Code division moving target indication radar

Abstract

A system and method for estimating moving object positions and tracks is disclosed. According to the method, during a plurality of transmit windows, a plurality of radar pulses is emitted, which each of the plurality of radar pulses encoded with a unique identifying code associated with a transmit time for each pulse. The encoding can reflect a timestamp of the pulse transmission. During each of a plurality of receive windows following the transmit windows, the transceiver receives a received radar signal and identifies, within the received radar signal, echoes of transmitted pulses by sequentially filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses. For each identified echo of a transmitted pulse, doppler filtering is performed on the identified echo to determine whether the echo is from a moving object by determining whether a doppler frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold. Positions and tracks over time are estimated based on the doppler.

Description

122350.00028CODE DIVISION MOVING TARGET INDICATION RADAR CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 738,072 of the same title filed on December 23, 2024, the entirety of which is incorporated herein by reference.FIELD OF THE INVENTION

[0002] This disclosure relates to systems and methods for resolving range ambiguities in moving target indication radar systems.BACKGROUND

[0003] Moving target indication (MTI) radar is a radar system and technique optimized for detecting moving targets, typically against a background of stationary clutter objects. MTI is useful, for example, in detecting moving targets against stationary ground clutter (e.g., aircraft against mountains, trees or buildings, or ships against ocean waves). MTI systems detect moving objects by detecting doppler shifts imparted by moving objects to reflected radar pulses. In MTI systems, pulses of a predetermined duration are transmitted periodically, at a predetermined pulse repetition frequency (PRF). Received pulses from stationary objects will exhibit no frequency shift (assuming a stationary transmitter), and received pulses from moving objects will exhibit a frequency shift. MTI techniques involve suppressing the portion of the received signal (i.e., the echoes) that have not been frequency shifted relative to the transmitted signal. This filtering may be thought of as high pass filtering of the doppler frequency (i.e. the frequency that is added or subtracted from the transmitted carrier frequency). Under these techniques, received frequencies that have very small shifts relative to the transmitted carrier frequency (i.e., from stationary or slowly moving objects) are suppressed, and received frequencies with large shifts relative to the carrier frequency are passed.

[0004] The situation is slightly more complicated where the transmitter / receiver is moving relative to all objects that will reflect returns. This will generally be the case when the radar is aboard aircraft or spacecraft. For such cases where the MTI radar is moving relative to the stationary objects (or ground clutter), knowledge of the radar relative velocity predicts where the 1QB\100090893.1122350.00028ground clutter will be relative to the transmitted frequency and the frequency shifts for determining stationary versus moving objects are measured from that point.

[0005] In a pulsed radar system using MTI, a number of pulses M, each of duration T, is transmitted. The interval between pulses, the pulse repetition interval (PRI), may be denoted as Tr. 1 / Tr is the pulse repetition frequency or PRF. For MTI processing, the number of pulses will typically be a handful of pulses (e.g., as little as two, but more commonly many more). The total time over which received pulses are analyzed for target detection is M*Tr, which is the coherent processing interval or CPI. According to MTI techniques, a pulse (i.e., pulse Ml) is transmitted, and the system receives and stores the echo signal. More precisely, the system typically receives and stores digitized values of the received echo signal. The samples are received and stored at a number of sample times after the end of the transmit pulse. Thus, each received sample corresponds to an echo being received from reflections by one or more object at a particular range (the range being determined by the delay between the pulse transmit time and time of the sample). This process is repeated for the subsequent transmit pulses in the CPI, and the result is a set of M received signal matrices, each reflecting echoes of a given pulse by objects at a range of ranges. As stated, the received signals are characterized both in terms of signal strength (amplitude) and phase.

[0006] The goal of MTI is to filter echoes having small doppler shifts relative to the transmitted carrier frequency and to pass the remaining signal (having higher doppler shifts, as from moving targets) for further analysis. This may be accomplished in a variety of ways. A common way is to delay a first received set of echoes by a time equal to the PRI, such that it is time-synched to the next received set of echoes, and then subtract or mix the two signals. The result of this process is suppression of portions of the received pulse waveform that are not changing between pulses, and amplification of the signals that are resulting from moving objects.

