System and method for resolving range ambiguity in bistatic radars

Abstract

Disclosed examples overcome the Doppler dilemma in bistatic radar. Bistatic measurements are collected of a first radar signal at a low pulse repetition frequency (PRF), providing more unambiguous range, and a second radar signal, with pulse-to-pulse phase coding, at a second PRF, providing more unambiguous Doppler. Range gates with targets are found in the first radar signal, and the folded range of the second radar signal, having the strongest return, is found for a first one of those range gates. The corresponding portion of the second radar signal is cohered, allowing target velocity estimation for a first target in the first range gate, and the cohered portion is filtered from the second radar signal. This is repeated for the folded range of the second radar signal, having the second strongest return, in a second one range gate. That second cohered portion allows for target velocity estimation for a second target.

Description

Docket No. OU-OOOl-WO-PCTSYSTEM AND METHOD FOR RESOLVING RANGE AMBIGUITY IN BISTATIC RADARSBACKGROUND

[0001] In radar, a low pulse repetition frequency (PRF) provides long maximum unambiguous range (i.e., the longest range before returns begin range-folding), but cannot simultaneously provide wide Doppler resolution in order to accurately estimate a target’s radial velocity. Conversely, a high PRF provides wide Doppler resolution, which enables estimating a target’s radial velocity without aliasing, but at the cost of a shorter maximum unambiguous range. Range-folding results in a measured value being reported as artificially low (essentially, a mod operation). For example, a target range of 185 miles will be observed at 85 miles when the maximum unambiguous range is 100 miles.

[0002] The trade-off between the maximum unambiguous range and the maximum unambiguous Doppler shift is named the “Doppler dilemma”. A good choice of PRF to achieve a large unambiguous range will be a poor choice to achieve a large unambiguous velocity and vice versa. The Doppler dilemma persists for bistatic radars, in which the transmitter and receiver are not collocated (i.e., located at the same place).BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The disclosed examples are described below with reference to the accompanying drawing figures listed below, wherein:

[0004] FIG. 1 illustrates an exemplary architecture that advantageously overcomes the Doppler dilemma in bistatic radar;

[0005] FIG. 2 illustrates an exemplary7bistatic radar configuration that may advantageously employ the architecture of FIG. 1;

[0006] FIG. 3 illustrates two exemplary radar signals, one with a lower pulse repetition frequency (PRF), and another with a higher PRF, as may' be used in examples of the architecture of FIG. 1;

[0007] FIG. 4A illustrates an unambiguous range and an ambiguous range;Docket No. OU-OOOl-WO-PCT

[0008] FIG. 4B illustrates an unambiguous Doppler shift and an ambiguous Doppler shift;

[0009] FIG. 5 illustrates an exemplary pules-to-pulse phase coding, as may be employed when using examples of the architecture of FIG. 1;

[0010] FIG. 6 illustrates further detail for exemplary processing that may be employed by examples of the architecture of FIG. 1 ;

[0011] FIG. 7 illustrates an exemplary' range-velocity estimate for two targets, as may be generated when using examples of the architecture of FIG. 1;

[0012] FIG. 8 illustrates exemplary combination of results output by multiple receivers in an exemplary network for receivers, as may occur when using examples of the architecture of FIG. 1;

[0013] FIGs. 9 and 10 illustrate flowcharts of exemplary operations associated with the architecture of FIG. 1 ; and

[0014] FIG. 11 illustrates a block diagram of a computing device suitable for implementing various aspects of the disclosure.

[0015] The various examples will be described in detail with reference to the accompanying drawings. Wherever preferable, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made throughout this disclosure, relating to specific examples, are provided for illustrative purposes, and are not meant to limit all implementations or to be interpreted as excluding the existence of additional implementations that also incorporate the recited features.DETAILED DESCRIPTION

[0016] Disclosed examples overcome the Doppler dilemma in bistatic radar. Bistatic measurements are collected of a first radar signal at a low pulse repetition frequency (PRF), providing more unambiguous range, and a second radar signal, with pulse-to-pulse phase coding, at a second PRF, providing a larger unambiguous Doppler shift. Range gates with targets are found in the first radar signal, and the folded range of the second radar signal, having the strongest return, is found for the first one of those range gates. The corresponding portionDocket No. OU-OOOl-WO-PCT of the second radar signal is cohered, allowing target velocity estimation for a first target in the first range gate, and the cohered portion is filtered from the second radar signal. This is repeated for the folded range of the second radar signal, having the second strongest return, in a second one range gate. That second cohered portion allows for target velocity estimation for a second target.

[0017] Aspects of the disclosure improve the operation of bistatic radar by overcoming the Doppler dilemma. The low PRF of the first radar signal provides a further maximum unambiguous range than does the high PRF of the second radar signal. The high PRF of the second radar signal provides a higher maximum unambiguous Doppler shift than the does the low PRF of the first radar signal. However, the integrated processing of the first radar signal with the second radar signal, described herein, enables (for at least two targets) determining target range using the further maximum unambiguous range of the first radar signal and determining target velocity using the larger maximum unambiguous Doppler shift of the second radar signal.

