Improving safety and efficiency
The Wide Area Augmentation System (WAAS) came online for general aviation users in 2003, and it is the greatest thing since, well, GPS itself. Actually, it's best to think of WAAS as GPS' fullest expression — at least as we can see it from here — and it promises to do nothing but improve flight safety and efficiency into the near future.
WAAS augments the accuracy and integrity of GPS signals over nearly the entire United States. The network incorporates 25 reference stations on the ground that listen to the satellite transmissions from the two dozen satellites in the primary GPS constellation, and two master stations whose computers fix any errors. Then three ground stations deliver a signal corrected for the normal atmospheric anomalies that show up in GPS emissions to two geostationary satellites, which send the signal to any WAAS-capable receiver, enhancing the GPS position accuracy to 1 meter horizontally and to 1.5 meters vertically.
Along with position accuracy, WAAS provides an integrity-monitoring feature that trumps the random autonomous integrity monitoring (RAIM) present in GPS units certified under the current technical standards order. If a WAAS or GPS signal isn't working correctly, the receiver tells you about it. That loss-of-integrity message may seem like a small deal, but it allows you to use the receiver as a standalone unit for IFR flight — no need for a backup nav system — because of the, yes, integrity of the process by which the signal is verified.
Beyond integrity, WAAS enables the development of precision approaches, even where before none was possible. Hardware requirements for establishing what's called an LPV (a WAAS-based approach with vertical guidance) are low compared with those for an ILS, and line-of-sight limitations of traditional nav systems are eliminated. This opens up a lot of airports previously underutilized in poor weather — those airports often nearest our homes and easiest to get into and out of. And recently the FAA dropped the lowest possible LPV minimums to match those of an ILS: 200 feet decision height and one-half-mile visibility. Those approaches that will first get lowest minimums are likely to be overlaid on existing ILS approaches — and some approaches will never get the lowest minimums, of course, because of terrain or airport constrictions.
In the next five years, look for the explosion of precision approaches coming to an airport near you, along with vertical navigation guidance for the nonprecision approaches in your quiver (see " Global Positioning System [GPS]," page 72). This helping hand also can be used when you're approaching an airport under VFR conditions — who among us wouldn't benefit from a suggested glidepath to the runway from time to time — especially at night?
The North American network covers only our part of the world, but another network is planned to cover the European Union. Galileo is a system comprised of 30 satellites with precision clocks that improve tenfold upon those currently in use, providing WAAS-like position accuracy and integrity. Other regional satellite networks should use state-of-the-art components that also will boost integrity to at least the current WAAS level.— JKB
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