PDS_VERSION_ID = PDS3 RECORD_TYPE = "STREAM" OBJECT = TEXT NOTE = "Known anomalies concerning this EPOXI dataset" PUBLICATION_DATE = 2011-06-30 END_OBJECT = TEXT END This file describes known anomalies concerning this EPOXI dataset as of 30 June 2011. Keywords in the Primary FITS Header =================================== - In the data file /DATA/2010/047/MV10021621_2000029_008.FIT, the target is incorrectly specified as "CANOPUS" by OBJECT. The correct target is "ACHERNAR". The corresponding PDS label correctly identifies the target (i.e., TARGET_DESC = "ACHERNAR"). - For data acquired before August 2010, the correct unit for NOMPXLSZ and RECPXLSZ is radian, not microradian as specified in the comment. - RANGE, CTRDIST, and RANGECEN have intentionally been set to the same value: the distance from the spacecraft to the center of the target. Anomalous Target-Spacecraft Velocities ====================================== Summary ------- The way geometry is calculated in the Science Data Center (SDC) pipeline for the EPOCh phase of the EPOXI mission yields some anomalously large values of the TARSCVX, TARSCVY, TARSCVZ, SCSUNRX, SCSUNRY and SCSUNRZ FITS header keywords and their corresponding keywords in the PDS label EPOXI:TARGET_SC_VELOCITY_VECTOR and SC_SUN_POSITION_VECTOR for extra-solar targets. Users are advised to ignore these values for extra-solar targets; note however that these keywords are generally of little use for such targets and little is lost by having them there. The awareness of this issue came about late in the archive process and so can only be documented in this delivery. The pipeline will be modified in the near future to resolve this issue; since integration of updated ephemerides into post-V1.0 dataset versions are a normal part of the archive process and all of these data will be re-run through the corrected pipeline, this minor inconvenience to PDS users is anticipated to be temporary. In the meantime, anyone needing resolution of the issue is welcome to contact Brian Carcich, SDC System Manager at Cornell University, for consultation. Details ------- Because of the velocity of the spacecraft relative to the solar system barycenter, there is a shift in the *apparent* position of any target; the correction for this is called stellar aberration. The stellar aberration correction is well documented elsewhere (e.g. see any astronomy text or the SPICE toolkit required reading files at http://naif.jpl.nasa.gov/); it will not be described here other than to say it does not change the length of calculated vectors but it does change their direction by up to about 100 (Vspacecraft / c) microradians for a typical mission's spacecraft velocity (30km/s barycentric relative). The correction was applied in the calculations to determine the apparent vector of the target with respect to the spacecraft. For convenience, the vector from the spacecraft to the target was then inverted to get the vector from the target to the spacecraft, and the corrected spacecraft-target and target-sun vectors were linearly combined to get the sun-spacecraft vector. Because the heritage of the pipeline geometric calculations is for bodies within the solar system (Spacecraft, Tempel 1, Earth, Moon, Sun, etc.), the finite but insignificant errors introduced by these conveniences have been accepted by users of the SDC and the archived DI datasets. Also because of that heritage, extra-solar targets (stars and extra-solar planetary systems such as the objects of the EPOCh phase of the EPOXI mission) have been modeled by the Deep Impact pipeline via fixed-position body proxies at a range of 100 light-years (ly). This simplified the geometric calculation as separate source code paths were not required to process the two types of targets. When that decision was made circa 2005, the stellar aberration correction algorithm used by the SPICE toolkit to calculate apparent velocity was a very rough approximation that did not take spacecraft acceleration into account. This approximation was adequate for most SPICE users' needs. It also did not cause any problems when used by the Deep Impact pipeline. In March 2008, the NAIF group at JPL implemented a better stellar aberration algorithm and distributed it with the SPICE toolkit version N0062. As a result, when this new version of SPICE was installed on a SDC pipeline in early 2009 in preparation for PDS and MAST archiving efforts, the effect of the spacecraft acceleration over the modeled 100 years of light travel time to the proxy bodies for extra-solar targets resulted in numerically correct, but anomalously large, apparent velocities. Note that the issue is with the extra-solar target model used and not with the SPICE toolkit: the SDC has conferred with NAIF to verify that these velocities are being calculated correctly by the SPICE toolkit routines for the target model used. The description of this anomaly was provided by Brian Carcich.