Re: DFH-3 Rkt Flashing & SGP4 thoughts
Fri, 21 Mar 1997 23:34:39 -0800

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     Sacramento, CA