Synchronizing Network Radar using All-in-View GPS-Disciplined Oscillators

Coherent network radar requires precise time, phase and frequency synchronisation. Time offsets result in bistatic range errors, while phase instability affects, amongst other things, the maximum coherent integration time. NetRAD, a coherent S-band pulsed-doppler network radar, developed jointly by University College London and University of Cape Town, was synchronised using low-cost GPS-disciplined oscillators (GPSDOs), developed at the University of Cape Town (UCT). This GPS synchronisation enabled coherent bistatic radar measurements with Tx-Rx baselines of up to 5km. Moreover, bistatic recordings were made of a lighthouse, serving as a large and static common view target, in Simon's Town, South Africa. The GPSDO epoch pulse enabled time synchronisation, in addition, to phase synchronisation. This feature is often absent in commercial GPSDOs and generates a synchronised edge, phase synchronous to the GPSDO output, at a pre-agreed future GPS time. This paper discusses selected features of the UCT GPSDOs such as a quick locking 2nd-order phase-locked loop filter, improving the lock time by a factor of four, and an epoch pulse capable of sub-10 ns time synchronisation. We measure the phase synchronisation performance of UCT GPSDOs over several hours and at various phase-locked loop time constants. The radar synchronisation performance is then analysed by using direct line-of-site radar transmissions as well as the lighthouse reflection. The bistatic time and phase synchronisation, as well as the Doppler spectra, are investigated.