METHODS AND SYSTEMS FOR IMPULSION ASSIGNMENT FROM SENSITIVITY EMISSIONERS

Description

BACKGROUND 1. Technical Field

[0001] This application relates to electronic support and radar warning systems and, more particularly, for electronic support and radar warning systems on tactical aircraft to detect threats that are radar emitters.2. Discussion of Related Art

[0002] Electronic Support (ES) and Radar Warning Receiver (RWR) systems have an increasing need to precisely identify potential threats including waveform and frequency agile emitters, as accurately and rapidly as possible. To identify distinct threats when many emitters are present requires that the RWR associate pulses with each emitter. For emitters that change waveform and / or frequency this may be challenging.

[0003] For pairs of antennas spaced many wavelengths apart, phase interferometry (PI) is capable of highly precise angle of arrival (AoA) measurements with less than 1° error. However, there are 2D / lambda possible AoAs (where D is the antenna separation and lambda the transmitter wavelength), which creates an ambiguous result for each pulse. Antennas spaced less than one-half wavelength apart have no ambiguities but less precise AoA measurements.

[0004] Time Difference of Arrival (TDOA) direction finding may be used to compute AoA over a span of 180° without ambiguity. The time delay between two antennas is monotonic with respect to geometric angle changing from 0 to 90 degree (and conversely from - 90 to 0), but TDOA provides relatively low AoA precision. Amplitude comparison direction finding generally provides a similarly coarse AoA precision. Improved AoA precision may be achievable using Frequency Difference of Arrival (FDOA) techniques, however FDOA techniques require stable emitter signal frequencies for long periods of time (e.g., 10ths of seconds, seconds, or many seconds depending on platform velocity and desired AoA precision), thus limiting the number of threats that can be simultaneously geolocated and limiting applicability to threats with a constant frequency. In comparison, an interferometer can provide precision AoA in a single pulse time (e.g., microseconds to milliseconds.)

[0005] U.S. Pat. No. 5,724,047 issued on March 3, 1998 to Lioio teaches a precise direction finding system making use of single pulses and TDOA and phase interferometry (PI). The method disclosed therein was directed to low frequencies (large wavelengths), and operates on an assumption that the correct PI AoA solution is the one closest to (the single, coarse) TDOA AoA. However, for antenna spacing of greater than 1.5 times the received wavelength, where three or more ambiguities exist, the AoA solution provided by that technique is more likely to be incorrect than correct. Moreover, that technique does not address the use of multiple pulses, or contemplate frequency agile emitters.

[0006] U.S. Pat. No. 6,313,794 issued on November 6, 2001 to Rose teaches a method of associating a single pulse from an agile emitter with previously detected pulses from that emitter in a time interval less than the pulse repetition interval (PRI) of the radar. Ambiguous phases from the previously detected pulses from the same agile emitter are stored. A single cos(aoa) from a subset of the stored ambiguous phases is estimated. A new ambiguous phase φ m at frequency f m , is detected. This frequency is different from at least one of the frequencies associated with the phases in the stored set. The phase measurement is made between two antennas spatially separated by distance d. A set of differenced phases is formed and corresponding differenced frequencies from the stored set, with at least one member of this set being the difference of the new ambiguous phase and frequency with one of the stored phases and its associated frequency. The phase cycle measurement ambiguity integer is measured resolving the phase difference formed from the new ambiguous phase utilizing this set of phase and frequency differences. The phase cycle measurement ambiguity integer is computed resolving the new ambiguous phase difference if the new pulse is from the same emitter as the stored set by utilizing the previously estimated cos(aoa) and newly measured frequency f m . The measured and computed ambiguity integers are compared. The newly detected pulse is associated with the previously stored pulses as being from the frequency agile emitter if the integers are equal.

