Evaluation of calibration methods for a helicopter-borne microwave scatterometer

The French frequency modulated continuous waves (FMCW) scatterometer ERASME mounted on small helicopter or aircraft has been designed as dualfrequency (C and X bands) and dualpolarization (HH, VV) to investigate simultaneously the vegetation and the soil responses in radar backscattering. It is operated as a forward looking radar with a large elevation beamwidth (± 10° at 3 db) to observe easily the same surface target over a large range of incidence angles during a single flight. By this ability, ERASME is a complementary research tool for intercalibration of airborne and spaceborne imaging Synthetic Aperture Radars like Radarsat and ASAR and has to be well calibrated in every configuration, both absolutely and relatively for comparisons at different incidence angles.

This paper evaluates different calibration methods to be applied to such an instrument. Absolute calibration within 1 dB is easily obtained by external calibration using metallic corner reflectors. But this method remains insufficient to get the antenna elevation aperture which is essential on natural distributed targets for antenna pattern correction, due to the severe constraint of a narrow azimuthal beam and flight parameters (pitch, roll, altitude) varying quickly in time and range.

The external calibration is strongly improved by using a statistical analysis of data obtained over natural targets which analyses the correlation between the processed data and the recorded flight parameters. This method appears promising, but its application on natural targets with random variations need specific statistical properties of the data set. It is operative for high antenna setting (here 38° incidence angle) and mostly over bare soils, with low of σ⁰ variances and σ² correlation length of the order of the correlation length of pitch. It provides the aperture range around the antenna axis and an accuracy of 0.5 dB upon erσ⁰ is achieved providing the antenna pattern correction are done.