-waveform value should be unity and the Q-waveform value should be zero.
- Demultiplexer units 211 and 212 thus extract calibration signal samples from the I and Q waveforms received from the quadrature downconvertors 205, 206, 207 and 208. These calibration signal samples are provided to automatic frequency control (AFC), automatic gain control (AGC) and synchronization (SYNC) units 213 and 214 where the calibration samples are compared with expected values and error signals produced.
- AFC automatic frequency control
- AGC automatic gain control
- SYNC synchronization
- the error signals are used for three purposes: (1) to apply AGC to the IF filter amplifiers 203 and 204 such that the magnitude ofthe calibration samples is controlled to the expected value; (2) to apply AFC or automatic phase control (APC) to the first and second local oscillators 20 Id and 202d such that the phase ofthe calibration samples is controlled to the expected value; and (3) to correct any sampling timing error in the demultiplexers 211 and 212 such that the calibration samples are sampled in the middle of their optimum sampling range. In this way, all phase and amplitude matching errors in the two feederlink channels from the satellite through respective ground processing channels is removed up to the outputs of the demultiplexers 211 and 212.
- AGC automatic phase control
- the outputs ofthe demultiplexers 211 and 212 each represent the composite of all signals received by one ofthe satellite receiving antenna 132 or 134, which may comprise many mobile transmitter signals operating on different channel frequencies.
- Digital channel splitting units 215 and 216 first split the signal from each antenna element into a number of signals from each element corresponding to different channel frequencies ⁇ v treat w 2 ....w n .
- the set of antenna element signals, for a particular frequency, for example w is then fed to a digital beamformer, shown as reference number 217 for the first satellite 128 and reference numeral 218 for the second satellite 130.
- the set of antenna element signals for frequency w n are fed to digital beamformers 219 and 220. Although only four beamformers 217, 218, 219 and 220 are shown, there is a separate pair of digital beamformers for each frequency w ⁇ , w 2 ...w n .
- the digital beamformers 217, 218, 219 and 220 combine the signals from each satellite antenna element using complex coefficients in order to create multiple, directive receiving beams that discriminate signals from different directions.
- the exemplary system first performs channel splitting followed by direction splitting, it is to be understood that this is merely a preferred arrangement.
- the operations of channel splitting and direction splitting can be reversed by use of a wideband beam former followed by channel splitting on a per-beam basis.
- the advantage ofthe preferred arrangement is that it allows staggering of beam directions on different channel frequencies (or even timeslots on a single frequency) in order to obtain higher spectral efficiency by greater frequency reuse on the ground, as is more fully described in the incorporated references.
- the transmitter 102 When the transmitter 102 is located in one ofthe receiving beams so formed, its signal emerges enhanced by the appropriate one ofthe beam outputs ofthe beamformer assigned to that transmitter frequency. For example, when the transmitter 102 is transmitting on the first frequency, or channel w , its signal emerges from one beam output ofthe beamformer 217 and a different beam output from the beamformer 218, as the beam number is not necessarily the same for the first and second satellites 128 and 130 as they illuminate the earth slightly differently because of their separation in space. Thus, the complex number streams Airri A 2 , A 3 .... and B consume B 2 , B 3 ...
- the streams A,, A 2 , A 3 ... and B l3 B 2 , B 3 ... may be fed to a diversity demodulation and decoder to extract transmitted information, as described in U.S. patent application serial no. 08/305,904, entitled "Diversity-Oriented Channel Allocation In A Mobile Communications System", to Dent and Ewerbring and filed on December 12, 1994, the disclosure of which is hereby incorporated by reference.
- the sample streams A may be stored, or certain characteristics of them, such as correlations, may be computed and stored for off-line processing.
- the determination of a transmitter's location does not need to be done in real time.
- the position can be deduced later, as required, by processing the stored sample streams retrospectively.
- the invention can comprise using a number of fixed transmitters on the ground having accurately known positions to assist in determining necessary parameters such as satellite position at a given time. Signals from such transmitters may be transmitted periodically and, after emerging from appropriate channel/beam outputs ofthe ground processing, can be stored for processing offline.
- the elements ofthe invention comprise transmitting a signal from the transmitter to be localized and receiving the signal at two separated receiving sites, the signal being downconverted, sampled and digitized at each site using synchronized or otherwise known local oscillator frequency and sampling time references.
- the respective signal samples are then communicated to a common location, such as a network processor, where they are processed to yield an accurate range difference from the transmitter to the respective receiving sites.
- the two separated receiving sites comprise two orbiting satellite relay stations that relay the received signals to a ground station.
- the satellites change the frequency received from the transmitter to a different frequency called the feederlink, on which the relayed signals are transported to the ground station.
- synchronization means 124 shall comprise means to synchronize the local oscillators on board respective spacecraft so that any phase difference therebetween is preferably zero, or at least predictable.
- Such means can comprise transmitting a pilot signal from the ground station to both satellites, which may be precompensated for Doppler shift separately for each satellite in order to account for satellite movement.
Abstract
Description
Claims
Priority Applications (6)
Application Number | Priority Date | Filing Date | Title |
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BR9813458-2A BR9813458A (en) | 1997-12-10 | 1998-11-20 | Process and system for determining the position of a mobile transmitter |
DE69812161T DE69812161T2 (en) | 1997-12-10 | 1998-11-20 | METHOD AND SYSTEM FOR DETERMINING THE POSITION OF A MOBILE TRANSMITTER |
AU15944/99A AU747967B2 (en) | 1997-12-10 | 1998-11-20 | Method and system for determining position of a mobile transmitter |
JP2000524685A JP3383797B2 (en) | 1997-12-10 | 1998-11-20 | Method and system for determining the position of a mobile transmitter |
EP98960317A EP1038188B1 (en) | 1997-12-10 | 1998-11-20 | Method and system for determining position of a mobile transmitter |
CA002313485A CA2313485A1 (en) | 1997-12-10 | 1998-11-20 | Method and system for determining position of a mobile transmitter |
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US08/988,129 US6243587B1 (en) | 1997-12-10 | 1997-12-10 | Method and system for determining position of a mobile transmitter |
US08/988,129 | 1997-12-10 |
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- 1998-11-20 BR BR9813458-2A patent/BR9813458A/en not_active IP Right Cessation
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AU1594499A (en) | 1999-06-28 |
DE69812161D1 (en) | 2003-04-17 |
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CN1281553A (en) | 2001-01-24 |
BR9813458A (en) | 2000-10-10 |
DE69812161T2 (en) | 2003-12-04 |
EP1038188B1 (en) | 2003-03-12 |
JP3383797B2 (en) | 2003-03-04 |
CN1311242C (en) | 2007-04-18 |
TW396691B (en) | 2000-07-01 |
WO1999030181A1 (en) | 1999-06-17 |
AU747967B2 (en) | 2002-05-30 |
JP2002504664A (en) | 2002-02-12 |
CA2313485A1 (en) | 1999-06-17 |
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