Airborne array aperture UWB UHF radar-motivation and systemconsiderations

This paper discusses the necessity, feasibility, and technology of FOPEN GMTI. It argues that this functionality may be one mode in a multi-function UWB UHF system, which jointly possesses the capabilities for air target MTI and high resolution FOPEN SAR. The radar platform may be a UAV or an aircraft, whereas, we propose to use the push boom type of antenna mounting previously adopted with the advantage for the CARABAS II UWB VHF SAR. Presently, the push booms will hold a set of UWB UHF antenna elements. This paper relates GMTI to SAR, extended from imaging stationary ground to the 4-parameter set of targets in linear and uniform motion relative to ground. It is recognised that this extended imaging problem depends on one new parameter, i.e., the SAR focusing velocity. The required signal processing may be tackled in an efficient manner by a hierarchical scheme based on iteratively merging subapertures and increasing the resolution. Rejection of stationary clutter and detection occurs on all levels of increasing resolution. This paper also provides a brief presentation of the Swedish FOA efforts to produce an experimental demonstrator of this multi-function radar system     

Analysis of foliage-induced synthetic pattern distortions

Phase and amplitude fluctuations induced by wave propagation through foliage limit the ability of a synthetic aperture radar (SAR) system to image a target under foliage. One-way measurements of these fluctuations were done at C-, L-, UHF band during the July 1990 Foliage Penetration Experiment using single frequency CW signal sources and the NASA/JPL SAR receiver. The phase and amplitude data are coherently integrated to create the synthetic azimuthal patterns that would result when attempting to image a point target obscured by foliage. The effect of synthetic aperture length frequency, and polarization on the attenuation and azimuthal response of foliage obscured targets is investigated     

Adaptive filters for detection of targets in foliage

In this paper, we examine three different target detection techniques (including a family of two-dimensional adaptive filters) applied to FOPEN data collected by a UHF foliage penetration radar     

Signal processing of wide bandwidth and wide beamwidth P-3 SAR data

This research is concerned with multidimensional signal processing and image formation with FOliage PENetrating (FOPEN) airborne radar data which were collected by a Navy P-3 ultra wideband (UWB) radar in 1995. The digital signal processors that were developed for the P-3 data commonly used a radar beamwidth angle that was limited to 35 deg. Provided that the P-3 radar beamwidth angle (after slow-time FIR filtering and 6:1 decimation) was 35 deg, the P-3 signal aperture radar (SAR) system would approximately yield alias-free data in the slow-time Doppler domain. We provide an analysis here of the slow-time Doppler properties of the P-3 SAR system. This study indicates that the P-3 database possesses a 50 deg beamwidth angle within the entire [215, 730] MHz band of the P-3 radar. We show that the 50-degree beamwidth limit is imposed by the radar (radial) range swath gate; a larger beamwidth measurements would be possible with a larger range swath gate. The 50-degree beamwidth of the P-3 system results in slow-time Doppler aliasing within the frequency band of [444, 730] MHz. We outline a slow-time processing of the P-3 data to minimize the Doppler aliasing. The images which are formed via this method are shown to be superior in quality to the images which are formed via the conventional P-3 processor. In the presentation, we also introduce a method for converting the P-3 deramped (range-compressed) data into its alias-free baseband echoed data; the utility of this conversion for suppressing radio frequency interference signals is shown     

ECCM capabilities of an ultrawideband bandlimited random noise imaging radar

Investigated here is high-resolution imaging of targets in noisy or unfriendly radar environments through a simulation analysis of the ultrawideband (UWB) continuous-wave (CW) bandlimited random noise waveform. The linear FM chirp signal was selected as a benchmark radar waveform for comparison purposes. Simulation of the recovery of various types of target reflectivity functions (TRFs) for these waveforms were performed and analyzed. In addition, electronic counter-countermeasure (ECCM) capabilities for both types of systems were investigated. The results are compared using the error between the interference (jamming)-free recovered TRF and the recovered TRF under noisy conditions as a function of the signal-to-interference/jamming ratio (SIR/SJR). Our analysis shows that noise waveforms possess better jamming immunity (of the order of 5-10 dB improvement over the linear FM chirp) due to the unique radar correlation processing in the receiver.     

