Global Navigation Satellite Systems (GNSS) — Page 324, Lesson 321

Let’s pick up with the Local Area Augmentation System — LAAS — and then move into the Satellite Based Augmentation Systems, SBAS. First, LAAS. The whole idea here is that a DGPS — a Differential GPS — ground station can only correct errors that are common to both the ground receiver and the aircraft. That works well when the aircraft is close to the ground station, because the signal paths through the ionosphere and troposphere are nearly identical. So LAAS is built specifically for the final approach phase, right around the aerodrome. The implementation requires a precisely surveyed site on the aerodrome. That means the exact position of the site is known to a very high accuracy. On that site sits a GPS receiver. It determines its GPS position, then compares that computed position with the known, surveyed position of the site. The difference between the two gives the error in the X, Y and Z coordinates — the three axes of the position. That error is then specially formatted and transmitted to approaching aircraft. Now, the system doesn’t just blindly broadcast corrections. It will detect any errors in the SV data — SV stands for Space Vehicle, the satellite — and either correct the error or, if it can’t, give a failure warning indication. So there’s an integrity element built in. The data is transmitted to the aircraft via a dedicated VHF link. VHF — Very High Frequency. That’s the radio band used for the correction broadcast. There’s also a pseudolite — a pseudo-satellite. That’s a ground-based transmitter that behaves like a satellite, providing range to the runway threshold using GNSS techniques. So it gives the aircraft an additional ranging source right at the critical point — the runway threshold. Now, the key advantage: when the aircraft is close to the DGPS site, the ionospheric and tropospheric transmission paths will be virtually identical between the ground receiver and the aircraft. So those errors — the ionospheric and tropospheric delays — are effectively eliminated. That’s what makes LAAS capable of providing the accuracy needed for Category IIIC type operations — the very lowest visibility, fully automatic landing conditions. Let me show you the layout. Now, moving to SBAS — Satellite Based Augmentation Systems. These use a wide area DGPS, abbreviated WADGPS, implemented through a wide area augmentation system, WAAS. So instead of correcting for a small local area like LAAS, SBAS covers a large region. There are four systems currently operating. Let me name them precisely: The European Geostationary Navigation Overlay System — EGNOS — declared operational in July 2004. The USA WAAS — the Wide Area Augmentation System — declared operational in July 2003. The Japanese Multifunctional Transport Satellite Augmentation System — MSAS. And the Indian Geo and GPS Augmented Navigation — GAGAN. The objectives of all four are more or less identical: to provide integrity monitoring and position enhancement to aircraft operating over a large area. The methods of implementation differ slightly between systems, but the end result to the user is the same — there will be full compatibility between the systems. The discussion in the text centres on EGNOS as the example for WADGPS, but the same principles apply to all SBAS. So the contrast to hold onto: LAAS is local, precise, for the final approach, using a surveyed site and a VHF link. SBAS is wide area, using geostationary satellites to broadcast corrections and integrity over a large region, with four operational systems worldwide.