[0007] There are several drawbacks with conventional MTI techniques. One is the problem of blind speeds or blind velocities. This occurs when the Doppler due to a target velocity is greater than the PRF. Consequently, that Doppler is aliased or folded into the 0 to PRF range and a lower than true velocity is estimated. Thus one is “blind” to velocities that generate Doppler shifts greater than the PRF.

[0008] The conventional method for dealing with blind speeds is to increase PRF such that the first blind velocity is too high to be practically achieved by targets of interest. Increasing PRF,2QB\100090893.1122350.00028and therefore decreasing the PRI and the overall CPI brings its own disadvantages, one of which is that it exacerbates the problem of range ambiguities.

[0009] As noted above, in pulsed radar systems, a sequence of electromagnetic pulses is emitted into some angular space or toward a specific azimuthal direction, reflections are received from objects along that direction, and the timing of the received pulses relative to the emitted pulses may be used to compute a range to the object. Difficulty occurs when pulses emitted at different times are received at about the same time, i.e., when there is an echo from a recent pulse that coincides with an echo received from an earlier pulse, or when echoes from an earlier transmit pulse Ml are received from some distant object during the receive window following a subsequent pulse M2. This may occur when there is another object along the emission beam direction that reflects an earlier pulse such that that pulse is received at about the same time as a more recent pulse from a nearer object. When this occurs, there is a range ambiguity, which is to say, it is difficult to distinguish between the two objects, and ghosting may occur (i.e., the system may report or display detection of either or both objects over time). If PRF is increased, to address Doppler ambiguities (i.e., blind speeds), the consequence is a shorter PRI. This means that the receive window between consecutive pulses is shortened, which increases the chances that an echo from an earlier pulse will be received during a subsequent receive window. This is especially the case if the targets of interest are a large distance from the transceiver.

[0010] Thus, in MTI radar systems there is a trade space between range and Doppler ambiguities and the pulse repetition frequency (PRF). As mentioned above, increasing the PRF will reduce “blind speeds” (Doppler ambiguities), but this will increase range ambiguities, while lower PRF will increase Doppler ambiguities but reduce range ambiguities. This tradeoff is further exacerbated at higher transmit carrier frequencies.

[0011] Two popular techniques for resolving these Doppler and range ambiguities are PRF staggering and pulse-to-pulse phase encoding. Both of these techniques come at the disadvantage of requiring processing over multiple pulses to resolve the ambiguities along with other disadvantages unique to each technique. Further improvement is warranted.SUMMARY OF THE INVENTION

[0012] Embodiments of the invention are directed to systems and methods for resolving information regarding the motion tracks of moving objects reflecting radar pulses that otherwise 3QB\100090893.1122350.00028would be ambiguous. In one aspect, transmit pulses are sequentially coded and received pulses are categorized according to whether they match pulses that were sent at predetermined times. This enables the system to determine that a received pulse was sent at a particular time. Additionally, the received pulse is analyzed for Doppler shift relative to the transmit pulse, according to conventional MTI methods. With this information, both Doppler and range, motion tracks originating at positions occupied by reflecting objects may be determined.

[0013] Embodiments of the invention are directed to a radar processing method, and a radar system having a computerized processor executing instructions to direct the system to carry out the method, and a computer program product encoding the instructions. The method is for detecting and estimating the positions and tracks of moving objects using a radar transceiver. The steps of the method include, during a plurality of transmit windows, emitting a plurality of radar pulses, each of the plurality of radar pulses encoded with a unique identifying code associated with a transmit time for each pulse. During each of a plurality of receive windows following the transmit windows, the transceiver receives a received radar signal and identifies, within the received radar signal, echoes of transmitted pulses by sequentially filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses. For each identified echo of a transmitted pulse, doppler filtering is performed on the identified echo to determine whether the echo is from a moving object by determining whether a doppler frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold. For each identified moving object, the range to the object is estimated on the basis of the difference between the transmit and receive times of an echo having the same unique identifying code. One or more positions of the object (i.e., tracks) are then estimated on the basis of the range estimate and a doppler shift associated with the object.

