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Late Ephemeris Updates |
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Spitzer does not carry ephemerides for Solar System targets on board the
spacecraft. Most Solar System observations are therefore commanded on
Spitzer using positions and tracking rates for the target that are
specified by the ground-system software using an ephemeris that is
typically generated 5 weeks prior to the Spitzer observation. The five
week delay from observing command sequencing to execution on the
spacecraft encompasses the time required to generate, finalize, check,
authorize and uplink the observing sequence to the spacecraft. (See How Scheduling
Works.) However, to successfully acquire the necessary data for the proposed science, some Solar System targets may require the use of positional and tracking rate information derived from an ephemeris that is generated much closer to the time of the Spitzer observation. If this is required for your observation, you may request a "Late Ephemeris Update" via Spot, at the time you submit your Spitzer observing proposal. As with all special constraints, the use of Late Ephemeris Update must be justified in your proposal. To implement a Late Ephemeris Update, the Spitzer Science User Support Team will work with the observer and JPL's Solar System Dynamics group to ingest additional ground-based astrometry for the target acquired less than 5 weeks prior to the Spitzer observation, and to recalculate and use the new ephemeris based on this data. Please note that Late Ephemeris Updates within 5 to ~2 weeks of the start of the week in which the observation is scheduled produce the least disruption to the scheduling process. Late ephemeris updates required within 2 weeks of the start of the week in which the observation is scheduled produce a significant disruption to the routine spacecraft commanding process and are considered equivalent in impact to a medium or high-impact ToO. Consequently a Late Ephemeris update requested within 2 weeks of the start of the week in which the observation is scheduled will need an extremely strong scientific justification, and must also be reviewed and approved by the SSC Director prior to implementation.
When to Request a Late Ephemeris UpdateYou should request a Late Ephemeris Update when you have, for example:
When Not to Request a Late Ephemeris UpdateIf your target is known to have poor positional accuracy at the time you submit your proposal to a Spitzer GO call, it is NOT necessary to request a late ephemeris update. However, in this case, it is your responsibility to ensure that the positional accuracy is adequate for the proposed observation prior to observation by Spitzer. (This needs to be at least 5 weeks out from when your observation might be scheduled; see How Scheduling Works.) Note that although we try to catch observations with poor positional accuracy before scheduling, we make no guarantees that we will catch every one. It is your responsibility to flag observations with uncertain positions, have a plan for providing an updated ephemeris within the cycle, and update the ephemeris if necessary. Please contact the SSC Helpdesk if you have any questions on how to meet and execute these requirements, preferably before you submit your proposal.Note that by their nature, medium- and high-impact ToO observations of moving targets will automatically include a "Late Ephemeris Update" as the most up-to-date ephemeris will be generated at the time of ToO activation, which will be within the 5 week period prior to observation. In this case, the Late Ephemeris Update does not need to be requested separately in Spot, and there is no additional overhead for this, beyond the (larger) overhead for a medium or high impact ToO.
How To Request a Late Ephemeris UpdateThe "Late Ephemeris Update" must be requested within the affected AORs at the time you submit your Spitzer observing proposal. This overhead must be applied to the first AOR in a group, chain, or sequence of AORs to be executed consecutively in the same observing session using a single science instrument. It does not, however, need to be applied to any shadow observations of the target, as the shadow retraces the path across the sky taken by the science observation and will use exactly the same ephemeris as the science observation. However, if you specify a follow-on observation (NOT a shadow) at a non-consecutive fixed time relative to the first observation that is longer than a week, e.g., multiple observations of a comet at different epochs near perihelion, then if a Late Ephemeris Update is required for at each epoch of observation, it must also be added to each AOR within the follow-on constraint.To request the late ephemeris update, using the guidelines above, determine the AOR to which it needs to be applied, open (modify) the AOR in Spot, and select the "Special" button at the bottom of the AOR. Then select "Late Ephemeris Change" from the pop-up window and click "OK". The Late Ephemeris Update incurs a "Special Overhead" to reflect the scheduling inefficiency induced. The current penalty incurred can be found in the Call for Proposals Appendix E. By selecting a Late Ephemeris Update in Spot, it will not only tag your AOR as requiring a Late Ephemeris Update, but will automatically add the appropriate Special Overhead Rate to the AOR duration.
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/documents/lateephem.html
