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

We're moving into the final piece of the GNSS picture now — the augmentation systems that fix the accuracy and integrity problems we've been talking about. Let's start with the Local Area Augmentation System, the LAAS. The LAAS is a ground-based system, and its whole job is to give aircraft approaching an aerodrome very precise corrections. The implementation starts with a precisely surveyed site on the aerodrome. That's critical — the site's exact position is known to a very high degree of accuracy. On that site sits a GPS receiver. This receiver determines its GPS position and compares it with the known position of the site. The difference between those two is the error, and that error is determined in the X, Y and Z coordinates — so we're talking about the three-dimensional position error in the Cartesian coordinate frame. That error is then specially formatted and transmitted to approaching aircraft. The system also monitors the satellite vehicle data — the SV data — and if it detects any errors in that data, it will either correct the error or give a failure warning indication. So it's not just broadcasting corrections; it's actively checking the integrity of the satellite information. Now, how does the data get to the aircraft? Through a dedicated VHF link. That's the transmission medium. And there's another component here — a pseudolite, which is short for pseudo-satellite. The pseudolite is provided to give range to the runway threshold using GNSS techniques. So it acts like an extra satellite on the ground, giving the aircraft a ranging signal to the runway threshold itself. Here's the key advantage of LAAS. When the aircraft is close to the DGPS site — and remember, DGPS is Differential GPS, the general principle of using a known ground position to compute corrections — the ionospheric and tropospheric transmission paths will be virtually identical. The signal from a satellite passes through the same atmosphere to both the ground station and the nearby aircraft. So those atmospheric errors are effectively eliminated. That's why LAAS has the potential to provide the accuracy necessary to achieve Category IIIC type operations — that's the highest category of instrument approach, essentially zero visibility landing capability. Now let's move to the wide-area version — Satellite Based Augmentation Systems, or SBAS. SBAS utilize a wide area DGPS, abbreviated WADGPS, implemented through a wide area augmentation system, WAAS. The idea is the same as LAAS — corrections and integrity monitoring — but over a much larger area. There are four systems currently operating. The European Geostationary Navigation Overlay System, EGNOS, declared operational in July 2004. The USA WAAS, 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 these systems 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. So an aircraft equipped for SBAS can use any of them. The discussion of WADGPS centres on EGNOS, but the same principles apply to all SBAS. So when we study EGNOS in detail, we're effectively studying the architecture of all four. Let me show you the LAAS setup so you can see how the ground station, the VHF link, and the pseudolite all fit together around the aerodrome. So to tie it together: LAAS is local, ground-based, uses a surveyed site and a VHF link, and can support Cat IIIC. SBAS is wide-area, uses geostationary satellites to broadcast corrections and integrity over a large region, and there are four operational systems — EGNOS, WAAS, MSAS, and GAGAN. Both are forms of differential GPS, just at different scales.