Patent Grant
10107892
Embodiments of the invention are in the field of signal processing and relates to techniques for locating a source of a signal by a plurality of receivers.
Various techniques are known in the art for determining the location of a signal source by receiving and processing the signal emitted from the source by a plurality of signal receivers.
For example, one such technique, known as Frequency Difference of Arrival (FDOA), also known as Differential Doppler (DD), provides for estimating the location of a radio signal emitter based on observations/receipt of the signal from other points/receivers. In the FDOA technique the receivers must be in relative motion with respect to the signal emitter. For example, in geolocation applications, the relative motion is sometimes achieved by using airborne receivers on an aircraft. This relative motion between the receivers results in different Doppler shifts in the signals that are received by the receivers. The location of the signal source is then estimated based on the locations and velocities of the receivers and the Doppler shifts in the signals received by the receivers.
A known disadvantage of the FDOA technique is that large amounts of data must be moved between receivers to a central location (processing center) to carry out the cross-correlation that is necessary to estimate the Doppler shift. The accuracy of the location estimate is related to the signal-to-noise ratio at each receiver point, and the geometry and vector velocities of the receivers points. Also, it is difficult to measure frequency of pulse-type signals to the level of accuracy needed to carry out the FDOA technique, because the frequency resolution is equal to 1/T, where T is the pulse duration.
Time Difference of Arrival (TDOA, also known as multilateration) is another known technique for determining the location of a signal source by a plurality of signal receivers. TDOA takes advantage of the fact that a transmitted signal will arrive at different times to receivers at different locations. According to this technique, a number of spatially separated receivers capture the emitted signal, and the time differences of the arrival (TDOAs) of the signal to the receivers are determined. The emitter's location is calculated by using the TDOAs and the location of the receivers.
When using the TDOA technique, the receivers and the emitter may be stationary, however the signal emitted from the emitter should generally be modulated (e.g., pulsed) to thereby enable identifying and measuring the time of arrival (TOA) of the modulation pattern to the different receivers, and determining the DTOAs of the modulation pattern in between different receivers. To this end, the TDOA technique requires that the two or more geographically separated receivers will be time synchronized with each other, in order to allow precise measurement of the TDOAs providing for determining the location of the emitter.
There is a need in the art for a novel technique for locating signal sources by utilizing a plurality of signal receivers. Conventional techniques, such as FDOA and TDOA indicated above, for locating a source of a signal detected by the plurality of receivers, are associated with various deficiencies which limit their use in various scenarios.
TDOA generally requires that the signal from the signal source is modulated/pulsed so that the time of arrival (TOAs) of the modulation/pulse can be identified at each of the receivers. Additionally, TDOA also requires that the receivers be time-synchronized so that TOAs of the modulation/pulses to each of them will be synced allowing to correctly determine the differential time of arrival of the modulation/pulses between the receivers.
Regarding FDOA, this technique requires transmission of large volumes of data from a plurality of respectively moving receivers to a central location (e.g., processing center), and also involves highly intensive computation of such data. For example, the FDOA technique utilizes measurement of the relative Doppler shifts between the signals received by a plurality of receivers, and requires that the receivers are moving at known velocities. According to this technique, data indicative of the signal received and sampled by each of the receivers is communicated to a processing center, at which the signals from the plurality of receivers are correlated to determine the relative Doppler shifts between them. Then, based on the relative Doppler shifts determined in this way and the known velocities of the receivers, the location of the signal source is estimated. However, conventionally, in order to achieve good accuracy, the above procedure is performed a plurality of time frames (e.g., in a plurality of time frames which may extend for relatively long total time durations, in the order of seconds). To this end, in the FDOA technique, a high volume of data is communicated from the receivers to the processing center and computationally intensive calculations are performed at the control center to correlate the signals received from the different receivers at each time frame to determine their relative Doppler shifts.
Embodiments of the present invention provide a novel technique, referred to in the following as differential phase technique, for locating of a signal source by detecting the signal from the signal source by a plurality of receivers, and processing the received signals to determine the differential phase that is the difference between the accumulated phases of the signals that are received by the different receivers, and accumulated over a certain time duration/interval (which may be, for example, in the order of seconds). Then, based on information indicative of the respective positions of the receivers at two time points (e.g., at the beginning and the end of the time interval), and the differences between the accumulated phases of the signals received by different receivers (i.e. based on the differential phase), the location of the signal source is accurately determined.
More specifically, Embodiments of the present invention rely on the understanding that the accumulated phase in a signal received by a receiver during a certain interval is a function of the duration of the time interval and the change in the distance between the receiver and the signal source during that time interval. By utilizing the accumulated phases of several receivers, differences between the changes of the respective distances between the receivers and the signal source may be determined, allowing estimating the location of the signal source.
As will be further described below, the technique according to one or more embodiments of the invention may be performed in conjunction with the TDOA technique to reduce the number of receivers required, and/or improve the accuracy of the location of the signal source, and/or resolve possible ambiguities in the location of the signal source.
According to the technique according to one or more embodiments of the invention, instead of monitoring the relative Doppler shifts between the signals received by different moving receivers as done in FDOA, and instead of, or in addition to, determining the different times of arrival of a pulse to the receivers as done in TDOA, the technique according to one or more embodiments of the invention monitors the accumulated phases of the signals received at each of the receivers.
As will be further described below, unwrapping/monitoring the accumulated phases of the received signals may be performed locally at each of the receivers, or at the processing center, or performed partially at the receivers and partially at the processing center.
For instance, determining the inter-pulse accumulated phase (which is the accumulated phase of each pulse of the signal that is received by a receiver) may be performed locally at the receiving receiver and may involve simple and lightweight inter-pulse unwrapping processing, which can be incorporated by suitable hardware/software at the receivers. It should be noted that communicating the inter-pulse accumulated phases that are accumulated at a certain time interval at the receivers involves communication of only small amounts of data (e.g., as compared to techniques such as FDOA where data indicative of practically the entire number of received signals is transmitted), thus does not require large data communication bandwidths and/or time. Then, at the processing center, the intra-pulse accumulated phase (which is the accumulated phase in between two or more successive pulses of the signal that were received by the receiver) may be determined, unambiguously, at the processing center, by carrying out an intra-pulse unwrapping processing in conjunction with the inter-pulse accumulated phases that are obtained from the different receivers.
