[Everyone] PSRCHIVE bug: channel frequencies parsed with insufficient precision

[Willem van Straten]

Hi everyone,

To date, PSRCHIVE has been loading the centre frequencies associated with
each frequency channel (as stored in the DAT_FREQ column of the SUBINT
table of PSRFITS files) using only single-precision floating point
numbers.  The fix for this bug was checked in about half an hour ago.  For
details, see

https://sourceforge.net/p/psrchive/bugs/418/

As a consquence of this bug, errors of the order of 1 part in 10^8 would
have crept into reference frequencies, which in turn would have led to
errors in applied phase shifts and derived arrival times.  For example, in
the case of the file provided at the above link, data could be dedispersed
to a reference frequency of "exactly" 704.062 MHz (double-precision
floating point), unloaded to a PSRFITS file, and then loaded from that file
with an erroneous reference frequency of 704.06201171875.  If these data
were then dedispersed to a different reference frequency (e.g. the centre
frequency of the band), the error of 11.71875 Hz in the erroneous reference
frequency of that channel would lead to a frequency-and-DM-dependent error
in the phase shift applied to that channel.  In this example, a DM of ~100
would correspond to an error of ~30 ns.

Of course, this bug would lead to greater errors at higher DMs and lower
frequencies.  It's surprising that the bug has gone undetected since 2004,
and serves as a excellent argument for greater code testing.

Please let me know if you detect any gravitational waves after updating
pipelines that use the psrchive tools.

Cheers,
Willem
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