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A new transportable frequency comb has been developed at the National Physical Laboratory. The transportable frequency comb is based around a MenloSystems FC1500 optical frequency synthesizer, which is capable of measuring any frequency between 500 and 2100 nm. Measurements made by the comb while off-sites are referenced to the SI second via a global positioning system (GPS)-disciplined Rapco 2804AR rubidium oscillator. All components of the system (except for the GPS antenna) have been integrated into a single mobile unit. Measurements of high-stability narrow-band optical sources using this system, with either the GPS-disciplined oscillator or a hydrogen maser as a reference, are described. Some of these measurements were made simultaneously with a hydrogen-maser-referenced Ti:sapphire-based comb. It was found that the GPS-referenced transportable comb has a fractional inaccuracy of better than 4 10-12 (averaged over several hours) when compared with the maser-referenced Ti:sapphire comb, and a fractional frequency instability of 1times10-12 for an averaging time of 10 s. It was also demonstrated that continuous frequency measurements are possible with this system over a period of 60 h or more.