Optionally, in cases where the signal source is a CW emitter, or otherwise not a pulsed emitter, the phases of the signal received from the emitter may be accumulated for extended time durations/intervals (in the order of seconds) so as to provide desired accuracy of location of the signal source. This may be done at the receivers, at the processing center, or by a combination of both, by using the same inter-pulse unwrapping processing indicated above. To this end, accumulating the phases for time intervals of several seconds may provide accuracy equivalent to, or better than, that achievable by carrying out a plurality of sessions for locating the signal source based on the FDOA technique, yet with much reduced signal processing and data communication requirements as compared to the FDOA technique.
In this regard, it should be noted that according to the technique of one or more embodiments of the present invention, the time intervals, during which the phases are accumulated at the different receivers, are not necessarily synchronized and may pertain to different times. This is advantageous over the TDOA technique which requires that the receivers are time synchronized. In this regard, it should be noted that the time intervals, during which the phases of the signals received by the different receivers are accumulated, should be of equal duration in order to resolve possible ambiguities in the estimation of the accumulated phases (e.g., in the intra-pulse accumulated phases). More specifically, this refers to cancelling out the term 2π*f*dt in the accumulated phases, where f is the signal's frequency and dt is the time interval.
According to the differential phase technique of one or more embodiments of the invention, after the accumulated phases at the different receivers are determined/estimated (e.g., at the receivers and/or in the processing center), they are then further processed by the processing center to determine the location of the signal source. More specifically, the location of the signal source is determined based on the difference between the phases accumulated at the different receivers and the positions of the receivers at the initial and final times of the time intervals during which phase accumulation (unwrapping) was performed. The processing for determining the location of the signal source involves a relatively lightweight computation, which does not require computationally intensive and data consuming processing (as opposed to other known in the art techniques such as FDOA which involves correlating the signals received by the plurality of receivers).
It should also be noted that the technique of one or more embodiments of the present invention obviates a need to monitor the trajectories of the signal receivers and their velocities along the trajectory, as would be required by techniques such as FDOA and that only data indicative of the positions of the receivers at two time points (e.g., at the beginning and the end of the time interval during which the phase is accumulated) is required to determine the location of the signal source.
Thus according to a broad aspect of one or more embodiments of the present invention there is provided a method for locating a signal source. The method includes:
According to another broad aspect of one or more embodiments of the present invention there is provided a system for locating a signal source, emitting a signal S. The system includes:
The source location processor determines a location of the signal source, such that the determined location satisfies that the differences between the changes, Δdn and Δdm, in the distances, dn and dm, from that location to the positions of the respective receivers during the respective time intervals Δtn and Δtm correspond to the distance differences {ΔDmn} that are associated with (e.g., computed from) the differential phase differences {ΔΔθmn}.
According to some embodiments of the present invention the first processing for determining said accumulated phase Δθn includes determining a difference between a phase θn(t0+Δtn) of the signal Sn at an end of the time interval Δtn and a phase θn(t0) of the signal at a beginning of said time interval Δtn. This operation may be performed for example by the phase accumulation modules of the system of one or more embodiments of the invention. To this end in certain implementations of the invention the phase accumulation modules apply first processing to unwrap the phase of said signal Sn by carrying out the following:
(i) dividing/segmenting the signal Sn received in the time interval Δtn into a plurality of time slots of durations shorter than the period T;
(ii) applying Fourier transform to signal portions of the signal Sn in each of the time slots to respectively determine signal phases of the signal portions modulus 2π;
(iii) processing the signal phases of the time slots to identify abrupt change of a signal phase of a successive time slot with respect to a signal phase of time slot preceding it, wherein a magnitude of the abrupt change satisfies the following: it is substantially larger than a noise level associated with said receiver Rcn, and it is larger than π;
(iv) adding multiples of 2π to the signal phase for the successive time slot and to signal phases of each of the time slots succeeding it; and
(v) repeating (iii) and (iv) for each pair of successive time slots.
According to some embodiments of the invention, the method and system are adapted to locate a signal source emitting signal Sn that includes or is constituted by continuous-wave (CW) signal component(s).
For example, the signal source may be a stationary signal source emitting a CW signal, and wherein the technique one or more embodiments of the invention enables determining the location of such signal source by utilizing a number of receivers that is greater than the number of spatial dimensions, with respect to which the location of the signal source should be determined. In such cases the one or more phase accumulation modules of the system are adapted to obtain information indicative of the signals {Sn} received from the signal source by a number of receivers {Rcn} that is at least the number of the spatial dimensions with respect to which the location of the signal source should be determined, plus one.
According to some embodiments, the method and system are adapted to locate a signal source emitting a modulated signal Sn (i.e., the signal Sn includes one or more signal sections modulated by at least one modulation pattern, such as frequency modulation, and/or one or more signal sections in the form of pulses). The technique of according to one or more embodiments of the invention includes identifying receipt timings by the receivers of the modulated signal sections and processing the receipt timings to remove/avoid ambiguities in determination of said differential phase differences ΔΔθmn. To this end the system according to one or more embodiments of the invention may include a time of arrival (TOA) module that is adapted to identify receipt timings of similarly modulated sections of the received signal.
In certain embodiments of the invention, specifically when locating a signal source of a modulated signal Sn is sought, the technique/method further includes processing the receipt timings of the modulated signal sections at the receivers (e.g., utilizing a multilateration module/processor) to determine differential time of arrival (DTOA) data which is indicative of the different times of arrival of the similarly modulated signal sections to the different receivers. Accordingly the DTOA data is indicative of the location of the signal source. To this end, according to some embodiments of the present invention, both the differential phase data ΔΔθmn and the DTOA data are used to determine the location of the signal source with reduced number of required receivers. In this case the minimal required number of receivers matches the number of spatial dimensions, with respect to which the location of the signal source should be determined.
