Local ties control in application of laser time transfer

Abstract

In many fundamental physical experiments time plays an important role. The standard way for the comparison of time and frequency is the application of GNSS signals and the Two-Way Satellite Time and Frequency Transfer - TWSTFT. This technique is based on radiofrequency signal transmission. Recently, there was a rapid increase of optical time comparison development, which uses the Satellite Laser Ranging network (SLR). Currently the French project T2L2 is in operation on board of Jason 2 and the European Space Agency project ELT in support of the Atomic Clock Ensemble in Space (ACES) is under development. The goal of both projects is the time synchronization with a precision below 40 ps rms and an absolute error well below 100 ps. Comparing the results of the optical time transfer with the GNSS time comparison requires unprecedented control of the local ties between the different observation techniques. One of the possible methods is the application of the Two Way Time Transfer (TWTT) on a single coaxial cable. Such a system can be implemented using two or more event timers, which are interconnected by a standard coaxial cable. The event timers are exchanging pulses between each other and time tagging them. Out of the measured result one can evaluate the difference of time scale represented by the event timers. It was shown that such a technique can be used for time transfer with a precision of a few picoseconds of rms and an absolute error below 20 ps for distances reaching several hundreds of meters. We have implemented this technique for establishing and monitoring the absolute time delays between the SLR system and the time keeping laboratory on the Geodetic Observatory Wettzell. The event timers were developed at the Czech Technical University. The measurement principal is based on SAW filter excitation. The Event Timers were located in different buildings 50 meters apart and with the help of the TWTT technique, the delay between them was measured. The second input sockets of the event timers were used to monitor the respective local timescale.