  In this paper we present a novel method of atomic clock synchronisation
combining digital error correction and phase tracking. Microwave broadcasts are
used to measure the difference in frequency between a pair of atomic clocks. We
use the fact that desynchronised atomic clocks give rise to a phase error in a
QPSK broadcast, which can be measured with high precision. The rate of change
of this phase error is used to find the relative error in frequency of
reference signals provided by two clocks, with an uncertainty in the region of
1e-15 Hz over a measurement time of 10 minutes. We find that the base stability
that can be measured with this technique is 2.66e-17 Hz, making it potentially
applicable to applications like measuring geodetic height to the centimeter
level.
