Review of Asar Performance and Product Quality Evolution after 5 Years of Operations

ENVISAT ASAR is successfully operating since March 2002 and resulting ASAR products are operationally distributed to the user community since December 2002. This paper provides an update of the ASAR performance, from the instrument status to the product quality assessment. 1. ASAR INSTRUMENT STATUS Routine monitoring of the 320 antenna T/R modules is daily performed using Module Stepping Mode (MSM). In MSM, the instrument samples all 320 T/R modules in less than one second for gain and phase. Several failures in the antenna sub-assembly, due to different reasons but resulting in any case in a certain number of T/R modules not operating, have been experienced since launch. The first failure in April 2002, was the loss of the power averaging module of PSU-1 in Tile C1. As a result, the four modules sharing the affected PSU are failed in transmit (both H & V) but still fully operational in receive. The second one in February 2003, with the complete loss of PSU-1 in Tile D2, resulting in the 4 T/R modules of this PSU being failed both in Tx and Rx, and in H & V polarisation. Later on, six individual T/R modules failures have been experienced (April 2004, May 2004, November 2004, January 2005, November 2005 and January 2007), three of them in H polarisation and three of them in V polarisation. Finally, the four modules sharing PSU-1 in Tile B3 have also been lost in February 2007. Fig. 1 shows the current status of the antenna, in H and V polarisation. Fig. 1. ASAR antenna modules failures in H (left) and V (right) polarisations. Modules highlighted in black have been lost in Tx only while modules in pink have been lost both in Tx and in Rx. In total, there are 15 TRMs failed for Tx-H, 15 for Tx-V and 8 for Rx H and V. Up to 6% of failures (i.e. 19 modules) in a given mode (e.g. Tx H) are allowed by design before there is an impact on the instrument performance. Although the failures experienced up to now have not significantly degraded the instrument performance, changes in the antenna radiation have been experienced and corrected on ground, resulting in new auxiliary calibration files. The antenna TRMs may not only fail but their gain and phase settings may also drift in time. A major gain & [hase correction activity was performed in September 2005, as described in more detail in [1].