Using TLEs to reverse time

Huston, Stu (A016174@MDCPO102.HB.MDC.COM)
Fri, 08 Nov 96 11:38:00 PST

Dear SeeSat-ers:

I have a work related problem which hopefully those more experienced with 
predictions than I can help with.

I need to be able to determine the position of a spacecraft (in this case, 
the NOAA weather birds) at times in the past.  I've downloaded the TLE 
archives and have a code which uses the SGP4 routines to determine the 
position.  Now suppose I have 2 elsets, with elset #2 at a later time than 
elset #1.  Also suppose that the time I need the spacecraft position is 
closer to #2 than #1.  Is it valid to use elset #2 to determine the 
position?  Assuming no maneuvers occurred between the time I'm trying to 
simulate and the epoch of elset #2, those orbital elements should be more 
representative than those of elset #1.  But, is there something inherent in 
the SGP routines that requires the epoch of the elset to be earlier than the 
epoch of the satellite?

I hope this is a legitimate request for this list - it may help someone 
trying to write their own prediction program.  If you want to reply to me 
individually, rather than to the entire list, please use the (god-awful) 
email address below.

Thanks in advance,

Stu Huston               |
McDonnell Douglas Aerospace   |  Phone: (714) 896-4787
A3-Y957 M/S H011C111          |  Fax:   (714) 896-2305
5301 Bolsa Ave.               |  email:
Huntington Beach, CA 92647         |
It's tough to make predictions, especially about the future. -- Yogi Berra