[0018] Example advantages of the teachings herein include improved range-Doppler resolution and accuracy for bistatic measurements, such as passive bistatic (or multistatic), a reduced (or eliminated) need to perform angle measurements (to the target) over time to calculate target speed, and a single receiver is able to generate range-velocity estimates. This reduces the need for a network of receivers, or improves accuracy when a network of receivers is available for use. Additionally, range determinations may be extended and target velocity may be measured more accurately.

[0019] Before further describing various embodiments of the apparatus, component parts, and methods of the present disclosure in more detail by way of exemplary description, examples, and results, it is to be understood that the embodiments of the present disclosure are not limited in application to the details of apparatus, component parts, and methods as set forth in the following description. The embodiments of the apparatus, component parts, and methods of the present disclosure are capable of being practiced or carried out in various ways not explicitly described herein. For example, the various apparatus and devices of the various embodiments described herein may be constructed using various off-the shelf components, such as PCBs, and other mechanical and electrical components which perform the same function as the particular components described herein. As such, the language used herein is intended to be given the broadest possible scope and meaning; and the embodiments are meantDocket No. OU-OOOl-WO-PCT to be exemplary, not exhaustive. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting unless otherwise indicated as so. Moreover, in the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the disclosure. However, it will be apparent to a person having ordinary' skill in the art that the embodiments of the present disclosure may be practiced without these specific details. In other instances, features which are well known to persons of ordinary skill in the art have not been described in detail to avoid unnecessary complication of the description. While the apparatus, component parts, and methods of the present disclosure have been described in terms of particular embodiments, it w ill be apparent to those of skill in the art that variations may be applied to the apparatus, component parts, and / or methods and in the steps or in the sequence of steps of the method described herein w ithout departing from the concept, spirit, and scope of the inventive concepts as described herein. All such similar substitutes and modifications apparent to those having ordinary skill in the art are deemed to be within the spirit and scope of the inventive concepts as disclosed herein.

[0020] All patents, published patent applications, and non-patent publications referenced or mentioned in any portion of the present specification are indicative of the level of skill of those skilled in the art to which the present disclosure pertains, and are hereby expressly incorporated by reference in their entireties to the same extent as if the contents of each individual patent or publication was specifically and individually incorporated herein.

[0021] Unless otherwise defined herein, scientific and technical terms used in connection with the present disclosure shall have the meanings that are commonly understood by those having ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular.

[0022] As utilized in accordance with the methods and compositions of the present disclosure, the following terms and phrases, unless otherwise indicated, shall be understood to have the follow ing meanings: The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and / or the specification may mean “one,” but it is also consistent with the meaning of “one or more.” “at least one,” and “one or more than one.” The use of the term “or” in the claims is used to mean “and / or” unless explicitly indicated to refer to alternatives only or when the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and / or.” The use of the term “at leastDocket No. OU-OOOl-WO-PCT one” will be understood to include one as well as any quantity more than one, including but not limited to. 2, 3. 4, 5, 6, 7, 8, 9, 10. 15, 20, 30, 40, 50, 100, or any integer inclusive therein. The phrase '‘at least one” may extend up to 100 or 1000 or more, depending on the term to which it is attached; in addition, the quantities of 100 / 1000 are not to be considered limiting, as higher limits may also produce satisfactory' results. In addition, the use of the term “at least one of X, Y and Z” will be understood to include X alone, Y alone, and Z alone, as well as any combination of X, Y and Z.

[0023] As used in this specification and claims, the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0024] The term “or combinations thereof as used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof’ is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA. ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AAB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context.

[0025] Throughout this application, the terms “about” or “approximately” are used to indicate that a value includes the inherent variation of error for the apparatus, composition, or the methods or the variation that exists among the objects, or study subjects. As used herein the qualifiers “about” or “approximately” are intended to include not only the exact value, amount, degree, orientation, or other qualified characteristic or value, but are intended to include some slight variations due to measuring error, manufacturing tolerances, stress exerted on various parts or components, observer error, wear and tear, and combinations thereof, for example. The terms “about” or “approximately”, where used herein when referring to a measurable value such as an amount, percentage, temporal duration, and the like, is meant to encompass, for example, variations of ± 20% or ± 10%, or ± 5%, or ± 1 %, or ± 0. 1 % from the specified value, as such variations are appropriate to perform the disclosed methods and as understood by persons having ordinary skill in the art. As used herein, the term “substantially” means that theDocket No. OU-OOOl-WO-PCT subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance occurs to a great extent or degree. For example, the term ■‘substantially” means that a thing possesses or occurs in an amount, duration, degree or other measure or parameter value that is 90% to 99% of which the thing is being compared to.