[0014] Inventive embodiments have certain advantages, chiefly, the ability to detect multiple moving objects over a wide field of view and a long distance, at the possible expense of high-fidelity data regarding any one object. Additional advantages will become clear to the person of ordinary skill in the art upon consideration of the following detailed description.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings described herein constitute part of this specification and include example embodiments of the present invention which may be embodied in various forms.4QB\100090893.1122350.00028

[0016] FIG. 1 is a conceptual block diagram of a pulsed MTI radar system according to an embodiment.

[0017] FIGs 2 and 3 depict side and top perspectives of a conceptual collection geography for a system including a radar transceiver and multiple targets.

[0018] FIG. 4 is a schematic diagram showing the timing of emitted and received RF pulses according to an inventive embodiment.

[0019] FIG 5 is a diagram outlining signal processing steps for each receive window according to an embodiment.

[0020] FIGs. 6-7 conceptually depict the simulated gain-loss pattern and SNR of a radar system implementing the disclosed.

[0021] FIG. 8 shows an exemplary waveform of an MTI pulse encoded with transmit time information according to one embodiment.DETAILED DESCRIPTION

[0022] The systems and methods described below related generally to pulsed radar systems, in particular, to moving target indicator pulsed radar systems. The operating units of such systems may be described below as modules in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like.

[0023] Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for example, comprise one or more physical or logical blocks of computer instructions which may, for example, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module.

[0024] Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different 5QB\100090893.1122350.00028programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network.

[0025] Reference to a signal bearing medium may take any form capable of generating a signal, causing a signal to be generated, or causing execution of a program of machine-readable instructions on a digital processing apparatus. A signal bearing medium may be embodied by a transmission line, a compact disk, digital-video disk, a magnetic tape, a Bernoulli drive, a magnetic disk, punch card, flash memory, integrated circuits, or other digital processing apparatus memory device.

[0026] The schematic flow chart diagrams (e.g., FIG. 5) included are generally set forth as logical flow chart diagrams. As such, the depicted order and labeled steps are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown.

[0027] Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of hardware elements and software modules to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. For example, FIG, 1 below shows a number of 6QB\100090893.1122350.00028modules of an MTT pulsed radar system. These modules may be implemented in hardware or software. For example, a reference signal generator may be a specific piece of circuitry, or may be a software module being executed on a programmable processor, or may be a combination of those elements. Where a radar is described as including mixers, filters, amplifiers and the like, these may be hardware circuit elements, or they may be software processes operating on digitized signals in a programmable processor. To that end, programmable processors described in connection with the embodiments below may execute the method steps described below by executing computer readable and executable instructions encoded and stored in data storage media.

[0028] This invention is described in preferred embodiments in the following description with reference to the Figures, in which like numbers represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0029] Where, “data storage media,” “storage”, memory” or “computer readable media” is used, Applicants mean an information storage medium in combination with the hardware, firmware, and / or software, needed to write information to, and read information from, that information storage medium. In certain embodiments, the information storage medium comprises a magnetic information storage medium, such as and without limitation, a magnetic disk, magnetic tape, and the like. In certain embodiments, the information storage medium comprises an optical information storage medium, such as and without limitation, a CD, DVD (Digital Versatile Disk), HD-DVD (High Definition DVD), BD (Blue-Ray Disk) and the like. In certain embodiments, the information storage medium comprises an electronic information storage medium, such as and without limitation, a PROM, EPROM, EEPROM, Flash PROM, compactflash, smartmedia, and the like. In certain embodiments, the information storage medium comprises a holographic information storage medium.

[0030] Reference is made throughout this specification to “signals.” Signals can be any time varying electromagnetic waveform, whether or not encoded with recoverable information. Signals, within the scope of this specification, can be modulated, or not, according to any modulation or encoding scheme. Additionally, any Fourier component of a signal, or combination 7QB\100090893.1122350.00028of Fourier components, should be considered itself a signal as that term is used throughout this specification.