According to certain embodiments of the present invention the first processing includes identifying similarly modulated sections in the signals Sn and Sm received by a pair Rcm, Rcn of receivers; and selecting the timings of the time intervals Δtn, Δtm of the pair Rcm, Rcn, during which to determine the accumulated phases Δθn and Δθm, in accordance with receipt timings of these similarly modulated sections.
For instance, in some cases, the signal Sn from the signal source is frequency modulated. The phase θn(t) of the signal Sn is therefore a non-linear function of time. Accordingly, in certain embodiments of the invention, the technique includes identifying matching modulation patterns in the signals Sn and Sm received by the different receivers, and determining a common reference point in the signals Sn and Sm. Then the timings of the time intervals Δtn, Δtm of the corresponding receivers Rcn, Rcm are set relative to the common reference point in their respective signals Sn and Sm. In cases where the signal transmitted from the signal source includes multiple repetitions of the similar modulation pattern, the common reference point is determined relative to same or different sections of the signal, which are modulated by the repeated modulation pattern that is received by the receivers Rcn, Rcm. This allows to determine the differential phase ΔΔθmn between the receivers while obviating a need to perform time synchronization in between the receivers Rcn, Rcm.
Yet in some embodiments of the present invention, the technique/method includes performing time synchronization between the receivers Rcn, Rcm and utilizing the time synchronization to identify the similarly modulated sections. For instance the time synchronization may be performed by processing data indicative of the signals Sn and Sm received by the receivers to determine a time delay associated with a best-fit between them and synchronizing the timings of said Rcn, Rcm based on said time delay.
As indicated above, in some embodiments a signal source, being a source of a pulsed signal, is considered. In such embodiments of the invention, the method/system is adapted to identify receipt timings of pulses of the signal by the different receivers and further utilizing differential time of arrival DTOA to determine the location of the signal source, thereby reducing the number of receivers required for identifying the location of the signal source.
Thus, the embodiments of the present invention provide a novel system and method for locating a signal source. The technique according to one or more embodiments of the invention is further described in more detail in the detailed description section below with references to the drawings illustrating various embodiments of the methods and systems of the invention.
In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
Reference is made together to
It should be noted that herein and in the following, except where explicitly denoted otherwise, the subscript indices (e.g., n, m and/or numerical indices) denote the index of the receivers Rc1-Rcn their respective signals, and processing parameters/properties associated therewith. Also, pairs of such subscript indices (e.g., m,n or mn) denote processing parameters/properties associated with the pair m,n of such receivers. Also it should be understood that notation of the curly brackets enclosing a certain element/parameter denoted with the sub-index (e.g., {Rcn}) are used herein to indicate a group/collection of such elements/parameters (e.g., the notation may designate a group including several or all of the receivers Rc1-Rcn).
According to one or more embodiments of the present invention at least some of the plurality of receivers Rc1-Rcn are carried by separate/different vehicular platforms (e.g., terrestrial- and/or airborne- and/or marine- and/or space-vehicles), and at least some of which are in motion during the operation of the system 200 for locating the signal source Src. To this end, the plurality of receivers Rc1-Rcn include at least two receivers and typically include three or more receivers capable of detecting the signal S emitted from a signal source Src, which is to be located. The number of receivers connected to (or otherwise in communication with) the system depends on the dimensionality of the space within which location of the signal source Src is desired (e.g., on whether determination of the location is required with respect to two or three dimensions). This is discussed in more detail below.
The signal processing system 215 includes phase accumulation system 220 configured and operable for applying a first processing to each of the signals S1-Sn received by the receivers Rc1-Rcn to determine for each signal, e.g., Sn, an accumulated phase value Δθn that corresponds to the phase of that signal S accumulated during the time interval Δtn by the respective receiver Rcn. That is the accumulated phase value Δθn corresponds to the accumulated unfolded (e.g., un-wrapped/non-cyclic) change of the phase of the signal Sn during the time interval Δtn, beginning at an initial time tinitn and ending at final time tfinn=tinitn+|Δtn| (where here |Δtn| indicates the duration of the time interval Δtn). The phase accumulation system 220 therefore determines the accumulated phases {Δθn} of the respective signals {Sn} from the receivers {Rcn} during their time intervals |Δtn|.
The phase accumulation system 220 includes one or more inter-pulse phase accumulator(s) 222 (and optionally also 222.2-222.n depicted in the figure) serving for determination (inter-pulse unwrapping) of the accumulated phase of continuous sections of the signal S received by the receivers (e.g., the accumulated phase during continuous sections such as pulses of the signal S or CW portions of the signal S). Optionally, for example for cases where the signal is pulsed and/or is not continuous, the phase accumulation system 220 also includes an intra-pulse phase accumulator 224 configured and operable for un-wrapping the phase of the signal received by each of the receivers at discontinued periods of the signal (e.g., at time intervals between pulses/CW sections of the signal and determining the intra-pulse accumulated phases of the signals received by each of the receivers. Then, by combining/adding the inter-pulse and the intra-pulse accumulated phases determined for each receiver, the accumulated phases Δθn of each of the receivers Rcn during the time interval of the period Δt is determined/estimated for each of the receivers. As indicated above, ambiguity of 2π*f*Δt may still exist in the accumulated phases Δθn determined this way, but since the time intervals {Δtn} are of equal duration for all the receivers, then this ambiguity is resolved in the following when calculating the differential phases between the receivers. It should be noted that inter-pulse phase accumulator(s) 222 may include a plurality of modules (e.g., 222 and 222.2-222.n) residing respectively near/at each receiver, such that the inter-pulse phase unwrapping is performed locally at each receiver position, or alternatively one (possibly more) inter-pulse phase accumulator 222 may reside at the processing center and may perform inter-pulse phase unwrapping there. In the former case, lower data rates are required for communicating data from the receivers to the processing center, since only the accumulated phases of each pulse need to be communicated, and not information about the entire signal received by the receivers, as in the latter case.
