Radio direction-finding using time of arrival measurements
First Claim
1. A method of determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the method comprises:
- receiving said signals at a plurality of mutually spaced wave-receiving elements,detecting the respective instantaneous amplitude of the signal received at each element,measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satisfactorily distinguished from noise and which is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation,determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of at least two of said plurality of pairs are sufficiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, andderiving a representation of the direction of incidence from the time difference(s) utilising the relationship
space="preserve" listing-type="equation">cos α
=cδ
t/dwhere α
is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements, δ
t is the time difference in respect of that pair of elements, and c is the free-space velocity of electromagnetic waves.
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Abstract
A method and a system for radio direction-finding by measuring the Time of Arrival (ToA) of the leading edge of signals from a distant source at two relatively closely spaced receiving elements. In order to give a good degree of immunity to multipath, the times at which the instantaneous detected amplitudes of the received signals first exceed a minimal threshold value such that received signals can be satisfactorily distinguished from noise is measured in such a manner that the measured time is not affected by multipath which involves more than a few meters additional path length for the indirect, delayed signal. A suitable timing circuit is disclosed.
By making ToA measurements on three coplanar, non-collinear receivers, directions of incidence in three dimensions can be determined.
A method and a system using both ToA and phase-difference measurements can provide the accuracy of interferometry but be simpler and cheaper.
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Citations
29 Claims
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1. A method of determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the method comprises:
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receiving said signals at a plurality of mutually spaced wave-receiving elements, detecting the respective instantaneous amplitude of the signal received at each element, measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satisfactorily distinguished from noise and which is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of at least two of said plurality of pairs are sufficiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and deriving a representation of the direction of incidence from the time difference(s) utilising the relationship
space="preserve" listing-type="equation">cos α
=cδ
t/dwhere α
is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements, δ
t is the time difference in respect of that pair of elements, and c is the free-space velocity of electromagnetic waves. - View Dependent Claims (2, 3, 4, 5, 6, 13, 14, 15, 16, 17, 18, 19)
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8. A system for determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the system comprises:
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plurality of mutually spaced wave-receiving elements, means for detecting the respective instantaneous amplitude of the signal received at each element, means for measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satisfactorily distinguished from noise and which is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, means for determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of at least two of said plurality of pairs are sufficiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and means for deriving a representation of the direction of incidence from the time difference(s) utilising the relationship
space="preserve" listing-type="equation">cos α
=cδ
t/dwhere α
is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements, δ
t is the time difference in respect of that pair of elements, and c is the free-space velocity of electromagnetic waves. - View Dependent Claims (7, 9, 10, 11, 12)
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- 20. A timing circuit comprising a clock pulse generator, a tapped delay device having a plurality of n mutually spaced taps, a latch coupled to the delay device for latching any signal on each of the n taps, and a decoding device coupled to the latch for producing a time representation signal from the signal(s) latched from the n taps, characterised in that an input signal to be timed is coupled to the input of the delay device, in that the clock pulse generator is normally operable to clock the latch, in that the circuit comprises inhibiting means responsive to the presence of a signal on at least one of the n taps when the latch is clocked to inhibit further clocking of the latch, and in that the decoding device is operable to produce the time representation signal corresponding to the interval between the time that said input signal reaches the tap nearest the input of the delay device and the preceding clock pulse.
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27. A method of determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the method comprises:
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receiving said signals at a plurality of mutually spaced wave-receiving elements; detecting the respective instantaneous amplitude of the signal received at each element; producing a first signal indicating when the detected amplitude at a first wave-receiving element exceeds a selected threshold value; producing a second signal indicating when the detected amplitude at a second wave-receiving element exceeds said selected threshold value; determining from at least the first and second signals at least the time difference between when the detected amplitudes at said first and second wave-receiving elements exceed said threshold value; and deriving a representation of the direction of incidence from the time difference utilizing the relationship cos α
=cδ
t/d where α
is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements, δ
t is the time difference in respect of that pair of elements, and c is the free-space velocity of electromagnetic waves.
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28. A method of determining the direction of incidence of electromagnetic wave signals from a distant source, the method comprising:
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receiving said signals at each of a plurality of mutually spaced wave-receiving elements; measuring the phase difference between the signals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substantially collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is ambiguous in the operating range of directions of incidence and the operating frequency range; determing the approximate direction of incidence of said signals from the times of arrival of corresponding wave signals received respectively at two of said plurality of elements, the line joining said two elements is parallel to and substantially coincident with the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the difference between the measured times corresponds to a range of angles of incidence whose magnitude is not greater than the magnitude of the range of angles of incidence corresponding to the unambiguous range of phase difference measurements on said one pair or said closest-spaced pair; and resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the approximate direction determined from the differences between the measured times.
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29. A system for determining the direction of incidence of electromagnetic wave signals from a distant source, comprising:
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a plurality of mutually spaced wave-receiving elements; means for measuring the phase difference between the signals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substantially collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is ambiguous in the operating range of directions of incidence and the operating frequency range; means for determining the approximate direction of incidence of said signals from the times of arrival the corresponding signals received respectively at two of said plurality of elements, the line joining said two elements is parallel to and substantially coincident with the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the difference between the measured times correspond to a range of angles of incidence whose magnitude is not greater than the magnitude of range of angles of incidence corresponding to the unambiguous range of phase difference measurement on said one pair or said closest-spaced pair; and means for resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the approximate directions determined from the difference between the measured times.
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Specification