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The performance of high-frequency surface wave radar (HFSWR) is known to suffer from external environmental interference and noise, such as cochannel radio-frequency interference from other radiating source, ionospheric clutter, lightning impulsive noise, etc. This paper experimentally evaluates the interference cancellation performance of various adaptive beamforming schemes with respect to the aforementioned three types of interferences in an attempt to find the most promising adaptive cancellation scheme in practical HFSWR environment. Index Terms--Adaptive beamforming, high-frequency surface wave radar (HFSWR), interference cancellation.

In this paper, a 3-D inverse synthetic aperture radar (ISAR) imaging method based on an antenna array configuration is proposed. The performance of conventional interferometric ISAR imaging system using three antennas is poor, as the positions of scatterers, which have the same range-Doppler value and projected onto the ISAR plane as a synthesis scatterer, cannot be correctly estimated. However, by using two antenna arrays perpendicular to each other, the system's ability to separate these scatterers can be improved. The criterion for the selection of a range unit that contains an isolated scatterer in the 2-D array domain for doing motion compensation is discussed. If there is no range unit which contains only an isolated scatterer, then radial and cross-range motion compensation has to be carried out by motion parameter estimation. Two cross-range motion parameters measurement algorithms, one based on array processing of the range profile and another based on correlation of ISAR images of different antennas, are proposed. The coordinates registration problem for the scatterers of a synthesis scatterer is also discussed. Simulation results have shown the effectiveness of the proposed methods. Index Terms--Antenna array, cross-range motion compensation, inverse synthetic aperture radar (ISAR), 3-D imaging.

The rotation of structures in a target introduces additional frequency modulations on the returned signals and also generates sidebands about the center Doppler frequency of the target. In other words, the body image will be contaminated due to the interference from the rotating parts. In this paper, an imaging method for moving targets with rotating parts is presented. The method is simple to implement and is based on the Hough transform (HT), which is widely used in image processing. Using the standard HT and an extended HT, we put forward a separation method by detecting the straight lines and the sinusoids on the spectrogram, respectively. A computer simulation is given to illustrate the effectiveness of the proposed method. Index Terms--Hough transform (HT), micro-Doppler, moving targets, rotating parts, spectrogram.

The performance of over-the-horizon radars (OTHRs) degrades significantly due to transient interferences such as lightnings, meteor echoes, and man-made impulse bursts. The conventional techniques used to suppress these transient interferences generally follow a simple time-domain procedure which first localizes the transient, sets the corrupted time samples to zero, and follows by data reconstruction via interpolations. However, for an OTHR operation with a short coherent integration time (CIT) of a few seconds, the transient interference may corrupt a significant portion of the received data such that data interpolation can become erroneous. In this paper, a new transient interference excision technique is proposed. It utilizes the adaptive time-frequency analysis technique to parameterize the radar signal. The transient interference can then be identified and subsequently removed via their characteristic parameters. No data interpolation is needed. The performance of the proposed technique is illustrated by simulated and experimental OTHR data. Index Terms--Adaptive Gaussian basis representation (AGR), over-the-horizon radar (OTHR), transient interference.

In this paper, a three-dimensional (3-D) interferometric synthetic aperture radar (ISAR) imaging method for moving targets is presented. This imaging method is based on the ISAR principle and the simple observation that all scatterers on a moving target move in tandem. The angular motion parameters in the cross-range directions could be estimated using the overall range profile of the moving target. Registration of the respective complex images at the two (or more) interferometric antennas can then be achieved via compensating the respective echoes at the raw data level, thus avoiding phase-unwrapping processing and image-resampling processing as required by conventional methods. Finally, a 3-D image of the moving target can then be reconstructed from the 3-D spatial coordinates of these scatterers. Furthermore, the method works well even for a target moving in heavily cluttered environments. Index Terms--Motion parameters estimation, synthetic aperture radar (SAR), moving target imaging.

In most interferometric inverse synthetic aperture radar (InISAR) systems, the pixels between two ISAR images derived from corresponding antennas usually do not register properly. As such, correct phase difference between two ISAR images could not be obtained. A three-dimensional (3-D) motion compensation method, or 3-D focusing, is put forward in this paper. With a multiple antenna pair configuration, the angular motion parameters both in the azimuth and pitching directions are accurately estimated without the usual phase-unwrapping processing. As such, there is no phase ambiguity in the inteferometric systems. The angle motion trajectory measurement here is based on the range profiles (or spatial spectrum) so that the 'angle scintillation' phenomenon can be effectively suppressed. The angular motion trajectory is then obtained by curve fitting of the spatial spectrum (range profile in the cross-range direction). The compensated ISAR images are registered accurately. Finally, the simulation data are used to illustrate the accuracy of the proposed method. Index Terms--Angle motion parameter, interferometric inverse synthetic aperture radar (ISAR), radar imaging, registration, three dimensions.

The range migration algorithm (RMA) is an accurate algorithm to process spotlight synthetic aperture radar (SAR) data. However, in applying the RMA to process the SAR data, especially for short-dwell data, the data length has to be extended in azimuth by zero-padding it to the length of the azimuth filter. On the other hand, the spectral analysis (SPECAN, which is a deramp followed by a fast Fourier transform) algorithm is efficient, since the data extension is unnecessary, but an assumption is made that all targets have the same FM rate and the same range cell migration. A novel algorithm is introduced here to combine the accuracy of the RMA and the efficiency of SPECAN. It involves bulk azimuth uncompressing the RMA processed data with a single linear FM filter, and then refocusing the data using SPECAN, all done without the azimuth extension in the RMA. Localized artifacts generated can be removed easily. Point target simulations, including one with a high squint angle of 60[degrees], were successfully performed to verify the algorithm. Index Terms--Aliasing, compression filter, linear FM rate, range cell migration, range migration algorithm (RMA), spectral analysis (SPECAN), synthetic aperture radar (SAR) processing.

