Minimize ASAR mission events

Full details of the ASAR acquisitions are provided in EOLI, (the Client for browsing the catalogue can be downloaded here).
Major events in ASAR operations include:

ASAR Data Subsystem Redundancy Switch (May 2003)
First signs of an on-board anomaly were found with the quality of ASAR data just after launch although no problems were detected from the instrument telemetry. Then a failure was found on the Q-branch of the on-board transmitter. ASAR switched down on 14 May 2003 to change the data subsystem (DSS) from side A to side B. Following the switch to DSS-B redundancy, ASAR operations resumed on 2 June 2003 with nominal data quality.

ASAR Antenna Reset#1 (September 2005)
A drift in the gain and phase of most TRMs was seen from the start of the mission. Due to the accumulated gain and phase drift for a large percentage of the TRMs, the first of two antenna resets was performed on 14th and 16th September 2005 in order to correct, as far as possible, the TRM drift.

ASAR Wide-Swath Burst Synchronisation (September 2006)
To perform interferometry with ASAR WS imagery, it is necessary for there to be an overlap in the bursts from the pair of products in each of the sub-swaths. If there is no overlap in the bursts (in azimuth), then it will not be possible to generate an interferogram (since there will be no correlation between the pair of products as the same point on the ground would be imaged by a different part of the azimuth beam). The acquisition start times for ASAR WSM imagery were altered in September 2006 so it started at discrete points around the orbit to maximise the chances of burst overlap between pairs of products . The revised planning strategy became operational from 17 September 2006 (orbit 23783). Rosich et al (‘ASAR Wide Swath Mode Interferometry: Optimisation of
the  Scan  Pattern  Synchronisation’,  Envisat  Symposium, 2007, ) provide details of the Burst Synchronisation optimisation.

ASAR AP Swath Modifications (May 2009)
Changes were made at the end of May 2009 to the ENVISAT ASAR alternating polarisation (AP) swaths characteristics. These were made to avoid unplanned shut-downs of the ASAR instrument that occurred since launch when acquiring AP data. Unplanned shut-downs are undesirable due to their impact on the possible degradation of the instrument hardware. The number of AP acquisitions was reduced significantly in early 2007 while the usage of IS5 was suspended at the end of 2006, both to reduce the occurrence of these unplanned shut-downs .

ASAR Antenna Reset#2 (March 2010)
The decision to perform a second ASAR antenna reset activity during early 2010 was based on the gain and phase drifts that has occurred over the previous 4+ years for each of the 320 antenna transmit and received modules (i.e. to bring the gain and phases close to pre-launch values).

Envisat Orbit Change (October 2010)
In order the extend the Envisat mission beyond its nominal lifetime of 5 years and because of limited remaining fuel on-board (used for orbit manoeuvres and attitude control), it was decided to reduce the orbit altitude of Envisat and cease some of the inclination orbit manoeuvres. The reduction in altitude was also the first step in placing Envisat in its end of life orbit. From the 22nd October 2010 the altitude of the Envisat orbit was reduced by about 17.4 km for the mission extension (Envisat 2010+). The Orbit Lowering Manoeuvre  was completed on 2nd Nov 2010   and  the mission extension phase was entered.


Tandem Missions
The first ENVISAT/ERS-2 SAR tandem mission took place from September 2007 to February 14, 2008.

The second ENVISAT/ERS-2 SAR tandem mission started on Nov. 23, 2008 and ended in late April 2009 (both spacecraft acquired data over the same area just 28 minutes apart). The second campaign was due to end in late January 2009, but could be extended through to late April 2009 due to the excellent performance of the satellites and the strong teamwork between the mission data teams at ESRIN, and ESOC.

The third tandem campaign occurred from February 2010 to April 2010.

The fourth tandem interferometric campaign, from July to October 2010, focused on low-lying coastal areas, such as New Orleans in USA and the Po River Mouth in Italy.

Leap Frog Mode
From 26-Nov-2010  to 09-Dec-2010  an experimental Leap Frog mode was used, whereby the swath used from one  wave mode imagette to another was switched between IS2 and IS4. This was repeated for 25-May-2011 to 06-Jun-2011.