****************************************************************************** DORISMail 09-Aug-2002 10:52:01 Message No 0198 ****************************************************************************** Author: "H. Fagard" Subject: Antenna drift at St Helena Dear Colleagues, We have been informed that due to some corrosion of its aluminum supporting plate, the DORIS antenna at St Helena (HELB) is out of vertical by a few centimeters. Therefore positionning and horizontal velocity results should be considered biased over an unknown time period (at least the last few months). The antenna tilt has been estimated to be approximately 3 to 4 degrees, meaning a 5 to 6 cm horizontal shift towards North-East for the 2 GHz phase center. The exact excentricity will be measured, and the plate replaced, when an IGN surveyor can travel to this remote location, which is planned for next December. As you may remember, this corrosion problem has already been noticed at other stations (Amsterdam and Chatham). It only affects a series of supporting plates made of anodized aluminum -that we have been using during a limited time and at only a few sites- if they are installed in a very humid environment. Other metals used for making the plates (marine aluminium and stainless steel) haven't been affected by corrosion up to now. And the very-high quality stainless steel (AISI 316L) we are using now should completely resist even in the most corrosive environments. Of course we are aware of the negative impact of such antenna drifts on the quality of the DORIS results, and are willing to replace the "bad" plates as soon as possible. Best regards, Herve Fagard -- ====================================================================== Herve FAGARD e-mail : herve.fagard@ign.fr Institut Geographique National 94165 SAINT-MANDE CEDEX PHONE : + 33 1 43 98 81 48 FRANCE FAX : + 33 1 43 98 84 50 ====================================================================== [ Part 2: "Attached Text" ] Please do not reply directly to this message, but send comments and suggestions to IDS.central.bureau@cls.fr