Integral Statevector vs. Spacetrack TLEs

From: Gerhard HOLTKAMP (
Date: Thu Nov 16 2006 - 14:32:29 EST

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    After the recent discussion on this site about the posted updated version of 
    the SGP4/SDP4 code I decided to run a few tests on my own. As ESOC Flight 
    Dynamics posts state vectors and TLEs for their own satellites on 
    and assuming that they know best where their satellites are this could serve 
    as a  reference to check SDP4 calculations of Spacetrack TLEs against. 
    A test I ran with XMM (1999-066A; 19500km x 101600km, 53.5 incl) showed a 
    marked improvement of the updated SDP4 code vs. the old one I had used (which 
    was essentially based on the original Report #3 with only some 
    modifications). The error stayed within 15 km over two orbits (4 days) which 
    for this kind of orbit I would find acceptable. With my old version it was 10 
    times as much.
    Moving on to INTEGRAL (2002-048A; 12800km x 149800km, 82.5 incl) produced a 
    surprise. For epoch 13-NOV-06, 16:00 UTC there were TLEs available from 
    Spacetrack as well as a state vector and TLEs from ESOC. Avaluating right at 
    that epoch I found a whopping difference of 7190 km between the state vector 
    and the respective Spacetrack TLEs! The ESOC TLEs agreed with the state 
    vector if I used the SGP4 rather than the SDP4 algorithm (with SDP4 I would 
    get a difference of 260 km - this simply means that ESOC generates TLEs via 
    the standard SGP rather than the SDP4 so SGP is what you have to use to 
    evaluate their TLEs). 
    The reason for this dismal performance does not seem be the SDP4 algorithm but 
    rather the underlying TLEs. Spacetrack posted TLEs for XMM, INTEGRAL, CXO and 
    a few other satellites on these high eccentricity  orbits always show epochs 
    at the exact hour. My guess is that US Spacecom tracks these satellites only 
    sporadically and in the meantime generates TLEs via numeric propagation of 
    their last tracking. I was told that ESOC recently had done some orbit 
    maneuvers with INTEGRAL (but none during the last two weeks) so it is 
    possible that the Spacetrack TLEs of three days ago were actually based on 
    tracking which was at least three weeks old. 
    Of course all those sats are not really on the most observed list so the 
    relevance of all this for SeeSat may be limited.
    Gerhard HOLTKAMP
    Darmstadt, Germany
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