NOTES TO ipac_att FILES RELEASED 5/13/97

1- This is the second release of the improved preflip data, and includes 
   the position reconstructions for 31 orbits in two packets from the 
   pre-flip period, namely 03312358, and 04020237.  Orbits included are
   210-240.  Previous reconstructions of these packets are superceeded and 
   should be replaced.  

2- Note the following improvements to the previous release:  

      (a)  A good reconstruction for orbit 212, previously marked as 
           dubious, has been obtained.  Orbits 216 & 224, previously 
           believed to be good, were discovered to be substantially in 
           error, due to G-angle changes in the middle of the CC segment, 
           and improved solutions have been obtained.

      (b)  It has been found valuable to break all orbits into two fit 
           segments, as was done earlier 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 segmentation is 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 
           0.6 arc min in-scan, and many are substantially better.  
           Typical attitude errors should be less, by a factor depending
           on the number of match stars (usually in the range 15-45) 
           used in the fit for each segment.

      (c)  A few segments remain problematic, particularly the very short
           AB segment of orbit 233.  THESE 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 the 
           rule in particular for the first ~400 sec of each orbit, 
           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 captions:

   Figure 1:  Plot of cross-scan (top) and in-scan (bottom) fit residuals 
   for the approximately 2130 match stars used in the 2 packets.  

   Figure 2a:  Histogram of cross-scan residuals dX.  RMS is 1.24 arc min.

   Figure 2b:  Histogram of in-scan residuals dY.  RMS is 0.51 arc min.