MSGP4 at Goddard

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
1 Nov 1997 12:53:08 -0800

Hello all,

Just an update for anyone following the SDP4 discussions.  I revisited
the Goddard source code which is basically their version of MSGP4 --
Merged Simplified General Perturbations (the merger being that of
SGP4 and SDP4).  Earlier I had reported that, like SkyMap 5.11,
Goddard's deep space code correctly handles solar-lunar perturbations
because it calculates perturbation offsets at epoch.  Now it appears
that I'm going to have to take that statement back.

I was fooled by the fact that they had an apparent structure set up to
retain the perturbation values at epoch.  I was wrong.  The "intent" of their
code is to have epoch offsets of PE0, PINC0, PL0, PGH0 and PH0
which they subtract from PE, PINC, PL, PGH and PH respectively.
The problem is, they set PE0 ... PH0 all to zero!  In other words, the
offsets do absolutely nothing.

According to Bruno Tilgner and David Irwin, the Goddard source code is
no longer available at:

http://seawifs.gsfc.nasa.gov/SEAWIFS/SOFTWARE/src/bobdays/sgp4sub.f

You can get as far as SOFTWARE, but access to /src is not allowed, so
I don't know if they've subsequently "fixed" their code.  It may be that they
no longer want to make their code publically available for export-control
reasons.  --Rob