Patent Application
20100097266
1. Field of the Invention
The invention relates generally to the field of passive coherent location (PCL).
2. Description of Related Art
Passive Coherent Location (PCL) is a bistatic or multistatic radar system, principally having a PCL receiver and one or more transmitters. Frequently, PCL radar systems rely on uncontrolled transmitters (i.e., emitters of opportunity), such as TV and FM radio transmitters, rather than deployed transmitters. By avoiding the need to broadcast, costs associated with PCL radar systems are considerably lower than a comparable conventional radar system.
PCL radar systems rely on radiation provided by remotely located transmitters at known locations. A portion of the radiation, known as the reference or direct path signal, is received directly from the transmitter. Another portion of the radiation, known as the scattered signal, is reflected from radar targets and received by the PCL receiver. The PCL receiver uses the direct path signal and the scattered signal to determine various properties of the target from which the scattered signal was reflected.
Methods and apparatus for performing passive coherent location (PCL).
In some aspects, the invention relates to a passive coherent location radar comprising a plurality of antenna elements, a plurality of analog-to-digital converters, and a plurality of frequency channelizers. Each antenna element outputs an RF signal. Each analog-to-digital converter is configured to receive the RF signal from a corresponding antenna element among the plurality of antenna elements, and output a digital signal, the digital signal being a digital version of said RF signal. Each frequency channelizer has a plurality of digital band-pass filters and configured to receive the digital signal from a corresponding analog-to-digital converter among the plurality of analog-to-digital converters, input the digital signal to each of the plurality of digital band-pass filters, and output a plurality of filtered signals, each filtered signal output from a digital band-pass filter among the plurality of digital band-pass filters.
In another aspect, the invention relates to a method of operating a passive coherent location radar system. The method comprising receiving an RF signal on each of a plurality of antenna elements, each antenna element corresponding to a physical channel; digitizing each RF signal; filtering each digitized RF signal with a plurality of digital band-pass filters, each digital band-pass filter corresponding to a frequency channel; determining a frequency-difference of arrival (FDOA) with respect to a corresponding reference signal for each filtered signal; and estimating a target state from the corresponding FDOA from each physical channel for each frequency channel.
In yet another aspect, the invention relates to a radar system comprising a physical channel and a processing unit. The physical channel comprises an antenna that outputs an RE signal; an amplifier configured to amplify at least a portion of the RF signal; an analog-to-digital converter configured to digitize the amplified RE signal; and a digital filter bank comprising a plurality of pass-band filters configured to receive the digitized RE signal and output a plurality of digital band-passed signals. The processing unit is configured to receive a digital band-passed signal, from the plurality of digital band-passed signals, output from a pass-band filter among the plurality of pass-band filters and estimate a target state.
The invention and embodiments thereof will be better understood when the following detailed description is read in conjunction with the accompanying drawing figures. In the figures, elements are not necessarily drawn to scale. In general, like elements appearing in multiple figures are identified by a like reference designation. In the drawings:
A passive coherent location (PCL) radar system is presented that utilizes a high speed, analog-to-digital converter (ADC) and a frequency channelizer. Direct path and scattered signals received from emitters of opportunity (e.g., TV and FM broadcasts) may be converted to the digital domain without analog spectral shifting.
The ADC has a very high Nyquist frequency, greater than 500 MHz in some embodiments, that enables capture of signals originating from many different types of transmitters to be captured (e.g., TV, FM, GSM). Each antenna may simultaneously capture signals from multiple transmitters broadcasting in different frequency bands. After analog-to-digital (A/D) conversion of the received RF signal, a digital filter bank, having a number of band pass filters, may be used to channelize the received signals. Each band-pass filter may correspond to a band of one of the uncontrolled transmitters (i.e., emitters of opportunity). The filtered signals may be processed to estimate target states.
An embodiment of the architecture for PCL radar system 200 is shown in
The RF signal 201-i received and output from each antenna 202-i is amplified and conditioned by the signal conditioner 204-i. The signal conditioner may optionally perform analog band-pass filtering, or perform up or down conversion using an analog mixer.
The conditioned signal is then digitized by ADC 206-i. ADC 206-i produces a digital version of the analog RF signal.