UWB radar detection of targets in foliage using alpha-stableclutter models

We address the problem of detection of targets obscured by a forest canopy using an ultrawideband (UWB) radar. The forest clutter observed in the radar imagery is a highly impulsive random process that is more accurately modeled with the recently proposed class of alpha-stable processes as compared with Gaussian, Weibull, and K-distribution models. With this more accurate model, segmentation is performed on the imagery into forest and clear regions. Further, a region-adaptive symmetric alpha stable (SαS) constant false-alarm rate (CFAR) detector is introduced and its performance is compared with the Weibull and Gaussian CFAR detectors. The results on real data show that the SαS CFAR performs better than the Weibull and Gaussian CFAR detectors in detecting obscured targets     

Doppler visibility of coherent ultrawideband random noise radar systems

Random noise radar has recently been used in a variety of imaging and surveillance applications. These systems can be made phase coherent using the technique of heterodyne correlation. Phase coherence has been exploited to measure Doppler and thereby the velocity of moving targets. The Doppler visibility, i.e., the ability to extract Doppler information over the inherent clutter spectra, is constrained by system parameters, especially the phase noise generated by microwave components. Our paper proposes a new phase noise model for the heterodyne mixer as applicable for ultrawideband (UWB) random noise radar and for the local oscillator in the time domain. The Doppler spectra are simulated by including phase noise contamination effects and compared with our previous experimental results. A genetic algorithm (GA) optimization routine is applied to synthesize the effects of a variety of parameter combinations to derive a suitable empirical formula for estimating the Doppler visibility in dB. According to the phase noise analysis and the simulation results, the Doppler visibility of UWB random noise radar depends primarily on the following parameters: 1) the local oscillator (LO) drive level of the receiver heterodyne mixer, 2) the saturation current in the receiver heterodyne mixer, 3) the bandwidth of the transmit noise source, and 4) the target velocity. Other parameters such as the carrier frequency of the receiver LO and the loaded quality factor of the LO have a small effect over the range of applicability of the model and are therefore neglected in the model formulation. The Doppler visibility curves generated from this formula match the simulation results very well over the applicable parameter range within 1 dB. Our model may therefore be used to quickly estimate the Doppler visibility of random UWB noise radars for trade-off analysis     

P-3 ultra-wideband SAR: description and examples

Under the Advanced Research Projects Agency (ARPA)/ASTO sponsorship, through a contract from the Naval Air Warfare Center (NAWC), the Environmental Research Institute of Michigan (ERIM) has developed an ultrawideband (UWB) very high frequency (VHF)/ultrahigh frequency (UHF) fully polarimetric airborne synthetic aperture radar (SAR) for studying the detection of foliage-obscured objects. The radar is installed in the NAWC P-3 testbed aircraft and takes advantage of existing ERIM-built multimode, fully-polarimetric X/L/C-band SAR hardware. This paper describes the radar and presents some examples of its capabilities including polarimetric imagery and two-pass interferometric surface height estimates     

Ultrawideband microwave-radar conceptual design

This paper outlines the special considerations that characterize the design of an UWB radar for the detection of low-altitude missiles over the sea. It discusses the factors which enter into the choice of frequency, and the selection of the transmitter, antenna, and receiver. Reviewed are signal processing issues concerning detection of UWB signals in noise and clutter, nondoppler MTI based on the pulse-to-pulse change in range due to target motion, measurement of target height based on multipath time delay, and target recognition. As the investigation progressed, the authors became disappointed with the available UWB technology, but encouraged about the potential advantages of UWB for this application. The chief limitation of UWB radar that must be overcome before applications are viable is its poor electromagnetic compatibility (EMC)     

RFI suppression for ultra wideband radar

An estimate-and-subtract algorithm is presented for the real-time digital suppression of radio frequency interference (RFI) in ultrawideband (UWB) synthetic aperture radar (SAR) systems used for foliage- and ground-penetrating imaging. The algorithm separately processes fixed- and variable-frequency interferers. Excision of estimated targets greatly reduces bias in RFI estimates, thereby reducing target energy loss and sidelobe levels in SAR imagery. Performance is demonstrated on data collected with the Army Research Laboratory's UWB rail SAR.     

