Re: SatSpy 2.5

Jim Varney (
Thu, 6 Nov 1997 08:38:52 -0800 (PST)

---Earl Needham <> wrote:
> At 06:49 AM 11/6/97 +0100, Bjoern Gimle wrote:
> >I agree on that - SGP4 should be OK in most programs with relatively
> >fresh elsets - corrected SDP4 needed only for old elsets of dead
> >geostationaries (or eccentric orbit satellites)
> 	I've been wondering about the differences in SGP4 and SDP4.  At least
> I have a clue.  Would someone "in the know" please cllue me in?

Very briefly: SGP4 uses some fairly simple models for the Earth's
"lumpy" gravity field and for atmospheric drag.  The models have
assumptions made that are optimized for LEO satellites.

SDP4 calls some subroutines that make adjustments for the "lumpiness" in
the Earth's gravity field that has the most effect on geosynchronous and
Molniya-type satellites.  In addition, SGP4 calculates perturbations
(little gravity tugs) due to the sun and moon.

The original intent of the SGP4/SDP4 algorithm is to have both routines
present in the program.  If the satellite has a period of less than 225
minutes, then SGP4 is supposed to be used; otherwise, the orbital
elements extracted from the elset gets passed to SDP4.

Jim Varney   121.398W  38.458N 8m   Sacramento, CA,

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