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

[Willem van Straten]

oops!  I forgot to thank Dick Manchester for doing the detective work that
led to the discovery of the bug!


On Wed, 22 Aug 2018 at 11:55, Willem van Straten <
vanstraten.willem at gmail.com> wrote:

> 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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