Pioneering Two-way Time Transfer Embedded within Communication Modems
Norman Adams, Gregory Weaver, Wesley Millard, Olukayode Okusaga, Eric Adles, Daniel Caughran, Johns Hopkins University Applied Physics Laboratory
Location: Room 307-309
Date/Time: Tuesday, Jun. 6, 2:10 p.m.
Two-way time-transfer is expected to be an essential part of next generation resilient time dissemination for military PNT service. There is broad interest across the DoD to integrate two-way time-transfer (TWTT) signals into communication modem applications, rather than implement TWTT with dedicated radiometric modems. The approach of embedding time transfer into communications subsystems is consistent with radios deployed for space exploration, which implement the closely related technique of satellite ranging. Satellite ranging is performed with either a PN code or a sequence of tones, and is often performed concurrently with data communications. The JHU/APL Frontier Radio is one such space radio. The present paper reports on a recently completed study in which two prototype Frontier Radios were modified to perform TWTT. Specifically, the modems were modified to perform time-difference measurements between their local timekeeping systems over crosslinks via geostationary bent-pipe communications, and report the measured reciprocal time offset.
To facilitate demonstration with existing DoD satellite Earth stations, a custom tunable L-band IF interface was designed and implemented. Existing firmware circuits from flight missions were repurposed for the TWTT modems. Software to estimate the time-difference from latched phase and time samples has been verified with an external PC and can be implemented in the embedded modem processor. The waveform that the Frontier Radio presently supports is optimized for deep-space communication and navigation. This waveform is not ideal for terrestrial time-transfer, but serves to demonstrate the approach. Furthermore, the waveform supports time-transfer with concurrent low-rate data. Performance was verified with a series of conducted link tests. Measurement precision < 1 ns RMS was verified for several link scenarios, including links with shared versus separate frequency allocations, links with and without frequency translation, and links with and without noise. Allan deviation and modified Allan deviation performance will be presented.