****************************************************************************** DORISMail 13-Aug-2002 11:08:18 Message No 0199 ****************************************************************************** Author: "H. Fagard" Subject: Antenna drift at St Helena (update) Dear Colleagues, On 9/08/02, I wrote: >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. In fact we will not be able to measure the excentricity resulting from the antenna tilt, because our local caretaker has plumbed the antenna on his own initiative, on July 31st. Therefore we can only provide the analysis centers with the following approximate information: - from January 1990 (SPOT-2 launch) to April 1997, the position of HELA (the first antenna, Alcatel type) should have been rather stable, except maybe over the last months of this period because of the support's corrosion. - as of from April 1997, the new antenna (Starec type) should have been stable for a few years, until corrosion started to cause a progressive antenna tilt, resulting in a phase center shift towards North-East, reaching 5 to 6 cm (estimated) in July 2002. Therefore data over the last year or more should be used with extreme caution, and estimated site velocities should be checked for a bias with respect to values derived from former years' data (HELA and first years of HELB). - on July 31st, 2002, the HELB antenna went back (within the accuracy of the plumbing carried out by our local caretaker) to its initial position. This should result in a several centimeter leap in time series, allowing the analysis centers to measure the antenna shift ;-) 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