[0007] U.S. Pat. No. 6,411,249 issued on June 25, 2002 to Rose teaches using feedback from RF carrier frequency measurements to disassociate the emitter angle-of-arrival component in the ambiguous phase measurement from the initially unknown phase measurement integer ambiguities; to then resolve the ambiguities; and finally to obtain the correct emitter AOA. U.S. Pat. No. 6,411,249 discloses converting the actual interferometer baselines on which the unassociated pulse phase measurements were made at different emitter frequencies to a baseline set for a single-frequency equivalent interferometer array. This conceptual array has the following property: the phase measurement that would be made on it at the fixed frequency for a signal at the same direction-of-arrival are identical to the actual phase measurements made on the physical array. Because of this equivalency, the conceptual array is called the E(equivalent)-array. But, whereas the physical array has antenna spacings that are invariant, the E-array "antenna" spacings change as a function of the RF carrier frequency measurement feedback, which depends on the particular residual pulses being tested. Thus, there is a different E-array, even for the same frequency agile emitter, depending on the particular residual pulse set used.

[0008] US 2017 / 242092A1 discloses systems, methods, and devices to estimate angle of arrival of wireless signals. An electronic device may include two or more antennas that receive a wireless transmission. The wireless transmission includes a first frequency signal at a first frequency and a second frequency signal at a second frequency. The electronic device includes angle of arrival logic that may determine one or more angles of arrival of the wireless transmission to the electronic device using phase difference on arrival based on each of the first and second frequency signals.SUMMARY

[0009] In view of the above, systems and methods of associating pulses which come from the same emitter, whether or not the emitter is frequency agile, without the aforementioned drawbacks would be beneficial. For example, an association method is desired that can quickly find which pulses are received from each emitter, including frequency agile emitters, across a broad range of frequencies. It would also be desirable to determine which pulses are not associated with an emitter and may be thinned from the analysis of that emitters characteristics. Further, it would be additionally desirable to provide such improved association to antenna systems already existing on many types of aircraft.

[0010] Association and thinning systems are described for multiple RF signals (e.g., signal pulses) relative to a system antenna array upon which the signal(s) are incident. In one embodiment, pairs of antenna elements (e.g. the antennas deployed on many aircraft) are employed for emitter identification.

[0011] The invention is defined in the appended claims. In an embodiment, the subject technology is directed to an advance warning system for detecting threats to a tactical aircraft, the system comprising: an antenna pair assembly for mounting on the tactical aircraft, the antenna pair assembly including: an antenna pair including a first antenna element and a second antenna element, each being configured for detecting: a first signal related to a potential threat, the first signal being at a first wavelength and a first frequency, a second signal at a second wavelength and a second frequency, and a third signal at a third wavelength and a third frequency; wherein the first and second antenna elements are spaced apart by more than one half of the first wavelength; a Radar Warning Receiver 'RWR' system in communication with the antenna pair assembly to receive the first, second and third signals, and configured to: compute a first frequency for the first signal, a second frequency for the second signal, and a third frequency for the third signal; compute a phase difference between the first and second antenna elements for each of the first to third signals; compute a frequency difference between the first frequency and the second frequency, and: if it is determined that the first and second frequency are within a threshold frequency difference and a phase difference between the phase differences between the first and second antenna elements for each of the first to second signals is less than the threshold phase difference, associate the second signal with the first signal as signals which come from the same emitter; if it is determined that the first and second frequency are within a threshold frequency difference and phase difference between the phase differences between the first and second antenna elements for each of the first to second signals is not less than a threshold phase difference, not associate the second signal with the first signal as signals which come from the same emitter; if it is determined that the first and second frequency are not within a threshold frequency difference, generate a set of ambiguous angle of arrival AoA for the first and second signals based on the frequency and the phase difference of the first and second signals, correlate the two sets of ambiguous AoA to determine if there is a common AoA, if there is a common AoA, generate a third set of ambiguous angle of arrival AoA for the third signal based on the frequency and the phase difference of the third signal, correlate the three sets of ambiguous AoA to determine if there is a common AoA for all three signals, and if there is a common AoA for all three signals, associate all three signals as signals which come from the same emitter, if there is no common AOA for all three signals, not associate the three signals as signals which come from the same emitter.

[0012] Such systems may comprise an integral component of, or provide AoA information to, Electronic Support (ES) and / or Radar Warning Receiver (RWR) systems (e.g., in the form of pulse descriptor words and / or emitter descriptor words.) As shown in FIG. 1, the direction finding system may comprise a software or firmware retrofit to upgrade legacy ES or RWR systems using existing antenna elements 10a-10d configured at widely spaced stations, for example, on wings 12, nose 14 and / or tail 16 of an aircraft 18. These positions may allow the simplest physical installation, provide the best unobstructed field of view around the platform.