[0026] As used herein any reference to "one embodiment" or "an embodiment" means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

[0027] As used herein, all numerical values or ranges include fractions of the values and integers within such ranges and fractions of the integers within such ranges unless the context clearly indicates otherwise. Thus, to illustrate, reference to a numerical range, such as 1-10 includes 1, 2. 3, 4, 5, 6. 7, 8, 9. 10, as well as 1.1, 1.2, 1.3, 1.4. 1.5. etc., and so forth. Reference to a range of 1 -50 therefore includes 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, etc., up to and including 50, as well as 1.1, 1.2, 1.3, 1.4, 1.5, etc., 2.1, 2.2, 2.3, 2.4, 2.5, etc., and so forth. Reference to a series of ranges includes ranges which combine the values of the boundaries of different ranges within the series. Thus, to illustrate reference to a series of ranges, for example, a range of 1-1,000 includes, for example, 1-10, 10-20, 20-30, 30-40, 40- 50, 50-60, 60-75, 75-100, 100-150, 150-200, 200-250, 250-300, 300-400, 400-500, 500-750, 750-1,000, and includes ranges of 1-20, 10-50, 50-100, 100-500, and 500-1,000. The range 100 units to 2000 units therefore refers to and includes all values or ranges of values of the units, and fractions of the values of the units and integers within said range, including for example, but not limited to 100 units to 1000 units, 100 units to 500 units, 200 units to 1000 units, 300 units to 1500 units, 400 units to 2000 units, 500 units to 2000 units, 500 units to 1000 units, 250 units to 1750 units, 250 units to 1200 units, 750 units to 2000 units, 150 units to 1500 units, 100 units to 1250 units, and 800 units to 1200 units. Any two values within the range of about 100 units to about 2000 units therefore can be used to set the lower and upper boundaries of a range in accordance with the embodiments of the present disclosure. More particularly, a range of 10-12 units includes, for example, 10, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11.0, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, and 12.0, and all values or ranges of values of the units, and fractions of the values of the units and integers within said range, andDocket No. OU-OOOl-WO-PCT ranges which combine the values of the boundaries of different ranges within the series, e.g., 10.1 to 11.5.

[0028] As used herein any reference to “we” as a pronoun may include laboratory personnel or other contributors who assisted in the laboratory procedures and data collection and is not intended to represent an inventorship role by said laboratory personnel or other contributors in any subject matter disclosed herein.

[0029] While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.

[0030] In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, components, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled may be directly coupled or may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.

[0031] The order of execution or performance of the operations in examples of the disclosure illustrated and described herein is not essential, unless otherwise specified. That is, the operations may be performed in any order, unless otherwise specified, and examples of the disclosure may include additional or few er operations than those disclosed herein. For example, it is contemplated that executing or performing a particular operation before, contemporaneously with, or after another operation is within the scope of aspects of the disclosure. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. When introducing elements of aspects of the disclosure or the examples thereof, the articles “a,” “an,” “the,” and “said” areDocket No. OU-OOOl-WO-PCT intended to mean that there are one or more of the elements. The term "exemplary" is intended to mean "an example of.”

[0032] Returning now to the description of several embodiments of the disclosure, reference is now made to the figures, FIG. 1 illustrates an exemplary architecture 100 that advantageously overcomes the Doppler dilemma in bistatic radar. A radar transmitter 210 transmits a radar signal 301 and a radar signal 302, which has pulse-to-pulse phase coding. In some examples, radar transmitter comprises a weather surveillance radar - 1988 Doppler (WSR-88D) and / or the pulse-to-pulse phase coding comprises SZ phase coding. Radar signal 301 and radar signal 302 are illustrated in further detail in FIG. 3, and an example of SZ phase coding is shown in FIG. 5. Radar signal 301 and radar signal 302 from a target 221 and a target 222, and the reflections are picked up by a receiver 201. Receiver 201 collects the reflections as bistatic measurements 601 of radar signal 301 and bistatic measurements 602 of radar signal 302. Receiver 201 performs processing 102, which is described in further detail below, along with the description of FIG. 6. FIGS. 2-5 will be described for context before returning to the description of FIG. 1.

[0033] FIG. 2 illustrates an exemplary bistatic radar configuration 200 that may advantageously employ architecture 100. A network of receivers 204 includes receiver 201, a receiver 202, and a receiver 203. Each of receivers is able to perform a version of processing 102 that will be described below for receiver 201. Target 221 is illustrated as an aircraft, and target 222 is illustrated as a storm (using a graphic of a cloud), possibly with wind and precipitation. Target 221 has velocity Vr, and target 222 has velocity V2.

[0034] A baseline distance, between radar transmitter 210 and receiver 201 is Rba. Receiver 202 is at a distance of D12 from receiver 201, and a distance D23 from receiver 203. None of radar transmitter 210, receiver 201, receiver 202, and receiver 203 is collocated with any of the others. In some examples, each of receiver 201, receiver 202, and receiver 203 comprises a passive bistatic radar (PBR) receiver that does not require a dedicated, co-operative radar transmitter. Target 221 it at a range RT1from radar transmitter 210 and a range RR11from receiver 201. Target 222 it at a range RT2(not shown) from radar transmitter 210 and a range RR21from receiver 202. The bistatic range is the sum of the transmitter-to-target range (distance) and the target-to-receiver range. For target 221 and receiver 201, the bistatic range Ren is:Docket No. OU-OOOl-WO-PCTRBII — TI + ^RII Eq. (1)The bistatic range RB2if°rtarget 222 and receiver 202 uses RT2in place of RT1and RR21in place of RR11in Eq. (1).

[0035] FIG. 3 illustrates radar signal 301 that has a PRF 311, the inverse of an inter-pulse repetition period (interval) T 321. Radar signal 301 that has a PRF 312. the inverse of an interpulse repetition period T 322. PRF 311 is lower than PRF 312, and in some examples, PRF 311 is between 250 Hertz (Hz) and 300 Hz, such as 290 Hz, and PRF 312 is between 900 Hz and 1300 Hz. such as 1200 Hz.

