XMM-Newton Science Analysis System


earthbarycen (barycen-1.22.1) [xmmsas_20230412_1735-21.0.0]

Introduction

The earthbarycen task converts times expressed in the local satellite frame to Barycentric Dynamical Time (TDB) at a given point on the Earth. In more detail, the task requires as main input a table in a data set which contains a column with times in XMM MET (Mission Elapsed Time) frame[1]. On this table, the following operations are carried out:

1. Check whether the earthbarycentric conversion has been performed already (based on value of attribute TIMEREF) and, if found true, exit

2. Correct all time tags stored in a specified column of the table

3. If a Good Time Interval extension exists, convert all interval start and stop times to TDB as well

4. Correct all time tags stored in the EXPOSU tables, if the parameter processexposutables is enabled.

5. Update the values of the attributes TIMEREF, TSTART, TSTOP, and TELAPSE where necessary

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 [1].

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 earthbarycen 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.