The signal processing system 215 also includes a differential phase processor 230 generally located at the processing center. The differential phase processor is configured and operable to apply second processing to the accumulated phases {Δθn} to determine differential phase differences ΔΔθmn=(Δθm−Δθn) between the accumulated phases, Δθm and Δθn, of the signals, Sm and Sn, received by two or more pairs {m,n} of the receivers, Rcm and Rcn. As indicated above, the differential phase differences ΔΔθmn may still include certain ambiguity being in the order of 2π*f*Δt where f is the signal frequency, yet this ambiguity is resolved if Δt is equal for the receivers Rcm and Rcn. After resolving this ambiguity, the differential phase difference ΔΔθmn between a pair of the receivers Rcm and Rcn, is indicative of a distance difference ΔDmn between the changes, Δdn and Δdm in the distances, dn and dm of the respective receivers Rcm and Rcn from the signal source Src during the time intervals Δtn and Δtm respectively. Accordingly, the signal processing system 215 also includes a source location processor 250 that obtains the differential phase differences {ΔΔθmn} of the two or more pairs {m,n} of receivers and is configured and operable to apply a third processing to the differential phase differences {ΔΔθmn} based on position data PD indicative of positions {Rn} of receivers and of changes {ΔRn} in their positions {Rn} during the respective time intervals {Δtn}, and thereby determine the location RSrc of the signal source Src. In this regard, it should be noted that the location RSrc of the signal source that is determined in this way, satisfies that the differences ΔDmn≡Δdm−Δdn between the changes Δdm and Δdn in the distances dm and dn from that location RSrc to the positions of the respective receivers Rm and Rn during the respective time intervals Δtm and Δtn and matches the distance differences {ΔDmn} that are computed based on the differential phase differences {ΔΔθmn}.
As will be readily appreciated by those versed in the art, there are various possible techniques for implementing the modules of the signal processing system 215. For instance, some of the modules and/or sub-modules of signal processing system 215 can be implemented by utilizing analogue signal processing means/circuits, and/or utilizing digital/computerized processing systems and/or by a combination of analogue and digital signal processing means/circuits. Components of the system which are implemented digitally may include or be associated with one or more digital processors, such as CPUs and/or DSPs for processing signals received, and with suitable samplers and/or analogue to digital converters (ADCs) for sampling the signals from the receivers and converting them to digital representation and/or possibly also with digital to analogue converters for converting digital signals to analogue forms in case the signals are processed by combination of analogue and digital means. As will be appreciated by those versed in the art, in case the system is implemented with analogue means, analogue circuits for implementing the operations described by method 100 may for example include a proper arrangement of signal amplifiers and signal frequency filters (e.g., band-pass filters) applying suitable amplification and/or filtering to the received signal to obtain desired frequency band thereof, signal mixers (e.g., homodyne/heterodyne) and possibly also local oscillators arranged to allow extraction of the phase of the signal, integrators and/or comparators configured and operable for generating signal indicative of the accumulated phase, and/or of the differential phase between the receivers. To this end, although the signal processing system 215 may be implemented by analogue means, in some embodiments it may be implemented in a more versatile manner by utilizing digital processing techniques.
For instance, in certain specific embodiments of the present invention, the receivers {Rcn}, which may be for example antenna modules, generate an analogue signal corresponding to the signal S respectively received thereby from the signal source Src. The receivers {Rcn} are associated with respective DACs converting analogue signals from the receivers into digital representations {Sn}, which are then fed to the signal processing system 215.
As indicated above, the receivers {Rcn} are generally located remotely from one another and move with respect to one another. However, the process of locating the signal source Src involves processing together of certain properties of the signals from multiple of the receivers {Rcn} (see the second and third processing indicated above and below). To this end, the vehicular platforms carrying the receivers, or some of them, may also carry respective data communication modules DTCs for communicating data indicative of certain properties of the signals {Sn} to the central processing center. The latter may also include a data communication module DTC for receiving the communicated data. As indicated above, in certain implementations the signal processing system 215 is implemented as a distributed system. For example certain stages of the processing, such as the first processing indicated above, is applied to each of the signals {Sn} received by the receivers {Rcn}, by utilizing suitable processing modules (e.g., 222) of the processing system 215 located adjacent to the respective receivers {Rcn} (e.g., at their vehicular platforms). For example, as shown in
In some embodiments of the present invention the signal processing system 215 also includes, or is in communication with, receivers positioning module 240 operable to provide position data PD indicative of positions of the receivers {Rcn} during the respective time intervals {Δtn}. For each receiver Rcn the position data PD may indicate its position (vector) Rn at two time points during the respective time interval, Rn(tninit) and Rn(tnfin), at least a first/initial and second/final time points within the time interval Δtn. Alternatively, equivalently, position data PD may be indicative of the position Rn of the receiver Rcn at one of the time points (e.g., Rn(tninit) or Rn(tnfin)) and the change in that position ΔRn during the time interval Δtn between tninit and tnfin.
As indicated above and shown in
Further details on the technique of one or more embodiments of the present invention for determining the location RSrc of the signal source Src will now be described with reference to method 100 of
In operation 105 of method 100 a number of at least two signal receivers {Rcn} capable of detecting the signal S emitted from the signal source Src that should be located, are provided. The receivers {Rcn} are mounted on respective moving platforms, which carry them along respective paths (curved or straight paths) PTH1-PTHn.
In
The signal S emitted from the source during the movement of the receivers is illustrated schematically in
Operation 110 includes providing data indicative of the signals S1, Sm and Sn (generally {Sn}), which are received and possibly sampled by the receivers Rc1, Rcm and Rcn (generally referred to as {Rcn}) during their movement along their respective paths (it is indicated that one of the receivers may also be stationary). The provided data of the signals S1, Sm and Sn may include only the part of those signals received by those receivers {Rcn} during respective time intervals {Δtn} during which the receivers move in between respective first positions {Rn(init))} to second positions {Rn(fin))} along their respective paths {PTHn}. For example in
It is noted, and also depicted in
As indicated above, operation 120 includes applying first processing to each signal Sn of the signals {Sn} to determine the phase Δθn that is accumulated during its respective time interval Δtn. As shown in
(1) change in the phase of the signal during the time duration (e.g., Δt) of the time interval Δtn; and
(2) the change in position Rn of the respective receiver Rcn in between first/initial and second/final times tn(init) and tn(fin) in the time interval Δtn, and more specifically the change Δdn of its distance dn from the signal source Src during that time interval, as depicted in the figure. More specifically the change Δdn of the distance dn of the nth receiver is given by:
Δdn=dn(tnfin)−dn(tninit)=|Rn(tnfin)−RSrc(tnfin)|−|Rn(tninit)−Src(tninit)| Eq. 1
To this end the accumulated change Δθn in the phase of the signal Sn received during the time interval Δtn by receiver Rcn is given by:
Δθn=2πfΔt+2πtΔdn/λ Eq. 2
where f is the frequency (e.g., carrier frequency) of the signal S emitted from the signal source Src, Δt is the duration of the time interval Δtn, λ is the wavelength of the signal S given by λ≡C/f (where C being the speed of light), and Δdn is the change in the distance dn of the nth receiver from the source Src during the time interval as given by Eq. 1.
