Ground Moving Target Indication Performance Research Articles

A Study on the Range Equation Modeling for Multichannel Medium-Earth-Orbit SAR-GMTI Systems

This paper studies the range equation modeling of a ground moving target for multichannel medium-Earth-orbit (MEO) synthetic aperture radar (SAR) ground moving target indication (GMTI), an issue which is challenging to tackle due to the non-linear motion of the radar platform and the Earth rotation. In the paper, the coordinates of the multichannel MEO SAR and the target, as well as the target’s range equation with respect to each channel, are developed. Moreover, an expression of concise form is derived for the target’s quadratic-approximated range equation, which will benefit the design of GMTI methods. Furthermore, theoretical analyses are conducted to reveal the dependency between the accuracy of the quadratic-approximated range equation and the parameters of the radar and the target. Numerical simulations are carried out to investigate the influence of the quadratic approximation of the range equation on the GMTI performance and to figure out the quadratic-approximated range equation’s scope of application.

Multi-Channel SAR System Design for HRWS and GMTI in Low PRF

High-resolution and wide-swath (HRWS) is an important challenge in synthetic aperture radar (SAR). Combined SAR with array, the azimuth dual-channel SAR configuration achieves high azimuth resolution and HRWS in low Pulse repetition frequency (PRF) by doppler band synthesizing technology. However, low PRF introduces ambiguity to ground moving target indication (GMTI). Interferometry is performed between two look data in the range frequency domain to resolve the ambiguity by decreasing the interferometry phase. As a result, the phase becomes insensitive to the radial velocity of moving targets. This condition brings difficultly in moving target detection. Two channels together with two equivalent channels are exploited fully to solve these problems. The four-channel interferometry is also conducted to improve the GMTI performance. Parameters, including system and motion parameters, are estimated unambiguously to complete HRWS imaging and GMTI. System parameter design is also considered to satisfy the requirements of HRWS and GMTI simultaneously. Simulations demonstrate the validity of the proposed methods.

A Clutter Suppression Method for Dual-channel Circular SAR-GMTI

Ground moving target indication (GMTI) technology is an important development direction in the field of SAR signal processing, and the performance of GMTI is determined by clutter suppression directly. Since the trajectory of airborne platform is a circular arc and the beam directions of antennas are time-varying in the circular SAR (CSAR) mode, the classical channel registration methods for the strip-map SAR are no longer applicable. Based on the measured data of a CSAR system, channel calibration and channel equalization are investigated to eliminate the phase and amplitude differences of the clutter between the channels. Further, according to the complementary property of displaced phase center antenna (DPCA) and along-track interferometry (ATI) technologies, we propose a clutter suppression method in the complex image domain by utilizing the interference phase of ATI to weight the DPCA amplitude. The process results of measured data prove a good capability for clutter suppression both in homogeneous and inhomogeneous areas.

Blind velocities mitigation for MIMO GMTI radar with Doppler division multiple access waveforms

Multiple-input multiple-output (MIMO) radar with multiple transmitters and multiple receivers can achieve a larger virtual antenna array and more system degrees of freedom; thus applying it to ground moving target indication (GMTI) radar can improve the performance of GMTI. Doppler division multiple access (DDMA) waveforms are approximately orthogonal providing good minimum detectable velocity (MDV) performance. However, in such DDMA systems, a sufficient pulse repetition frequency (PRF) design freedom is required. Furthermore, these waveforms suffer from blind velocities which are serious problems, especially in radar systems with high carrier frequency or low PRF. This paper analyses the blind velocities problem and show that blind velocities are relative to variation of the PRF and/or the carrier frequency. Variable PRF techniques are widely used in conventional GMTI radar including multiple PRFs and variable pulse repetition intervals (PRI). Combined with the characteristics of the DDMA MIMO GMTI radar, this paper proposed two methods to mitigate blind velocities: “multi-PRF DDMA” which employs multiple PRFs over successive coherent processing intervals, and “PRI-dithered DDMA” which employs nonuniform sampling by dithered PRI in slow time. Simulation results demonstrate that both the methods are effective ways to mitigate blind velocities in DDMA MIMO GMTI radar systems.

