The role of time and frequency in the wide Area Augmentation System (WAAWS)

Abstract

Time and frequency play a very important role in four different areas within the Wide Area Augmentation System (WAAS). Because of this, the WAAS will become a primary means for time distribution and synchronization within the US. The WAAS is a satellite based augmentation system (SEAS) which collects data from the NAVSTAR Global Positioning System (GPS), provides an independent ranging signal from a geostationary satellite to supplement the navigation signals from GPS, provides differential corrections to improve the navigation capability of the GPS, and provides the offset of WAAS Network Time (WNT) from Coordinated Universal Time (UTC), as well as providing system integrity information to the user. The accuracy and precision attainable by the WAAS is critically dependent on the performance of the time and frequency sub-systems within these areas of the WAAS. This paper discusses the manner in which the data recording system time tags its observations. Accurate recording of the time of observations is essential in order to measure the timeliness (latency) of all transmitted information that is based on algorithms which derive their input from data collected from a network of 25 WAAS Reference Stations (WRS). Each WRS has three independent, free-running cesium beam frequency standards. The WAAS, through its Master Stations (WMS), maintains its own local time scale, called WAAS Network Time (WNT), and is steered to a real-time estimate of GPS Time. Only an estimate of GPS Time can be made because GPS Time is affected by Selective Availability (SA). It is necessary to have an uniform time scale as a reference in order to derive differential corrections in order to improve the accuracy obtainable from the GPS Standard Positioning Service. This paper discusses the manner in which WNT is derived and kept close to GPS Time. The navigation signals, transmitted from the WAAS geostationary satellite (GEO), can be used as a supplemental ranging source to GPS and must be emitted in synchronization with GPS navigation signals in order to make them compatible within a common navigation solution. The manner used to control the transmission of the WAAS navigation message from the GEO will be discussed. The secondary mission of the WAAS is to provide UTC.