Using spaceborne transmitter and airborne receiver, the space–air bistatic radar (SABR) system has advantages of wide coverage, antistealth, and anti-interference. However, the SABR encounters severe clutter spreading in spatial–temporal domain since it works in down-looking mode, which would cause serious performance degradation in moving target detection. Moreover, SABR encounters several practical factors, such as the separated transmitter and receiver, the non-side-looking array configuration, wide Earth coverage, and Earth rotation, which would induce range-dependent and high-order nonlinear spatial–temporal-coupled ground/sea clutter. Under this circumstance, this article first investigates the general space-time clutter model of SABR for arbitrary transmitter–receiver geometry and array configuration. Then, the high-order nonlinear coupling relationship of clutter curve is derived, where the expressions of the spatial frequency and Doppler frequency are derived with respect to the eccentric anomaly of bistatic iso-range ring. In the sequel, the spatial–temporal-coupled and range-dependent characteristics of the clutter in SABR is analyzed and discussed in detail for three typical geometrical configurations with high/medium/low elliptical orbit satellite as transmitter and aircraft as receiver. On the basis of the above analysis, an effective clutter compensation approach based on subspace rotation is proposed to eliminate the range dependence of the clutter. Numerical examples of different bistatic geometries are conducted to analyze the spatial–temporal coupling relationship of clutter and validate the effectiveness of the proposed compensation method.

Frequency diverse array (FDA) introduces frequency increment across array elements, resulting in increased controllable degrees-of-freedom (DOFs) in spatial and frequency domains. As different sub-band waveforms are transmitted by different elements simultaneously, it is capable of obtaining a large synthesized bandwidth at the receiver, thus improving the resolution of radar imaging. In this article, a general bandwidth synthesis approach is proposed, which includes flexible shift of each sub-band waveform as well as combination of all these sub-band waveforms in frequency domain. Since the sub-band waveforms can be flexibly shifted in frequency domain, we can obtain an arbitrarily synthesized bandwidth, both wideband and narrowband waveforms. In the proposed approach, the general bandwidth synthesis mechanism is revealed. The analytical expressions of the bandwidth-synthesized signals are derived with an arbitrary shift in frequency domain, resulting in multiresolution synthetic aperture radar (SAR) imaging, which provides the potential for realizing multimode SAR imaging simultaneously. Several specific cases regarding the bandwidth-synthesized signals, including overlapped and non-overlapped, are provided to further explain the bandwidth synthesis approach. The range resolution, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR) performance are analyzed. Nonuniform frequency shifts are applied to alleviate the PSLR degradation due to non-overlapped sub-band synthesis. Numerical simulation results of point target and distributed scene target are provided to demonstrate the effectiveness of the proposed approach.

Video synthetic aperture radar (SAR) has been found very useful in ground moving target indication (GMTI) and tracking. The dynamic shadows in video SAR imagery sequences indicate the real positions of moving targets, which can be utilized in target detection and tracking. Unfortunately, the shadow-based method often fails when the shadows are not sufficiently developed. On the other hand, the traditional energy-based GMTI methods exhibit performance degradation when SAR images of a moving target are distorted or smeared. Neither of these two methods can stand alone to provide robust detection and tracking of moving targets. This article presents a joint processing framework for video SAR GMTI and tracking by combining the target shadow and echo energy information, which effectively lowers the false alarm and miss-detection rate. This approach is very suitable for real-time surveillance of ground moving targets in a single-channel video SAR system. The proposed approach has been verified by using the simulated and the real video SAR data.

Beamforming plays a crucial role in synthetic aperture radar (SAR) for interference mitigation and ambiguity unaliasing. In this article, a series of novel beamforming methods for SAR systems is proposed based on nulling the transceive beampattern accurately in frequency diverse array (FDA)-multiple-input and multiple-output (MIMO) scheme. In general, these methods are implemented by assigning artificial interferences with prescribed powers within the given rectangular regions in the joint transmit-receive spatial frequency domain. In specific, according to the predefined null depths, closed-form expressions of artificial interference powers are first formulated. Then, iteration algorithms are developed to update the interference-plus-noise covariance matrix and the designed weight vector. In such a way, a trough-like transceive beampattern with arbitrarily distributed broadened nulls is formed in the joint transmit-receive spatial frequency domain. As a result, interferences mixed in signals received by SAR can be suppressed effectively. Numerical simulations and experimental results are provided to corroborate the effectiveness of the proposed methods.