[0031] The methods and systems described herein are usable with radar systems, such as pulsed radar systems. An exemplary radar system is described in reference to FIG. 1. The radar system of FIG. 1 (all components apart from target 105) may include one or more modules including antenna 110, transmitter electronics 115, a signal generator 120, receive electronics 125, a mixer or adder 130, an analog to digital converter 135, digital signal processing (DSP) electronics 150, a programmable processor 140 and associated memory or storage 145, and one or more output devices 155. FIG. 1 shows one conceptual example of a radar system, however, the methods and configurations described herein may be usable with other hardware configurations. For example, digital signal processing (e.g., frequency domain transforms, filtering, etc.) and signal generation functions may be performed by the processor. In such cases, the radar may also include a DAC downstream of the processor or signal generator as part of or prior to the transmitter 115.

[0032] The radar system of FIG. 1 may be installed in a stationary ground installation or in any type of vehicle, such as aircraft, missile, spacecraft (e.g., satellite), etc. The purpose of the radar system of FIG. 1 is to search for and track targets (e.g., target 105) for navigational, collision avoidance or military purposes.

[0033] In the system of FIG. 1, transmission and receive portions of the radar are shown as part of a single apparatus, but these portions may be divided between a transmitter and a receiver. As shown in FIG. 1, a radar system includes a includes a transmitter 115 in electronic communication with a signal generator 120. The transmitter 115 drives an antenna 110. In practice, the transmitter 115 and the antenna 110 may each include multiple elements arranged in an array, i.e., transmitter 115 may comprise an array of transceiver elements driving one or more individual antenna elements. Antenna 110 may have any number of configurations. It may be designed to have high transmit or receive gain in a particular direction or directions. It may be a single element, or multiple, spaced-apart elements. It these later cases, it may be a phased array radar, where array processing is performed to steer and tailor gain of the transmit or receive pulses. In the case where the antenna is multiple elements, these elements may be located on the same or different vehicles, and some may be stationary. The antenna element(s) may be stationary or rotating. Additionally, various radar transceiver configurations are usable with the inventive methods described herein, including systems using conventional single-element antennas (e.g., rotating parabolic antennas)8QB\100090893.1122350.00028and phased array antennas. In the event that the radar transceiver is a phased array radar, the antenna elements are typically arranged in a 2-D grid, and the individual transceiver elements may apply signal processing to the received transmission signal, which is received by all transceiver elements for broadcast through the associated antenna elements. The primary processing applied to the transmission signal is a phase shift, which enables the antenna array to tailor the gain (both and transmit and receive) in various directions, thereby enabling a steerable transmit beam.

[0034] Signal generator 120, which may include a voltage-controlled oscillator, supplies a waveform (i.e., a time-varying electrical signal) to the transmitter 115. Signal generator 120 is in electronic data communication with programmable processor 140, which itself, is in electronic data communication with memory 145. Processor 140 may supply data regarding and / or to be encoded onto the radar transmit signal. Generally speaking, antenna 110 transmits a radar signal (typically a series of pulses of a frequency modulated signal) received from the transmitter 115 and transmits the radar signal in a radiation pattern such that the radar system may search for and track targets (e.g., 105). Antenna 110 may receive a portion of the reflected signal and send the received signal to receiver electronics 125.

[0035] Target 105 may include ground vehicles, terrain, watercraft, aircraft, weather and other similar reflectors of a transmitted radar signal. Target 105 may move laterally or radially relative to the location of the radar system and antenna 110. Targets with movement that include a radial component may have a Doppler effect on the frequency of the reflected radar signal.

