MORE REFERENCE STATIONS FOR SBAS COVERAGE
The Arctic is a sensitive environment, and thus navigation should have high integrity. For this reason, we are interested in extending the SBAS to serve this region. At present, none of the three operational SBAS provides meaningful service in the far North. In fact, Figure 1 shows the current SBAS availability coverage with vertical alert limit (VAL) equal to 35 meters, and horizontal alert limit (HAL) equal to 40 meters. Figure 1 is based on two of the currently operating SBAS: the U.S. WAAS and the European EGNOS. The locations of the WAAS and EGNOS reference stations are shown in Figure 2. The indicated lack of integrity coverage in the Arctic is due to too few reference stations.
Of course, we can extend integrity into the Arctic by adding reference stations to those shown in Figure 2. We include the reference stations of the Russian SDCM (System of Differential Correction and Monitoring) and Japanese MSAS systems, and add five new reference stations, whose locations are shown in Figure 3. We assume all these references stations provide the same measurement quality as current WAAS reference stations. We also assume that there is continuous user connectivity, that is, the user is always able to receive the SBAS corrections. Although SBAS GEO coverage is limited in the Arctic, there are other ways to maintain the connectivity, such as using low Earth orbit (LEO) satellites. This topic is addressed in more detail in the next section.
Figure 4 shows the horizontal and vertical availability with user connectivity and the reference stations in Figure 3. Again, we set VAL to 35 meters and HAL to 40 meters. Availability has been improved from no availability coverage to greater
FIGURE 1 Current integrity availability in the Arctic with VAL = 35 m and HAL = 40m.