Late Ephemeris Updates
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 Spitzer 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 must request a "Late Ephemeris Update" in your AORs in Spot, at the time you submit your Director's Discretionary Time (DDT) observing proposal. All Late Ephemeris Update observations must be submitted as DDT proposals. 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.
Late ephemeris updates are not supported in GO proposals during the Warm Mission. Any observations requiring a late ephemeris update must be submitted via a DDT proposal.
When to Request a Late Ephemeris Update
You should request a Late Ephemeris Update when you have, for example:
- a target that has a high probability of behaving unpredictably during the 5 weeks prior to its observation, as might be the case for an active comet near perihelion, or
- a target that, just prior to observation by Spitzer, has relatively few observations and a correspondingly large positional error, as might occur for a newly discovered object that has been requested as a "no-impact" (more than 5 weeks warning of desired observation date) Target of Opportunity (ToO) observation.
When Not to Request a Late Ephemeris Update
If 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 Spitzer 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. During the warm mission all medium and high-impact ToO observations must be submitted via DDT.
How To Request a Late Ephemeris Update
In the warm mission all late ephemeris update observations must be submitted as part of a DDT proposal.
The "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.