Re: SDP algorithm

Bjoern Gimle (
Fri, 21 Mar 1997 07:53:21 +0100

>Greetings all,
>I've just completed coding and initial debugging of the SDP4 algorithm for
>inclusion in SkyMap.  A word of warning to anyone who used Spacetrack Report
>#3 to code the Deep Space routines -- the code is flawed!  4 variables are
>passed to the deep space routines which the programmer obviously intended to
>retain their values upon subsequent calls.  Trouble is, these variables are
>DUMMY variables within DEEP.  Thus on subsequent calls, the variables will
>have random, unpredictable values.
...>SGP and HANDE were each off by about 38 minutes time along-track, and
>0.4-degrees crosstrack.  SDP4, on the other hand, was off by only 20 SECONDS
>time along-track, and 0.0025-deg crosstrack!  20 seconds off with 5-month-old
>ephemerides -- that's SOME improvement!
This is interesting news! I have at least twice questioned the differences
between TrakStar, SeeSat, and SkyMap, which all claim to implement SGP4.
I used them for Interbol-1 and Mars-96 predictions, and found extreme
differences, starting in the latter case from an orbit generated from
state vectors by Vec2Tle, which is using SGP/SGP4.

TS Kelso's reminded me that SGP4 is not designed to work with highly
elliptic orbits, but did not mention that TrakStar also uses SDP4
(but this is obvious from the program header!)

Now I am beta-testing a new SGP4/SDP4 prediction program, and found the
similar differences on the pseudo-elset I generated with Vec2Tle and Pelcor
for Hale-Bopp (posted here Mar.5), which is not high excentricity,
when viewed for Mar-Apr as if it were in Earth orbit. 
SkyMap and SeeSat SGP4 are OK, but TrakStar predicts positions with
no apparent relation between direction and range, and RA 1:40 Dec.+15 yesterday.

Hale-Bopp pseudo elsets, RMS=.0068
1 97000U          97062.00000002  .00000000  00000-0  00000-0 0    14 
2 97000  45.8766 279.3278 2581797  44.1676   3.7176  0.00305198    12

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