| Date: | 2004-02-20 00:24:00 | |
| Sender: | Frank Lemoine <flemoine@santafe.gsfc.nasa.gov> | |
| Subject: | [SLR-Mail] No. 1189: GFO-1 tracking during upcoming full-sun orbit regime. | |
| Author: | Frank Lemoine | |
| Content: | ******************************************************************************** SLR Electronic Mail 2004-02-20 00:24:00 UTC Message No. 1189 ******************************************************************************** Author: Frank Lemoine Subject: GFO-1 tracking during upcoming full-sun orbit regime. Dear colleagues, This message is to advise that the GEOSAT Follow-On spacecraft will suspend the collection of altimeter data, and enter sun-acquistion mode from approximately February 22 to April 2, 2004. GFO will shortly enter a full-Sun regime, inducing abnormally high temperatures in the spacecraft. It is believed the performance of the momentum wheels degrades at high temperatures. To prevent the loss of the spacecraft (which is now six years old!), the payloads will be turned off and GFO will be placed in a 2-wheel sun acquisition mode (solar arrays perpendicular to the Sun using only the two ´good´ momentum wheels). Flaky momentum wheel performance in the last full sun regime during the autumn of 2003 was identified as the cause of the spacecraft entering contingency mode. In sun-acquistion mode, tracking will be possible only intermittently, and we advise the stations to temporarily suspend tracking GFO. As per our agreement with NOAA and the US Navy, we have agreed to suspend delivery of the medium precision orbits over this time period. Accordingly, the delivery of IRV predictions will also be suspended. We will resume IRV predictions based on the Doppler data on approximately April 2. We are fortunate that due to orbit geometry considerations, the full-Sun regime to be experienced by GFO in the next month will be the last time the spacecraft will face this problem for several years. The next periods of face-on (high beta prime) orbit geometry will occur at the equinoxes where eclipses will occur at the high latitudes. Please do not hesitate to contact us if you have any questions or concerns. We will apprise the stations and the ILRS of any change to the dates of GFO down time. In our orbit group, we plan to use this down time in the production of the medium precision orbits, to update the models and procedures for the production of the precise ephemerides (POE´s). Thank you for your continued support of this mission. Regards, Frank Lemoine Space Geodesy Branch, Code 926 NASA Goddard Space Flight Center From: flemoine@santafe.gsfc.nasa.gov ******************************************************************************** |