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

Right, let's pick this up with the Satellite-Based Augmentation System — SBAS. This is the wide-area cousin of the local system we just looked at, and it's built from three distinct segments. First, the space segment. This is made up of the GPS and GLONASS constellations, plus geostationary satellites — the SVs. Now, a key fact about those geostationary satellites: they 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 keeps them fixed over one spot on the equator, which is exactly what we need for a broadcast relay. Second, the ground segment. This comprises reference stations — the RS — regional control stations — the RCS — and a master control station — the MCS, which is also called the navigation earth station, or NES. So you'll see MCS and NES used interchangeably. Third, the user segment. That's simply all who use the service — every suitably equipped GPS receiver. Now let's look at how the ground segment actually works, because that's where the intelligence lives. The reference stations are established within a region to measure two things: the accuracy of the satellite data, and the ionospheric and tropospheric effects on the satellite transmissions. Just like with LAAS, the RS are precisely surveyed sites — they contain 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. Here's the clever part. Each reference station determines its GPS position from the satellite data. But here's the thing — the RS already knows its own position, because it's precisely surveyed. It also receives the satellite ephemeris — that's the precise orbital data — plus the clock time and any clock error corrections. So it back-calculates the true position and time at the satellite, and from that it determines the range error for each satellite. It also determines if there are significant errors that render any of the satellites' information unusable — and that's what provides an integrity check on the whole system. Now, a crucial point about why this works over a wide area: 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 the whole principle of a wide-area system — the errors are common to a large region, so one set of corrections serves everyone in that region. So the data — the satellite errors and the integrity assessment — is sent via the RCS to the MCS. In this case, the MCS is located at NATS at Gatwick. There it's formatted for use by suitably equipped GPS receivers. The data is then sent to Goonhilly Down, where it's uplinked for broadcast on the East Atlantic and Indian Ocean INMARSAT geostationary satellites' navigation broadcast channels. The GPS receivers incorporate the data into their calculations, and achieve both enhancement of position and failure warning. Now, one important limitation to finish on. Whilst the accuracy of GPS will be greatly enhanced by WADGPS — that's Wide Area Differential GPS, the general term for this kind of system — it cannot, and is unlikely to, achieve the accuracy required for Category I type operations. Those will continue, for the foreseeable future, to require the provision of LAAS. To give you a sense of the numbers: 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 gets you a big accuracy boost and integrity, but for the precision approach work — Category I and below — you still need the local area system.