Multi Phase Center Processing of Ice Sounding Radar Signals for Across Track Surface Clutter Cancellation: Final Report

Kurzfassung

ESA’s POLarimetric Airborne Radar Ice Sounder demonstrator (POLARIS) – built, maintained and deployed by the Technical University of Denmark (DTU) - operates at P-band and features a multi-phase-center antenna for the purpose of surface clutter suppression. The first data suitable for the development and demonstration of surface clutter cancellation methods were acquired in February 2011 during the IceGrav campaign in Antarctica. The purpose of the present study was to investigate and compare different methods for surface clutter cancellation and to implement a Processing Tool to augment the along-track POLARIS processor developed by ESA, thus improving bedrock detectability. Five algorithms have been selected for implementation and analysis: the standard beamformer, two different clutter nulling algorithms, the optimum beamformer and the MVDR beamformer. Prior to the analysis with real POLARIS data the algorithms have been implemented in a simulation environment, allowing for a performance analysis concerning the different input parameters. These included pulse bandwidth, aircraft roll, cross-track terrain slopes, as well as POLARIS antenna patterns. The improved performance of the nulling, MVDR and optimum beamformer techniques for surface clutter suppression compared to standard beamformer has been demonstrated and requirements on the knowledge of slope and roll parameters have been quantified for nulling and optimum beamformer algoorithms. By means of simulation it was shown that the relatively large separation of the POLARIS phase centers (~lambda) is responsible for regions of high noise scaling around +/- 45deg off-nadir angle, which turned out to be annoying in particular for the application of the nulling techniques. The optimum beamformer and the MVDR instead are able to better handle these angular regions. The clutter suppression performance has also been demonstrated with the available POLARIS data from the IceGrav 2011 campaign. For all 4 available data takes a clear improvement in clutter suppression performance could be achieved, independent of the signal bandwidth. It was shown that, in terms of clutter suppression, the advanced methods (nulling, optimum beamformer and MVDR) outperform the standard beamformer by up to 15 dB. However, it was also shown that a major limitation for the performance of nulling and optimum beamformer is the requirement of a highly accurate ice surface model. For the purpose of this study preliminary TanDEM-X DEM data were used to avoid the limited accuracy and artefacts of ASTER2 GDEM data. However, comparison with Laser altimetry data revealed uncertainties in the used TanDEM-X data due to the limited calibration performed. Also the different penetration depth of P-band, confirmed by Direction-of-Arrival estimates of clutter angles from the radar data, turned out to be an issue for consideration. In summary the MVDR technique proofed to be an easy to implement and the most robust technique since it does not require inputs on the ice surface topography, leading to the best clutter suppression results. These findings have been confirmed by simulation also for some selected spaceborne scenarios. The present document is the Final Report summarizing the study results including a basic description of the implemented Processing Tool for multi-phase center surface clutter cancellation (MPC Processing Tool). This report is divided into six major parts: The selected algorithms for surface clutter suppression are summarized in section 2. Section 3 includes a summary of the algorithm’s performance analysis based on simulated data. The developed MPC processing tool is described in section 4. Selected processing results of POLARIS data are shown in section 5. The suitability of the derived surface clutter cancellation algorithms for spaceborne scenarios will be addressed in section 6. Conclusions are presented in section 7, including suggestions for future deployment of the POLARIS sensor.