Global Navigation Satellite Systems (GNSS) — Page 324, Lesson 324

I want to walk you through the three segments that make up SBAS — Satellite-Based Augmentation Systems — because this is the heart of how GNSS gets its accuracy and integrity upgraded for aviation use. Let's start with the space segment. This comprises the GPS and GLONASS constellations, plus geostationary satellites. Now, geostationary SVs — that's "space vehicles" — have an orbital period of 24 hours, and they're found only in equatorial orbits at an altitude of 35,800 km. That's what makes them geostationary: they sit over the same spot on the equator, so they can continuously broadcast augmentation data down to a region. Next, the ground segment. This has three layers: reference stations, which we call RS; regional control stations, RCS; and a master control station, MCS — sometimes called a navigation earth station, or NES. Then finally the user segment, which is simply all who use the service. Let me explain how these ground stations work together, because the chain of data flow is critical. The RS are established within a region to measure two things: the accuracy of the SV data, and the ionospheric and tropospheric effects on the SV transmissions. Just like with LAAS — the Local Area Augmentation System — the RS are precisely surveyed sites. That means their exact position is known to a very high accuracy. Each RS contains a GPS receiver and an accurate atomic clock. Each RS is linked to an RCS, and the RCS is linked in turn to the MCS, or NES. Now here's the clever part — the actual error computation. The RS determines its GPS position from the SV data. But since the RS already knows its own true position, and it receives the SV ephemeris — that's the satellite's orbital position data — plus the clock time and any clock error corrections, it can back-calculate the true position and time at the SV. From that, it determines the range error for each SV. It also determines if there are significant errors that render any of the SVs' information unusable — and that's the integrity check on the system. A key point here: this range error will not deviate significantly over a considerable range — more than 400 km. Neither will the relative effects of the ionospheric and tropospheric propagation. That's why a regional system works: the errors measured at one surveyed site are valid across a wide area. The data — the SV errors and the integrity assessment — is sent via the RCS to the MCS, which is located at NATS at Gatwick. There it's formatted for use by suitably equipped GPS receivers. Then the data is sent to Goonhilly Down, where it's uplinked for broadcast on the East Atlantic and Indian Ocean INMARSAT geostationary SVs' navigation broadcast channels. So the geostationary satellites are the broadcast link back down to the users. When the GPS receivers incorporate this data into their calculations, they achieve two things: enhancement of position, and failure warning. That's the whole point of SBAS — better accuracy and integrity. Now, one important limitation to understand. While the accuracy of GPS is greatly enhanced by WADGPS — Wide Area Differential GPS — it cannot, and is unlikely to, achieve the accuracy required for Category I type operations. Those precision approaches will continue, for the foreseeable future, to require LAAS. The best decision height achieved to date is about 300 feet, and that's unlikely to be improved upon in the near future. So SBAS gives you a big boost, but it's not the final word for the most demanding precision approaches — that's still LAAS territory.