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/JPL missions that span our solar system from the study of the Sun's coronal mass ejections (STEREO) to the examination of the outer planets and the Kuiper Belt objects (New Horizons). 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 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 all-in-view time transfer. The TFL's clocks are also incorporated into the formulation of International Atomic Time (TAI). Recently, the TFL Master Clock was transitioned from a cesium-beam frequency standard to a hydrogen maser, and the frequency adjustments of UTC (APL) are now performed with a high-resolution offset generator. These changes have greatly improved the stability of UTC (APL) and have also improved our ability to steer to UTC.