Chapter 2: Required files
Tempo2 requires certain files in order to run correctly. Most of these files are provided with the download and are discussed in the following sections (sections 2.1 to 2.5). Tempo2 also requires a parameter file containing the pulsar timing model and a set of pulsar arrival times. The formats for these files are discussed in chapters 3 and 4 respectively.
2.1 Clock correction files
Times of arrival provided to tempo2 are recorded against local observatory clocks. These times differ from those recorded against a uniform clock, firstly because observatory clocks are typically maintained in approximate synchrony with Coordinated Universal Time (UTC), which itself is not uniform, and secondly because they deviate from ideal UTC owing to deviations in uniformity in the underlying frequency standard (usually a hydrogen maser). The ultimate aim of the clock correction process is to transform all site arrival times to a chosen realisation of TT (terrestrial time), which in an ideal realisation is a uniform clock ticking SI seconds on the geoid. By default this is TT(TAI), which (since 1971) differs from UTC by a constant offset plus an integral number of leap seconds. Alternative realisations of TT can be specified using the CLOCK keyword in the parameter file.
The clock correction process proceeds entirely on the basis of linear interpolation of user-supplied tabulations of the difference between named pairs of clocks, as a function of Modified Julian Day (MJD; the fame in which the MJD is measured is not specified; it is assumed that clock offsets and drift rates are small enough that if $t^\prime = t + f(t)$ then $t \sim t^\prime - f(t^\prime)$.) These files reside in the directory
# UTC(PKS) UTC(GPS) 10
Non-comment lines consist of a sequence of pairs of MJDs and offsets (in seconds) specifying the difference between the second and first clocks as a function of date. For example:
50844.72917 -7.49068e-07 50845.77083 -7.47637e-07 50846.81250 -7.46650e-07 ...
The spacing of the dates need not be any specific value, or even be regular. For most purposes roughly daily values are suitable.
All files ending in
CLK_CORR_CHAIN pks2gps.clk gps2utc.clk utc2tai.clk tai2tt_tai.clk
This parameter may be specified multiple times. Tempo2 will attempt to apply each path in the order in which they were specified (which may fail if the MJD of the TOA is outside the range of component tables).
If no applicable pre-defined paths are found, tempo2 finds the "best" possible path using all of the available tables. Here "best" means the path for which the sum of badness values is minimised. Tie-breaking is arbitrary. This path is then appended to the global list of pre-defined paths. Since tempo2 always checks this list before attempting automatic path construction, subsequent transformations will always use this path if it is applicable, even if the MJDs of some TOAs would have allowed for a "better" path. Caution is therefore advised in using the automatic path construction feature when multiple paths exist.
2.1.1 Updating clock corrections
The distribution of tempo2 includes several useful files containing corrections based on the BIPM's Circular T (offsets between UTC and its various realisations, as well as the GPS clock) and the IERS Bulletin C (announcing leap seconds). A suite of ancillary software is available on this website which provides, among other things, a means for parsing Circular T to update the relevant clock correction files (
2.2 Earth orientation parameters
To compute the Roemer delay, the position of the observatory must be known. This depends not only on the Earth's orbit, but on the Earth's orientation and rotation. The necessary parameters are obtained by interpolation of the "C05" series of Earth Orientation Parameters (EOPs) from the IERS. The file
2.3 Time ephemeris
The pulse arrival times at the observatory are transformed to the arrival time at the solar system barycentre (SSB). In this transformation the Einstein delay, which described the combined effect of gravitational redshift and time dilation due to the motion of the Earth and other bodies, must be taken into account. This transformation converts the site arrival time from TT to a coordinate time at the SSB, known as Barycentric Coordinate Time (TCB). Optionally, for backward compatibility with tempo1 the user may also choose to use a scaled version of this frame in which the mean drift relative to TT is divided out: this is nominally (but incorrectly; see the tempo2 paper II) referred to as TDB. This is accomplished by specifying "UNITS TDB" in the parameter file.
The Einstein delay is computed using a polynomial approximation to the numerical evaluation of the time dilation integral as provided in Irwin & Fukushima (1999). It lives in
2.4 Planetary ephemeris
In order to correct that arrival time to the solar system's barycentre, tempo2 requires a solar system ephemeris. By default the JPL ephemeris DE200 is chosen. Different JPL ephemerides may be selected using the EPHEM_FILE command in the parameter file. For example,
would select the DE405 JPL ephemeris. If the full-path is defined from
In order to install the new DE421 ephemeris the following routine was carried out:
1. ftp ssd.jpl.nasa.gov 2. cd pub/eph/planets/ 3. get the correct ascii files 4. also get /pub/eph/planets/fortran/asc2eph.f 5. emacs asc2eph.f and uncomment the PARAMETER (NRECL = 4) line (also can set start and end dates if required - in JD) 6. f77 -o asc2eph asc2eph.f 7. cat header.421 ascp1900.421 | asc2eph 8. This produces the binary file JPLEPH that can be moved to, e.g., DE421.1950.2050
2.5 Observatory definitions
It is necessary for tempo2 to know the coordinates of the observatory. In the original tempo1, a file (
For full accuracy, observatory coordinates should be specified in the International Terrestrial Reference System. Geodeteic coordinates (as optionally used by tempo1, given as latitude and longitude in degrees in the form dddmmss.ss and height in metres) may be specified, in which case tempo2 will detect this and convert them to the ITRF on the assumption that they refer to the GRS80 geoid. The converted coordinates are displayed and execution is halted for the user to add the converted coordinates to the observatories database. The current observatory list is given below in Table 2.1.
Table 2.1: Observatory details