Continuous Observation of Navstar Clock Offset from the DoD Master Clock Using Linked Common-View Time Transfer

Abstract : Analysis of the on-orbit Navstar clocks and of the Global Positioning System (GPS) and National Imaging and Mapping Agency (NIMA) monitor station reference clocks is performed by the Naval Research Laboratory (NRL) using both broadcast and postprocessed precise ephemerides. The precise ephemerides are produced by NIMA for each of the GPS space vehicles from pseudorange measurements collected at five GPS and seven NIMA monitor stations spaced around the world. That the time reference for the NIMA Washington, DC, monitor station is the DoD Master Clock has enabled synchronized time transfer every 15 minutes via Linked Common-View Time Transfer from the DoD Master Clock to the eleven monitor stations. Summing the offset of a space vehicle clock from a monitor station time reference with the offset of the monitor station time reference from the DoD Master Clock yields the offset of the space vehicle clock from the Master Clock for the period during which the space vehicle was in view of the monitor station. Repeating this procedure for each of the monitor stations produces continuous overlapping observations of the offset of the Navstar clock from the DoD Master Clock. Following this procedure for the Navstar 29 cesium clock for 118 days during which there were no anomalies in either the space vehicle clock or the Washington, DC monitor station time reference yielded a measurement noise with a standard deviation of 1.1 nanoseconds. This was reduced to an estimated measurement precision of 641 picoseconds by averaging overlapping measurements from multiple monitor stations at each observation time. Analysis of the low-noise clock offset from the DoD Master Clock yields not only the bias in time of the space vehicle clock, but focuses attention on structure in the behavior of the space vehicle clock not previously observable.