[0013] These and other features and advantages of the systems and methods will be apparent from this disclosure. It is to be understood that the summary, drawings, and detailed description are not restrictive of the scope of the inventive concept described herein.BRIEF DESCRIPTION OF THE DRAWING

[0014] In the illustrations of the accompanying figures, like components may be given the same reference characters, regardless of whether they are shown in different examples. The illustrations of various elements are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the examples. Various views are provided, and reference may be made to the following figures. Figure 1 is an illustration of an exemplary aircraft configured with multiple antenna elements for receiving emitter RF signals from possible threats such as a surface-to-air-missile in accordance with the subject technology. Figure 2 is a functional block diagram of an exemplary system for improved identification of threat emitters in accordance with the subject technology. Figure 3 is an exemplary graphical user interface displaying multiple sets of emitter data in accordance with the subject technology. Figure 4 is a flow diagram of an exemplary method for improved association of pulses based on ambiguous angle of arrival in accordance with the subject technology. Figures 5A and 5B are graphs illustrating exemplary combs for first and second pulses, respectively, in accordance with the subject technology. Figure 6 is a flow diagram of an exemplary method for improved association of pulses using multiple hypothesis in accordance with the subject technology. Figure 7 is another flow diagram of an exemplary method for improved association of pulses using multiple hypothesis in accordance with the subject technology. Figure 8 is still another flow diagram of an exemplary method for improved association of pulses using multiple hypothesis in accordance with the subject technology. Figure 9 illustrates the geometric relations among an incident RF signal wavefront from a source emitter and received at the antenna elements in accordance with the subject technology. DETAILED DESCRIPTION

[0015] In the following description, numerous specific details are set forth. In the other instances, details well known to those skilled in the art may not be set out so as not to obscure the disclosed embodiments. It will be apparent to those skilled in the art in the view of this disclosure that modifications, substitutions and / or changes may be made without departing from the scope of the embodiments. The RF signals that are described in the following exemplary embodiments may be referred to as pulsed signals and / or pulses; however this is by no means intended to limit the scope of the disclosure to such signals. As used herein, the term signal may refer to a pulsed or continuously transmitted waveform originating at an emitter but it is noted that interchangeability of such terms exists. In the case that the emitter emits pulsed signals, some or all of a pulse may be received and processed. In the case of a continuous wave signal, the signal may be received for an initial and subsequent intervals. For illustration, the following description refers to pulsed signals.

[0016] In brief overview, the subject technology enhances performance of Electronic Support (ES) and / or Radar Warning Receiver (RWR) systems. In particular, the subject technology is useful for identifying each possible threat amidst a plurality of agile emitters. For example, signals from possible threats and other sources can be associated or thinned so that useful information can be more quickly and accurately determined to enhance safety. U.S. Patent Application Serial No. 15 / 492,694 filed April 20, 2017 discloses finding methods and systems using interferometric time delay of arrival.

[0017] In one application, the subject technology is deployed on an aircraft 18 such as shown in Figure 1. The subject technology may be integral to the aircraft 18 upon initial assembly or a firmware retrofit to upgrade legacy ES or RWR systems. The aircraft 18 has an antenna assembly 20 that may include one or more pairs of antenna elements 10a-10d, which are not drawn to scale for illustration. The antenna elements 10a-10d are preferably widely spaced on the aircraft 18. For example, the antenna elements 10a-10d are on wings 12, nose 14 and / or tail 16 of the aircraft 18. These positions may allow the simplest physical installation, and provide the best unobstructed field of view. The antenna elements 10a-d are separated by a distance s, which is typically many meters. The aircraft 18 also includes a RWR system 100 that communicates with the antenna assembly 20 to provide identification of threat emitters 50 such as surface-to-air-missiles (SAM), other airplanes, ground structures, and the like.

