The Doodson X0 filter is a simple filter designed to damp out the main
tidal frequencies. It takes hourly values, 19 values either side of
the central one. A weighted average is taken with the following
weights
(1010010110201102112 0 2112011020110100101)/30.
If the initial values are at a higher frequency than hourly, they are
first averaged to give hourly values.