  We carried out a 26-day comparison of five simultaneously operated optical
clocks and six atomic fountain clocks located at INRIM, LNE-SYRTE, NPL and PTB
by using two satellite-based frequency comparison techniques: broadband Two-Way
Satellite Time and Frequency Transfer (TWSTFT) and Global Positioning System
Precise Point Positioning (GPS PPP). With an enhanced statistical analysis
procedure taking into account correlations and gaps in the measurement data,
combined overall uncertainties in the range of $1.8 \times 10^{-16}$ to $3.5
\times 10^{-16}$ for the optical clock comparisons were found. The comparison
of the fountain clocks yields results with a maximum relative frequency
difference of $6.9 \times 10^{-16}$, and combined overall uncertainties in the
range of $4.8 \times 10^{-16}$ to $7.7 \times 10^{-16}$.