Foliage attenuation and backscatter analysis of SAR imagery

The ability of a synthetic aperture radar (SAR) to detect targets under foliage is in part determined by the attenuation suffered by radiation propagating through the foliage and the backscatter from the foliage. MIT Lincoln Laboratory made measurements of foliage attenuation and backscatter using the NASA/JPL-UHF, L-, C-band fully-polarimetric SAR in July 1990. In this experiment, a 48 km² forested area near Portage, Maine was imaged. Twenty-seven 8 ft trihedral corner reflectors were arrayed throughout the imaged area in order to measure foliage attenuation. Ground truth was recorded at the time of the experiment in order to correlate the attenuation and backscatter results with foliage biophysical properties. The probability densities for foliage attenuation and for backscatter are determined as functions of frequency, polarization, and depression angle     

Bayesian edge detector for SAR imagery using discontinuity-adaptive Markov random feld modeling

Synthetic aperture radar（SAR）image is severely affected by multiplicative speckle noise,which greatly complicates the edge detection.In this paper,by incorporating the discontinuityadaptive Markov random feld（DAMRF）and maximum a posteriori（MAP）estimation criterion into edge detection,a Bayesian edge detector for SAR imagery is accordingly developed.In the proposed detector,the DAMRF is used as the a priori distribution of the local mean reflectivity,and a maximum a posteriori estimation of it is thus obtained by maximizing the posteriori energy using gradient-descent method.Four normalized ratios constructed in different directions are computed,based on which two edge strength maps（ESMs）are formed.The fnal edge detection result is achieved by fusing the results of two thresholded ESMs.The experimental results with synthetic and real SAR images show that the proposed detector could effciently detect edges in SAR images,and achieve better performance than two popular detectors in terms of Pratt＇s fgure of merit and visual evaluation in most cases.     

Multichannel whitening of SAR imagery

The presence of speckle in synthetic aperture radar (SAR) imagery makes image interpretation more difficult and worsens the performance of algorithms designed to detect objects in the imagery. Image processing techniques to reduce speckle usually do so at the expense of spatial resolution. Multichannel whitening is one image processing technique that reduces image speckle while maintaining spatial resolution. Multichannel whitening is applied to imagery recorded during a foliage penetration experiment undertaken by MIT Lincoln Laboratory using the NASA/JPL UHF, L-, C-band fully polarimetric SAR in July 1990. In this experiment, a 50 km² forested area near Portage, Maine was imaged. Twenty-seven 8 ft trihedral corner reflectors were arrayed throughout the imaged area beneath the foliage in order to measure foliage attenuation. The detection performance for corner reflectors under foliage is compared for the raw data and whitened data, and the predictions of a product model for the degree of speckle reduction are compared with the data     

A technique for enhanced slant range resolution for SAR systems inknowledge-based environment

Synthetic Aperture Radar (SAR) is an airborne (or spaceborne) radar mapping technique for generating high resolution maps of surface target areas including terrain. High resolution is achieved by coherently combining the returns from a number of radar transmissions. The resolution of the images is determined by the parameters of the emissions, with more data giving greater resolution. A requirement of the Microwave Radar Division's SAR radar is to provide classification of targets. This paper presents a technique for enhancing slant range resolution in SAR images by dithering the carrier centre frequency of the transmitted signal. The procedure controls the radar waveforms so they will optimally perform the classification function, rather than provide an image of best quality. It is shown that a Knowledge-Based engineering approach to determining the waveform of the radar gives considerably improved performance as a classifier of targets (of large radar cross-section), even though the corresponding image is degraded     

Programmable ultrawideband signal generator for electromagnetic susceptibility testing

