[Smwg] Response for AI 2019-0205-01 (topocentric vs center-of-earth occultation event)

Barkley, Erik J (3970) erik.j.barkley at jpl.nasa.gov
Mon Apr 15 18:48:05 UTC 2019

CSSM Colleagues,

In our last teleconference, I agreed to take the lead with regard to action item 2019-0205-01 which has to do with how to express occultation start/stop events - please see below for a copy of the action item.

The issue essentially arose as part of the PIF prototyping and in particular with regard to data exchange from ESA to NASA. The current PIF structuring requires that an aperture be referenced for expression of start or end occultation events. For the ESA information produced, it turns out that occultations are in reference to the center of the earth which makes the aperture reference indeterminate. For the NASA generated information, the current system involved in producing prototype information is topo-centric meaning that the occultation is calculated for that location on earth, which in turn means that an aperture reference can be provided.

Taking the worst case into account (an equatorial tracking station -- where Earth's radius is the widest -- and an occultation that occurs exactly when the ground station is on the equatorial limb) my rough calculations are that we would have a difference of essentially 2/100 of a second in terms of event time.  For planning purposes I don't think this will be significantly important, but I also think it is important to make sure that the our systems understand what is being calculated.

The proposal, is quite simple but I think is effective:  for the occultation event, we allow a reserved keyword for the aperture reference which is "Earth-Center".   I propose that this be a keyword relative to the standard and not something that is registered in SANA. I think this has the benefit of not requiring any significant change to the structuring of the draft standard but at the same time is clear as to the basis of the calculation.  It will have to be made clear in the standard that calculations for this event are topo-centric unless the aperture reference is stated as being "Earth-Center".  But that is just some verbiage to add to the current draft standard.

Please let me know if you have comments or further thoughts on this.

Best regards,




Determine how to represent occultations that are earth-centered vs topographic- centered (is aperture made optional?)




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