Ron Lee wrote: >B. I am excited about Rob Matson's implementation of SDP for Skymap. >I have used Skymap and SGP4 to view Molniya satellite (eccentric). >Is this a manifestation of his statement? Also I assume that SDP would >work better with Molniya's and older elsets much as Rob's recent test >with Superbird A. IMHO, neither SGP4 nor SDP would work well with a Molniya. SGP4 is a general perturbation theory. This basically means that simplifying assumptions are made regarding the earth's figure, drag effects, the atmosphere model, etc. so that a general set of equations can be derived that work at all times. The authors of SGP4 did this because they wanted a good tradeoff between accuracy and speed. SDP4 is a special perturbation program. The equations calculate a small shift in a satellite's position due to each perturbing force over a small increment of time. To get a prediction, the small shifts are calculated one by one and summed up between epoch and prediction. This is also called numerical integration. If you look at the SDP4 algorithm, it features a numerical integrator to calculate the shifts in position for lunar and solar perturbations. Every model has its boundaries, beyond which they become unreliable. SGP4 is optimized for low earth orbits. SDP4 is optimized for mid- and high earth orbit objects. So the Molniyas are not a good fit for either model. The trick is to use elsets as close to epoch as possible to keep the models from introducing too much error. _______________________________________________________ Jim Varney jamesv@softcom.net Sacramento, CA