Combination of T2L2 and GPS-CP Data: Towards an improvement of Time Transfer accuracy

The Time Transfer by Laser Link (T2L2) experiment, developed by both OCA and CNES, performs ground to ground time transfer over intercontinental distances. The principle is derived from laser telemetry technology with dedicated space equipment designed to record arrival time of laser pulses at the satellite. Using laser pulses instead of radio frequency signals, T2L2 allows realizing some links between distant clocks with expected time stability of a few picoseconds and accuracy better than 100 ps. On the one hand, several experiments demonstrated the capability of the T2L2 system to perform time transfer with a residual accuracy in the 100 ps range [1]. Nevertheless laser operations are weather dependent and do not allow continuous comparison of remote clocks. On the other hand GPS time transfer is a worldwide, all weather and fully operational system. GPS data processing has been widely improved since the first common view / single satellite time transfer and GPS Carrier Phase techniques now allow continuous comparison with accuracy in the nanosecond range. However, carrier phase data are ambiguous and several method were developed in order to reduce them. Performances of these methods are widely demonstrated for frequency transfer and with continuous data. But some ambiguities remain as soon as there are gaps in the data and for accurate time transfer. This work investigates the possibility to combine GPS-CP time transfer using single-difference integer ambiguity resolution [2] and T2L2 data in order to improve the resolution of the remaining GPS-CP ambiguities, in case of interruption in the data, and the accuracy of the time transfer. First we introduce the method and then we present some first results for common view time transfer in between European stations.