Ensemble time in GNSS - Performance requirements and algorithm tests

Abstract

Any Global Navigation Satellite System (GNSS) relies on a highly stable and reliable System Time which has to meet high performance requirements to enable GNSS services suitable for navigation and timing communities. The challenge is to guarantee this high performance continuously. The Kalman filter algorithm implemented in GPS, called GPS Composite Clock, is a mature method to generate such a highly robust System Time. The algorithm estimates the time offsets of every individual clock to the so called implicit mean, which is a common component in all clock estimates. The common component offers the functionality of System Time and is understandable as a weighted average out of all ensemble clock readings. GPS Composite Clock performance is analyzed by simulations of a “light” GNSS configuration with 10 satellite Rubidium-clocks including deterministic drift, 6 ground Cesium-clocks and 2 ground Active Hydrogen Masers. Besides evaluating the stability of an error-free clock constellation to define the regular performances the behavior of the algorithm is investigated considering different operational scenarios: exclusion of clocks from the GNSS ensemble and occurrence of clock feared events (frequency steps in satellite Rubidium-clocks and ground H-masers).