[0018] Referring now to Figure 2, a functional block diagram of an exemplary RWR system 100 for improved identification of multiple frequency agile threat emitters in accordance with the subject technology is shown. It is envisioned that a large variety of different hardware and software configurations can adequately implement the subject technology. For example, the RWR system 100 may include multiple signal processors implemented in hardware, firmware, as one or more computer programs having computer-executable instructions or code running on one or more computers, or as any combination thereof. One or more users can interface with the RWR system 100 using any suitable display (e.g., screens in a cockpit, CRT screens, televisions, computer monitors, laptops, tablets computing device, smart phones, personal digital assistant (PDAs) and / or other displays and computing devices, etc.). Typically, the RWR system 100 and the users (e.g., pilots, navigators and the like) are co-located in the aircraft 18. Of course, users may also be located remotely from the aircaft 18 (e.g., a drone) and the RWR system 100. The RWR system 100 preferably includes processors with common local oscillators (LO) with a low noise LO / clock. Wide band antenna are preferred so that the subject technology works over very wide bandwidths. In one embodiment, the frequency dependent phase errors are calibrated.

[0019] An exemplary user interface 300 is illustrated in Figure 3. User interface 300 may provide information for display, such as real-time display of multiple emitter data 305, 310 and AoA plots 315, 320.

[0020] In certain embodiments, the antenna elements 10a-d may comprise directional antennas having known gain patterns that are configured to point in different directions. Amplitude comparison direction finding techniques may be employed, in addition to TDOA and PI direction finding techniques, to further adjust the TDOA error boundary pulse signals.

[0021] Still referring to Figure 2, the RWR system 100 includes a plurality of modules. A module is a functional aspect, which may include software and / or hardware. Typically, a module encompasses the necessary components to accomplish a task. It is envisioned that the same hardware could implement a plurality of modules and portions of such hardware being available as needed to accomplish the task. Those of ordinary skill will recognize that the software and various processes discussed herein are merely exemplary of the functionality performed by the disclosed technology and thus such processes and / or their equivalents may be implemented in different embodiments in various combinations without materially affecting the operation of the disclosed technology.

[0022] The RWR system 100 receives data from the antenna assembly 20 into an RF distribution / conversion module 102. The RWR system 100 also includes a digitization module 104, a multi-antenna comparison / fusion module 108, a de-interleaver and clusutering module 110, an identification module 114, a tracking module 116, a mission data file (MDF) module 112, a geolocation and WS correlation module 118, an AEF module 120, and a scheduler module 122.

[0023] The RWR system 100 can calculate a plurality of parameters including a precision angle of arrival (AoA) estimate of pulse signals incident on the antenna elements 10a-d. Hence, although not shown in Figure 2 explicitly, the RWR system 100 can calculate such as well as a probability intercept (PI), a time difference of arrival (TDOA), a phase difference, ratios and the like as needed to accomplish the systems and methods described herein.

[0024] The flowcharts herein illustrate the structure and the logic of the present technology, possibly as embodied in an RWR system, which may include computer program software for execution on a controller computer, digital processor or microprocessor. Those skilled in the art will appreciate that the flow charts illustrate the structures and functions of the elements, including what may be logic circuits on an integrated circuit, that function according to the present technology. As such, the present technology may be practiced by machine components that render flowchart steps in a form that instructs the RWR system to perform a sequence of function steps corresponding to those shown in the flow charts.

[0025] Referring now to Figure 4, a flow diagram illustrating a method 400 of associating pulses based on ambiguous angle of arrival using the RWR system 100 is shown. The RWR system 100 and antenna elements 10a-d capture a plurality of pulses from multiple frequency constant and frequency agile emitters 50. The RWR system 100 may use signals from all of the antenna elements 10a-d or a subset such as antenna elements 10a, 10b in the following example. The method 400 does not utilize assistance from coarse AoA but does set union ambiguous AoA test.

[0026] At step 405, signals are received at the two antenna elements 10a, 10b. The antenna elements 10a, 10b feed the signals to the RWR system 100. For each pulse, the RWR system 100 measures the frequency (f) and phase difference (Δφ). The RWR system 100 may also measure the Time Difference of Arrival (TDOA). Processing of the pulses can determine other signal features such as times of arrival (TOA), peak amplitude (A), TDOA error (T err ), geometric angle of arrival (AoA) and the like. The TDOA may be measured using leading edge envelope detection for simple pulsed signals, and pre-detection correlation for phase and frequency modulated signals. The TDOA error (T err ) is the angle error in the TDOA measurement.