This article presents a novel radar imaging approach through an electronically displaced-phase-center antenna (eDPCA). In this approach, a reflectarray is used to directly scan a beam across a parabolic reflector, where the antenna phase center is displaced electronically. The proposed eDPCA-based imaging radar can provide a very high frame rate and does not depend on the motion between the target and the radar platform. The feasibility of the eDPCA-based imaging radar is investigated and validated through a designed example eDPCA at 157 GHz and the simulation experiments for a point target and a complex target, respectively. The effects of practical impairments such as errors in phase centers and position-dependent fluctuations in antenna patterns on the imaging quality are examined. Furthermore, by applying the precompensation method suggested in this article, the imaging procedure could be performed by using classic synthetic aperture radar (SAR) algorithms.

In this paper, we propose a long-time coherent integration method for a maneuvering target with the first-, second-, and third-order range migrations, and the complex Doppler frequency migration. In this method, after range compression, the echo signal is first transformed into the range-frequency and Doppler domain based on series reversion, and then an azimuth matched filtering procedure is implemented in the 2-D frequency domain. It can eliminate the coupling effects between range and azimuth jointly caused by the radial velocity, radial acceleration, and radial acceleration rate of a moving target. Due to the linear transform property, the proposed method can work well under low signal-to-clutter and noise ratio. In addition, the Doppler ambiguities, when target azimuth spectrum either is within a pulse repetition frequency (PRF) or spans over neighboring PRF bands, can be well solved. Both simulated and real synthetic aperture radar data processing results are provided to validate the effectiveness of the proposed algorithm.

The image processing for diving highly squinted synthetic aperture radar (SAR) mounted on a maneuvering platform with nonlinear trajectory is challenging due to the spatial variance in the range envelope along azimuth, which is caused by the inherent range dependence of squint angle, platform acceleration, and the range cell migration (RCM). In order to deal with the spatial variance that leads to defocusing in SAR imaging, a range perturbation approach is proposed in the frequency domain to mitigate the azimuth-dependent RCM curvatures, so that the RCM can be uniformly compensated for in the 2-D frequency domain. Moreover, this approach is totally interpolation free, and thus the computation load is reduced dramatically. The effectiveness of the proposed approach is confirmed and demonstrated via simulations.

For radar target detection, the clutter distribution model needs to be identified first. The goodness of fit (GoF) between the original data and the assumed distribution can be used to choose the proper distribution model. Generally, the GoF is obtained using data histogram and theoretical distribution curve, and then the distribution model is judged via GoF. However, when the sample number is small, the histogram is rough and fluctuating, affecting the analysis of GoF. For the small sample, the graphical characteristic is obtained with the sample data to choose the most fitting distribution to the data in this paper. The graphical characteristic is acquired by a simpler process, that is, the original data are directly set as the test statistics, avoiding computing and sorting of other statistics. In this paper, the real airborne circular synthetic aperture radar data under different scan angles are analyzed using the GoF corresponding to histogram and graphical GoF, respectively. The results show that when the sea-clutter data histogram is close to two distributions, a more fitting distribution model may not be obtained by traditional GoF, but can be acquired by graphical representation. In addition, the sea data with different sight angles have different match properties. It is seen that the sea data are closer to the Rayleigh distribution in side-looking mode than that in big squint-angle mode, while the Weibull distribution and K distribution show equal fitting performance to sea clutter under variant radar sight angles.

In the conventional space-time signal model, the statistical amplitude of the clutter is assumed to be a specific distribution. For sea clutter, a pulse-to-pulse correlation matrix is added to the temporal covariance matrix to describe the motion of the sea surface. However, such sea clutter model cannot reflect the property of the sea clutter. In this paper, a space-time model for the sea clutter is presented based on the physical sea surface model, in which the clutter amplitude is deterministic instead of statistic. The reflectivity and the radial velocity of the sea clutter for any specific position and time are computed based on the physical sea surface. Both the aforementioned two factors vary with time, which corresponds to the time variation of the sea surface. Moreover, the spatial channel decorrelation is modeled, which has an effect on the spatial covariance matrix of the sea clutter. The simulated angle-Doppler spectra and the signal-to-clutter-plus-noise-ratio loss show that the reflectivity of the sea clutter makes the space-time-adaptive-processing performance degrade, and the radial velocity of the sea clutter results in a more significant spread of the clutter power spectrum in sea state 4 than sea state 2. The Doppler spectrum can be acquired by the space-time model proposed in this paper instead of the experimental Doppler model for sea clutter. For a given sea state, the low range resolution has a more prominent effect on the power spectrum because of severer spatial channel decorrelation.