[0036] FIG. 4A illustrates ranging considerations 400, with an actual range 401 shown without range ambiguity concerns (i.e., an unambiguous range), and an ambiguous range 403, which is the remaining distance of actual range 401 after performing a modulo operation of actual range 401 with a maximum unambiguous range 402 for a radar signal with a given PRF. For a radar signal, the maximum unambiguous range RBmax is:^Bmax ~CEq. (2) where c is the speed of light, and T is the inter-pulse repetition period, such as T 321 or T 322. For 290 Hz (radar signal 301), the maximum unambiguous range is 642 miles, and for 1200 Hz (radar signal 302), the maximum unambiguous range is 155 miles.

[0037] FIG. 4B illustrates a plot 410 of Doppler shift 411 as a function of target velocity' 412 (radial velocity). An actual Doppler shift 414 (unambiguous Doppler shift) is illustrated, along with a maximum unambiguous Doppler shift 415 for a given PRF, and an ambiguous Doppler shift 416 that has been aliased (a modulo operation of actual Doppler shift 414 with maximum unambiguous Doppler shift 415). For a radar signal, the maximum unambiguous velocity VBmaxis determined using the Doppler shift, and is given by:where A is the wavelength of the transmitted waveform, and T is the inter-pulse repetition period, such as T 321 or T 322.

[0038] FIG. 5 illustrates an exemplary SZ phase coded pulse train, as may be employed when using examples of architecture 100. SZ phase coded pulse train is spread across an amplitude graph 510 and a phase graph 520. A family of phase codes, known as SZ phase codes and named after their developers Sachidananda and Zmic, are used in weather radars, such as aDocket No. OU-OOOl-WO-PCTWSR-88D. An advantage of SZ phase codes is that energy' from noncohered trips is spread out into a number of deterministic sidelobes evenly across the spectrum, preventing bias of mean velocity. This enables a relatively simple Discrete Fourier Transform (DFT)-based algorithm to notch out strong returns and estimate the mean velocity of multiple trips, avoiding the computational complexity' of computing more sophisticated adaptive fdters. In some examples, at least radar signal 302 is pulse-to-pulse phase coded (e.g., SZ phase coded, such as SZ-1 or SZ-2, or L-coded). For SZ-2 phase coded signals the SZ-2 algorithm to recover spectral moment estimates from overlaid target echoes. Other phase codings have similar recovery algorithms.

[0039] SZ phase coded pulse train 500 is obtained by applying a phase shift to successive pulses in an ordinary radar pulse train. Specifically, an SZ(n / M) phase code is parameterized by the integers n, M, and q, along with the arbitrary' phase offset c which can be any real number. With these parameters, the phase code ipkis periodic with period M, and the phase of the first M pulses is given by(j e No

[0040] Graph 510 has SZ phase coded pulse train 500 plotted as a function of amplitude versus time, using a time axis 501 and an amplitude axis 502. Amplitudes for the different pulses are constant at a value of 1.0 in a normalized range. There are nine pulses shown: a pulse 511, a pulse 512, a pulse 513, a pulse 514, a pulse 515, a pulse 516, a pulse 517, a pulse 518, and a pulse 519, in sequence. Graph 520 has pulses 511-519 of SZ phase coded pulse train 500 plotted as a function of phase versus time, using time axis 501 (synchronized with graph 510) and a phase axis 503. Phases for the different pulses range between -180 degrees and +180 degrees (although not reaching these limiting values). The nine pulses have a phase 521 for pulse 511 (0 degrees, so on axis 502), a phase 522 for pulse 512, a phase 523 for pulse 513, a phase 524 for pulse 514, a phase 525 for pulse 515, a phase 526 for pulse 516, a phase 527 for pulse 517, a phase 528 for pulse 518, and a phase 529 for pulse 519, in sequence.Docket No. OU-OOOl-WO-PCT

[0041] FIG. 6 illustrates further detail for processing 102, as performed by receiver 201 (and also receivers 202 and 203). FIGS. 1 and 6 should be viewed together. Collected bistatic measurements 601 of radar signal 301 and collected bistatic measurements 602 of radar signal 302 are provided to a range gating function 104 that determines a set of range gates 604 for radar signal 301, and also a set of trips 606 for radar signal 302. A detection function 106 operates on set of range gates 604 to determining a set of range gates 610 having detected targets. Detection function 106 may use thresholding (e.g., Neyman-Pearson detection) or constant false alarm rate (CFAR) detect on.

[0042] Set of range gates 610 shows three range gates having detected targets: a range gate 611, a range gate 612, and a range gate 613. Range gate 611 corresponds to a strongest return 631 within radar signal 302, range gate 612 corresponds to a second strongest return 632 (i.e., next strongest after strongest return 631) within radar signal 302, and range gate 613 corresponds to a third strongest return within radar signal 302. Estimating target velocities for the strongest two returns in returns in radar signal 302 (e.g., corresponding to range gate 611 and range gate 612) is reasonably accurate, although estimating target velocity for the third strongest return 633 (e.g., corresponding to range gate 613) is generally less accurate.

