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I am doing orbit propagation for a geostationary satellite which requires the deep space corrections of SDP4. See my previous question SGP 4 for Geostationary Satellite.

I am using SGP4 on STK, but, I don't know how check to find out for sure if SGP4 in STK already contains the SDP4 deep space correction or not. I use STK 11.2. How can I check this?

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    $\begingroup$ I added it. Thanks for your suggest $\endgroup$
    – Elisa Fitri
    Commented Aug 28, 2019 at 12:12

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I can confirm that the STK SGP4 propagator contains the deep space corrections in SDP4. Source: I worked at Analytical Graphics, Inc (AGI) for 25 years and the astro team sat 2 desks away from me :). In the early days of STK we called it MSGP4 ("merged" SGP4) to try to make it obvious that SGP4 and SDP4 were "all in one", but that caused more confusion than it solved. So we switched to calling it SGP4 (sometime in the early 2000's if I recall) to be consistent with everyone else's terminology.

The version that Vallado et. al. produced was done while he was working at AGI and rolled up a bunch of tweaks that we had found along with some from others). We didn't have access to the "official" source code but some of our customers did and confirmed that the Vallado version matched "very nicely" with the official one. Later on Air Force Space Command (AFSPC) started releasing the "official" SGP4 in the form of DLLs (not source). We built an interface in STK to let users who had the "official" DLL drop it in the proper directory and STK would call that if they wanted to (which is why you see options in the various STK APIs that allow you to specify which SGP4 propagator to use).

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  • $\begingroup$ FWIW, don't hesitate to call/email AGI's tech support when you have questions about STK. Even if you are just using the free version they will help you out. $\endgroup$
    – Tom Johnson
    Commented Dec 17, 2021 at 17:21
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This is a partial answer, and I think it would be good to have confirmation from someone who knows more about this than I do.

After my previous answer I did some more searching, and found History of the SGP4 Propagator on the STK website.

It seems that this happened a lot earlier than I'd originally thought!

In the mid- to late 1970s, the SGP4 was modified to address deep space requirements. The incorporation of deep space algorithms into the SGP4 was developed primarily by Air Force Captain Bruce Bowman and Richard Hujsak of the 14th Aerospace Force/Air Defense Command/NORAD. The current SGP4 propagator, and then, is really an SGP4/DP4 propagator."

I assume that the SDP4 deep space correction is automatically implemented for TLEs with mean motions of 6.4 per day (225 minute period), and that there are no settings or controls to turn this on or off. TLEs are generated expecting standard SGP4 behavior, so my guess is that this all happens hidden from view and you don't have to turn anything on or off.

But I'd feel better if someone were to add to this answer or write a new one with confirmation.

Update:

STK's page SGP4 Propagator says:

By default, STK utilizes the CSSI SGP4 routine, Version 2008-11-03

I don't know what version that is exactly, but for CSSI see http://www.centerforspace.com/downloads/. Hat tip to @brethvoice for commenting that this link is currently broken. I'm not really familiar with this and will appreciate any help finding a new one.

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    $\begingroup$ yeah, i found the same. CSSI use orbital data from NORAD SGP4/DP4. It means SGP4 in STK also contain SDP4. $\endgroup$
    – Elisa Fitri
    Commented Aug 28, 2019 at 13:27
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    $\begingroup$ @ElisaFitri okay that's great news! $\endgroup$
    – uhoh
    Commented Aug 28, 2019 at 13:32
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    $\begingroup$ @uhoh I emailed CSSI using the contact address on the bottom of this page asking for answers to this question. In the meantime, your link is broken, so if you know of a better source of information, please post to /edit your answer. $\endgroup$
    – tdMJN6B2JtUe
    Commented Jun 22, 2021 at 14:13
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    $\begingroup$ @brethvoice Thanks for the ping! I've made a note about it, perhaps something will come up. $\endgroup$
    – uhoh
    Commented Jun 22, 2021 at 14:27
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    $\begingroup$ Answer by email from dav (David A Vallado, [email protected]) truncated to fit in one comment block: "The original 1980 version of SGP4 differentiated the near-Earth and deep-space portions of the code, calling them SGP4 and SDP4. Future revisions removed the separate nomenclature, but maintained the two approaches in the code because different force models are used for each approach. The code can be found here, along with the paper I did discussing the various aspects of the code, including removal of GOTOs, test cases, etc." celestrak.com/publications/AIAA/2006-6753 $\endgroup$
    – tdMJN6B2JtUe
    Commented Jul 21, 2021 at 13:15

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