Observation_Time
The time at which the data comprising an atomic data set were originally recorded. If the duration of the data in the atomic data unit is large compared with the search time resolution, the Observation_Time is to be understood to correspond to the center (mid-point) of the observation(s), weighted as appropriate. For purposes of the Data Model, Observation_Time is given in calendar-clock form, e.g.2004.03.08_16:25. Times are assumed to be UTC. The time resolution is one minute, so for much data the conversion from say start time of an exposure to Observing_Time should not matter. Likewise the conversions between UTC and other units such as ET, TAI, and GPS should not be a matter of much concern. A data match is assumed to include all data from 30 seconds before the target time to 30 seconds after, inclusive (closed at both ends), so that a data Observation_Time can in principle fall into two adjacent target times. Note that since Jan 1, 1999, TAI = UTC + 32 sec, and GPS = UTC + 13 sec.