Investigation into a GPS Time Pulse Radiator for Testing Time-Stamp Accuracy of a Radio Telescope

The MeerKAT radio telescope under construction in South Africa is required to tag the arrival time of a signal to within 10 ns of Coordinated Universal Time (UTC). The telescope has a local maser clock ensemble, compared to UTC by dual-band GPS receivers, and transferred to the digitizers of the array by an optical fiber system. In order to verify the accuracy of the end-to-end time tagging of samples, a portable instrument was constructed that transmits a periodic time signal. This GPS time pulse radiator (GTR) is mounted 10 m away from the telescope L-band feed horn, and radiates a broadband signal of −20 dBm. The signal is modulated by turning off at each UTC second, using the 1PPS output of a GPS receiver. The recorded voltage stream of the telescope is searched for the time signal and the corresponding timestamp compared to its expected value. While less accurate than the masers and dual-band GPS, this technique is simple and the instrument is easily characterized. Laboratory tests of the GTR showed its RF pulse to be at $1.65\pm 0.1 \ \mu \mathbf{s}$ after the UTC second. Tests on the telescope revealed a $13.0\pm 0.3 \ \mu \mathrm{s}$ deviation from the expected timestamp value. This was later found to be due to a buffer in the digitizer FPGA, and confirmed by pulsar timing. The GTR concept allows simple, independent testing of a radio telescope pulsar timing system. Future work is planned to improve the shape of the transmitted signal, to study GPS timing errors and correction techniques, and to process the received data using a more standard pulsar timing pipeline.