[0027] At step 410, a first pulse from a potential threat is identified and processed to store the frequency f 1 and phase difference Δφ 1 . A second pulse is also identified and processed to store a respective frequency f 2 and phase difference Δφ 2 .

[0028] At step 415, the RWR system 100 determines a difference (f diff ) between the frequency f 2 and frequency f 1 . If f diff is less than a threshold frequency difference (f e ), the method 400 proceeds to step 420. If f diff is more than a threshold frequency difference (f e ), the method 400 proceeds to step 435.

[0029] At step 420, a phase difference (Δφ diff ) between the phase differences Δφ1, Δφ2 is determined. In other words, the phase differences Δφ 1 , Δφ 2 are compared directly to decide if the pulses associate. If the phase difference Δφ diff is less than a threshold (Δφ e ), then the RWR system 100 associates the pulses at step 425. If the phase difference Δφ diff is more than the threshold (Δφ e ), then the RWR system 100 does not associate the pulses at step 430.

[0030] Returning to step 415, where the method 400 may proceed to step 435, the RWR system 100 generates two sets of ambiguous AoA. The first set of ambiguous AoA (AoA 1J ) is based on frequency f1 and phase difference Δφ 1 of the first pulse. The second set of ambiguous AoA (AoA 2k ) is based on frequency f 2 and phase difference Δφ 2 of the second pulse.

[0031] At step 440, the RWR system 100 evaluates the sets of ambiguous AoA (AoA 1J , AoA 2k ) to determine if there are any common angles. A common angle is any angle from set J and set K whose difference is below a user selected error threshold (AoA e ). If the sets of ambiguous AoA (AoA 1J , AoA 2k ) have no common members, then the method 400 proceeds to step 445 and the RWR system 100 does not make any associations. For the sets of ambiguous AoA (AoA 1J , AoA 2k ) that have a common member, then the method 400 proceeds to step 450

[0032] At step 450, the RWR system 100 reduces false associations by testing additional pulses. If the number of associated pulses is less than three, the method 400 proceeds to step 460 to test additional pulses. At step 460, another pulse (e.g., a third pulse) is tested and the method 400 loops back to step 440. If no other pulse associates with the pair of pulses with different frequencies and a common angle, the association is removed by proceeding to step 445. If the association is verified with a third pulse, then the method 400 proceeds to step 455, where the pulses are associated. Thus, the false associations are reduced by repeating through steps 440, 450, 460.

[0033] Still referring to step 450, the RWR system 100 may reduce false associations by correlating the comb of ambiguous AoAs with the current comb when the two frequencies of the pulses are different. Such a correlation is shown graphically in Figures 5A and 5B, which are graphs of probability versus AoA in degrees for the first and second pulses, respectively. As can be seen from Figures 5A and 5B, the delta phase Δφ of widely spaced antenna elements 10a, 10b results in combs, 502, 512 of multiple ambiguous AoAs, where the number of ambiguous AoAs is limited by TDOA / amplitude A direction finding (DF).

[0034] Still referring to Figures 5A and 5B, probability graph 500 is for the first pulse and probability graph 510 is for the second pulse. If the second pulse has the same comb as the first pulse, the potential threat emitter is probably not frequency agile and the combs / pulses can be associated as from the same emitter. The combs only need to be compared if the frequencies are of the two pulses are different. If the combs have no common angles, the emitters are different and the pulses should not be associated. This may occur when a second emitter is at a fortuitous angle and frequency.

[0035] If the comb of the second pulse has one "tooth" that matches, then the method 400 proceeds to further test the second pulse for association. The second pulse may have a matching tooth because the emitter has changed frequency (e.g., an agile emitter) or because there is another emitter at a fortuitous angle and frequency. The method 400 utilizes correlation to reject a high percentage of what would be incorrect associations, say more than 95%. Multi hypothesis can further be used to eliminate all ambiguities such as by testing three pulses from the frequency agile emitter if the frequency agile emitter is still emitting (see step 460 of method 400 as noted above). However, for the initial evaluation of two pulses, after the successful correlation at step 450 of Figure 4, the method 400 proceeds to step 455 to make the association of the pulses.

