Performance Assessment of a Local Area Monitoring System in Asia Pacific Region

摘要

The civil aviation organizations worldwide are implementing Satellite Based Augmentation Systems (SBAS) for the civil aviation applications. Currently, users in several regions around the world are benefited from the enhanced GNSS (Global Navigation Satellite System) service provided by SBAS. For example, in Taipei FIR (Flight Information Region), GNSS users can receive SBAS messages from Japanese MSAS and Indian GAGAN. Among them, Japanese MSAS was commissioned for aviation use on September 27, 2007, and Indian GAGAN is in test mode since July, 2007. However, aviation users in Taipei FIR are not allowed to use the SBAS messages from both systems, because they are not designed for Taipei FIR. Therefore, there is a navigation service gap for aviation users around the Asia- Pacific region. The objective of this paper is to use a local area monitoring system to evaluate the GNSS capability for the civil aviation applications in Taipei FIR. The evaluation criterion is the VPL (Vertical Protection Level) calculation under different phases of flight. VPL is the confidence bound of the post-correction vertical positioning and it is defined in the augmented GNSS flight operation standard (i.e., RTCA DO-229D). As a result, this paper will use a local area monitoring system in Taipei FIR to monitor GNSS satellites over this region. This local area monitoring system will then generate the regional differential (correction) messages for GNSS. This paper evaluates these regional differential messages from this local area monitoring system and the correction messages from MSAS or GAGAN based on their vertical positioning performance (i.e., VPL values) and maintain high level of confidence which is similar to that of MSAS or GAGAN at the same time. According to the evaluation result, the local area monitoring system will alter its GNSS augmentation messages to optimize its vertical performance (based on VPL calculation). The local area monitoring system in this paper will have two advantages in comparison with the conventional approach: first, in order to generate correction for error due to satellite ephemeris and clock, the conventional SBAS approach requires the great diversity in geographical distribution of reference stations and it is a difficult task for many small countries in Asia Pacific region. For our approach in this paper, we could complete the task by taking the advantage of the MSAS or GAGAN corrections for satellite ephemeris and clock. Second, with the local GNSS monitoring and messages from the MSAS or GAGAN, this local area monitoring system could generate better correction model for error due ionosphere. Therefore, the development of this local area monitoring system can be extended to other neighboring countries to further enhance its performance in the future.