[0043] Set of trips 606 for radar signal 302 may have multiple trips per range gate of set of range gates 604 (and set of range gates 610). In the example of FIG. 6, four trips within set of trips 606 per range gate within set of range gates 604 are shown. A selection function 108 identifies strongest return 631, first. Strongest return 631 is within one of the trips of radar signal 302, and is shown as a folded range 621 that within the set of trips that correspond to range gate 611. This is the return from target 221, manifesting as strongest return 631 in folded range 621 of radar signal 302, w hich corresponds to range gate 611 in radar signal 301.

[0044] A cohering function 110 coheres radar signal 302 for folded range 621, for example by using the phase coding conjugate to remove phase coding. This produces cohered second radar signal 112 for folded range 621. The phase coding conjugate can be obtained from the phase code by multiplying each phase in the code by negative one. That is, if the phase of the kthpulse is ik, a phase of —may be applied to cohere the data for range gate 611. Similarly, a phase of —can be applied to cohere the data for range gate 612.Docket No. OU-OOOl-WO-PCT

[0045] Cohered second radar signal 112 is used to estimate a target velocity 116 (i.e., bistatic target velocity) for detected target 221 with a velocity estimation function 114. Estimating target velocity 116 may involve pulse pair processing, and or use: v = ^ ^Ryy[l] Eq. (5) where v is the bistatic velocity (i.e., bistatic target velocity 116), is a wavelength of the second radar signal, T is an interval of PRF 312, and / ?yy[l] is the first lag of the slow time autocorrelation.

[0046] The slow time autocorrelation is defined in the usual understanding of the term as applied to discrete-time signals. That is. if y[n] is the cohered slow-time data for some range gate, the slow time autocorrelation at lag I iswhere y[n] is the complex conjugate of y [n],

[0047] Cohered second radar signal 112 is also filtered out of radar signal 302 (i.e., bistatic measurements 602) to enable a selection function 124 to identify second strongest return 632. Second strongest return 632 is within one of the trips of radar signal 302. and is shown as a folded range 622 that within the set of trips that correspond to range gate 612. This is the return from target 222, manifesting as second strongest return 632 in folded range 622 of radar signal 302, which corresponds to range gate 612 in radar signal 301. Selection function 124 operates similarly to selection function 108 identifying folded range 621.

[0048] This filtering is performed by a filtering function 120 that outputs a signal 122. Signal 122 is second radar signal 302 minus cohered second radar signal 112 (e g., the cohered portion of second radar signal 302 for folded range 621). In some examples, the filtering comprises zeroing DFT bins centered around target velocity 116.

[0049] Zeroing DFT bins to filter the strongest return signal can be achieved by determining which DFT bin the velocity estimate 114 belongs in. Then, a predetermined number of bins of the DFT of the cohered signal 112 can be set to zero before taking the inverse DFT to yield the filtered signal 122.Docket No. OU-OOOl-WO-PCT

[0050] A cohering function 126 coheres radar signal 302 (i.e., the filtered version) for folded range 622, for example by using the phase coding conjugate to remove phase coding. This produces cohered second radar signal 128 for folded range 622, similarly to cohering function 110 producing coheres second radar signal 112. Cohered second radar signal 128 is used to estimate a target velocity 132 (i.e., bistatic target velocity) for detected target 222 with a velocity’ estimation function 130. Velocity estimation function 130 may operate similarly to velocity estimation function 114. Although FIGS. 1. 6, 9, and 10 describe estimating target velocities for two targets, some examples may estimate target velocities for a different number, such as one, here, or more than three.

[0051] An output generation function 140 receives range gate 611, target velocity 1 16, range gate 612, and target velocity’ 132 and generates a range Doppler map 142 as an intermediate product. This may then further processed into a range-velocity’ estimate 700, which is shown in FIG. 7. In some examples, velocity estimation stops after processing second strongest return 632, and third strongest return 633 is not processed to estimate a third target velocity.

[0052] FIG. 7 illustrates an example of range-velocity estimate 700. Range-velocity estimate 700 has an indication 701 of target 221 in range gate 611 with target velocity 116, and an indication 702 of target 222 in range gate 612 with target velocity 132. Indication 701 of target 221 shows a relatively high velocity7because target 221 is an aircraft, whereas target 222 is a weather mass (e.g., cloud, wind, precipitation) in this example (see FIG. 2).

[0053] FIG. 8 illustrates combining the results of receivers 201 and 202. If network of receivers 204 has more receivers, the illustrated arrangement may be extended. As shown, receiver 201 collects bistatic measurements 601 of radar signal 301 and bistatic measurements 602 of radar signal 302, and performs processing 102. Processing 102 determines range gate 611 and target velocity’ 116 for target 221, along with range gate 612 and target velocity 132 for target 222, as described above. This enables generation of range- velocity estimate 700 having indication 701 for target 221 and indication 702 for target 222.

