Applying a Composite Pattern Scheme to Clutter Cancellation with the Airborne POLARIS Ice Sounder

Abstract

The European Space Agency’s (ESA) POLarimetric Airborne Radar Ice Sounder demonstrator (POLARIS) – built, maintained and deployed by the Technical University of Denmark – operates at P-band and features a multi-phase-center antenna for surface clutter suppression. Data for the development and demonstration of surface clutter cancellation methods were acquired in February 2011 during the IceGrav campaign in Antarctica. The POLARIS radar uses a four-element cross-track antenna to control clutter suppression. The 0.96 m element spacing – 1.4 wavelengths – is significantly greater than the spacing required to avoid grating-lobe ambiguities. The ESA funded the present study to investigate and compare different methods for surface clutter cancellation and to implement a software tool to augment the along-track POLARIS processor developed by the ESA, thus improving bedrock detectability. Within this study, Cranfield University carried out an investigation of the performance of a composite pattern approach to the POLARIS clutter suppression. The scheme exploits the principle of pattern multiplication, whereby the required composite four-element array is produced by the convolution of smaller sub-arrays. This has the advantage that it is computationally much simpler to work with two-element arrays and generate nulls in preferred directions, which are retained in the angular response of the final four-element composite array. Other approaches that consider all four elements need to seek solutions to a set of simultaneous equations. The work considered the performance of the scheme with reference to varying input parameters, which included pulse bandwidth, aircraft roll and cross-track terrain slopes. The work started by recognizing that a four-element array can produce three primary nulls (plus higher-order nulls due to the wide element spacing). A general excitation for this particular case was developed to make best use of all the available information at each array element. The optimum solution was found to have two nulls placed towards the clutter angles required, but with the third always placed towards the endfire direction. This positioning of the third null was found to maximize the gain in the nadir direction. Because of grating-lobe effects arising from the large element spacing, placing nulls at the 45° clutter angles caused a null to be also placed at nadir, leading to a singularity at this depth. However, it was found that by degrading nulling performance at this angle the nadir-singularity could be avoided.