Thus, in operation 120 of method 100 of
By inverting Eq. 2, the change in the distance Δdn to the source Src can be expressed in terms of the accumulated phase Δθn as follows: Δdn=(C/2πf)Δθn−CΔt. By combining this with Eq. 1 above, a relation between the accumulated phase Δθn and the positions Rn and RSrc of the receiver Rcn and the signal source Src can be obtained as follows:
|Rn(tninit+Δt)−RSrc(tninit+Δt)|−|Rn(tninit)−RSrc(tninit)|=(C/2πf)Δθn−CΔt Eq. 3
where the time interval Δt during which the phase is accumulated is given by:
Δtn=[tninit,tnfin]=[tninit,tninit+Δt]
It is noted that the location of the signal source Src cannot be generally resolved from Eq. 3 directly. This is because of the large ambiguity which may be included in the value of the accumulated phase Δθn. For instance, considering using time intervals of duration Δt=1 millisecond for the duration for locating a signal source of a signal S having frequency f=10 GHz, the ambiguity in the accumulated phase Δθn would be in that case very large, in the order of 2π*f*Δt˜2π*107 radians. With such large ambiguity it is practically impossible to resolve the location of the signal source by using Eq. 3 directly. However this is solved according to the technique of an embodiment of the present invention by measuring the accumulated phases {Δθn}, which are accumulated during equal time durations Δt at different receivers {Rcn}. This is followed, as described in more detail below, by calculating the differences ΔΔθmn (referred to herein as differential phases) between the accumulated phases Δθm, Δθn of different pairs {m,n} of receivers (see Eq. 4 below) and determining the location of the signal source Src from these differential phases. In this way the ambiguity of the accumulated phases {Δθn} is cancelled out/reduced significantly when calculating the differential phases {ΔΔθmn}, and the location of the signal source Src can be accurately determined.
Thus, operation 130 includes obtaining the accumulated phases {Δθn} (e.g., {Δθ1, Δθm and Δθn}) determined for the signals {Sn} of the plurality of receivers and applying a second processing to determine differential phase ΔΔθmn between the accumulated phases, Δθm and Δθn, of the signals, Sm and Sn, received by several pairs {m,n} of the receivers Rcm and Rcn. The number of different pairs {m,n} of the receivers for which the differential phase ΔΔθmn should be determined depends on the dimensionality of the space within which location of the signal source Src is desired and on whether only the location of the source Src should be determined. For each such pairs {m,n} of receivers, the differential phase ΔΔθmn is determined as follows:
ΔΔθmn=(Δθm−Δθn) Eq. 4
And more specifically by substituting Eq. 2 into Eq. 4:
ΔΔθmn=2π/λ(ΔDm−Δdn)+2πf·(|Δtm|−|Δtn├). Eq. 5
As indicated above, the time intervals Δtm and Δtn of each pair {m,n} of the receivers for which the differential phase is calculated are of equal durations, namely |Δtm|=|Δtn|≡Δt. Thus ambiguities in the accumulated phases Δθm, Δθn are resolved. Accordingly, the second term in Eq. 5 nullifies and the following relation is obtained for each pair {m,n} of receivers for which the differential phase is computed in 130:
ΔΔθmn=2π/λ(Δdm−Δdn). Eq. 6
Substituting Eq. 1, the differential phase ΔΔθmn is obtained in terms of the positions of the pair {m,n} of receivers, Rcm and Rcn, and the position of the source Src at initial and final times tminit, tmfin and tninit, tnfin in the respective time intervals, Δtm and Δtn.
ΔΔθmn=2π/λ[(|Rm(tmfin)−RSrc|−|Rm(tminit)−RSrc|)−(|Rn(tnfin)−RSrc|−|Rn(tnnut)−RSrc|)] Eq. 7
Therefore method 100 includes operation 140 in which the position data PD is provided (e.g., obtained from positioning modules 240, associated with the receivers, and/or monitoring their respective positions). As indicated above, the position data PD includes data indicative of position (vector) of each receiver Rcn in at least two, initial and final, time points, tninit and tnfin, within the respective time interval Δtn of the receiver; namely providing Rn(tninit) and Rn(tnfin). This is equivalently indicative of the position Rn of the receiver Rcn at one of the time points (e.g., Rn(tninit)) and the change in that position ΔRn during the time interval Δtn.
In view of the above, the only remaining unknown variable left in Eq. 7 are those related to the location of the signal source: RSrc. Thus in operation 250 a third processing is applied to determine the location RSrc of the signal source based on the differential phase ΔΔθmn obtained for different pairs {m,n} of the receivers in operation 130. This is achieved by solving a set of Eq. 7 above for different pairs {m,n} of the receivers {Rcn} while utilizing the positions {Rn} of the receivers {Rcn} at their respective time intervals as obtained in operation 140 and also utilizing the differential phases for different pairs of receivers ΔΔθmn obtained in operation 130.
