Time and Frequency Activities at the JHU Applied Physics Laboratory

Abstract

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 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, evaluation of our ultra-stable oscillators (USOs), frequency references 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, PTB and UTC is maintained via GPSP3 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 5 nanoseconds of UTC 95% of the time. The APL timescale incorporates three hydrogen masers and three cesium atomic clocks. The timescale includes variable weighting for each clock that allows for the adjustment of the variation in clock performance due to aging and environment effects. As an example, this year, aging-induced effects lead to anomalies in the performance in Maser#1. By precisely altering the weight of Maser#1 relative to Maser#2 and Maser#3, we were able to maintain the overall performance of the time scale. In this work, we will present details on this variably-weighted timescale. We will also explain how to select weights so as to maximize the benefits of fusing data from multiple clocks, while minimizing the impact of anomalies in an individual clock