[RADS] Envisat moved to new orbit

Remko Scharroo remko at altimetrics.com
Tue Oct 26 03:54:19 CEST 2010

Dear RADS users,

On 22 October Envisat moved to its new orbit. It can be best described as a "near-repeat" with 431 orbits in 30 days. 

The orbit pattern will not drift longitudinally, but the inclination will keep drifting from about 98.5º currently to 98.4º at the end of the mission. By allowing a slight variation of the node at the same time, the drift of the orbit is kept zero at 38ºN on descending tracks and 38ºS on ascending tracks. This was done to optimise the repeatability of daytime SAR observations over land (most land mass being in the Northern Hemisphere).

This orbit design feat has resulted in a minimal ground track drift without using any manoeuvres that is approximately linear in time. The maximum drift per cycle is at 50ºS (descending tracks) and 50ºN (ascending tracks) and amounts to about 0.6 km (westward and eastward, resp). It is smaller everywhere else.

In conclusion, Envisat will not stay within a 1-km deadband which one would consider a "repeat track", yet it does not drift enough to be considered a geodetic mission. A drift of 15 km  at the equator over the remaining expected live time is far inferior to the approximately 100 km track spacing.

Please refer to
for more information.

The implication for RADS is that you will soon see a directory n1/c appear with the Envisat data in the new orbit. Although ESA announced that the new data would not be flowed out to the users until 1 November, it looks like it is already arriving (but still in the n1/b directory as I have not updated the software yet).

If, in the near future you want to use the Envisat data in the new orbit you will need to:
1) Sync the data/nml directory; it contains the new setup for Envisat Phase C
2) Sync the data/n1/c directory; it will contain the new Envisat data
3) Run RADS programs with the option sat=n1/c to get the new data


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