[0036] Generally, received signals from the antenna are passed to one or more signal processing elements which may include one or more programmable microprocessors which direct the transceiver elements and also perform various other tasks such filtering, application of phase shifts, and integration of received signals. The processors may execute computable readable and executable instructions stored in volatile or non-volatile storage of a computing device, which may cause the processors to implement the various signal processing and other methods described below. In the specific example of FIG. 1, receiver electronics 125 receive the radar return signal from antenna 110. Receiver electronics 125 may include filters and amplifiers, not shown. The received, and optionally amplified, signal is provided to a mixer 130 which mixes the return signal with a portion of the transmitted signal received from signal generator 120. The resulting mixed signal is a beat signal that, in frequency domain, has components resulting from the time delay of the trip from the antenna to the target, as well as any Doppler shift resulting from relative radial 9QB\100090893.1122350.00028motion between the target and the and the radar, or more specifically, antenna 110. The resulting signal from the mixer is provided to a ADC, which samples the signal at a sufficiently high frequency (i.e., above the Nyquist frequency for the highest frequency component expected). The receive electronics may also include optional signal processors 150, which may be DSP processors configured to frequency filter to perform Fourier transforms on the digital signal output by the ADC

[0037] One or more programmable processors 140 may receive the processed digital signal for analysis. The one or more processors may be one or more microprocessors, a controller, an ASIC, FPGA or the like. As noted above, the processor 140 may execute programming instructions stored in memory 145 to, at least, implement the analysis and measuring steps described below. The processor 140 may also execute stored programming instructions to execute the encoding steps described below. Additionally, the processor may execute digital filtering and frequency domain transformation and analysis rather than those functions being carried out by discrete components such as DSP module 150.

[0038] FIGs. 2 and 3 are diagrams showing a side (FIG. 2) and top (FIG. 3) view of a geometrical arrangement between a radar transceiver such as the one conceptually depicted at FIG.1 (“radar”) and a plurality of targets (targetl, target2, target3). The transceiver is a moving transceiver, which may be arranged on, for example, an aircraft or satellite and is moving approximately parallel to the surface of the earth with a velocity V, as shown in FIG. 3. Targets targetl, target2, and target3 are also assumed to be moving. The surface of the earth may be approximated as a plane, although that assumption is not limiting on or required by the methods to be discussed. The transceiver is a distance of DO above the surface of the earth along nadir and is a lateral distance (measured along the surface) of DI, D2, and D3 to targetl, target2 and target3 respectively. Relative to the radar’s heading (shown by the vector V), the bearings to targets 1, 2 and 3 are given by 01, 02 and 03, respectively.

[0039] In view of the geometry shown in FIGs. 1 and 2, the range from radar to the targets is given by the following, where R0 is the height of the radar above nadir.R = R + O>R = R + Df10QB\100090893.1122350.00028

[0040] And the Doppler equations are given byDrvvR1= ——cos01

[0041] According to one embodiment, pulses are encoded at different transmit times in order to detect targets along a large range swath without incurring range ambiguities.. Received pulses are then analyzed to identify their encoding, and the system classifies the received pulse on the basis of the time at which the pulse was transmitted. This method is illustrated in FIG. 4, which shows sequentially sent pulses, 1, 2 and 3, each of which is encoded with a unique identifying code. An example waveform of a pulse with added digital encoding is shown at FIG. 8. An example of information encoded onto a pulse would be a time stamp or a numerical order in a sequence. The encoded echoes from the various targets are then received, and the received pulses may be analyzed to recover their encoding, and thus a determination may be made as to the transmit time of each encoded pulse. Combined with array processing to determine direction this can identify range and direction of moving targets that are returning the pulses.

[0042] The encoding of the pulses may be any encoding sufficient to uniquely identify each pulse within a CPI upon receipt. Thus, the method as described is not limited to any particular encoding scheme. In one embodiment, a binary sequence B is generated and subdivided into n parts, and one subpart of B (Bl, B2,...Bn) is encoded into each transmit pulse in sequence (i.e., the first pulse Ml is encoded with Bl, M2 and encoded with B2, etc.). Upon receipt, the received pulses may be analyzed to determine which transmit pulse it corresponds to. Thus, each transmit pulse is encoded with a unique identifier which is usable to determine the transmit time of a received pulse.

