Refining Monitor Station Weighting in the GPS Composite Clock

摘要

The two closely linked missions of the Global Positioning System (GPS) are precise positioning for navigation accuracy and precise time transfer. Positioning has typically received the most attention; however, the ability to derive a precise position depends on having a stable and reliable time scale as a starting point. Throughout the history of GPS, its time scale has slowly evolved through many phases. The most significant change came with the introduction of the Composite Clock in 1990. Additionally, in recent years the 2nd Space Operations Squadron (2 SOPS) has tuned the Composite Clock to take better advantage of its contributing timing sources. In addition to reviewing the Composite Clock's tuning history, the authors will discuss the recent implementation of unique tuning for each of the GPS monitor station frequency standards. As pointed out in a previous paper regarding satellite clock-unique tuning, no two satellite frequency standards exhibit the same performance. This also holds true for monitor station frequency standards. However, other factors must be considered when deriving process noise values (tuning parameters) for ground system contributors to the Composite Clock. Mainly because the values chosen for each clock determine the size of its vote in contributing to GPS time, one must look not only at clock performance, but environmental factors as well. An excellent clock placed in an environment with significant temperature and humidity fluctuations can be very detrimental to GPS time if that clock is weighted heavily in the Composite. This newly adopted concept of representing monitor station clocks based on individual performance and environmental history has significantly improved the stability and reliability of GPS Time. The authors discuss several samples that reveal the robustness of the new tuning as well as the current implicit long-term weighting given to each frequency standard that contributes to the Composite Clock.