[0036] In view of the above, it is envisioned that additional embodiments could employ additional techniques to improve the speed and / or accuracy of the analysis. For example, data thinning could be used. A PRI test could determine if pulses should be associated. The solution may then be output, with a computed pulse repetition interval (PRI), graphically as shown in Figure 3. Fading memory and / or explicit motion compensation over long intervals could be used to update TDOA and interferometric AoA error function.

[0037] Pulse repetition frequencies (PRFs) from typical emitters are on the order of 10 3< to 10 5< pulses per second. Embodiments of the disclosed methods may collect 1 to several hundreds of pulses, so acquisition times may be on the order of 10s to 100s of microseconds. Signals at a lower carrier frequency may require many fewer pulses, leading to acquisition times on the order of 100s to 1000s of microseconds.

[0038] Higher frequency RF signals generate greater numbers of ambiguities. However, emitters of such higher frequency signals also tend to have higher pulse repetition rates, providing greater numbers of additional available pulse signals in a short interval for collection and analysis. Both approaches benefit by including TDOA and / or amplitude DF. Bias between AoA derived from these methods can also be removed.

[0039] Referring now to Figure 6, a flow diagram illustrating another method 600 of associating pulses based on ambiguous angle of arrival using the RWR system 100 is shown. Similar to method 400, the method 600 is performed by the RWR system 100 and antenna elements 10a-d. The method 600 does utilize assistance from coarse AoA and does set union ambiguous AoA test. For brevity, similar steps between methods 400, 600 are referenced with similar numbers so that the following description can largely focus in the distinctions between the methods 400, 600.

[0040] At step 605, the antenna elements 10a, 10b feed the signals to the RWR system 100 to measure the frequency (f), phase difference (Δφ) and the Time Difference of Arrival (TDOA). At step 610, pulses from potential threats are identified and processed to store the frequency f n , phase difference Δφ n , and TDOA n for two pulses. At step 612, the RWR system 100 determines a difference (TDOA diff ) between the TDOA 2 and TDOA 1 . If TDOA diff is more than a threshold TDOA difference (TDOA e ), the method 600 proceeds to step 614 without associating the pulses. If TDOA diff is less than the TDOA e , the method 600 proceeds to step 615.

[0041] At step 615, the RWR system 100 determines a difference (f diff ) between the frequency f 2 and frequency f 1 . If f diff is more than a threshold frequency difference (f e ), the method 600 proceeds to step 635.

[0042] If f diff is less than a threshold frequency difference (f e ), the method 600 proceeds the method 600 proceeds through steps 620, 625 and 630 in a manner very similar to that described above with respect to method 400, steps 420, 425, 430.

[0043] Returning to step 615, where the method 600 may proceed to step 635. At step 635, the RWR system 100 generates two sets of ambiguous AoA constrained by TDOA bounds. In other words, when generating the sets of ambiguous AoA, as visualized in Figures 5A and 5B, only AoA angles closer to the TDOA angle estimate than the TDOA error estimate are considered. From step 635, the method 600 proceeds through steps 640, 645, 650, 655, 660 similarly to corresponding steps 440, 445, 450, 455, 460 of Figure 4.

[0044] Referring now to Figure 7, a flow diagram illustrating still another method 700 of associating pulses based on ambiguous angle of arrival using the RWR system 100 is shown. Similar to methods 400, 600, the method 700 is performed by the RWR system 100 and antenna elements 10a-d. The method 700 does not utilize assistance from coarse AoA but does utilize a joint probability ambiguous AoA test. Again, for brevity, similar steps between methods 400, 700 are referenced with similar numbers so that the following description can largely focus in the distinctions between the methods 400, 700. Steps 705, 710, 715, 720, 725, 730, 735 are basically the same as steps 405, 410, 415, 420, 425, 430, 435 of method 400 so that discussion in this section is omitted.