Improved MIMO radar GMTI via cyclic-shift transmission of orthogonal frequency division signals

ABSTRACTMinimum detectable velocity (MDV) and maximum detectable velocity are both important in ground moving target indication (GMTI) systems. Smaller MDV can be achieved by longer baseline via multiple-input multiple-output (MIMO) radar. Maximum detectable velocity is decided by blind velocities associated with carrier frequencies, and blind velocities can be mitigated by orthogonal frequency division signals. However, the scattering echoes from different carrier frequencies are independent, which is not good for improving MDV performance. An improved cyclic-shift transmission is applied in MIMO GMTI system in this paper. MDV performance is improved due to the longer baseline, and maximum detectable velocity performance is improved due to the mitigation of blind velocities via multiple carrier frequencies. The signal model for this mode is established, the principle of mitigating blind velocities with orthogonal frequency division signals is presented; the performance of different MIMO GMTI waveforms is analysed; and the performance of different array configurations is analysed. Simulation results by space–time–frequency adaptive processing proves that our proposed method is a valid way to improve GMTI performance.

Simultaneous High-Resolution Wide-Swath SAR Imaging and Ground Moving Target Indication: Processing Approaches and System Concepts

In this paper, a novel method for enabling ground moving target capability for high-resolution wide-swath (HRWS) synthetic aperture radar (SAR) systems is presented. Such HRWS SAR systems are generally operated with very low pulse repetition frequencies (PRFs). With the novel method, moving target signals highly aliased in Doppler can be reconstructed and the targets’ motion parameters and actual positions can be estimated with high accuracy. A special matched reconstruction filter bank (MRFB) is introduced and incorporated into two novel low PRF ground moving target indication (GMTI) algorithms. Owing to the application of the MRFB, GMTI and HRWS SAR imaging can be performed simultaneously without the need of a separate high PRF system operation mode, which, so far, was necessary for achieving a reasonable GMTI performance. With the MRFB, the advantages of the HRWS imaging principle, i.e., the wide swath and the high resolution are preserved for both HRWS SAR imaging and GMTI. Potential applications are wide-area land and maritime traffic monitoring, where the benefit of the preserved high resolution could, for instance, be used for generating high-quality inverse SAR (ISAR) images for target recognition purposes.

Design Considerations of PRF for Optimizing GMTI Performance in Azimuth Multichannel SAR Systems With HRWS Imaging Capability

Using multichannel in azimuth to suppress the Doppler ambiguities allows for high-resolution wide-swath (HRWS) synthetic aperture radar (SAR) imaging, thus overcoming the minimum antenna area constraint of the conventional space-borne SAR. If the degrees of freedom in azimuth are used for the clutter suppression, the ground moving target indication (GMTI) can be achieved. Therefore, a space-borne multichannel SAR system for GMTI has the potential to offer HRWS imaging capability to some extent. However, here, the GMTI performance may suffer from the Doppler ambiguity caused by the undersampling. This paper focuses on the analysis of the influence of pulse repetition frequency (PRF) on GMTI performance in Doppler ambiguity. The multichannel signal models of the clutter and the moving target with Doppler ambiguity are derived in complex image domain. Considering the Doppler ambiguity and the multichannel spatial ambiguity, the influence of PRF on the output signal-to-clutter-plus-noise ratio after adaptive clutter suppression and the signal-to-noise ratio loss after SAR imaging is discussed. Accordingly, the design considerations of PRF for optimizing GMTI performance in multichannel SAR systems with HRWS imaging capability are given, along with a simulation example. Finally, the real-data experiments verify the theoretical investigations.

A Generalization of DPCA Processing for Multichannel SAR/GMTI Radars

This paper generalizes the well-known displaced- phase-center antenna (DPCA) method for efficient ground moving target indication (GMTI) with a two-channel synthetic aperture radar (SAR) to any multichannel SAR/GMTI radars independent of the number of receive channels. This processing method called extended DPCA (EDPCA) is derived in this paper and is applied to data acquired with the Canadian RADARSAT-2 satellite. The expected GMTI performance of RADARSAT-2 after EDPCA processing is compared to results achieved with measured RADARSAT-2 data recorded during several trials in order to validate the developed theory.

Optimum SAR/GMTI Processing and Its Application to the Radar Satellite RADARSAT-2 for Traffic Monitoring

This paper derives an optimum processing method for ground moving-target indication (GMTI) with a multichannel synthetic aperture radar (SAR) system. This method enables efficient detection of moving objects and accurate estimation of their parameters and does not require any knowledge of the street network. The processing is applied to data acquired with the Canadian RADARSAT-2 satellite. Results of the performed trial are compared with the expected GMTI performance of the radar in order to validate the theory.

