Stu Huston wrote: >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? No, there is not. The time since epoch may be negative. However, some popular prediction programs like TRAKSAT refuse to calculate the position for a point in time before the epoch. Hope this helps. Bruno Tilgner 100533.2016@compuserve.com