[0054] Receiver 202 collects bistatic measurements 801 of radar signal 301 and bistatic measurements 802 of radar signal 302, and performs its own version of processing 102 as processing 804. Processing 804 (for receiver 202) determines a range gate 811 and a target velocity7821 fortarget 221, along with arange gate 812 and atarget velocity’ 822 fortarget 222. This enables generation of a range-velocity estimate 870 having an indication 871 for targetDocket No. OU-OOOl-WO-PCT221 and an indication 872 for target 222. A combination function 830 merges data from receivers 201 and 202 (e.g., merges range-velocity estimate 870 with range-velocity estimate 700 to produce a composite report 832.

[0055] FIG. 9 illustrates a flowchart 900 of exemplary operations associated with examples of architecture 100. In some examples, at least a portion of flowchart 900 may be performed using one or more computing devices 1100 of FIG. 11. Flowchart 900 commences with radar transmitter 210 transmitting radar signal 301 at PRF 311 and radar signal 302 at PRF 312 in operation 902. In operation 904, receiver 201 collects bistatic measurements 601 of radar signal 301 and bistatic measurements 602 of radar signal 302.

[0056] Operation 906 determines set of range gates 610 having detected targets, using radar signal 301. Detection may use CFAR or thresholding with Neyman-Pearson detection. Operation 908 uses radar signal 301 and radar signal 302 (i.e.. collected bistatic measurements 601 and 602) to determine, from among set of range gates 610 having detected targets, range gate 61 1 for which folded range 621 of radar signal 302 has strongest return 631. Operation 910 coheres radar signal 302 for folded range 621, for example by using the phase coding conjugate to remove phase coding.

[0057] Operation 912 uses cohered second radar signal 1 12 for folded range 621 (from operation 910) to estimate target velocity 116 for detected target 221 of folded range 621. In some examples, target velocity 116 comprises wind velocity or aircraft velocity, and estimating target velocity 116 comprises performing pulse pair processing. Some examples use Eq. (4) in operation 912. Operation 914 removes cohered second radar signal 1 12 (i.e., the cohered portion of radar signal 302 for folded range 621) from radar signal 302. In some examples, this comprises zeroing DFT bins centered around target velocity 116.

[0058] After filtering a second target velocity may be determined. Operation 916 uses radar signal 301 and radar signal 302 (i.e., collected bistatic measurements 601 and 602) to determine, from among set of range gates 610 having detected targets, range gate 612 for which folded range 622 of radar signal 302 has second strongest return 632 (after strongest return 631). That is, with the energy from target 221 removed, the energy from target 222 may now be processed. Operation 918 coheres radar signal 302 for folded range 622 (similarly to operation 910).Docket No. OU-OOOl-WO-PCT

[0059] Operation 920 uses cohered second radar signal 128 for folded range 622 to estimate target velocity 132 for detected target 222 of folded range 622. Target velocity 132 may also comprises wind velocity or aircraft velocity and may be performed similarly as velocity estimation for target velocity! 14. Operation 922 generates range-velocity estimate 700 having indication 701 of range gate 611 with target velocity 116 and indication 702 of range gate 612 with target velocity 132.

[0060] Operation 924 repeats operations 904-922 for other receivers in network of receivers 204, such as receiver 202 and receiver 203. In the next pass through flowchart 900, operation 904 now includes receiver 202 collecting bistatic measurements 801 of radar signal 301 and bistatic measurements 802 of radar signal 302. Operation 922 then generates range-velocity estimate 870 having indication 871 of range gate 811 with target velocity 821 and indication 872 of range gate 812 with target velocity 822.

[0061] Upon completing the additional passes through operations 904-922 for other receivers, operation 926 combines data from multiple receivers and / or multiple range-velocity estimates (e.g., range-velocity estimate 700 and range-velocity estimate 870) into composite report 832.

[0062] FIG. 10 illustrates a flowchart 1000 of exemplary operations associated with architecture 100. In some examples, at least a portion of flowchart 1000 may be performed using one or more computing devices 1100 of FIG. 11. Flowchart 1000 commences with operation 1002, which includes collecting, by a first receiver, bistatic measurements of a first radar signal at a first PRF and a second radar signal at a second PRF. wherein the first PRF is lower than the second PRF, and wherein the second radar signal comprises pulse-to-pulse phase coding. Operation 1004 includes determining, using the first radar signal, a set of range gates having detected targets.

[0063] Operation 1006 includes, using the first radar signal, determining, from among the set of range gates having detected targets, a first range gate for which a first folded range of the second radar signal has a strongest return. Operation 1008 includes cohering the second radar signal for the first folded range. Operation 1010 includes, using the cohered second radar signal for the first folded range, estimating a first target velocity for a detected target of the first folded range. Operation 1012 includes removing, from the second radar signal, the second radar signal for the first folded range.Docket No. OU-OOOl-WO-PCT

[0064] Operation 1014 includes, using the first radar signal, determining, from among the set of range gates having detected targets, a second range gate for which a second folded range of the second radar signal has a second strongest return. Operation 1016 includes cohering the second radar signal for the second folded range. Operation 1018 includes, using the cohered second radar signal for the second folded range, estimating a second target velocity for a detected target of the second folded range. Operation 1020 includes generating a first rangevelocity estimate having a first indication of the first range gate with the first target velocity and a second indication of the second range gate with the second target velocity.

