2.11 ASAR Characterisation and Calibration

2.11.1 Introduction

Unlike the ERS AMI-SAR that operates a passive phased-array antenna, the ENVISAT-1 ASAR encompasses an active one that consists of 320 subarrays each one connected to a transmit/receive module (TRM). The antenna layout is of 32 rows of 10 subarrays. The subarrays are grouped in 2 x 10 tiles of 16 subarrays each (see figure 1). Each TRM has two transmit chains, one for horizontal and one for vertical polarisation, and one receive chain for both. The three chains are independently programmable in amplitude and phase to provide the required elevation beam patterns. The eight swaths required by the ASAR operation modes are dealt with by the antenna by means of the definition of eight sets of beam coefficients. The beam coefficient set consists of 32 transmit and 32 receive complex coefficients for each polarisation, corresponding to the 32 antenna rows in the across-track or elevation direction.

Any drift in the gain and phase characteristics of the TRMs can distort the antenna beams. Deviations in antenna pattern and antenna gain will potentially contribute to radiometric errors in the SAR image. The ASAR calibration scheme is, therefore, different to that used for ERS.

In-orbit beam calibration of an active phased-array antenna is a major task and it will include measurements in special modes, like module stepping and external characterisation, as well as acquisitions over rain forest. A sophisticated antenna model will combine the various data together with the pre-flight characterisation and provide the elevation and azimuth patterns for the ground processor.

Monitoring any instrument gain drifts requires a separate calibration network, to couple out part of the transmit signal or to inject chirp signals into the receiver chain. This data will be included into the high-rate data stream and will be analysed by the ground processor in order to estimate the necessary gain drift corrections.

The absolute overall system gain can be most accurately determined from the image response of point targets with high and well-known Radar Cross Sections (RCS). ( for the derivation of backscattering coefficients and RCSs in ASAR products see below 2.11.5. ) ASAR high precision transponders will be deployed in the Netherlands and will serve as the main external calibration targets. A special transponder operation mode and well-characterised distributed targets will be used for the low-resolution Global Monitoring mode.