This article provides a brief review of radar space-time adaptive processing (STAP) from its inception to state-of-the art developments. The topic is treated from both intuitive and theoretical aspects. A key requirement of STAP is knowledge of the spectral characteristics underlying the interference scenario of interest. Additional issues of importance in STAP include the computational cost of the adaptive algorithm as well as the ability to maintain a constant false alarm rate (CFAR) over widely varying interference statistics. This article addresses these topics, developing the need for a knowledge-based (KB) perspective. The focus here is on signal processing for radar systems using multiple antenna elements that coherently process multiple pulses. An adaptive array of spatially distributed sensors, which processes multiple temporal snapshots, overcomes the directivity and resolution limitations of a single sensor.

Space-time adaptive processing: a knowledge-based perspective for airborne radar

We derive the nonhomogeneity detector (NHD) for non-Gaussian interference scenarios and present a statistical analysis of the method. The non-Gaussian interference scenario is assumed to be modeled by a spherically invariant random process (SIRP). We present a method for selecting representative (homogeneous) training data based on our statistical analysis of the NHD for finite sample support used in covariance estimation. In particular, an exact theoretical expression for the NHD test statistic probability density function (PDF) is derived. Performance analysis of the NHD is presented using both simulated data and measured data from the multichannel airborne radar measurement (MCARM) program. A performance comparison with existing NHD approaches is also included.

Statistical analysis of the nonhomogeneity detector for non-Gaussian interference backgrounds

For air- and spaceborne MTI radar the clutter Doppler frequency depends on range for all array geometries other than linear sidelooking. This applies particularly to forward-looking arrays. The range dependence of the clutter Doppler causes specific problems in clutter rejection through STAP (space-time adaptive processing) techniques. One aspect is the lack of training data required for adapting the STAP filter. Another aspect is that the STAP filter becomes range dependent. A technique for compensating for the range dependence is proposed. The efficiency of the method is verified by use of clutter data measured with the multichannel radar AER II (airborne experimental radar).

Doppler compensation in forward-looking STAP radar

Space-time adaptive processing (STAP) holds tremendous potential for the new generation airborne surveillance radar, in which the phased array antennas and pulse Doppler processing mode are adopted. A new STAP approach using the multiple-beam and multiple Doppler channels is presented here for airborne phased array radar. The approach with space-time multiple-beam (STMB) architecture is robust to array errors and has very low system degrees of freedom (DOFs). Hence, it has low sample support requirement and it is very suitable for the practical planar phased array radar under nonhomogeneous clutter environments. Meanwhile, a new nonhomogeneous detector (NHD) based on the correlation dimension (CD) is also proposed here, which is used as an effective method to screen tracing data prior to detection processing. It can further improve the performance of the STAP approach in the severely nonhomogeneous clutter environments. Therefore, a scheme that incorporates the correlation dimension nonhomogeneity detector (CD-NHD) with the STMB is recommended, which we term CD-NHD-STMB. The experimental simulation results indicate that: 1) the STMB processor is robust to array element error and has high performance under nonhomogeneous clutter environments; 2) the CD-NHD is also effective on the nonhomogeneous clutter. As a result, the CD-NHD-STMB scheme is robust to array element error and nonhomogeneous clutter, and therefore available for airborne phased array radar applications.

Robust space-time adaptive processing for airborne radar in nonhomogeneous clutter environments

A high-pulse-repetition-frequency (PRF) radar can handle the high Doppler frequencies of clutter echoes received by a fast-moving airborne radar. However, high-PRF radar causes range ambiguity. In addition, the clutter is range dependent when the airborne radar works in a forward-looking geometry. The range ambiguity and range dependence will lead to severe performance degradation of the traditional space-time adaptive processing (STAP) methods. In this paper, a vertical frequency diverse array (FDA), which applies frequency diversity in the vertical of a planar array, is explored to circumvent the range ambiguity problem in STAP radar. A range-ambiguous clutter suppression approach is devised, which consists of vertical spatial frequency compensation and pre-STAP filtering. In the vertical-FDA radar, the vertical spatial frequency depends not only on the depression angle but also on the slant range. By using this characteristic, the range-ambiguous clutter can be separated in the vertical spatial frequency domain, and then, clutter suppression is achieved for each separated range region. As a result, both problems of range ambiguity and range dependence are solved. Simulation results are provided to demonstrate the effectiveness of the proposed method.

Space-Time Adaptive Processing With Vertical Frequency Diverse Array for Range-Ambiguous Clutter Suppression

Space-time adaptive processing (STAP) is a crucial technique for the new generation airborne radar with high air-to-ground performance. The authors study this technique applied to airborne early warning (AEW) radars with various array orientations, which include the sideways looking array radar (SLAR) and non-sideways looking array radar. First, the ground clutter is characterised by showing the angular-Doppler spectra and Doppler-range relation for various array orientations. Secondly, the impact of various array configurations and pulse repetition frequencies (PRFs) on the STAP performance by the use of a multiple Doppler channels joint adaptive processing (M-CAP) approach is analysed. Thirdly, a 3-Doppler-channel processing scheme, in coordination with the various PRF modes, is recommended to solve the STAP problem for practical AEW radars with various array orientations. Finally, some examples are given by simulation to show the performance of the 3-Doppler-channel processing scheme for various cases of interest and to validate the theoretical analysis.

Space-time adaptive processing for airborne radar with various array orientations

Space-time adaptive processing (STAP) has been widely discussed for airborne radar systems to improve the system performance of detecting targets. This is especially true for airborne early warning (AEW) radar, which should find long-range and small radar cross section (RCS) targets such as the stealth aircraft and missiles. However, in existing airborne radar literature, STAP is mainly considered for clutter and jamming rejection in side-looking airborne radar (SLAR) applications. There have been fewer discussions on airborne radar with non-side-ways looking array radar (non-SLAR). The STAP of non-SLAR such as forward looking array radar is also very important and can not be avoided for airborne radar to detect targets in all directions. The STAP of the non-SLAR is studied here. A scheme has been proposed, which is processed by the way of STAP combined with multiple staggered medium pulse repetition frequencies (PRFs). We further study the selection of PRFs in order to make the scheme more available for non-SLAR radar. We analyze two typical non-SLAR cases, i.e., inclined-sideways looking array and forward looking array. We examine this scheme by comparing the performances of three processing systems under the criteria of range-velocity blind zone minimization. Computer simulation results show the multiple-PRFs STAP scheme is feasible for non-SLAR and can be applied to phased-array AEW radar systems.

Yong-Liang Wang

Papers

Robust space-time adaptive processing for airborne radar in nonhomogeneous clutter environments

Space-time adaptive processing for airborne radar with various array orientations

STAP with medium PRF mode for non-side-looking airborne radar