Subject: FW: RE : RE : Corrosion a Ascension (complements)
From: "Frank G. Lemoine" <[email-masked]>
To: <[email-masked]>, IDS_Governing_Board <[email-masked]>, IDS Central Bureau <[email-masked]>
Cc: <[email-masked]>, <[email-masked]>, <[email-masked]>, <[email-masked]>
Date: Mon, 05 Feb 2007 19:26:16 +0000

Dear Colleagues-

The attached document, prepared by Christian Jayles,
 summarizes the discussions and
 stability issues concerning the DORIS station at
 Ascension Island. Much detailed discussions and studies
 were initiated by a comparison of the GPS and DORIS
 ITRF2005 velocities for this station, which showed
 a difference wheras the DORIS ITRF2000 velocity
 agreed with the ITRF2005 velocity (analysis from Laurent
 Soudarin).

Herve Fagard contacted the local hosts at Ascension who
 furnished a photograph verifying a corrosion of
 the base plate (see attachment). The heavy
 corrosion at the site has always been a major
 concern, acc. To herve Fagard, rather
 than the periodic turn-off's of the
 station beacon transmissions due to Ariane launches.

Ascension Island does host a GLOSS tide gauge, however
 there has been a data gap due to instrument issues
 which has now been addressed
 in the words of Phil Woordworth (message dated
 January 3, 2007. 10:21 UT)

 " ..... the Ascension sea level data stopped in
   2001 because of a hole in a pipe which wasn't fixed for ages but a year
   or two ago a new gauge was installed.....  Delayed mode data are in

   www.pol.ac.uk/psmslh/phase2

   At the same time we added an OTT radar tide gauge so there are 2 there
   now and both are near real time - the data go to the GLOSS real time
   centre at Hawaii.

   Also when things go wrong with it, in principle we should be able to fix
   them almost at once as it is simple to fly there. If things are not
   fixed it is our fault (as per post-2001). So what I am saying is that it
   as good a site and a gauge as any."

In the same message,Phil Woodworth pointed out that sites on the island
 might be undergoing differential movement, a good argument
 for a local GPS network.

A check of the IGS website will reveal that the data for Ascension GPS
 has not been available for some months. Dave Stowers (JPL) confirmed that
 there were data communication issues, and that it takes time to
 resolve them. However the IGS plans to maintain this site as part
 of its network.

The discussion with Ascension, and the colocation of geodetic stations
 highlights important issues:

 (1) The quality of the respective sites. Do local conditions mean
  that a colocation on paper is not really a colocation in practice?
 (2) Item one is closely related to the distance between the
     geodetic stations (GPS, DORIS, tide gauge) as well as issues
     of multipath, visibility masks.
 (3) Of the three instruments 2/3 have or have/had communications
   issues.

Having focused energetically on the issue of the Ascension DORIS
 site, we look forward to station upgrades and further updates.

Regards,

Frank Lemoine
IDS Analysis Coordinator

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