4.6 Ephemeris Uncertainty [Ephem_Uncert]

The <value> for ephemeris uncertainty is the distance along its trajectory that the target is expected to be from its ephemeris position, in kilometers (KM), or seconds of time (S). The latter reflects the fact that in general, the least well known parameter in an ephemeris is the perihelion time. This parameter is required for any moving target used in an exposure with the REQuires EPHEMeris CORRection Special Requirement (see REQuires EPHEMeris CORRection <id>).

Note: A realistic estimate of ephemeris uncertainty is needed to schedule the time necessary to repoint the telescope to the improved position when it is known. It will not be possible for STScI to apply a correction larger than the specified uncertainty.