RXTE 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 RXTE FITS files, one will find the following quantities, either as header keywords or table columns.

Time         Time stamp, in MET seconds (i.e., the spacecraft clock reading)
TIMEZERO     First order clock correction (~3.4 s)
MJDREF       The MJD(TT) of 1994.0(UTC): 49353 d + 60.184 s
TIMESYS      Time system; either TT or TDB
CLOCKAPP     Boolean: any correction applied?
TIERABSO     Absolute time accuracy

TT           Terrestrial Time
TDB          Barycentric Dynamical Time
TCG          Geocentric Coordinate Time
TCB          Barycentric Coordinate Time

Time can be expressed in the following ways:

MET (accuracy 4 s)      Time
MET (accuracy 100 µs)   Time + TIMEZERO
MET (accuracy 5 µs)     Time + TIMEZERO + clockCorr;
MJD(UTC)                MET + 49353 d - leap seconds
MJD(TT)                 MET + 49353 d + 60.184 s
MJD(TDB)                TT + baryCorr(TDB) = TT + tau + (TDB - TT)
MJD(TCB)                TT + baryCorr(TCB) = TT + tau + (TCG - TT) + (TCB - TCG)

MOC Clock Calibration

The MOC (Missions Operations Center) performs clock calibrations several times a day. It uses the USCCS method which involves tagging of a round-trip signal, claiming an accuracy of 1 µs, deteriorated to 5 µs by uncertainties in the White Sands ground clock.

For various reasons, we believe this technique to have yielded an accuracy of 8 µs for RXTE observations made prior to 1997-04-29 (mission day 1214). From that day onward, a formal accuracy of 5 µs is appropriate.

Scientific Verification

Using the pulsars PSR B1509-58 and PSR B0531+21 (the Crab pulsar), we have verified the MOC's absolute clock calibration to an accuracy of 1 ms. This the best that can be done, using cosmic sources. Hence, we feel justified in accepting the clock calibrations by the MOC to the quoted accuracy of 5 or 8 µs (see above).

In addition, the RXTE instruments have intrinsic delays which need to be included in high-precision clock corrections. These are:

PCA:       16 - 20 µs
HEXTE:      0 -  1 µs

Barycenter Corrections

The pathlength corrections (tau) applied by the Ftool fxbary are based on inherited software and ephemeris; they are accurate to better than 1 µs. The pathlength correction calculation in fasebin uses new software, based more directly on the JPL DE-200 ephemeris, and is accurate to a fraction of a µs.

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.

Clock Correction Tools

If an absolute timing accuracy of 100 µs is sufficient, the FITS files contain all that is needed, provided that CLOCKAPP is true and TIMEZERO is non-zero.

If one needs to do better than that, there are two XTE-GOF tools that incorporate the higher accuracy:

1. The Ftool fasebin automatically applies high-accuracy clock corrections, provided that it has access to a current version of the file tdc.dat.

2. The web tool xTime always provides all clock corrections, whenever it does a time conversion.

If one is using one's own software, one can obtain a C function that calculates the clock correction. xCC.c is available through the web.

All these tools rely on the same ASCII file, tdc.dat, which contains the clock correction coefficients and needs to be kept up-to-date.

A service of the Laboratory for High Energy Astrophysics (LHEA) at NASA/ GSFC
HEASARC Director: Dr. Nicholas E. White, white@adhoc.gsfc.nasa.gov, 301-286-8443
Technical Rep: Sherri Calvo, sherri.calvo@gsfc.nasa.gov, 301-286-5668