The digitized signal is received by frequency channelizer 208-i, Frequency channelizer 208-i has a digital filter bank 207-i having a number of digital band-pass filters (e.g., band-pass filters 209-A-i, 209-B-i, and 209-C-i). The pass-band filters may be used to provide channel equalization or conditioning as well as bandwidth reduction. In this example, three band-pass filters are illustrated in each filter bank. The filter bank of a frequency channelizer, however, may have any number of band-pass filters.
In some embodiments, the filter banks of each frequency channelizer may be identical or similar in each physical channel. A frequency channel is shared by the signals output from pass-band filters having the same, or similar, filter characteristics (e.g., center frequency, bandwidth). In some embodiments, each physical channel provides a signal on a frequency channel. For example, the signals sharing a frequency channel (e.g., 103.2 to 103.4 MHz) may each correspond to the scattered signal received by the respective antenna elements for a common transmission source (e.g., FM radio station 103.3). In the example embodiment, each of band-pass filters 209-A-1, 209-A-2, 209-A-3, and 209-A-4 has the same passband characteristics and thus the outputs correspond the same frequency channel. Similarly, pass-band filters 209-B-i (i=1, 2, 3, 4) may correspond to another frequency channel, and filters 209-C-i (i=1, 2, 3, 4) may correspond to yet another frequency channel.
In some embodiments, the filter banks of each physical channel have the same (or sufficiently similar) filter characteristics and signals from different physical channels, filtered by band-pass filters having the same passband, may be processed together.
Filtered output signals sharing the same frequency channel are processed by the same PCL processing unit 221-X (X=A, B, C). In the example embodiment of PCL radar system 200, there are three band-pass filters in each filter bank, and there are three corresponding PCL processing units. The outputs of band-pass filters 209-A-i, 209-B-i, and 209-C-i are output to PCL processing units 221-A, 221-B, and 221-C, respectively.
Each PCL processing unit 221-X has a parametric estimation bank 222-X and a parameter estimator 228-X. The parametric estimation bank 222-X has a number of parametric estimators. The parametric estimators 222-C-i are labeled explicitly for parametric estimation bank 222-C.
Each PCL processing unit 221-X identifies the direct path signal 223-X (e.g., direct path signal 111,
The isolated direct path signal 223-X is fed into the parametric estimation bank 222-X. The parametric estimation bank 222-X has parametric estimators for each physical channel 210-i that provides a signal corresponding to the frequency channel the PCL processing unit 221-X is processing.
Each parametric estimator in the parametric estimation bank 222-X provides estimates of signal properties. Such signal properties may include, for example, frequency-difference of arrival (FDOA), time-difference of arrival (TDOA), and angle-of-arrival estimates for each target. These estimates are based on the signal scattered from targets (contained in the band-pass filtered signal) and the direct path signal 223-X.
The signal properties determined by each parametric estimator in the parametric estimation bank 222-X are provided to the parameter estimator 228-X. The parameter estimator estimates a target state, which may include, for example, a target's rectangular cross section, position, trajectory (bearing), speed, acceleration, type, and the like.
The target state is provided to output device 250 which may present a representation of the target states. Output device 250 may perform post processing to present a cohesive representation of the theater of operation including targets. Various estimation techniques, as are known in the art, may be used to resolve discrepancies of target properties estimated independently of on another. In some embodiments, output device 250 provides a visual display of the theater of operation. The theater of operation may for example, be the region in space where targets are being detected. In some embodiments, output device may also receive information from other intelligence systems (e.g., satellite imagery, additional radar systems) and provide a display to convey the totality of intelligence.
In some embodiments, beam forming may be used to isolate targets and/or direct path transmissions.
In some embodiments, multiple beam formers may be used simultaneously for each frequency channel to individually isolate target signals and the direct signal path. For example, a first beam former for a frequency channel may isolate the direct path signal, and a second beam former for the frequency channel may isolate a target signal. In some embodiments, time slots are used by beam former 211-X to beam form for multiple targets and/or sources.
The PCL processing units 322-X receives the beam formed signals from each physical channel individually from the corresponding beam former 211-X.
Having generally described some embodiments of the PCL radar system 200 with reference to
The transmitters (e.g., transmitters 110, 120, 130 in
The antenna array described above may consists of a plurality of antenna elements 202-i. The antenna elements may be dipole antennas, linear antennas, phased arrays, dish antennas, or any of many other suitable antenna designs. The antenna elements need not be of the same type.
The antenna elements may be arrange in any suitable manner.
