Arrival Time Correction
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For a given event file, gtbary tries to convert every "time" in your event file, including header keywords such as TSTART and TSTOP, and the contents of START and STOP columns in GTI extensions. As a result, for gtbary to work properly on your event file, a spacecraft file (a.k.a. FT2 file) given to gtbary must cover wider range of time than your event file.
For example, gtbary will fail to convert a TSTOP keyword value in EVENTS extension of your event file if the TSTOP value is later than the last spacecraft data available in your spacecraft file, and it will not convert any TIME column values in that extension. In addition, because GTI extension usually has the same TSTOP value as EVENTS extension, gtbary will not convert any START and STOP column contents in GTI extension either. In such a case, it results in an output file identical to an input file, with an output message shown below.
In order to avoid this problem, it is not a good idea to have any kind of time in your event file exactly on the boundary of the time coverage of your spacecraft file. This is because you never know for sure whether the time will be interpreted as being inside of the coverage or outside, because of rounding in computation. In an event file some times are written as a binary number, such as the contents of TIME column in EVENTS extension, and those of START and STOP columns in GTI extension, while others are written as a character string, such as TSTART and TSTOP. It is not guaranteed whether those will be converted perfectly into an internal expression in a computer so that equality of double-precision variables holds in the code.
In summary, it is recommended to make a spacecraft file that covers a time range wide enough to contain everything in an event file in order to avoid being trapped in this issue.
As a result, even if the TUNIT# is set to 'km' (for example), gtbary reads the numbers in the SC_POSITION column and uses them as if they were in meters.
Please make sure your spacecraft data file contains the spacecraft positions expressed in meters, if you feel that it is causing a problem. Since the spacecraft is flying around at about 7000 km from Earth center, numbers in the SC_POSITION column are likely to be a couple of millions, unless otherwise intended. Common mistakes are to have them three orders of magnitude smaller than intended, by putting numbers in kilometers.
Photon arrival times are usually converted in the order shown in the table below, one step at a time. Note that each conversion step also has a common name, and that additional information about arrival time conversion is included.
Conversion | Convert from | Convert to | Light travel time | Time system conversion |
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Geocentric correction | Mission Elapsed Time | Geocentric time | ±23 ms at maximum | From mission-dependent time system to TT |
Barycentric correction | Geocentric time | Barycentric time | ±500 s at maximum | From TT to TDB |
Binary demodulation | Barycentric time | Binary-demodulated time | Depends on binary parameters | None |
Note: In principle, everything after a geocentric conversion depends on the pulsar location since the pulsar location is needed to calculate photon travel time between the spacecraft and the geocenter.
Owned by: | Masaharu Hirayama hirayama@jca.umbc.edu |
James Peachey James.Peachey-1@nasa.gov |
Last updated by: Masaharu Hirayama 06/05/2009 |