karn@mouton.UUCP (07/20/84)
A new set of Keplerian elements has been posted to net.ham-radio. In response to a request by DJ4ZC at the England technical meeting, I have added a new entry to each set, the "Ref perigee" time. This is the time of the perigee nearest to the reference epoch of the element set. It is expressed in the "AMSAT-DL standard" time format, i.e., decimal days since 00:00:00 UTC 1 Jan 1978. For added convenience, a human-readable time format follows on the next line. The perigee chosen may be either before or after the reference epoch, depending on which is closer. This is a derived element in that it is not part of the NASA/NORAD sets, but is calculated from the epoch time, mean motion and mean anomaly at epoch. Apparently there are many users of a Sharp hand calculator tracking program which requires perigee times and these calculators cannot be easily reprogrammed. Users of the many W3IWI program variants do not require this information and may safely ignore this new entry. Those using the Ref perigee time should note that the other orbital elements remain valid at the Epoch time, NOT at the Ref perigee time. For most applications the rates of change of the elements over this time difference (e.g., the secular perturbations in RAAN and Argument of perigee) are small enough to preclude any significant error. 73, Phil