[0045] At step 737, the method 700 defines a probability distribution (Prob(AoA)) for each AoA,using a comb function, such as that shown in Figures 5A and 5B, to determine the probability distribution (Prob(AoA)).. At step 739, the RWR system 100 multiplies the two probability distributions to yield a joint probability distribution. The resulting product, the joint probability distribution, will have peaks only where the peaks of the two probability distributions match. Then the method 700 proceeds to step 740.

[0046] At step 740, the RWR system 100 evaluates the joint probability distribution Prob(AoA) to determine if any AoA angles have a probability greater than a probability threshold Prob e . If the joint probability distribution has no angles AoA with probability greater than Prob e , then the method 700 proceeds to step 745 and the RWR system 100 does not make any associations. If the joint probability distribution has angles AoA with probability greater than Prob e , then the method 700 proceeds to step 750.

[0047] At step 750, the RWR system 100 reduces false associations by testing additional pulses (e.g., three or more). If the number of associated pulses is less than three, the method 700 proceeds to step 760 to test additional pulses. At step 760, another pulse (e.g., a third pulse) is tested in the same manner by defining a probability distribution thereof and returning to step 739. If no other pulse associates with the pair of pulses with, the association is removed by proceeding to step 745. If the association is verified with a third pulse, then the method 700 proceeds to step 755, where the pulses are associated. Thus, the false associations are reduced by repeating through steps 739, 740, 750, 760.

[0048] Referring now to Figure 8, a flow diagram illustrating still another method 800 of associating pulses based on ambiguous angle of arrival using the RWR system 100 is shown. Similar to methods 400, 600, 700, the method 800 is performed by the RWR system 100 and antenna elements 10a-d. The method 800 is very much a combination of the methods 600, 700. Again, for brevity, similar steps between method 800 and the other methods 400, 600, 700 disclosed herein are referenced with similar numbers so that the following description can largely focus in the distinctions between the methods. Steps 805, 810, 812, 814, 815, 820, 825, 830, 835 are basically the same as steps 605, 610, 612, 614, 615, 620, 625, 630, 635 of method 600 so that detailed discussion in this section is omitted. Steps 837, 839, 840, 845, 850, 855, 860 are basically the same as steps 737, 739, 740, 745, 750, 755, 760 of method 700 so that detailed discussion in this section is omitted.

[0049] Referring now to Figure 9, the geometric relations among an incident RF signal wavefront 900 from a source emitter and received at the antenna elements 10a, 10b are shown. The wavefront 900 includes a first signal component 905a and second signal component 905b. The path difference between the signal components 905a, 905b may be expressed as d = s * sinθ, where θ as shown comprise is the angle formed by a line drawn from the first antenna 10a normal to the signal path of the second signal component 905b. The time difference of arrival (TDOA) between the signal components 905a, 905b may be expressed as Δ t = s ∗ sinθ c , where c is the speed of light. The phase difference of arrival may be given as Δ φ = 2 π ∗ s ∗ sinθ λ . However the measured phase difference will be between 0 and 2π, Δ φ = mod 2 π ∗ s ∗ sinθ λ , 2 π . It is understood that TDOA and PI AoA measurements each contain uncertainty. With regard to TDOA error, the uncertainty is proportional to a time measurement error estimate associated with the measurement equipment, and may be empirically derived or may result from analysis of the equipment's design. The range of TDOA AoA solutions comprises a single solution plus or minus the associated error (which may have a Gaussian shape.) For example, the TDOA error is proportional to the baseline distance s and is roughly independent of pulse signal frequency, and may be expressed as σ sinθ = C S ∗ σ t . The PI AoA solutions also contain an uncertainty due to phase measurement errors, which may be expressed by σ sinθ = λ s σ φ 2 π . However, the largest uncertainty of PI solutions is due to a modulo (2π) phase measurement error, which may be expressed by Δ sinθ = λ s . That is, the number of possibly correct AoA solutions resulting from PI direction finding techniques increases with signal carrier frequency (or goes inverse to signal wavelength) and increases with wider antenna baseline spacing. When s is wider than λ pulse 2 , the set of PI AoA estimates has approximately 2 s λ ambiguous results ("ambiguities") over the full hemisphere from -90° to 90° (or sin θ from -1 to 1) . If any other technique allows the angles to be constrained to ±Δθ degrees; then the number of ambiguities can be reduced to 2 s λ Δθ 90 .