[0043] The invention is not limited as to the nature of the sequence or the manner or method of encoding. In one embodiment, a random or pseudorandom number generator (which may be a process running on the processor described above in reference to FIG. 1) is used to generate a random number, which may be a random binary number. That number is then chunked or subdivided, and one chunk or subdivision is encoded onto each transmit pulse such that the 11QB\100090893.1122350.00028pulse is identifiable with knowledge of the number. Frequency filtering may be applied to naturally generated numbers. In other embodiments, the pulse may be generated from an analog noise source where time chunks of its output are both transmitted and recorded for subsequent correlation against receive windows. An example of an analog noise source is a Pasternack PE85NS4001 Surface Mount (SMT) Pin Packaged Noise Source Module. In the case where such a noise source is used, the noise signal would be recorded in order to correlate it with the received pulses later on.

[0044] In one embodiment, phase-shift keying may be used to encode the identifying number into each pulse. In one embodiment, BPSK (binary phase-shift keying) may be used, where the input bits are determined by an LRS (linear recursive sequence). This method of generating the number has the advantage that it does not repeat for a very long time relative to the PRI. Whatever number generation, chunking and encoding scheme is used, it is advantageous and preferred that each successive pulse is encoded with the next set of bits from the number, and that the number be sufficiently random such that each sequence is nearly orthogonal in terms of correlation one pulse to the next. PN or pseudo noise code has this property. FIG. 4 illustrates how this sort of encoding is used.

[0045] Referring here to FIG. 4, it can be seen that an echo of a first-sent pulse, which is encoded with code 1, is received first from a close object during a first receive window. In a second receive window, another code 1 echo is received from a more distant object, along with a code 2 echo from the first object. In another receive window, another code 1 echo is received from a third, more distant object, a code 2 echo is received from the second object, and a code 3 echo is received from the first, closest object. The time delays between transmit and receive for the various coded pulses may be used to determine the ranges (Rl, R2 and R3) to various objects, and measuring the doppler shift of the received pulse relative to the transmitted pulse may determine the relative velocity between the radar and the target that returned the echo along the vector connecting the radar and each target.

[0046] The system may determine the position and tracks of the objects as follows:• Measuring delay between sent and received pulses gives the range• Knowing height above ground plane, the offset distance for each object from nadir is calculated (i.e., DI, D2, D3)• Measure Doppler gives velocity to target12QB\100090893.1122350.00028• assume target velocity is small compared to radar velocity, correct for later if necessary• With other known values, calculate bearing relative to radar heading • Obtain range and bearing estimate per target

[0047] The method set forth above may be implemented by the following steps being performed by a programmable processor during each receive window:• For each previously transmitted code• Compute CAF for that code• Detect target and estimate delay / Doppler• Use delay / Doppler to estimate target position

[0048] After each receive window, the processor may store the estimated target position at the receive window, and multiple estimates may be used to estimate target tracks. This method will now be described in additional detail:

[0049] A radar transceiver generates a number, which may be a binary sequence. The number is then divided into sequential chunks of bits. Each sequential chunk of bits is assigned to a corresponding sequential transmit pulse. A first transmit pulse encoded with a first portion (chunk) of the binary sequence and is transmitted. Received signals are received and recorded during a first receive window. A second transmit pulse is encoded with a second portion (chunk) of the binary sequence, and is transmitted. Received signals are received and recorded during the second receive window. This process repeats over the processing interval, with M pulses having been sent encoded with portions of the number, such that each transmit pulse is uniquely identifiable according to its sequence in the string of transmitted pulses.

[0050] For signals received in each receive window, the system processes the signals as follows:Compute a cross-ambiguity function for each previously transmitted pulse (Ml, M2, etc.) (this has the effect of filtering all received pulses except the pulse being searched for in the data at the moment);13QB\100090893.1122350.00028For each detected pulse (M 1 , M2, etc.) detect targets using conventional Doppler detection methods;Estimate delay (i.e., range) and Doppler for each detected target;Use the estimate of the delay and Doppler to estimate target position.

[0051] With an estimate of target positions over the processing interval, an estimate of target tracks can be generated.

[0052] It will be noted that the method described above requires substantial processing capacity. Assuming that the processing interval is 20 pulses long, for each of the 20 receive windows, CAFs must be built for every previously sent pulse, which means 20 by the last pulse. For the signal that corresponds to each previously sent pulse, MTI doppler processing must then be applied. One way to do this is to build another series of CAFs corresponding to expected doppler shifts. And this process with typically be repeated for every azimuthal direction being scanned.

