The Advantages of Local Monitoring and VHF Data Broadcast for SBAS

In Space Based Augmentation Systems (SBAS), many reference receivers are placed throughout a large region, and their measurements are combined to create a set of corrections for each ranging satellite as well as for ionosphere delays. These corrections as well as confidence parameters are broadcast to the user via Geostationary satellites. Unfortunately, today’s SBAS systems do not necessarily achieve the highest levels of service desired for aviation applications. The U.S. Wide Area Augmentation System (WAAS) is capable of guiding an aircraft to within 250 feet of the ground, just shy of the CAT I decision height of 200 feet and well short of a CAT III autoland operation. The accuracy of WAAS system has been consistently observed to be better than 2 m (95%) throughout the U.S, which is within the accuracy requirements for both CAT I and CAT III operations. However, the error bounds reported by WAAS are several times larger due to the possible impact of unobserved anomalies on WAAS users. One approach to improving SBAS performance is to add a GBAS-like local monitor at airports for which CAT I or better service is desired. This local monitor could be used to identify times when the SBAS performance is such that it permits these more demanding operations. Because the local monitor is within 10 km of the airplane and the runway, it can detect local error sources that SBAS cannot. This in turn leads to tighter confidence parameters than SBAS can broadcast. Because the local monitor is independent of the broadcast corrections, it is not possible for the local monitor to introduce an actual measurement error. At worst, it could miss an existing SBAS error and fail to provide an integrity warning. However, this would require simultaneous offsetting failures of two independent systems. As such, it may be possible to design the system with less stringent multipath and receiver failure requirements than those that apply to “traditional” GBAS. Once a local presence at a particular airport is established, the local monitor would broadcast the SBAS corrections and reduced confidence bounds over a local VHF data broadcast designed to fit within the existing GBAS standards. In this way, it is similar to the Australian Ground-based Regional Augmentation System (GRAS) with the addition of local monitoring to provide improved local performance. This paper introduces the Local Area Monitor (LAM) concept, explains how it is envisioned to work, and discusses its advantages and disadvantages relative to the existing SBAS, GBAS, and GRAS means of supporting GNSS-based precision approach.