In
As yet another example of an antenna array configuration,
In some embodiments, ADC 204-i is an Advanced Digital Receiver Processor (ADRP™) manufactured by Lockheed Martin Corporation. The ADRP may also serve as the frequency channelizer. In some embodiments, the ADC is capable of A/D conversion of 512 MHz baseband signals.
In some embodiments, an analog front end is used to up mix to an intermediate frequency. In some embodiments the intermediate frequency is up to 1152 MHz.
The frequency channelizers 208-i each have a filter bank 207-i having a number of band-pass filters. The pass-band filters may be of any suitable configuration. Each filter may have any suitable band-pass shape (amplitude and phase) and the relationship between the filters (e.g., spectral spacing) may take any suitable form. The band-pass filters may be tunable as a group or independently tunable. Tuning alters a filters transfer function. The transfer function may be defined by a center frequency and a bandwidth. In some embodiments, the band-pass filters each have independently adjustable bandwidths and/or center frequencies. In some embodiments, the pass-bands of the pass-band filters in a filter bank are spectrally adjacent to on another. In some embodiments, the filters all have same bandwidth. Collectively, the filter bank may only cover a portion of the spectrum captured by the ADC. In some embodiments, the filter bank comprises 120 pass-band filters each having 2 MHz spacing. In some scenarios, multiple band-pass filters may correspond to a single transmitter.
The PCL processing unit 221-X may provide target state estimates for each target based on the information available within one or more frequency channels. A target's state may define the target's rectangular cross section, position, trajectory (bearing), speed, acceleration, type, and other properties of the target.
Line tracking may be used to track the return from a target over time. Line tracking may be used to reduce and reject false target identifications (false alarms).
The components of the PCL radar system 200 may be communicably coupled using any suitable technology. The components of the system need not be collocated. For example, components downstream of ADC 206-i may receive information via a digital communication network such as a fiber optic network, Ethernet, coaxial cable network, telephone, satellite, or any suitable digital communication technology.
Components may be implemented using software, hardware, or a combination of both. Some components may be implemented as digital signal processors (DSPs). Some components may be implemented in the same microprocessor (chip). Some components may be implemented as an application-specific integrated circuit (ASIC).
In step 505, RF signals are received on each of a plurality of antenna elements. The antenna elements may form an antenna array. Each antenna element corresponds to a unique physical channel.
In step 510, signal conditioning is performed on each physical channel to each RF signal. Conditioning may include selectively amplifying the RF signal. The signal conditioner may optionally perform analog band-pass filtering, or channel equalization to improve the quality of the reference signal.
In step 515, the RF signals of each physical channel is digitized. The RF signals may be digitized by an ADC such as ADC 206-i in PCL radar system 200 (
In step 520, the RF signals of each physical channel are digitally filtered by a filter bank. The filter bank may be part of a frequency channelizer such as frequency channelizer 208-i. The filter bank may comprise a plurality of band-pass filters. The pass-band filters may be used to provide channel equalization or conditioning as well as bandwidth reduction. A signal is output from each band-pass filter the filter bank.
Thus for each physical channel there may be multiple band-pass filtered output signals. The band-pass filters of one physical channel may each have a similar frequency response as a corresponding band-pass filter in each of the other physical channels. A frequency channel is shared by the output signals of a set of band-pass filters having a common frequency response but residing in distinct physical channels.
In step 525, beam forming is performed for each frequency channel on the band-pass filtered output signals originating from each physical channel corresponding the same frequency channel.
In step 530, signal properties are determined. The signal properties are determined from the filtered, and optionally beam formed, frequency channelizer output signals. For example, signal properties may include FDOA, TDOA, and angle-of-arrival information.
In step 535, the target state is estimated. The target state may include information such as the target's rectangular cross section, position, trajectory (bearing), speed, acceleration, type, and the like.
In step 540, post processing is performed to present a cohesive representation of the theater of operation. Discrepancies of the estimates of a target's state from the various channels may be resolved. Various estimation techniques, as are known in the art, may be used to resolve discrepancies in the target state estimated independently of on another.
In step 545, the target scenario is presented. For example, the target scenario may be displayed on a display as is known in the art.
The steps of method 500 may be performed in any suitable order or any suitable way. In some embodiments, each component continuously processes an inflowing stream of data.
In some other embodiments, data is sampled for a period of time and then processed. For example, such an embodiment may be used if the data rate of A/) conversion exceeds the processing speed of the down stream components.
Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.
