# SGP4 on Systems Tool Kit (STK); how to check if SDP4 deep space correction is implemented?

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?

• I added it. Thanks for your suggest Aug 28 '19 at 12:12

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.

• yeah, i found the same. CSSI use orbital data from NORAD SGP4/DP4. It means SGP4 in STK also contain SDP4. Aug 28 '19 at 13:27
• @ElisaFitri okay that's great news!
– uhoh
Aug 28 '19 at 13:32