**Previous message:**Steve Newcomb: "8539 obs Nov 7"**In reply to:**Bram Dorreman: "How to determine N-dot from two values of N?"**Next in thread:**Björn Gimle: "Re: How to determine N-dot from two values of N?"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

Bram Dorreman wrote: > As with Skymed 4 rocket the N-dot value from the TLE I used to find > it, had a small negative value. Just as the day before when I was too > late to have a search action. I was thinking: If one has two values of > N it should be possible to calculate the N-dot value from them. My > only question is: What is a suitable algorithm for this. You can calculate the mean ndot/2 between a pair of elsets from their values of epoch (t) and mean motion (n): ndot/2 = (n2 - n1) / (t2 - t1) / 2 Some judgement is required to select suitable elsets, especially soon after launch, when they may not yet have settled down to realistic values. It is best to avoid using erratic-looking elsets, and to consider spans of at least one day. Looking at all of the data of 10060B / 37217, up to and including the epoch 10312.07014429 elset, it appears that a realistic ndot/2 is about 0.003 rev/d^2. Finding ndot/2 will be sufficient for the older SGP model, but nearly everyone has adopted the current model, SGP4, which requires the B* decay term. For the elset and mean ndot/2 in question, I estimate a corresponding B* value of 33702-3, expressed in TLE format. There is a simple equation relating B* and ndot/2, which can be added to SGP4 with a bit of surgery: http://www.satobs.org/columbia/SGP4decayterms.pdf Ted Molczan _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l

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