Evaluating an off-the-shelf white rabbit system to synchronise network radar via optic fibre

Abstract

Networks of coherent sensors, such as radar, require precision time and frequency synchronisation for their operation. In this paper, we evaluate the White Rabbit technology as a commercial off-the-shelf (COTS) synchronisation solution for GNSS-denied environments. White Rabbit (WR) was originally developed for the Large Hadron Collider project and provides sub-nanosecond synchronisation via optic fibre. WR is based on Ethernet and associated standardised network technologies. We test WR as a drop-in replacement for existing GPS-disciplined oscillators (GPSDOs). These GPSDOs were developed at the University of Cape Town (UCT) and were used to synchronise NetRAD across baselines of 5 km. NetRAD is a network radar developed jointly at University College London (UCL) and UCT. The UCT GPSDO uses a GPS-derived pulse per second (PPS) signal as the time reference. The internal oven stabilised local oscillator (OCXO) is phase-locked to this GPS PPS signal. To assess the WR performance, we substitute the GPS PPS signal with a WR generated PPS signal. Thus, the internal OCXO is WRN-synchronised providing a low phase-noise 10 MHz sinusoidal frequency reference. The 10 MHz sinusoid is used as a reference to a 100 MHz Wenzel PLO. A dual-mixer time difference (DMTD) system was developed to quantify the synchronisation performance of such WR-disciplined oscillators (WRDOs) and other systems. For 100 MHz, this DMTD system has a noise floor of below 1ps and an Allan Deviation of less than 1.96 × 10−13 at 1 second. The long term phase and frequency stability of the WRDOs are measured across a fibre baseline of 10 km over a period of a few hours. The WRN performance is compared to the existing GPSDOs and COTS Fine-Delay cards.