Time comparison accuracies relay on the accurate calibration, stability and reproductibility of the measurement equipement employed; the most important of them is the GPS receiver and the GPS antenna. To verify that BIPM has been carring out relative calibration trips in the past specially for the calibration of Two Ways Satellite Time Transfer (TWSTT) earth stations. As result of this, adjustments have been included on some links of the participant laboratories. Unfortunately this calibration trips did not include all the European time laboratories, so there are laboratories whose links had not been verified.
One of the most important influences on the accuracy of a GPS link, is the temperature dependence of the antenna. Although several studies have been done additional comparisons are needed to estabmish a method to determine, in a relative way, the performance of the GPS receiver antennas at the European time laboratories.
NMi Van Swiden Laboratorium will act as pilot laboratory and the Real Instituo y Observatorio de la Armzda will provide a TTR-5 model GPS receiver to circulate, as a transfer standard, around the European time laboratories in a continous way. The transfer standard will be initially calibrated by the pilot laboratory and again each time when the transfer standard has been circulated through six time and frequency laboratories or at least each three months.