Under Phase I of the Defense Advanced Research Projects Agency (DARPA) Networking in Extreme Environments (NETEX) initiative, Multispectral Solutions, Inc. (MSSI) was tasked with the development of a general-purpose, ultrawideband hardware simulator capable of reproducing a wide variety of ultrawideband (UWB) waveforms. The simulator, with UWB outputs from baseband through millimeter wave, was to be used for the purpose of electromagnetic susceptibility testing of legacy military radio, radar, and positioning systems. The ultimate goal of this portion of the Phase I program was the quantitative determination of those UWB parameters (e.g., frequency, power, pulse rate, pulse shape, dithering, etc.) which had the least impact on the operational performance of legacy designs. This describes the development of the MSSI NETEX UWB simulator (BFP1000).     

Detection of moving targets in wideband SAR

A likelihood ratio is proposed for moving target detection in a wideband (WB) synthetic aperture radar (SAR) system. WB is defined here as any systems having a large fractional bandwidth, i.e., an ultra wide frequency band combined with a wide antenna beam. The developed method combines time-domain fast backprojection SAR processing methods with moving target detection using space-time processing. The proposed method reduces computational load when sets of relative speeds can be tested using the same clutter-suppressed subaperture beams. The proposed method is tested on narrowband radar data.     

一种孔径和频率二维稀疏的步进频SAR成像方法

 步进频率信号(SFWs)在不增加雷达系统瞬时带宽的情况下能够获得高的距离向分辨率的同时,也存在着抗干扰能力较差及其等效重复频率较低的问题,并且在方位向积累时间内由于雷达载机工作状态的变化,会导致方位向的数据录取不完整。针对上述问题,提出一种孔径和频率二维稀疏的步进频合成孔径雷达(SAR)成像方法。首先,分析了稀疏步进频率信号(SSFWs)的SAR成像模型,然后基于压缩感知理论完成距离向成像处理。其次,针对稀疏孔径的回波数据,通过构造成像算子和压缩感知重建模型的方法实现其距离徙动校正和方位压缩处理,进而获得二维成像结果。相比于传统的步进频率信号SAR成像,利用所提方法能够在少量的频率资源和雷达回波数据情况下实现准确的SAR成像。最后,通过对仿真和实测的步进频率雷达数据进行成像处理,验证了所提方法的有效性和可行性。     

噪声调制连续波雷达信号波形射频隐身特性

雷达信号波形的射频(RF)隐身性能是雷达系统能否适应现代战场环境的重要因素,雷达射频隐身信号波形设计是现代雷达系统设计中的重要课题.首先,在介绍随机噪声信号雷达原理的基础上,基于Schleher截获因子阐述了噪声调制连续波雷达信号波形的射频隐身特性.然后,分析了高斯噪声相位和频率调制连续波雷达输出自相关函数和高斯噪声相...     

Imaging of point scatterers from step-frequency ISAR data

A high-resolution method for imaging of point scatterers from step-frequency inverse synthetic-aperture radar (ISAR) data is presented. An analysis of the noise sensitivity of the method is provided to show the high performance of the method. Simulation results are given to show the robustness of the method against model errors     

Prescreening during image formation for ultrawideband radar

Standard radar image formation techniques waste computational resources by full resolving all areas of the scene, even regions of benign clutter. We introduce a multiscale prescreener algorithm that runs as part of the image formation processing step for ultrawideband (UWB) synthetic aperture radar (SAR) systems. The prescreener processes intermediate radar data generated by a quadtree backprojection image former. As the quadtree algorithm iterates, it is resolving increasingly finer subpatches of the scene. After each quadtree stage, the prescreener makes an estimate of the signal-to-background ratio of each subpatch and applies a constant false alarm rate (CFAR) detector to decide which ones might contain a target of interest. Whenever the prescreener determines that a subpatch is not near a detection, it cues the image former to terminate further processing of that subpatch. Using a small database of UWB radar field data, we demonstrate that the prescreener is able to decrease the overall computational load of the image formation process. We also show that the new multiscale prescreener method produces fewer false alarms than the conventional two-parameter CFAR prescreener applied to the completely formed image