Re: DFH-3 Rkt Flashing & SGP4 thoughts

Matthew Francey (
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.