The ABC of XTE
A Time Tutorial
A Time Tutorial
Postscript version of this chapter
- Introduction
- Time and Date Systems
- Useful Relations
- Other Useful Formulae
- RXTE FITS Tables
- Barycentering
- Changes
This chapter aims at collecting brief definitions of the various time systems in use, as well as simple formulae for doing transformations. Information was gleaned from a number of sources, like the Explanatory Supplement to the Astronomical Ephemeris (1992), a paper by Soffel and Brumberg in Celestial Mechanics and Dynamical Astronomy 52, 355 (1991), and several papers in the Proceedings of the 6th European Frequency and Time Forum (ESA
SP-340, 1992).
The last section describes the time as it is kept in the RXTE FITS tables.
Information is available on absolute time calibration.
See also the USNO Time Scales page.
The vectors xe and ve denote the barycentric position and velocity of the earth's center of mass, and x is the barycentric position of the observer. The quantity P represents periodic terms which can be evaluated using analytical formulae. For observers on the Earth's surface, these are diurnal, with a maximum amplitude of 2.1 microseconds; for spacecraft, they presumably depend solely on the orbit.
The origins of coordinate times have been arbitrarily set so that these times all coincide with TT at the geocenter of 1977.0 (TAI).
This approximates to:
At an altitude of 500 km, the clock would loose approximately 8.5 ms per year, unless its frequency were adjusted.
The time stamps presented by RXTE FITS tables as generated by XFF are in TT.
It has been agreed that the Spacecraft Clock Seconds (SCCS), or Mission Elapsed Time (MET), will represent true elapsed seconds since January 1, 1994, at 0h0m0s UTC, which corresponds to MJD = 49353.0 (UTC).
Raw time stamps in the FITS tables created by XFF will be on this (SCCS or MET) time system. However, FITS times are to be taken as the sum of a Time value and the values of the MJDREF and TIMEZERO keywords. The MJDREF keyword will incorporate the transformation from UTC, via TAI, to TT (equaling an offset of 60.184 s). Thus, for January 1, 1994, at 0h UTC:
MJD (Jan 1, 1994, 0h UTC) = 49353.0 (UTC) = 49353.000696574074 (TT)
The TIMEZERO keyword will provide a nominal clock correction that ensures an absolute accuracy of 60 µs for the time stamps.
The goal is to provide reasonably accurate absolute time with a minimum effort. Generically, the value of TIMEZERO may be described as "the fractional part of UTCF".
[UTCF is an institutional dinosaur that can be found in the telemetry. Its purpose is to provide a conversion to UTC: when the sum of SCCS and UTCF is divided by 86400, the integer part of the result provides the number of UTC days since January 1, 1993, at 0h UTC, while the fractional part indicates the current UTC. Thus, UTCF is the sum of the number of seconds in 1993 (31,536,001) and the clock correction, minus the number of leap seconds since January 1, 1993.]
To summarize the times in the FITS tables:
Thus, to compute the modified Julian days (MJD) from raw XTE FITS
files, referred to the TT time system, one should use the
following formula,
In order to recover the most precise absolute times, the fine clock
corrections found in the tdc.dat file must be applied. See the RXTE Absolute Timing page for
more details about fine clock corrections, and which tools incorporate
these. See the Timing Digest
for a figure of fine clock corrections as a function of time. The
maximum attainable absolute time precision, using the tdc.dat file
corrections, is about 2.5 microseconds for most of the mission
(ref. Jahoda et al. 2006, ApJS, 163, 401).
It may be possible to reach even higher accuracies, to the level of a few microseconds, but this will require a knowledgeable guest observer fine-tuning the clock corrections "by hand". No SOC support will be extended for this, beyond providing the guest observer with all data readily available to the SOC.
The relevant FITS table keywords are summarized below:
For high precision timing work, one often has to account for the fact
that the spacecraft and earth are moving through the solar system.
Since the speed of light is finite, the arrival time of a photon from
a target will be delayed or advanced, depending on the position of the
observatory within the solar system. The amount of delay or advance
varies appxroximately sinusoidally with time, with a period of one
year and an amplitude of about 8 minutes. Any astrophysical observed
variations from the target will be advanced or delayed by the same
amount.
Removing these observatory-related variations is known as
barycentering. Photon arrival times are adjusted so they represent
the effective arrival time to a ficticious observer, located at the
solar system barycenter. The solar system time scale is known as
Barycentric Time. The software tasks 'fxbary', 'faxbary' and
'barycorr' are capable of performing this correction on RXTE data.
The adjusted barycentered times will be written to either the 'TIME'
or 'BARYTIME' column, depending on the options used.
After barycentering, the 'TIME' or 'BARYTIME' column contains the
photon arrival time in TDB seconds, since the reference epoch, MJDREF,
now also expressed in the TDB system. Thus, to compute the MJD time
from barycentered files, referred to the TDB system, one should use
one of the following formulae,
After barycentering, the contents of the FITS table will be transformed,
as follows:
NOTE:
The time stamps present in RXTE FITS tables as generated by XFF are in
TT.
Introduction
Time and Date Systems
Useful Relations
Other Useful Formulae
RXTE FITS Tables
MJD(TT) = (MJDREFI+MJDREFF)+(TIME+TIMEZERO)/86400
Keyword Value Unit Comment
TTYPE1 'TIME' s Column 1 contains raw time in SCCS
TIMESYS 'TT' Defines TT as fundamental time system
MJDREF 49353.000696574074 d 1994.0 (UTC) in TT
TIMEZERO frac(UTCF) s Clock correction
Various transformations between MET, TT, and UTC in a variety of formats
(seconds, calendar date/time, year-day-time, JD, MJD) can be performed
using the xTime web tool.
Barycentering
MJD(TDB) = (MJDREFI+MJDREFF)+(BARYTIME+TIMEZERO)/86400 (or)
MJD(TDB) = (MJDREFI+MJDREFF)+(TIME+TIMEZERO)/86400
depending on task the settings used. If the TIMEZERO keyword is not
present, the default is zero.
Keyword Value Unit Comment
TTYPE1 'TIME' or 'BARYTIME' s Column name depends on task settings
TIMESYS 'TDB' Time system now in Barycentric Time
MJDREF 49353.000696574074 d 1994.0 (UTC) in TDB
TIMEZERO 0 or undefined s Should have been absorbed into time column
Changes
The ABC of XTE is written and maintained by the RXTE GOF. Please email xtehelp@athena.gsfc.nasa.gov if you have any questions or comments. This particular page was last modified on Thursday, 22-Jan-2009 20:33:43 EST.