Time and Frequency Activities at the JHU Applied Physics Laboratory
Mihran Miranian, Erika A. Sanchez, Jeffrey F. Garstecki, Richard A. Dragonette, Gregory L. Weaver, JHU/APL
Location: Regency B
The Time and Frequency Laboratory (TFL) at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) provides support to multiple NASA/APL missions that span our solar system from the upcoming Parker Space Probe, set to directly explore the Sun’s corona, to the on-going mission of the New Horizons expedition into the Kuiper Belt. This support includes providing precise time and frequency to the integration and testing of flight hardware, frequency reference for spacecraft ranging and communications via the APL satellite communications facility, and the time-stamping of ground-receipt telemetry packets from various spacecraft.
The TFL’s ensemble of three high performance cesium standards and three hydrogen masers are integrated to form the APL timescale that is the basis for estimating UTC-UTC(APL) and for evaluating the performance of the individual clocks. Traceability to the USNO, NIST, and UTC is maintained via GPS Common-View and Precise Point Positioning (GPSPPP) time transfer. The TFL’s clocks are also incorporated into the formulation of International Atomic Time (TAI). The TFL Master Clock is a hydrogen maser and the frequency adjustments of UTC(APL) are performed with a high resolution offset generator. This combination of hydrogen maser and high resolution offset generator along with our UTC-UTC(APL) estimation algorithm has made it possible to maintain UTC(APL) within 10 nanoseconds of UTC 95% of the time.
In 2017, the TFL furthered its use of GPSPPP to show improved time transfer resolution in collaboration with the United States Naval Observatory (USNO) to within 100 ps. In the process, we identified an algorithm for minimizing the phase transients associated with day boundaries in carrier phase estimation, and explored the relationship to P3 code methods for smoothing techniques. Our examination of GPSPPP time transfer using the available RINEX data from both US and international metrology laboratories has given us the confidence to consider using these data for remote clock inclusion into our APL timescale generation. We will show the quality of our GPSPPP measurement through select comparison of UTC(APL) to the metrology labs examined in our work.