Message-Id: <200505131056.j4DAu7Q19164@cnes.fr>
From: Tavernier Gilles <gilles.tavernier@cnes.fr>
To: ids.analysis.forum@cls.fr
Subject: Low elevation ENVISAT DORIS data
Date: Fri, 13 May 2005 12:56:03 +0200
MIME-Version: 1.0
Content-Type: text/plain

Dear colleagues,

As already mentioned in DORISMail # 0366 a new DORIS flight software was uploaded on-board ENVISAT on October 12, 2004 (start of arc 122). The receiver is now operating in the so called "self-programming mode" instead of the "waiting mode". This increases the number of measurements which are performed, especially at low elevation. For most stations the minimum elevation was then set to 12 degrees, with a few exceptions (new stations with a cut-off at 8 degrees). The POD team noticed an increase in the residuals level, which is due to low elevation measurements; you should have also experienced such an increase in residuals around that time.

Since then the stations coordinates used on-board for measurements programming were updated on week 14 (4 to 8 April 2005). At that time the minimum elevation was set to 8 degrees for all stations. These low elevation measurements will be available starting with the next ENVISAT measurements delivery (last delivery : 145 arc 5 cycle 35 22 mar. 05 to 28 mar. 05). However, as the measurements are edited at CNES using a 10 degree cut-off, only the measurements between 10 and 12 degrees will appear as usual measurements (with the editing indicator bit35 equal to 0). Measurements between 8 and 10 degrees will have a preprocessing indicator bit35 equal to 1. To use them you need to reset the editing flag and then pre-process the data in order to get rid of bad measurements.

We intend to change the CNES POD configuration to lower the cut-off to 8 degrees. All data would then appear as normal data (indicator 35 = 0 if measurement OK, indicator 35 = 1 if measurement NOK). 

It would  be interesting to assess the impact of lower elevation measurements on stations coordinates determination : it could maybe explain the different behaviours observed during recent comparisons between Analysis Centers.

We expect feed-back from the IDS Analysis Centers on this matter.

On behalf of the Central Bureau,

Gilles TAVERNIER           
_____________________________________________________
CNES
DCT/PO/AL - Bpi 2002             Phone :  33 - (0)5 61 27 37 76
18, avenue Edouard Belin         Fax :      33 - (0)5 61 28 25 95
31 401 TOULOUSE CEDEX 9   Email : Gilles.Tavernier@cnes.fr 
_____________________________________________________