  We used Precise Point Positioning, a well-established GPS carrier-phase
frequency transfer method to perform a direct remote comparison of two optical
frequency standards based on single laser-cooled $^{171}$Yb$^+$ ions operated
at NPL, UK and PTB, Germany. At both institutes an active hydrogen maser serves
as a flywheel oscillator; it is connected to a GPS receiver as an external
frequency reference and compared simultaneously to a realization of the
unperturbed frequency of the ${{}^2S_{1/2}(F=0)-{}^2D_{3/2}(F=2)}$ electric
quadrupole transition in ${}^{171}$Yb${}^+$ via an optical femtosecond
frequency comb. To profit from long coherent GPS link measurements we
extrapolate over the various data gaps in the optical clock to maser
comparisons which introduces maser noise to the frequency comparison but
improves the uncertainty from the GPS link. We determined the total statistical
uncertainty consisting of the GPS link uncertainty and the extrapolation
uncertainties for several extrapolation schemes. Using the extrapolation scheme
with the smallest combined uncertainty, we find a fractional frequency
difference $y(\mathrm{PTB})-y(\mathrm{NPL})$ of $-1.3(1.2)\times 10^{-15}$ for
a total measurement time of 67 h. This result is consistent with an agreement
of both optical clocks and with recent absolute frequency measurements against
caesium fountain clocks.