[0065] FIG. 11 illustrates a block diagram of computing device 1100 that may be used as any component described herein that may require computational or storage capacity. Computing device 1100 has at least a processor 1 102 and a memory 1104 that holds program code 1110, data area 1120, and other logic and storage 1130. Memory 1104 is any device allowing information, such as computer executable instructions and / or other data, to be stored and retrieved. For example, memory 1104 may include one or more random access memory (RAM) modules, flash memory modules, hard disks, solid-state disks, persistent memory devices, and / or optical disks. Program code 1110 comprises computer executable instructions and computer executable components including instructions used to perform operations described herein. Data area 1120 holds data used to perform operations described herein. Memory 1104 also includes other logic and storage 1 130 that performs or facilitates other functions disclosed herein or otherwise required of computing device 1100. An input / output (I / O) component 1140 facilitates receiving input from users and other devices and generating displays for users and outputs for other devices. A network interface 1150 permits communication over external network 1160 with a remote node 1170, which may represent another implementation of computing device 1 100. For example, a remote node 1 170 may represent another of the above-noted nodes within architecture 100.Additional Examples

[0066] An example system comprises: a processor; and a computer-readable medium storing instructions that are operative upon execution by the processor to: collect, by a first receiver, bistatic measurements of a first radar signal at a first PRF and a second radar signal at a second PRF, wherein the first PRF is lower than the second PRF, and wherein the second radar signal comprises pulse-to-pulse phase coding; determine, using the first radar signal, a set of range gates having detected targets; using the first radar signal, determine, from among the set ofDocket No. OU-OOOl-WO-PCT range gates having detected targets, a first range gate for which a first folded range of the second radar signal has a strongest return; cohere the second radar signal for the first folded range; using the cohered second radar signal for the first folded range, estimate a first target velocity for a detected target of the first folded range; remove, from the second radar signal, the second radar signal for the first folded range; using the first radar signal, determine, from among the set of range gates having detected targets, a second range gate for which a second folded range of the second radar signal has a second strongest return; cohere the second radar signal for the second folded range; using the cohered second radar signal for the second folded range, estimate a second target velocity for a detected target of the second folded range; and generate a first range-velocity estimate having a first indication of the first range gate with the first target velocity’ and a second indication of the second range gate with the second target velocity.

[0067] An example method of wireless communication comprises: collecting, by a first receiver, bistatic measurements of a first radar signal at a first PRF and a second radar signal at a second PRF, wherein the first PRF is lower than the second PRF, and wherein the second radar signal comprises pulse-to-pulse phase coding; determining, using the first radar signal, a set of range gates having detected targets; using the first radar signal, determining, from among the set of range gates having detected targets, a first range gate for which a first folded range of the second radar signal has a strongest return; cohering the second radar signal for the first folded range; using the cohered second radar signal for the first folded range, estimating a first target velocity for a detected target of the first folded range; removing, from the second radar signal, the second radar signal for the first folded range; using the first radar signal, determining, from among the set of range gates having detected targets, a second range gate for which a second folded range of the second radar signal has a second strongest return; cohering the second radar signal for the second folded range; using the cohered second radar signal for the second folded range, estimating a second target velocity for a detected target of the second folded range; and generating a first range-velocity estimate having a first indication of the first range gate with the first target velocity and a second indication of the second range gate with the second target velocity.

[0068] One or more example computer storage devices has computer-executable instructions stored thereon, which, upon execution by a computer, cause the computer to perform operations comprising: collecting, by a first receiver, bistatic measurements of a first radar signal at a firstDocket No. OU-OOOl-WO-PCTPRF and a second radar signal at a second PRF, wherein the first PRF is lower than the second PRF, and wherein the second radar signal comprises pulse-to-pulse phase coding; determining, using the first radar signal, a set of range gates having detected targets; using the first radar signal, determining, from among the set of range gates having detected targets, a first range gate for which a first folded range of the second radar signal has a strongest return; cohering the second radar signal for the first folded range; using the cohered second radar signal for the first folded range, estimating a first target velocity for a detected target of the first folded range; removing, from the second radar signal, the second radar signal for the first folded range; using the first radar signal, determining, from among the set of range gates having detected targets, a second range gate for which a second folded range of the second radar signal has a second strongest return; cohering the second radar signal for the second folded range; using the cohered second radar signal for the second folded range, estimating a second target velocity for a detected target of the second folded range; and generating a first range- velocity estimate having a first indication of the first range gate w ith the first target velocity and a second indication of the second range gate with the second target velocity.