It should be noted that in certain embodiments of the system and method of the present invention the signal source Src is assumed and/or is known to be stationary. In this case, the velocity VSrc of the signal source needs not to be determined and/or it is assumed zero VSrc=0. In such cases the only unknown variable that needs to be determined by the set of Eq. 7 is the stationary vector location of the signal source Src. It is noted that the number M of independent pairs of receivers that can be matched given that there are N receivers is M=N−1. Accordingly, for a given number of N receivers {Rcn}, a set of up to M linearly independent equations such as Eq. 7 may be processed in operation 150 to determine the location RSrc of the signal source Src. As the location RSrc is a vector, the number of unknown variables in Eq. 7 actually depends on the dimensionality D of the space within which it should be determined. Thus considering the space dimensionality D, the number V of unknown variables is therefore in this case: V=D. Therefore V is the minimal number of independent pairs of receivers for which differential phase should be computed in 130 in order to solve the location of the signal source. The maximal number M of linearly independent equations, similar to Eq. 7, that can be obtained when utilizing N receivers, is M=N−1. Therefore in case of the stationary signal source, the number N of required receivers should satisfy M≥V, that is:
N>D. Eq. 8
Thus, for two or three space dimensions D=2 or D=3, only a number of N=3 receivers or N=4 receivers are required respectively. Possibly additional receivers can be used to locate the signal source with improved accuracy.
In view of the above, operation 150 applies a third processing to determine the location RSrc, of the signal source Src, based on the position data PD of the receivers {Rcn} and the differential phase differences {ΔΔθn} determined for two or more pairs {m,n} of receivers. More specifically, according to some embodiments of the present invention, in 150 a set of at least V linearly independent equations similar to Eq. 7 obtained for at least V independent pairs {m,n} of the receivers {Rcn} are processed/computed and solved to determine the location RSrc, of the signal source Src.
Optionally, in certain embodiments of the present invention, method 100 further includes operation 160 which inter-alia provides for further reducing the number N of receivers required for locating the signal source Src. This is achieved by determining the location RSrc of the signal source Src using multilateration (otherwise also known and further referred to in the following as differential time of arrival (DTOA)), in combination with the differential phase technique described above with reference to operations 110 to 150 of method 100. Indeed as will be appreciated by those versed in the art, DTOA techniques utilize several receivers (e.g., {Rcn}) at different known locations (e.g., {Rn}) to estimate the location RSrc of a signal source Src based on the differential time of arrival at which a signal S emitted from the signals source Src reaches the different receivers, even without having information regarding the time at which the signal S was transmitted/emitted from the signal source Src.
However, to achieve that, the signal S should be modulated by a certain modulation pattern, such as a pulse. This is illustrated for example in
Certain embodiments of the system 200 of the present invention, include multilateration processor 260, such as that illustrated for example in
In this connection, it should be understood that in case the signal S from the signal source is a pulsed signal which includes one or more pulses, indexed j, then the TOAs τ1j . . . τnj of the arrival of any pulse PLj (indexed j) to the n different receivers {Rcn} may be recorded, by identifying the rise time of the pulse at the receiving path/channel of the receivers. In case the signal source emits a modulated CW signal S (i.e. not pulsed), the TOAs {τ1 . . . τn} of the signal S to the n different receivers {Rcn} may be determined by cross correlating the signals {S1 . . . Sn} received by different receivers to determine/measure the relative time difference between their reception times. It is understood that in the former case, where the signal is pulsed, the TOAs may be optionally determined by a plurality of modules 264.1-264.n which may be optionally included/located at/near the receivers Rc1-Rcn, and adapted to process the signals S1-Sn respectively received thereby to identify their profile (rise and/or fall times) and thereby determine the times of arrival τ1j . . . τnj of pulse j by the different receivers. However, when the signal is a modulated CW signal, then cross correlation should be performed, and therefore the data indicative of the received signals S1-Sn is communicated to the processing center. Such cross correlation can be performed by a DTOA module, which resides at the processing center and is suitably configured and operable to perform the above described cross-correlation operations. It is understood that the cross correlation can also be used to determine the DTOA of a pulsed signal.
The multilateration processor 260 may also include a differential time of arrival (DTOA) analyzer 268 that is adapted to receive the times of arrival τ1j . . . τnj of the one or more pulses j and utilize these to determine one or more relations indicative of the location RSrc of the signal source Src relative to the locations {Rn} of the receivers {Rcn}. To this end, in some embodiments of the present invention, the TOA module(s) 264 may be a plurality of distributed modules arranged/placed at/adjacent to the respective receivers, and the DTOA analyzer 268 may be configured at a central processing center and adapted to obtain data indicative of the times of arrival τ1j . . . τnj from the TOA module(s) 264 and process this data to determine a relation between the locations of the receivers and the location of the source. Such configuration may reduce the required data transmission bandwidth between the receivers and the central processing center as only the times of arrival τ1j . . . τnj need to be transmitted between them in this case (and not the complete information on the received signals as may be the case when the TOA module(s) 264 are centralized in the central processing center).
The use of the DTOA technique in operation 160 performed by the system 200 and method 100 is exemplified in more detail in the following with reference to the flow chart of
Operations 162 and 164 are typically performed by the TOA module(s) 264 (by modules 264.1-264.n) located adjacent to the receivers {Rcn}. In 162 a modulated signal S from the signal source Src including one or more modulation patterns (e.g., pulses) {PLj} is received by the receivers {Rcn}. In 164 the corresponding signals {Sn} respectively obtained by the receivers {Rcn} are processed (e.g., sampled and analyzed) to identify at least one modulation-pattern/pulse PLj therein (such identification can be performed by cross-correlation with signals received by other receivers and/or by identifying predetermined modulation patterns, such as the rise/fall time of a pulse). Then, the times of arrival (TOAs) {τjn} of the at least one pulse/modulation pattern PLj at two or more of the receivers {Rcn} are recorded. As indicated above, in some implementations (e.g., when the signal is pulsed) the TOAs {τjn} may be determined by TOA modules 264 near/at the receivers {Rcn} and then be communicated from the respective receivers {Rcn} to the DTOA analyzer 268 which may reside at a central processing center. Alternatively, e.g., in the case of modulated CW signal S, the DTOAs {Δτjmn} may be determined by DTOA module residing at a central processing center.
In operation 165, sync data indicative of time synchronization of the receivers may be obtained and used to process and synchronize the times of arrival {τn} obtained by the receivers {Rcn}. The sync data may be data indicative of a time differences/lags between the clocks of the different receivers and a certain reference clock. The sync data may be obtained by any suitable known in the art time sync technique. Accordingly the times of arrival {τn} may be synced by adding thereto the corresponding time lag.
