Analysis of Horizontal Positioning Error Distribution of the Chinese Area Positioning System

The Chinese Area Positioning System (CAPS) is a satellite-based system that transfers ground-generated navigation information to receivers via transmission of commercial geostationary communication satellites. During the test stage, CAPS only provides positioning signals from geostationary satellites and a receiver needs an incorporated barometric altimeter for altitude information to aid positioning. In order to describe the distribution of CAPS horizontal positioning errors, we define EDOP (Dilution of Precision East) and NDOP (Dilution of Precision North) of the altitude-aided algorithm. According to the calculations of EDOP and NDOP based on the CAPS constellation, the EDOP and NDOP values of CAPS are symmetric about the Equator, and are symmetric about the mean orbital longitude of the three geostationary satellites. The minimum of EDOP is reached around the meridian of the mean orbital longitude and the EDOP value increases as the receiver moves farther away from the mean. The NDOP is larger than the EDOP at any point in the CAPS coverage area. As the receiver moves from lower latitudes to higher latitudes, the north-south distances between satellites increase in the user coordinate frame and the NDOP decreases. We carry out simulation analysis by adopting six CAPS observation stations (Beijing, Shanghai, Changchun, Xi'an, Urumqi, and Kunming) and the constellation in the test stage. The results show that the EDOP and NDOP can be used to estimate the enlargement of User Equivalent Range Error (UERE) caused by the constellation, and explain the reason why the east-west precision is better than the north-south precision in CAPS positioning. The analysis of observed data from the six observation stations further shows that the EDOP and NDOP can be used to estimate the positioning precisions in the east-west and north-south directions.