The role of time and frequency in the mtsat satellite-based augmentation system (MSAS)

摘要

The MTSAT Satellite-based Augmentation System (MSAS) is a satellite-based augmentation system (SBAS) for navigation. The MSAS is one of several applications that shares the use of the Multifunctional Transport Satellite (MTSAT), a geostationary earth orbiting (GEO) satellite, which will be launched by the Japancese Ministry of Transport in August 1999. The MSAS design is based on the development of the US Wide Area Augmentation System (WAAS) and therefore may be familiar to those who are acquainted with the WAAS. Time and frequency play a critical role in four areas within MSAS, and these areas are the focus of this paper. Firstly, MSAS performs several functions. One-it collects data from the NAVSTAR Global Positioning System (GPS); two - it provides an independent ranging signal from MTSAT to supplement the navigation signals from the GPS constellation; three - it provides differential corrections to improve the navigation capability of the GPS; four-it provides the offset of MSAS Network Time (MNT) from Coordinated Universal Time (UTC); and five - it provides system integrity information to the user. With these functions, the MSAS can become the primary means of time distribution and synchronization within Japan and other areas under the footprint of the MTSAT. The accuracy and precision attainable by MSAS is critically dependent on the performance of the time and frequency subsystems in the four areas of discussion. The first area of discussion is the manner in which the data recording system tages its observations. Accurate recording of the time of observations is essential in order to measure the timeliness (latency) of all transmitted information. This information is based on algorithms that derive their input from data collected from a network of eight data collection sites, including four Ground Monitoring Stations (GMS) and four Monitor and Ranging Stations (MRS). Each GMS/MRS has three independent, free-running cesium-beam frequency standards. The second is the discussion of the method of deriving the MNT and keeping it close to GPS Time. The MSAS, through the Master Control Stations (MCS), maintains MNT, its own local time sacle, 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 essential to have a uniform time scale as a reference in order to derive differential corrections that improve the accuracy that can be obtained from the GPS Standard Positioning Service (SPS). The third topic of discussion is the manner in which the MSAS navigation message from the MTSAT is controlled and transmitted. The signals transmitted from the MTSAT, which can be used as a supplemental ranging source to GPS, must be emitted in synchronization with GPS signals in order to make them compatible within a common navigation solution. The last area of discussion is the secondary mission of the MSAS. Like the WAAS (1), it is to provide UTC. It does this by comparing MNT, which is synchronized to GPS Time, with UTC, as maintained by a UTC provider. For the WAAS, it is the US Naval Observatory (USNO). Although the Japanese Civil Aviation Bureau (JCAB) has yet to make a decision to provide the time distribution service, the procedures for obtaining, calculating, and transmitting the offsets of MNT from UTC, as part of the MSAS navigation message, is described.