In 166, the differential time of arrival (TDOA) Δtjmn between the times of arrival of at least one pulse PLj to the one or more pairs {m,n} of the receivers is determined/computed. The DTOA may be computed in operation 166 (e.g., by DTOA analyzer 268) as follows:
Δτjmn=τmj−τnj Eq. 9
As described in the following, according to various embodiments of the present invention, the DTOA data, including data indicative of the differential times of arrival Δτjmn of one or more pulses to different receivers, may be further used to carry out one or both of the optional operations 168 and 169 to provide one or more of the following:
(i) optional operation 168 carrying out the third processing (operation 150 of method 100) to determine the location of the signal source Src while utilizing the differential times of arrival Δτjmn=τjm−τjn of the pulse PLj to the receivers Rcm and Rcn in addition to the differential phases ΔΔθmn between pairs of the receivers, Rcm and Rcn. This provides for reducing the number of receivers required for determining the location of the signal source, and/or for improving the accuracy of locating the signal source. As described more specifically below, in this way, two or three receivers may suffice to determine the location RSrc of the signal source Src in two or three dimensions; and/or
(ii) optional operation 169—utilizing the differential times of arrival Δτimn, Δτjmn of two or more pulses PLi and PLj to resolve ambiguities, which may arise in determination of the differential phases ΔΔθmn in the second processing (operation 130 of method 100). In this way, the location of the signal source may be determined unambiguously and with improved reliability.
Turning now to optional operation 168, a relation between the DTOA Δτjmn and the locations Rm and Rn of the pair {n,m} of receivers Rcm and Rcn and the location RSrc of the signal source Src may be expressed as follows:
Δτjmn=(1/C)[|Rm(τj)−RSrc(τj)|−|Rn(τj)−RSrc(τj)|] Eq. 10
where here changes in the locations Rm, Rn of the receivers during the propagation time of the pulse S in between the signal source RSrc and the respective receivers is considered to be negligible (this assumption is plausible since the receivers are moving with velocities much lower than the speed of light C of the signal). Accordingly, the time τj in Eq. 10 indicates the nominal time of the emission/receipt of the pulse j.
In 168 the relation expressed for example in Eq. 10 is used to determine the location RSrc of the signal source Src. In this regard it should be noted that operation 168 may be actually be included and/or performed in the scope of the third processing described above (e.g., in the scope of operation 150 described above) to determine the location RSrc of the signal source Src based on the differential phase ΔΔθmn relation expressed in Eq. 7 in conjunction with the DTOA relation Δτjmn expressed in Eq. 10 for several pairs of the receivers {Rcn}. In some embodiments of the present invention, which concern locating signal sources emitting modulated/pulsed signals, the location RSrc of the signal source Src may be determined based on both the differential phase ΔΔθmn relation expressed in Eq. 7 in conjunction with the DTOA relation Δτjmn expressed in Eq. 10, thus further reducing the number of receivers that are required for determining the location of the signal source. For a given number N of receivers, the number of independent pairs is as indicated above N−1 and therefore the maximal number M of linearly independent equations, similar to Eq. 7 and Eq. 10 is M=N 1. Considering space dimensionality D, the number V of unknown variables is associated with determination of the vector location RSrc of the signal source: V=D. Thus, solving a set of a number of at least 2M≥V of linearly independent equations is required to obtain the location of the source. Accordingly, the required number N of receivers should satisfy:
2N>D+1 Eq. 11
That is, a number of N=2 or N=3 receivers may suffice for determining the vector location RSrc of the signal source Src in two or three dimensions (D=2 or D=3) respectively.
Turning now to optional operation 169, it should be noted, and also indicated above, that the differential phase difference ΔΔθmn, which is computed in operation 130, is indicative of a difference Δdn−Δdm between the changes Δdn and Δdm occurring during a certain time interval Δt, in the distances, dn, and dm of the respective receivers Rcm and Rcn from the signal source Src. In some cases, e.g., when the signal source emits a CW signal, the differential phase difference ΔΔθmn that is accumulated over a relatively long time interval Δt, for example in the order of one second, may be computed in order to facilitate highly accurate and reliable determination of the location of the signal source with improved SNR. In other cases, for instance when the signal is pulsed, it may be sufficient to calculate the differential phase over shorter durations in the order of a few milliseconds.
As will be further explained in more detail below, in cases where the signal S is modulated (e.g., S may be a pulsed signal), then operation 120 for computing the accumulated phases Δθm and Δθn at the receivers Rcm and Rcn may yield ambiguous results when the PRI is large and accordingly the differential phase ΔΔθmn computed in 130 may become ambiguous. This might impair the accuracy and reliability of the location of the signal source determined in such scenarios.
However, according to some embodiments of the present invention, operation 169 above is performed to resolve such possible ambiguities and utilize the DTOA data determined in 166 to disambiguate the differential phase ΔΔθmn=Δθm−Δθn between the accumulated phases Δθm and Δθn accumulated by the pairs {n,m} of the receivers Rcm and Rcn, thereby unambiguously resolving their respective differential phase ΔΔθmn and accurately and reliably determining the location of the signal source. This is based on the understanding that the differential phase ΔΔθmn, which is indicative of a difference Δdn−Δdm between the changes Δdn and Δdm in the distances, dn and dm, of the respective receivers Rcm and Rcn from the signal source Src during the time interval Δt, corresponds to the difference ΔΔτijmn=Δτjmn−Δτimn between the differential times of arrival Δτjmn, Δτjmn of two pulses PLi and PLj (which respectively occur at the beginning and the end of a time interval Δt′) to the receivers Rcn and Rcm. More specifically, the differential phase ΔΔθmn (ddphase) over the time interval Δt should be proportional to the differential times of arrival of the pulses PLi and PLj occurring at the beginning and end of the pulses in the dwell within the time interval Δt′. More specifically, the differential phase ΔΔθmn should satisfy the following relation:
where here f represents the frequency of the signal S. It can be shown that the relation of Eq. 12 is valid since by replacing the differential time of arrival expressed in Eq. 10 above into Eq. 12, with Δt′=Δt, the differential phase Eq. 7 is obtained.
