Design of a low cost, compact round-trip delay measurement system for radio telescope time transfer applications

The architecture of a precise fibre optic time transfer system for the MeerKAT radio telescope is presented. Preliminary measurement results achieved, on the pre-production units of the timing interface for L-Band Digitisers, are shown. The time transfer system links the central telescope atomic clock ensemble and the L-Band Digitiser units which are located at each of the 64 MeerKAT dishes, separated by up to 10 km. Timing is encoded via radio frequency (RF) modulation of an optical carrier at the time transmitter. A portion of the light is reflected back at the Digitiser units and demodulated at the time transmitter. The Digitiser demodulates the timing signal which is sampled in parallel with astronomical signals at 1712 Megasamples per second (MSPS), thereby accurately timestamping astrophysical signals. Initial test results show that the interfacing system is capable of transferring time to the Digitiser units with <;1 ns accuracy over ~10 km. The absolute cable delay measurement is shown to be traceable to the primary International System of Units (SI) via an independent verification method. The system's timing demodulators show low drift (<;100 ps peak drift when using 20 s moving averages) over 50° C temperature changes. The whole system is shown to track cable excursions induced in an environmental chamber. The interfacing system shows promise for accurate absolute timestamping of data to perform precision time domain science like absolute pulsar timing measurement with possible applications in geodesy and detection of low frequency gravitational waves.