EUSAR 2021 Tutorial: "GMTI with Multi-Channel SAR" with Prof. Dr.-Ing. Joachim Ender

EUSAR 2021 Tutorial
GMTI with Multi-Channel SAR
Prof. Dr.-Ing. Joachim Ender
Air- or space-borne radar/SAR systems with only a single antenna have severe difficulties to recognize slowly moving targets against the ground clutter. Here, ‘slowly’ here has to be understood relative to the platform velocity, so fast cars are slow compared to the velocity of a SAR satellite. The reason is that the relative radial velocity of the target then is covered by the interval of relative radial velocities of fixed ground scatterers illuminated by the antennas main beam (‘endo-clutter’ targets).
The ambiguity between radial velocity and direction can be resolved by introducing a redundancy in form of two or more phase centers displaced in flight direction and the related number of receive channels.
The lecture starts with the treatment of the easy to understand ‘short CPI case’, i.e. during the coherent integration time the signal of a moving or non-moving scatterer does not leave its range/Doppler cell. For this case classical techniques as displace-phase-center antenna (DPCA) are explained as well as the probabilistically motivated space-time adaptive processing (STAP). We will compare the performance in detection and DOA-estimation for pre- and post-Doppler STAP in dependence on the number of channels and temporal samples.
On this fundament, we pass over to the SAR case with its long temporal azimuth signals including non-linear frequency modulation and range curvatures. First, we study the effects of target motion on the SAR image, followed by the treatment and analysis of the classical two-antenna along-track-interferometry (ATI) and the transition of STAP to the SAR case.
Beyond STAP new GMTI algorithms were developed using the benefits of compressive sensing (CS). They are based on the property that in the Doppler-direction plane ground clutter is concentrated on the clutter ridge and along this continuously distributed, while moving target signals appear beside the ridge, and are sparse.
We will further address the special challenges of GMTI with space based SAR systems. Small objects like cars or small ships have a low RCS and therefore need long integration times, they are principally much slower than the platform, clutter ambiguities are introduced by low PRFs due to large swath widths. Space based systems are usually equipped with at most of two along-track channels. To increase the GMTI performance, subaperture switching modes can synthesize a higher number of spatial channels.
Along the whole tutorial, we illustrate the presented techniques with real data examples obtained by the airborne Fraunhofer radars AER-2 and PAMIR, and the Canadian space based SAR system RADARSAT-2.
The tutorial was held at the "European Conference on Synthetic Aperture Radar (EUSAR)" on 29 March 2021 by Prof. Dr. Joachim Ender (former director of Fraunhofer FHR).
Conference homepage: www.eusar.de/