[0053] An exemplary processing method is illustrated in FIG. 5, which illustrates the substantial processing capacity needed to implement the methods described. FIG. 5 schematically illustrates processing steps implemented at each receive window, that is, after each transmission of an encoded pulse, the method involves:computing a cross ambiguity function for each transmitted code;array processing;detecting moving targets and estimating delay / doppler to each detected target; using the delay / doppler / and direction to estimate target position

[0054] Applicants have implemented the methods set forth above in simulated environments to achieve good results. To simulate a collection geometry, a commercial SAR satellite was chosen as a model for the emitter / col lector. Using the publicly available TLE (two-line element) sets, a realistic model was constructed to examine the feasibility of the concept. Code was written to propagate the TLE project the antenna beam pattern onto a point that is cross-track from ground track of the satellite at specific range. The size of the antenna (modelled as parabolic dish of a certain size) was chosen such that the fall-off of the beam nearly matches the increase in 14QB\100090893.1122350.00028gain as R~4gets smaller closer to the ground track. This provides for a fairly flat gain-loss pattern on the surface of the earth until the fall-off of the antenna pattern out paces the R~4gain (since there’s a minimum range at the nadir point) approaching the ground track. This provides for a very large area that is greater than 6 dB SNR while also providing for a less than -10 dB SNR at the nadir point under the satellite.

[0055] If this concept is used over the open ocean, then this geometry provides for a very low clutter environment with very bright targets. The area to expect high clutter environment would be right at the nadir point. Figure 6 and 7 show this pattern for our model collector (green is greater than 6 dBSNR, whereas red is less than -10 dB SNR, magenta track is the ground-track of the collector and red tracks are separated 500 microseconds in two-way range). The area that is all above 6 dB SNR is an ellipse that is approximately 1300 km by 90 km (semi-major by semiminor axis) covering about 117,000 square kilometers.

[0056] An augmentation to this concept is to receive the signal with an antenna array such that the receive antenna can be made more directive than the transmit antenna. This can be accomplished by only using one element of the array to transmit and receiving on all elements or phasing the elements on transmit to deliver a wide beam pattern while receiving all elements of the array into separate channels. The concept then is to take the separate channels and knowledge of where the transmitted codes should show up in each receive window and combine the channels for each direction of the expected range ambiguities then match-filter process each of those combinations for each transmitted code expected to possibly appear in that range ambiguity in that direction. The advantage of this augmentation is a directivity gain on receive. This is a variation on a technique where directivity is applied on both transmit and receive and that directivity is used to sort range ambiguities. With this technique, the transmitted coded pulses are used to sort range ambiguities.

[0057] This augmentation would allow this concept to possibly be applied to a ground-based scenario where the transmitted pulses are somewhat omni-directional into the surrounding airspace. The received channels would then match-filter processed for each transmit code and the direction would be determined by which phasing of the channels maximizes the correlation. The order of operation here is reversed from the previous scenario where the collection geometry constrains the direction and the phasing can be determined prior to match-filter processing.15QB\100090893.1122350.00028

[0058] While one or more embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.QB\100090893.1

Claims

122350.00028CLAIMSWhat is claimed is:

1. A method for detecting and estimating the positions and tracks of moving objects using a radar transceiver, comprising:during a plurality of transmit windows, emitting a plurality of radar pulses, each of the plurality of radar pulses encoded with a unique identifying code associated with a transmit time for each pulse;during each of a plurality of receive windows following the transmit windows,receiving a received radar signal;identifying, within the received radar signal, echoes of transmitted pulses by sequentially filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses;for each identified echo of a transmitted pulse, doppler filtering the identified echo to determine whether the echo is from a moving object by determining whether a doppler frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold;for each identified moving object, estimating the range to the object on the basis of the difference between the transmit and receive times of an echo having the same unique identifying code;generating an estimate of one or more positions of the object on the basis of the range estimate and a doppler shift associated with the object.