Analysis of the Multimode Feedhorn Concept for Multichannel SAR-GMTI

This paper presents the theoretical and experimental performance evaluation of a novel and innovative concept to add a multichannel ground moving target indication (GMTI) capability to a synthetic aperture radar (SAR) reflector antenna with a standard single feed. This approach exploits the capability to achieve a phase center displacement by combining signals obtained from different electromagnetic excitation modes of the feedhorn. While, in the past, this so-called multimode concept has been mainly analyzed in simulation and by various antenna laboratory measurements, several flight experiments have been recently conducted to verify and demonstrate its GMTI performance. The theoretical and experimental analysis provides qualitative and quantitative insights into the effectiveness of multimode SAR-GMTI applications, such as the SAR image-based along-track interferometry (ATI) or post-Doppler GMTI approaches based on space-time adaptive processing. In addition, it introduces practical lessons learned, along with an example of the detection and parameter estimation for a large vessel using ATI. This paper finishes with an examination of the expected performance improvement when the transmitted modes are alternated between subsequent pulses. This toggling results in a time-multiplexed three-channel GMTI system with significantly enhanced estimation performance.

Experimental Verification of SAR-GMTI Improvement Through Antenna Switching

Recently, theoretical investigations have shown that switching between parts of the transmit and/or receive aperture in a pulse-to-pulse cycle may improve the ground moving target indication (GMTI) performance of a two-channel radar system. This paper intends to verify these findings experimentally for the switching concept for the first time. Accordingly, six different GMTI modes were implemented in the system PAMIR (phased-array multifunctional imaging radar) of the Fraunhofer Institute for High Frequency Physics and Radar Techniques (Fraunhofer FHR) including three switching modes. An experiment was conducted using six cooperative vehicles following a predefined scenario to compare the GMTI performance and, in particular, the positioning accuracy of the different GMTI modes. The results of this experiment as well as the underlying signal processing are presented in this paper. The analysis is based on a multilook constant false-alarm rate detection scheme. Due to the heterogenous nature of the considered scene, the well-known test statistic for homogeneous terrain had to be extended accordingly, and a novel test statistics is proposed.

Improved Space-Based Moving Target Indication via Alternate Transmission and Receiver Switching

Ground moving target indication (GMTI) by space-based radar systems requires special antenna and data acquisition concepts to overcome the problem of discriminating target signals from clutter returns. Owing to the high satellite speed, the clutter contains a broad mixture of radial velocities within the antenna beam, leading to a large Doppler spread. Effective clutter suppression can solely be achieved by multiple aperture or phase center antennas. For space-based systems, however, the number of receiver channels connected to subapertures is very limited due to their weight, volume, and high data rates (current systems such as TerraSAR-X and RADARSAT-2 possess only two). This classical along-track interferometry architecture, in which the antenna is split into two halves, achieves only suboptimum GMTI performance. This paper presents and statistically analyzes an innovative approach to create additional independent phase centers to improve the GMTI performance considerably. The extra degrees of freedom are created by cyclical phase and amplitude switchings of the transmit/receive modules for transmitter and receiver between pulses, hence trading Doppler bandwidth for extra spatial diversity. In the first part of this paper, different strategies of spatial-temporal diversity are introduced and analyzed for realistic system parameters with respect to ambiguities and detection performance. The second part is concerned with the elaborate theoretical analysis of the relocation improvement for the spatial diversity approach.

Wide-Area Traffic Monitoring With the SAR/GMTI System PAMIR

This paper presents a wide area traffic monitoring experiment under real conditions, using the scan-MTI mode of the airborne radar sensor PAMIR. This flexible GMTI (Ground Moving Target Indication) mode was designed in order to rapidly monitor wide areas for moving targets. The scan operation enables the detection of targets from different aspect angles with a high revisit rate. The parameters (e.g., radial velocity, signal-to-noise ratio, and positioning accuracy) of the detected vehicles are investigated and compared to the expected theoretical GMTI performance. It will be shown that the scan-MTI mode is particularly adapted to perform an efficient wide-area traffic monitoring.

A new approach for ground moving target indication in foliage environment

Ground moving target indication (GMTI) in foliage environment is a challenging problem, because the strong echoes from dense foliage environment and big attenuation of the radar microwave often make it difficult for traditional time frequency distributions to clearly and locally represent the Doppler frequency. This paper introduced a new approach to improve the performance of GMTI. This so-called Hilbert Huang Transform (HHT) method composes of empirical mode decomposition (EMD) and Hilbert transform technique. EMD is applied to extract target signal from interference and the associated Hilbert transform calculates the instantaneous Doppler frequency, representing a novel Hilbert spectrum in time frequency domain. Both simulative and experimental signals are used to validate the efficiency of HHT. This adaptive approach offers higher signal to disturbance ratio and higher resolution in the time frequency distribution.