Chandra Absolute Timing Accuracy

Definition of Terms

To properly understand the various elements involved in establishing absolute time, one has to be aware of the different components and time systems. In the Chandra FITS files, one will find the following quantities, either as header keywords or table columns.

Time systems: MET (Mission Elapsed Time) is counted as elapsed seconds since 1998.0 (TT). MJD (Modified Julian Day) is defined as JD - 2400000.5. A more detailed description of time scales and time systems can be found in the Chandra Time Tutorial.

Time can be expressed in the following ways:

Clock Calibration

The DSN (Deep Space Network) performs clock calibrations about twice a week. The accuracy is required to be better than 100 µs. We have performed a scientific calibration of absolute time in Chandra data and concluded that it is possible to achieve an absolute time accuracy of about 4 µs. See the note on absolute time calibration.

Barycenter Corrections

The pathlength corrections (tau) applied by the tool axBary are accurate to better than a fraction of a µs, based on JPL ephemerides DE-200 or DE-405. The tool faseBin uses the same code.

The values for TDB-TT that these tools derive are not to be trusted to better than a few µs. However, if one were to improve on these, one should really switch from using TDB to TCB.

Sampling and the Position of Time Stamps

In Chandra FITS files, three types of data are encountered:

In the last two cases, it may be possible to be more precise, but this requires detailed knowledge of the subsystem, as well as particulars of the data, and no general rule can be given.