From davem@seal.cea.berkeley.edu Thu Sep 7 12:03:07 1995 To: patr@cea.berkeley.edu, bretts@cea.berkeley.edu cc: jvv@cea.berkeley.edu, patj@cea.berkeley.edu, mrh@cea.berkeley.edu Subject: Star Tracker Blockage problem Content-Length: 1989 X-Lines: 48 Hello all, HDM/EUVE/0080/95 During the Eps Eri observation, the Moon was viewed by ST 2. This shut off the Star Tracker (as it is supposed to). However, John Vallerga and I went thru the orbital mechanics of it and it appears that for certain attitudes (RA, Dec & Rolls) the Moon could shut down a Star Tracker (ST) for as much as 68 hours. When this happens, the science data gets corrupted. I don't believe that the science planning software checks for star tracker blockage other than during arrival. For a full Moon, the ST's will shut down when the Moon is around 17 degrees off the ST boresight, as it was, ST 2 shut down due to a partially lit Moon at around 12 degrees. This gave us an additional 5 hours of science data without ST 2 blockages. We slewed to another target in the middle of the Eps Eri observation, 5 1/2 hours after ST 2 shut down. If we had not slewed, the Moon would of been in ST 2 for around 57 hours. 1) Can our software take out the effects. 2) If not, do we need to modify our checkconstraint software to remove attitudes which put the Moon in a ST for more than 4-5 hours? 3) In worse cases, we will end up doing a TOO to move the Moon out of a ST. ------- Forwarded Message (From EUVE ACS Engineer Mark Heidenreich) On Tue, 5 Sep 1995, Dave Meriwether wrote: > Hey now Mark, here's some possibly useful tidbits. > If we had stayed on Eps Eri instead of slewing to the comet, > the moon would of been in ST2 for 57 hours. At most, the moon > could linger in a ST for 68 hours. We don't really check for > this. If a ST was to stay shutoff for 68 hours, would we need > to roll? Absolutely, if we don't get updates in a ST for about 4 to 5 hours, our attitude "knowledge" (how for off from where we know we are) begins to get pretty high. If this gets too high, I assume that you can't be too sure of where you are getting your data from, so the science data is probably not as good. Mark ------- End of Forwarded Message