NOTES TO ipac_att FILES RELEASED 4/28/97

1- This release includes the position reconstructions for three packets from 
   the pre-flip period, namely 04030028, 04031330, and 04050109.  Previous
   reconstructions of these packets, the most recent being 9/96, are superceded
   and should be replaced.  

2- The following improvements to the previous release are noteworthy:  

      (a)  Good reconstructions for several orbits previously marked as dubious            
           or poor have been obtained.  These include in particular orbits
           256, 257, 259, 274, 275, and 283.   Orbit 260 & 275, previously
           believed to be good, were discovered to be substantially in error,
           and good solutions have been obtained.

      (b)  It has been found valuable to break all orbits into two fit 
           segments, as was previously done for postflip but not preflip.  
           The segmentation normally occurs at point "D", and at the
           sunrise point of each orbit, the two segments being denoted "AB"
           for the first and "CC" for the second.  The result of this segmenta-
           tion has been a reduction in cross-scan errors of typically 2-4 
           arcmin.  Almost all fit segments now have RMS residuals of less than 
           1.5 arc min cross-scan and less than 0.5 arc min in-scan, and many
           are substantially better.  Typical attitude errors should be much
           less, by a factor of roughly the square-root of the number of match 
           stars (usually in the range 15-45) used in the fit.

      (c)  A small number of orbits remain problematic, especially 271-273.
           THESE ORBITS HAVE BEEN MARKED WITH A 99.0' POSITION UNCERTAINTY.

   ****** The users are warned that those time intervals, where the ******
   ****** boresight pointing uncertainties are set to 99.0', should ******
   ****** be used only with great attendant risk.  The uncertainty  ******
   ****** field M U S T  A L W A Y S  be consulted before use!!!    ******

      (d)  Portions of some other orbits are also flagged, typically when
           no or few secure match stars could be found.  This was especially 
           common for the interval between point D and sunset.

3- Methods of reconstruction described in previous releases have generally
   been used, and quality checks previously described have been applied.


   Figure caption:

   Plot of cross-scan (top) and in-scan (bottom) fit residuals for the 
   approximately 2750 match stars used in the 3 packets.  RMS residual is 
   < 1.5 arcmin cross-scan, and < 0.5 arcmin in-scan.