Design and Evaluation of an Open-Loop Receiver for TWSTFT Applications

The two-way satellite time and frequency transfer (TWSTFT) technique provides uncertainty at the nanosecond level for time differences measured over an intercontinental baseline. The time-difference results occasionally reveal daily variations of a few nanoseconds due to multiple effects. These daily variations are called diurnals, and they increase the uncertainty of the time-difference measurements. Since a time difference is obtained by combining two time-of-arrival (TOA) measurements of two receivers, the receivers may contribute the diurnals if they are not reciprocal. To improve reciprocities, a TWSTFT receiver based on the open-loop (OL) method was proposed and implemented on a software-defined radio receiver (SDR). For evaluation, the OL-based SDR was compared with a conventional receiver in a short-baseline TWSTFT experiment. The experimental results indicate that the diurnals disappeared with the OL-based SDRs while a diurnal of +/- 0.2 ns could be observed with conventional receivers. These results imply that the diurnals partially depend on TOA measurement methods used in the TWSTFT receiver.