In accordance with NASA/OGIP recommendations, the value of the TIMEREF keyword is either LOCAL or SOLARSYSTEM depending on whether the times are expressed in local satellite time or TDB. Note that the conversion to TDB does not affect the actual time system in use, i.e., all times keep being referred to the fixed mission reference time 1998-01-01T00:00:00 TT .
The conversion of a satellite time tag to TDB requires the knowledge of the satellite's position at that time. Therefore, the task needs access to the orbit file in the ODF which covers the time period in question. This access is done transparently via the ODF access layer which, however, needs to be pointed to the directory containing the ODF .
In addition to the satellite position data, the time conversion algorithm in use by barycen requires the barycentric vectors for the time period in question to be known a priori. This information is comprised in the JPL ephemeris data set in FITS format which is part of the barycen package.
XMM-Newton SOC -- 2021-11-30