Re: Propagation Models

Jim Varney (jvarney@mail2.quiknet.com)
Sat, 24 Aug 1996 19:47:37 -0700

Thanks to Rob Matson and Bjoern Gimle for their replies to my note on
propagation models.

Rob Matson wrote:

>SkyMap has implementations of SGP, SGP4 and HANDE, so it is possible to
>compare the model accuracies against some recent decays.  For example, an
>older element set could be propagated to the epoch of a more recent set (taken
>as truth), and the various model offsets could then be compared.  HANDE should
>do a better job.  SGP has zonal harmonics J2 and J3, and assumes the drag
>effect on mean motion to be quadratic in time, resulting in a cubic variation
>in mean anomaly w/time.  The drag effect on eccentricity is modeled in such a
>way that perigee height remains constant.
>
>HANDE, on the other hand, was developed by Hoots (1983) and includes zonal
>harmonics J2, J3 and J4 for its gravitational model, and the dynamic
>atmosphere of Jacchia (1970) for its atmospheric model.  It includes lunar and
>solar effects as well as resonance terms for satellites with 1/2 day and 1 day
>periods.

Very interesting.  The only drawback would be the need to store two sets of
elements, HANDE and TLE, in order to run both HANDE- and SGPx-based programs.
Storage is hardly an issue in this era of gigabyte hard drives.

Out of curiosity, how do the HANDE and TLE elsets compare?  What are the
elements
that appear in HANDE that do not appear in TLE's, and are any elements in TLE's
dropped and do not show up in HANDE?  Is solar flux one of the HANDE elements?

Barring a USSPACECOM switch to HANDE, another approach would be to develop a
prediction program that includes both a SGPx propagator _and_ a numerical
integrator.  Such a "dream" program would use SGPx for most objects, but 
would call the integrator when a decaying object is encountered and would
start auto-generating its own TLEs accordingly.  In fact, such a program
could generate a TLE with each time step and feed it to the SGPx propagator.
This would effectively nullify larger errors caused by the SGPx models because 
all predictions would be at or very close to epoch.  Obviously you
would still need fresh elsets based on actual observations on perhaps a
daily basis.

To implement this you just need to add just a few lines of code to Skymap ;)

 -- Jim


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Jim Varney      |  121^ 23' 54" W,  38^ 27' 28" N   |           Sacramento, CA
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