2. The method of claim 1, further comprising refining estimates of an object’s range or position based on measurements over subsequent receive windows.

3. The method of claim 1 , wherein filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses comprises applying 17QB\100090893.1122350.00028a plurality of cross-ambiguity-functions computed for each previously transmitted pulse including its unique identifier to the received radar signal and identifying peaks in the result.

4. The method of claim 1, wherein doppler filtering the echo to determine whether the echo is from a moving object, by determining whether a doppler frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold comprises computing a plurality of cross-ambiguity -functions each corresponding to a doppler shift to a filtered received radar signal and identifying peaks.

5. The method of claim 1, wherein the unique identifying codes for transmitted pulses are generated by a random number generator.

6. The method of claim 1, wherein the unique identifying codes for transmitted pulses are generated by generating a random bitstream, dividing the bitstream into indexed segments, and then encoding each segment, sequentially, into transmitted pulses.

7. The method of claim 1, wherein phase-shift keying is used to encode the unique identifying code into each transmitted pulse.

8. The method of claim 1, wherein the transceiver is a sky-based transceiver.

9. The method of claim 1, wherein the transceiver is a ground-based transceiver.

10. The method of claim 1, wherein the transceiver includes a directional antenna, and further comprising configuring the antenna to transmit pulses into a relatively wide transmit beam, and configuring the antenna during the receive windows to preferentially receive a radar signal along a relatively narrow directional beam oriented in a particular direction.

11. A radar system having transceiver, an antenna, a signal generator and a programmable processor in communication with non-volatile memory, the non-volatile memory including computer readable instructions operable upon execution to cause the programmable processor to direct the radar system to detect and estimate the positions and tracks of moving objects, by performing a method comprising:18QB\100090893.1122350.00028during a plurality of transmit windows, emitting a plurality of radar pulses, each of the plurality of radar pulses encoded with a unique identifying code associated with a transmit time for each pulse;during each of a plurality of receive windows following the transmit windows,receiving a received radar signal;identifying, within the received radar signal, echoes of transmitted pulses by sequentially filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses;for each identified echo of a transmitted pulse, doppler filtering the identified echo to determine whether the echo is from a moving object, by determining whether a doppler frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold;for each identified moving object, estimating the range to the object on the basis of the difference between the transmit and receive times of an echo having the same unique identifying code;generating an estimate of one or more positions of the object on the basis of the range estimate and a doppler shift associated with the object.

12. The system of claim 11, wherein the method further comprises refining estimates of an object’s range or position based on measurements over subsequent receive windows.

13. The system of claim 11 , wherein the method further comprises filtering the received radar signal for pulses having the same unique identifying code as each of the previously transmitted pulses comprises applying a plurality of cross-ambiguity-functions computed for each previously transmitted pulse including its unique identifier to the received radar signal and identifying peaks in the result.

14. The system of claim 11, wherein the method further comprises doppler filtering the echo to determine whether the echo is from a moving object, by determining whether a doppler 19QB\100090893.1122350.00028frequency shift of the echo relative to a transmit frequency exceeds a predetermined threshold comprises computing a plurality of cross-ambiguity-functions each corresponding to a doppler shift to a filtered received radar signal and identifying peaks.

15. The system of claim 11, wherein the unique identifying codes for transmitted pulses are generated by a random number generator.

16. The system of claim 11, wherein the unique identifying codes for transmitted pulses are generated by generating a random bitstream, dividing the bitstream into indexed segments, and then encoding each segment, sequentially, into transmitted pulses.

17. The system of claim 11, wherein phase-shift keying is used to encode the unique identifying code into each transmitted pulse.

18. The system of claim 1, wherein the transceiver is a sky-based transceiver.

19. The system of claim 1, wherein the transceiver is a ground-based transceiver.

20. The method of claim 11 , wherein the transceiver includes a directional antenna, and further comprising configuring the antenna to transmit pulses into a relatively wide transmit beam, and configuring the antenna during the receive windows to preferentially receive a radar signal along a relatively narrow directional beam oriented in a particular direction.20QB\100090893.1

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