Re: DFH-3 Rkt Flashing & SGP4 thoughts
Matthew Francey (mdf@presence.lglobal.com)
Sat, 22 Mar 1997 08:55:46 GMT
Jim Varney:
>[general vs. special perturbations]
Minor nit: technically, the distinction between "general" and
"special" perturbations is only in how the problem is solved, not the
underlying model.
> If you look at the SDP4 algorithm, it features a
>numerical integrator to calculate the shifts in position for lunar and
>solar perturbations.
It also includes 12 and 24hr resonance effects. And actually
*looking* at the FORTRAN SDP4 code is not recommended. A most
unsettling experience it can be. :-/
The integrator in SDP has an annoying property: it computes perturbations
(and derivatives) on an evenly spaced grid in time, and extrapolates
(using the derivatives) as needed. The extrapolation isn't perfect -- it
rarely mates up with the "next" grid point. This results in discontinuities
in the state-vector estimates when you cross a grid-line (in time).
A more "formal" approach (using, say, Encke's method) would likely
be a little slower, but may result in "smooth" state-vector
estimates. Hm.