In many cases, the differential phase and the differential times of arrival is measured/computed for one or more dwells of pulses (dwell is a sequence of pulses). In this case Eq. 12 above may be represented as follows
where ΔΔθmni is the measured differential phase over the time interval Δti of the first dwell i and ΔΔθmnj is the measured differential phase over the time interval Δtj of the second dwell j.
Thus, in operation 169, the expression of Eq. 13 above is used to set-bounds-to/estimate the possible values that the differential phase ΔΔθmn can acquire. Accordingly, in some embodiments of the present invention, operation 169 is incorporated/included in 130, in which the relation of Eq. 12 is used to resolve and disambiguate the differential phase ΔΔθmn calculated in 130.
This may be achieved for example by modifying the differential phase ΔΔθmn, expressed in Eq. 6 above, to read as follows:
ΔΔθmn=2π/λ(Δdm−Δdn)+2πZ Eq. 14
where Z is an integer number that is selected such that the differential phase ΔΔθmn is within the boundaries given by the errors in the DTOA measurements set by Eq. 12.
In the above example the DTOA positioning technique provides for resolving possibly ambiguities in the differential phase ΔΔθmn. Alternatively, or additionally, information of other positioning techniques (e.g., from the differential Doppler technique) may also be used in the scope of the embodiments of the present invention for resolving this ambiguity in the differential phase ΔΔθmn.
Reference is now made together to
Operation 122 is carried out to estimate the inter-pulse accumulated phase. That is, in other words, to estimate the accumulated phase of a certain modulated section, such as a pulse, of the signal Sn being received by a certain receiver Rcn Operation 122 may include sub-operations 122S, 122F, and 122P that may be implemented according to some embodiments of the present invention by modules 222S, 222F, and 222P of the inter-pulse phase accumulation module 222 of system 200.
In operation 122S, the signal Sn received in a certain time interval Δt is sampled/divided into a plurality of samples corresponding to time slots TS1-TSL of durations shorter than half the period T (being one over the frequency f) of the carrier wave of the signal.
Then in operation 122F portions/samples {TSL} of the signal Sn corresponding to the time slots are transformed into the frequency domain (e.g., via Fourier transform) and their respective phases {θL} modulus 2π are determined. The phases are therefore bounded within a range of 2π (e.g., in the range [0, 2π] or [−π, π]). The phases {θL} modulus 2π of the signal portions (samples) {TSL} obtained in that way for a CW/pulsed signal are illustrated by the blackened points in
Operation 122P includes unfolding (unwrapping) the phases {θL}, to unwrap the modular representation of the phases such that each phase θL will present the actual phase of the signal S accumulated from the beginning of the time interval Δt until the time of the respective time slot TSL. The unwrapped phases are illustrated by the hollow circles in
Operation 122P may be carried out by various techniques. For example in some cases sub operations (i) to (iii) are carried out as described in the following:
It should be noted that in some cases, specifically in cases where the signal Sn received in the time interval Δt is divided to separate sections/pulses, then the unfolding operation 122 may be carried out separately on each of the sections/pulses to unfold their phase, and determine the phase profile of each section/pulse separately. This is illustrated for example in
As indicated above, the operation 122 may be implemented near/at each/some of the receivers and/or at the processing center, depending on the implementation of system 200.
In view of the above, optional operation 124, which is also referred to herein as intra-pulse phase accumulation, may be carried out in cases where phase profiles, such as PP1 and PP2, are determined independently for different sections, such as P1 and P2 of the signal Sn during the time interval Δt. The purpose of optional operation 124 is to obtain data indicative of the phase profile PPn of the signal Sn during the entire time interval Δt, from which the accumulated phase Δθ during the time interval Δt can be determined/estimated. In 124 the phase profile PPn for the entire time interval Δt is obtained by fitting the phase profile PP1 and PP2 onto a common line. As indicated above, this may be performed at the processing center. For example, the phase profile PP1 of the first pulse/modulated-section P1 of the received signal Sn is used to construct the guideline GD illustrated in the figure
In operation 128 the phase profile PP is used to determine the accumulated phase Δθn that is accumulated during the time interval Δt.
It should be however noted that in cases where operation 124 is conducted, some ambiguity might be introduced in the thus determined accumulated phase Δθn. This is because the added phase value Dθ which is used to match and fit the phase profile(s) may be ambiguous and may actually supplement 2πk value to the accumulated phase where k is any integer value (positive, negative, or zero). To this end, in optional operation 128.1, the ambiguity is optionally removed by utilizing the DTOA technique as described above with reference to Eq. 13. Alternatively, or in addition to Eq. 13, the ambiguity in the differential phase ΔΔθmn may be also reduced by other techniques (e.g., by using additional positioning techniques other than the DTOA). More specifically, the ambiguity in the accumulated phase Δθn is manifested in an ambiguity in the differential phase ΔΔθmn, which is computed in operation 130 of method 100. Thus, by utilizing additional positioning techniques, such as DTOA (see Eq. 12 above) and/or other additional positioning techniques, this ambiguity can be resolved.
It should be noted that in certain cases, for instance when the signal S is an FM signal, the time intervals Δtm, Δtn should be synchronized in order for the accumulated phases Δθm, Δθn to correspond to the same section of the signal and/or to two similarly modulated sections of the signal S. In such cases, optional operation 128.2 may be carried out for synchronizing the time intervals Δtm and Δtn (e.g., utilizing cross-correlation between the signals received by the different receivers), so that the time intervals Δtm and Δtn correspond to the times of receipt of similarly modulated signal sections by the respective receivers.
| Number | Date | Country | Kind |
|---|---|---|---|
| 240281 | Aug 2015 | IL | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20140300516 | Min et al. | Oct 2014 | A1 |
| Entry |
|---|
| European Search Report for European Application No. 16179029.0 dated Dec. 20, 2016. |
| Number | Date | Country | |
|---|---|---|---|
| 20170030996 A1 | Feb 2017 | US |
