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Session A2: Atomic Clocks and Timing Applications 1

Wireless Time Dissemination over a Network of Mobile Handheld Radios, Enabling GPS Hot Start and Timing Holdover in Degraded Environments
Daniel T. Goff, Robert B. Alwood, William P. Lounsbury, Steven T. Seppala, Wilbur L. Myrick, ENSCO, Inc.; Nhut Vo, US ARMY CERDEC
Location: Room 307-309
Date/Time: Tuesday, Jun. 6, 2:50 p.m.

Relative measurements of time and frequency enable advanced capabilities in positioning, navigation, and timing (PNT) as well as communications systems. Round-trip time-of-flight radio frequency (RF) transactions combined with two-way time transfer (TWTT) measurements using band-limited communications enable relative measurements of time, frequency, distance, and velocity, which can be used to actively synchronize time across a network of untethered platforms. Integration of this measurement system with an accurate timing reference, such as GPS or an atomic clock, allows for the rapid dissemination of accurate time to members of the wireless network where GPS is denied, degraded, or otherwise unavailable. This work presents a rapid, accurate, and precise time transfer method leveraging software defined radios (SDRs), that is used to disseminate accurate time from a chip scale atomic clock (CSAC) GPS disciplined oscillator (GPSDO) across a wireless network. Hot Start of an ICD-153 GPS device is also demonstrated using the RF TWTT measurements and data transfer capability implemented on the SDRs.
Round-trip time of flight and TWTT measurements provide physical measurements of the offset in time, and phase and frequency offset between local timing sources, in addition to distance and relative velocity between platforms. Round-trip measurements allow frequency offsets due to Doppler velocity to be separated from local timing reference frequency and phase offsets. This capability allows the underlying measurements to function for both static and dynamic systems with varying clock generation stabilities. A timing, communications, and ranging (TCR) element provides these measurements on a hand-held, resource-constrained SDR platform. The TCR element also contains a control filter that enables synchronization of the local timing reference to an accurate external reference source.
Previous work has demonstrated robust, relative time and frequency synchronization using independent, round-trip transactions between TCR elements. In this work, dissemination of accurate timing information is demonstrated across a network of untethered TCRs, without active control of the TCR reference timing source. Each TCR contains a field-programmable gate array (FPGA)-based timestamp generation block that has inputs to digitally control both the frequency and the phase offset of the timestamp output. This timestamp architecture allows for closed-loop time synchronization between TCRs, without the need to discipline their local timing source. A 1-pulse-per-seccond (PPS) reference waveform is generated from the synchronized timestamp generator, and together with the ICD-153 interface protocol, the TCR elements are able to transfer accurate and precise timing information to GPS receivers or other systems and sensors.
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Atomic Clocks and Timing Applications



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