[Spice_discussion] Questions about the Two-Body Propagator, PROB2B.f

Samuel Dupree sdupree at speakeasy.net
Tue Mar 14 20:25:44 PDT 2017

I have some question concerning the two-body propagator PROP2B. I'd like 
to use it in performing an initial validation the DIVA (from MATH77) and 
TOMS Algorithm 670 (from the ACM Transactions On Mathematical Software) 
numerical integrators using the two-body problem. My questions follow.

 1. How was PROP2B validated?
 2. In reviewing the code I saw that PROP2B uses a modified version of
    the method documented in Danby's book on celestial mechanics. I'm
    also reviewed the methods developed by Goodyear (documented in the
    Astronomical Journal and elaborated in a NASA report) and by
    Shepperd (documented in the journal Celestial Mechanics). The
    question I have here is what are the "pros" and "cons" when each
    method is compared against the others?
 3. What kind of numerical precision (8 digits, 10 digits, 12 digits ,
    etc.) can I expect from PROP2B over a one day propagation? two days?
    propagation out to 8 days?
 4. Lastly, are there any "gotchas" I need to pay attention to when
    using PROP2B to compare two-body propagated position and velocity
    vectors with their counterparts from a numerical integration? For
    example, does one get better numerical accuracy by by propagating
    from the epoch point to each later time point in turn, or is it
    better to use PROP2B to propagate from each time point to the next?
    Any run-time trade-offs between these two approaches to pay
    attention to?

Looking forward to hearing from you.

Sam Dupree.

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