[Smwg] Response for AI 2016-0408-22 (Sun Alignment Angle)

Barkley, Erik J (3970) erik.j.barkley at jpl.nasa.gov
Mon May 9 20:59:53 UTC 2016

CSSM Colleagues,

At the Cleveland meetings, I took an action item to "Propose definition for sun alignment angle for planning book".

>From my perspective, the point of such a measurement is to indicate conjunction (in an angular sense, not radial) of a spacecraft that will potentially be tracked such that solar scintillation can be accounted for.    Too much solar scintillation and the tracking antenna and or the spacecraft can essentially be "blind" such that there really is no point in trying to communicate/track the spacecraft at that time.

I believe there are two options that can be considered. The first option and the one I prefer is with a topo-centric (or aperture) reference meaning the angle subtended from the sun to the spacecraft relative to the tracking antenna. Another option is a geocentric reference which I think is less accurate. I think this becomes more important when tracking is near to the horizon as the topo-centric reference will likely have the biggest difference relative to geocentric measurement at that time. The two pictures below illustrate the different scenarios for Earth-based tracking stations. For tracking station in space I think we in fact have no choice but to make this relative to the aperture.

Given the above, the proposed definition is something along the lines of "the predicted angular separation between the sun and the spacecraft at the indicated time, in reference to the tracking aperture".

As understand it, the important thing is not so much the alignment, but rather the separation, so perhaps this could be indicated as the Aperture-Sun-Separation Angle, but that may have an unfortunate abbreviation as in the <Bleep> angle.  So perhaps this could be the Sun-Separation Angle ?

Best regards,

        [cid:image004.png at 01D1A9FB.0EEF6330]

[cid:image006.png at 01D1A9FB.0EEF6330]Option A: Preferred - Aperture/Topo-centric reference

Option B: Geo-centric reference
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