In this paper, a new easy-to-implement variant of stepped-frequency signal format is presented for ground clutter cancellation application. A sequence of pulse-groups, where each pulse-group consists of two pulses of the same carrier frequency, is transmitted. The carrier frequency of each subsequent burst is stepped at [DELTA]f. Simple first-order cancellation of ground clutter can be carried out by comparing the returns of the two adjacent pulses, as the clutter movement (due to wind, etc.) can be assumed negligible within this interval (typically < 1 [micro]s). A computer simulation is given to illustrate the effectiveness of the proposed method. Index Terms--Clutter cancellation, inverse synthetic aperture radar (ISAR) imaging, stepped-frequency signal.

Based on the relative differential box-counting algorithm and the gliding-box algorithm, a novel method for estimating the lacunarity features of grayscale digital images is proposed in this paper. Four nature texture images are used to test the performance of the novel lacunarity measure. Comparisons with published methods show that the proposed method can efficiently describe texture images, and provide accurate classification results. Real synthetic aperture radar (SAR) images analyses are found to have different lacunarity values for different regions. We show that good result can be obtained with appropriate lacunarity parameters applied to SAR images segmentation. Index Terms--Image segmentation, lacunarity, synthetic aperture radar (SAR), texture analysis.

A new method of phase error estimation that utilizes the weighted least-squares (WLS) algorithm is presented for synthetic aperture radar (SAR)/inverse SAR (ISAR) autofocus applications. The method does not require that the signal in each range bin be of a certain distribution model, and thus it is robust for many kinds of scene content. The most attractive attribute of the new method is that it can be used to estimate all kinds of phase errors, no matter whether they are of low order, high order, or random. Compared with other methods, the WLS estimation is optimal in the sense that it has the minimum variance of the estimation error. Excellent results have been obtained in autofocusing and imaging experiments on real SAR and ISAR data. Index Terms: Autofocus, inverse synthetic aperture radar (ISAR), synthetic aperture radar (SAR), weighted least-squares (WLS).

In this paper, a high squint subaperture (HSS) algorithm was developed from the perspective of a stripmap synthetic aperture radar (SAR) system and has the ability to focus SAR data at extremely high squint angle of 55 [degrees] with less than 1.27% mainlobe expansion or even higher squint angles if larger mainlobe expansion can be tolerated. The unique characteristic of this algorithm is that the subapertures are formed by multiplying the received signal with a set of overlapped complex conjugated reference signals where the chirp rate varies in the azimuth direction. This is done to solve the range focus problem that arises after the azimuth input signal undergoes the range walk removal procedure. As the HSS algorithm only involves short fast Fourier transforms (FFTs) and avoids interpolation, it is computationally efficient and allows for small data buffers and facilitates hardware real-time implementation. The distinct feature of this algorithm is its simplicity of implementation, which is vital in real time processing and motion compensation when the squint angle may be changing continuously. Index Terms--High squint SAR, subaperture approach.

New applications of nonlinear chirp scaling (NLCS) have been found in processing monostatic and bistatic SAR data acquired in the strip-map mode. When the effects in impulse response broadenings due to quadratic range migration are negligible for short radar wavelengths, a time domain range cell migration correction (RCMC) can be applied. After the correction, the FM rates of targets confined in a range gate will differ from each other. A nonlinear chirp perturbation function can be applied to each range gate to equalize the targets' FM rates before azimuth compression. This algorithm is analyzed in the paper and is supported by point target simulation experiments.

A new technique for inversed synthetic aperture radar (ISAR) ranging, which resembles the principle of the Vernier measuring system, is presented in this paper. In this technique, the transmitted ISAR pulse comprises a train of chirp subpulses with uniformly stepped up center frequencies. The return ISAR echo is first processed, using hardware, to determine a coarse estimate of the target range. Further refinements of the range estimate are achieved through software processing, consisting of two stages of discrete Fourier transform operation. The ranging accuracy can be increased without the need for increased bandwidth, but at the expense of a slight increase in computational complexity. Numerical evaluation shows that a noiseless system is capable of achieving high-ranging accuracy, of the order of millimeters, even in the presence of dispersion and target motion. From computer simulations, the proposed system is also found to be robust against additive system noise and frequency jitter under practical conditions. Index Terms--Inverse synthetic aperture radar (ISAR), radar ranging, super resolution.

Propagation of electromagnetic waves in forest environments is examined in which both the transmitting and receiving points are located in the trunk layer of a four-layered anisotropic forest model. This propagation model considers the forest as a horizontally stratified, anisotropic media of canopy and trunk, bounded by ground below and air above. The electromagnetic fields are obtained using dyadic Green's functions in their eigenfunction expansion forms for an anisotropic four-layered geometry. Analytical results are found for the fields, which consist primarily of three wave modes: a direct wave, multiply reflected waves, and lateral waves. These field constituents are compared, and their domains of preponderance are calculated; it is found that the lateral wave plays a major role in communication at large distances. Radio losses for typical forest are calculated to illustrate numerical application of the forest model. Index Terms - Anisotropic media, dipole antennas, electromagnetic propagation, Green function, nonhomogeneous media.