Multiple-Path Linked Common-View Time Transfer (LCVTT)

摘要

Analysis of the on-orbit Navstar clocks and of the GPS monitor site reference clocks is performed by the Naval Research Laboratory using post- processed precise ephemerides. The ephemerides are produced by the National Imagery and Mapping Agency (NIMA) for each of the GPS space vehicles from pseudorange measurements collected at five U.S. Air Force (USAF) and at ten NIMA GPS monitor sites spaced around the world. The time reference at the NIMA Washington, D.C., site, which is co-located with the U.S. Naval Observatory precise-time site, is the DOD Master Clock. Hence, it is possible to transfer time via linked common view every 15 minutes from the DOD master clock to the remaining USAF and NIMA GPS monitor sites. Linking sites serially, although logically simple and computationally efficient, was found to suffer several disadvantages. The most serious of these was the accumulation of gaps in the time transfer to a remote site. A secondary disadvantage was the dominance in the linking process of the short-term noise from a single noisy site. Using multiple common-view paths overcomes both of these defects by supplementing the link that propagated the gap in the data and by providing multiple independent measurements that can be averaged to reduce the measurement noise. An additional benefit of the multiple-path method is an improvement in the continuous coverage 2) of the Navstar space vehicle clocks, i.e., in referencing the observations of a space vehicle clock by each of the monitor sites back to the DOD Master Clock. Improvement was manifested by an increase in the number of estimates obtained of the offset of the space vehicle clock from the DOD Master Clock and by a decrease in the short-term noise of these estimates.