[0069] Alternatively, or in addition to the other examples described herein, examples include any combination of the following:- the first target velocity comprises wind velocity;- the second target velocity comprises wind velocity;- the first target velocity comprises aircraft velocity;- the second target velocity comprises aircraft velocity;- the first radar signal and the second radar signal are transmitted by a radar transmitter that is not collocated with the first receiver;- collecting, by a second receiver, not collocated with the first receiver or the radar transmitter, bistatic measurements of the first radar signal at the first PRF and the second radar signal at the second PRF;- generating a second range-velocity estimate having a third indication of a third range gate with a third target velocity and a fourth indication of a fourth range gate with a fourth target velocity;- the first receiver comprises a PBR receiver that does not require a dedicated, cooperative radar transmitter;- the radar transmitter comprises a WSR-88D;Docket No. OU-OOOl-WO-PCT- the first PRF is between 250 Hz and 300 Hz;- the second PRF is between 900 Hz and 1300 Hz;- the pulse-to-pulse phase coding comprises SZ phase coding;- the pulse-to-pulse phase coding comprises SZ-2 phase coding;- the pulse-to-pulse phase coding comprises L-coding;- the first radar signal comprises pulse-to-pulse phase coding;- determining the set of range gates having detected targets comprises performing thresholding or CFAR detection;- the thresholding comprises Neyman-Pearson detection;- cohering comprises using a phase coding conjugate to remove phase coding;- estimating a target velocity for a detected target comprises performing pulse pair processing;- estimating a target velocity for a detected target uses:where v is the target velocity, X is a wavelength of the second radar signal, T is an interval of the second PRF, and ^Ryyis a slow' time autocorrelation;- removing the second radar signal for the first folded range comprises zeroing DFT bins centered around the first target velocity; and- combining data from multiple range-velocity estimates into a composite report.

[0070] Having described aspects of the disclosure in detail, it will be apparent that modifications and variations are possible without departing from the scope of aspects of the disclosure as defined in the appended claims. As various changes may be made in the above constructions, products, and methods without departing from the scope of aspects of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

Docket No. OU-OOOl-WO-PCTCLAIMSWHAT IS CLAIMED IS:

1. A method comprising: collecting, by a first receiver, bistatic measurements of a first radar signal at a first pulse repetition frequency (PRF) and a second radar signal at a second PRF, wherein the first PRF is lower than the second PRF, and wherein the second radar signal comprises pulse-to- pulse phase coding; determining, using the first radar signal, a set of range gates having detected targets; using the first radar signal, determining, from among the set of range gates having detected targets, a first range gate for which a first folded range of the second radar signal has a strongest return; cohering the second radar signal for the first folded range; using the cohered second radar signal for the first folded range, estimating a first target velocity for a detected target of the first folded range: removing, from the second radar signal, the second radar signal for the first folded range; using the first radar signal, determining, from among the set of range gates having detected targets, a second range gate for which a second folded range of the second radar signal has a second strongest return; cohering the second radar signal for the second folded range; using the cohered second radar signal for the second folded range, estimating a second target velocity for a detected target of the second folded range; and generating a first range-velocity estimate having a first indication of the first range gate with the first target velocity and a second indication of the second range gate with the second target velocity.

2. The method of claim 1, wherein the first target velocity and / or the second target velocity comprises wind velocity.

3. The method of claim 1 or 2, wherein the first target velocity and / or the second target velocity comprises aircraft velocity.

4. The method of any of claims 1-3, wherein the first radar signal and the second radar signal are transmitted by a radar transmitter that is not collocated with the first receiver.Docket No. OU-OOOl-WO-PCT5. The method of claim 4, further comprising: collecting, by a second receiver, not collocated with the first receiver or the radar transmitter, bistatic measurements of the first radar signal at the first PRF and the second radar signal at the second PRF; and generating a second range-velocity estimate having a third indication of a third range gate with a third target velocity’ and a fourth indication of a fourth range gate with a fourth target velocity.

6. The method of any of claims 1 -5, w herein the first receiver comprises a passive bistatic radar (PBR) receiver that does not require a dedicated, co-operative radar transmitter.

7. The method of any of claims 1 -6, wherein the radar transmitter comprises a weather surveillance radar - 1988 Doppler (WSR-88D).

8. The method of any of claims 1-7, wherein the first PRF is between 250 Hertz (Hz) and 300 Hz.

9. The method of any of claims 1-8, yvherein the second PRF is betw een 900 Hertz (Hz) and 1300 Hz.

10. The method of any of claims 1-9, yvherein the pulse-to-pulse phase coding comprises SZ phase coding.

11. The method of any of claims 1-9, wherein the pulse-to-pulse phase coding comprises L-coding .

12. The method of any of claims 1-1 1, wherein determining the set of range gates having detected targets comprises performing thresholding or constant false alarm rate (CFAR) detection.

13. The method of claim 12, wherein the thresholding comprises Neyman-Pearson detection.

14. The method of any of claims 1-13, wherein cohering comprises using a phase coding conjugate to remove phase coding.Docket No. OU-OOOl-WO-PCT15. The method of any of claims 1-14, wherein estimating a target velocity for a detected target comprises performing pulse pair processing.

16. The method of any of claims 1-15, wherein estimating a target velocity for a detected target uses:where v is the target velocity, X is a wavelength of the second radar signal, 7' is an interval of the second PRF, and ^-Ryyis a slow time autocorrelation.

17. The method of any of claims 1-16, wherein removing the second radar signal for the first folded range comprises zeroing Discrete Fourier Transform (DFT) bins centered around the first target velocity.

18. The method of any of claims 1-17, further comprising: combining data from multiple range-velocity estimates into a composite report.

19. A system comprising: a processor; and a computer-readable medium storing instructions that are operative upon execution by the processor to: perform the method of any of claims 1-18.

20. One or more computer storage devices having computer-executable instructions stored thereon, which, upon execution by a computer, cause the computer to perform operations comprising: the method of any of claims 1-18.

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