Claims

1. An advance warning system for detecting threats (50) to a tactical aircraft (18), the system comprising: an antenna pair assembly (20) for mounting on the tactical aircraft, the antenna pair assembly including: an antenna pair (10a, 10b) including a first antenna element (10a) and a second antenna element (10b), each being configured for detecting: a first signal related to a potential threat, the first signal being at a first wavelength and a first frequency, a second signal at a second wavelength and a second frequency, and a third signal at a third wavelength and a third frequency; wherein the first and second antenna elements are spaced apart by more than one half of the first wavelength; a Radar Warning Receiver 'RWR' system (100) in communication with the antenna pair assembly to receive the first, second and third signals, and configured to: compute (405, 605) a first frequency (fn) for the first signal, a second frequency for the second signal, and a third frequency for the third signal; compute (405, 605) a phase difference (Δφ1, Δφ2, Δφ3) between the first and second antenna elements for each of the first to third signals; compute (415. 615) a frequency difference between the first frequency and the second frequency, and: if it is determined (420, 620) that the first and second frequency are within a threshold frequency difference (fe) and a phase difference (Δφdiff) between the phase differences (Δφ1, Δφ2) between the first and second antenna elements for each of the first to second signals is less than a threshold phase difference (Δφe), associate (425, 625) the second signal with the first signal as signals which come from the same emitter; if it is determined (420, 620) that the first and second frequency are within a threshold frequency difference (fe) and a phase difference (Δφdiff) between the phase differences (Δφ1, Δφ2) between the first and second antenna elements for each of the first to second signals is not less than the threshold phase difference (Δφe), not associate (430, 630) the second signal with the first signal as signals which come from the same emitter; if it is determined that the first and second frequency are not within a threshold frequency difference (fe), generate (435, 635) a set of ambiguous angle of arrival AoA for the first and second signals based on the frequency and the phase difference of the first and second signals, correlate (440, 640) the two sets of ambiguous AoA to determine if there is a common AoA, if there is a common AoA, generate (460, 460) a third set of ambiguous angle of arrival AoA for the third signal based on the frequency and the phase difference of the third signal, correlate (440, 640) the three sets of ambiguous AoA to determine if there is a common AoA for all three signals, and if there is a common AoA for all three signals, associate (455, 655) all three signals as signals which come from the same emitter, if there is no common AOA for all three signals, not associate (445, 645) the three signal as signals which come from the same emitter.

2. An advance warning system as recited in Claim 1, wherein to correlate the two sets of ambiguous AoA to determine if there is a common AoA, a common AoA is when a difference between the AoA for the first signal and the AoA for the second signal is below a user selected error threshold (AoAe).

3. An advance warning system as recited in Claim 1, wherein the RWR system is further configured to determine (612) a difference, TDOAdiff, between a Time Difference of Arriva, TDOA2, for the second signal and a Time Difference of Arrival, TDOA1, for the first signal, wherein if TDOAdiff is not less than a threshold TDOA difference, TDOAe, no association (614) of the first and second signals as signals which come from the same emitter occurs.

4. An advance warning system as recited in Claim 1, wherein to generate (635) a set of ambiguous angle of arrival AoA for the first and second signals based on the frequency and the phase difference of the first and second signals, wherein a number of ambiguous angle of arrival AoA for the first and second signals based on the frequency and the phase difference of the first and second signals is limited by time difference of arrival, TDOA, and / or amplitude direction finding (DF).

5. An advance warning system as recited in Claim 1, wherein to correlate the two sets of ambiguous AoA to determine if there is a common AoA, the RWR system: defines a probability distribution (Prob(AoA)) for each AoA of the first and second signals; multiplies the two probability distributions to yield a probability distribution product; and evaluates the probability distribution product to determine if the probability distribution product has any angles with joint probability greater than a probability distribution threshold Probe to determine if there is a common AoA between the first and second signals / pulses.

6. An advance warning system as recited in Claim 1, wherein the first signal is received from an RF emitter.

7. An advance warning system as recited by any preceding claim, wherein the first signal is a first pulse, the second signal is a second pulse, and the third signal is a third pulse.