****************************************************************************** DORISMail 25-Jun-2004 08:59:22 Message No 0326 ****************************************************************************** Author: Tavernier Gilles Subject: Jason-1: switch over to back-up DORIS receiver Dear colleagues, Considering the continuously increasing effect of radiations (in particular over the South Atlantic Anomaly) on the DORIS Ultra Stable Oscillator frequency and following the comments made by the SWT POD team in Arles and the recommendation made more recently by the International DORIS Service during the Paris Meeting in May, the Jason-1 project team has decided to switch over to the back-up DORIS receiver. POD ephemeris are still compliant with the requirements but the number of eliminated DORIS measurements become sufficiently large to warrant this change. As you know, stations coordinates estimation was much more affected, especially in and around the SAA area, due both to measurements degradation and to the increasing number of measurements being eliminated during pre-processing. The switch over operations are scheduled on Monday 28th, they should last for about 14 hours. During this period no OSDR will be produced, however the off-line altimeter products should be produced. This schedule has been decided to profit by experts availability and to avoid any emergency action during the summer holiday period when those experts may not be available to overcome any difficulty. As usual, when a DORIS receiver is turned on, an Ultra Stable Oscillator (USO) heating phase is to be expected. As long as the USO frequency is not perfectly stabilized, the measurements quality may not be compliant with the requirements involving a lower quality of the DORIS orbit. Jason-1 DORIS measurements deliveries to the Data Centers may be suspended until the USO frequency is stabilized. The duration of the phenomenon is difficult to predict : - from several weeks, as experienced when receivers were turned on after launch, - to a few days as experienced when switching over to the back-up DORIS receiver: - in December 1998 for TOPEX/POSEIDON: the 9 days lack of measurements was mainly due to the incident prior to the switch over - and apparently for ENVISAT a few days ago. We hope that the second oscillator will be less sensitive to radiations and we are looking forward to reading Analysis Centers results using measurements performed with the back-up receiver. Best regards, 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 _____________________________________________________ Please do not reply directly to this message, but send comments and suggestions to IDS.central.bureau@cls.fr