Fermi Gamma-ray Space Telescope

Ephemeris Computation Utility (gtephem) Tutorial

The gtephem tool is used to present the user with the best ephemeris available in a pulsar database for a given pulsar and instant of time. Note, the default solar system ephemeris for the FSSC Science Tools is JPL DE405 but not all pulsar ephemerides use this, some use JPL DE200. As such, it may be helpful to try the following examples with the command line option 'solareph="JPL DE200" ' if no valid ephemeris is found for your pulsar.

Known Issues: Some differences have been noted between output on Windows and output on Linux. The values are not qualitatively far off, but it is not yet known whether the differences are significant, or indicate a problem in the code.


Sample File. To try the examples in this section, you can download the following simulated data file. The simulated data is for demonstration purposes.

Also see:

Tutorial SciTools References
Pulsar Analysis Tutorial: Collect Ephemeris Information of Your Pulsar gtephem
Ephemeris Data File gtpulsarDb

» SciTools References

How To Find A Spin Ephemeris For A Particular Pulsar At A Particular Time

This tool can be used to check whether an ephemeris is available for a particular pulsar at a particular moment in time. This is accomplished by setting parameter strict=yes, which causes gtephem to consider only ephemerides whose interval of validity includes the given time. If an ephemeris is available, the tool will compute and display an extrapolated ephemeris for that given moment in time, in a manner similar to that shown in the next section. If no ephemeris is available, the tool will produce output similar to the following:

How To Compute (Extrapolate) A Spin Ephemeris For A Particular Pulsar At A Particular Time

To find an ephemeris for a particular pulsar at a particular time, ignoring the interval of validity of the candidate ephemerides, gtephem can be invoked with parameter strict=no. (This option can be omitted because no is the default value.)

How To Compute (Extrapolate) A Binary Ephemeris For A Particular Pulsar At A Particular Time

If orbital parameters for a binary orbit of a pulsar of interest are included in an ephemerides database file, the tool will use them to compute orbital effects on photon arrival times (light-travel time, gravitational time dilation, etc.), apply them when it estimates a spin ephemeris at a given moment in time, as shown in the following example. Unlike spin parameters there is no validity interval associated with orbital parameters, so the strict=yes option does not have any effect on search and computation of orbital parameters.

Getting More Information on Computation

If gtephem is invoked with chatter parameter set to 4 or larger, it will display a brief summary of loaded ephemerides, and spin and orbital ephemerides that are used to compute an extrapolated spin ephemeris at a particular moment in time, together with an extrapolated ephemeris at the given time. This feature shows you additional information on computations being performed, which may help you understand the tool. Also, the extra information printed may contain a clue to diagnose certain types of problem. So, try a high chatter when you are unsure on the tool's behavior.

Last updated by: Masaharu Hirayama 08/24/2009