The barycentric corrections are applied directly to the RAW_TIMETAG list in the input file, the time axis or the header components as appropriate.
The satellite Orbit file should be produced by either ATT_POS_SAT for ROSAT WFC data or XRTORB for ROSAT XRT data.
If the Orbit file is available and used, the default update period to the calculation is the frequency of the updated orbit data. This applies to VMS only.
The algorithm used is based on Stumpff Astron & Astrophys 1979.