Plane & Pilot
Tuesday, March 23, 2010

What’s Up With WAAS?


WAAS is the third generation of GPS, and it makes all the difference


The Navstar satellites’ unusually high orbit assures that all satellites can “see” nearly half of the earth most of the time. Hence, a 12-channel GPS receiver with good line-of-sight reception will “read” as many as 11 signals all the time, many from satellites high above the horizon. Accordingly, signal integrity often is excellent.

I’ve used basic GPS for almost 20 years, and it has served me well. In one instance, it saved my life. I executed a totally irregular, basically zero-zero approach into Narsarsuaq, Greenland, using a Garmin 530 and 430, plus my Garmin 296 portable backup. Faced with unforecast fog that extended practically to the ground and no options other than to crash or try something dumb, I chose the latter.

I selected the largest possible scale to give me maximum definition, slowed the airplane to 100 knots with wheels and flaps extended, and started up the 42-mile Tunugviarfik Fjord toward Narsarsuaq Airport, totally blind at 100 feet. I followed the fjord contours suggested by the three GPS units, with occasional glimpses of water and icebergs below. I turned final for an unseen runway about a half mile out, broke out at 50 feet and plopped the airplane onto the ground a few seconds later. (I thought I might be in trouble for that approach, but the tower operator’s only question when I cleaned up the left seat, calmed down and made my way up to the airport office, was, “Are you planning to go on to Goose Bay tonight?”)

The accuracy of the original GPS was deliberately downgraded to 100 meters by the military, a process called selective availability. This was intended to keep anyone—except them—from stuffing a cruise missile into a particular window in a building in Baghdad, an interesting paradox since only the military had cruise missiles. The government eventually stopped corrupting the GPS signal in the late ’90s, and standard GPS was finally allowed to operate with an error of only 10 meters.

As accurate as it was, the initial GPS did suffer some errors. Clock error and sunspot activity were two factors that affected GPS accuracy. The first upgrade was called differential GPS (DGPS). This corrected those errors to the range of three to five meters.

DGPS is simple in concept, though more difficult in execution. It’s a series of ground stations at known lat/long coordinates that monitor the GPS signal, note any error in the position reported by the satellites, then transmit a correction to the entire Navstar system.

The third generation of GPS is called WAAS. The Wide Area Augmentation System is an extension of DGPS, only to much finer tolerances. The FAA hopes to replace the current VHF/ILS system with 3-D precision GPS approaches (including a pseudo glideslope) in the next few years, and WAAS is part of the improvement necessary to provide adequate accuracy. GPS/WAAS approaches, technically referred to as LPV (localizer performance with vertical guidance), will offer the same 200-1/2 minimums as standard CAT I ILS procedures.

WAAS is a U.S. initiative that’s operable in North America and consists of 25 ground stations. These account for satellite orbit oscillations, clock drift and atmospheric aberrations, and provide position correction to the GPS signal down to less than three meters both vertically and horizontally, easily within the tolerances of the current ILS approach. Several other countries and international cooperatives—the Japanese Multi-Functional Satellite Augmentation System (MSAS), the Indian GPS-Aided Geo Augmented Navigation (GAGAN) and the Euro Geostationary Navigation Overlay Service (GNOS)— eventually will provide similar service overseas. Other countries also are pursuing full-on vertical guidance GPS, and it will become the rule for instrument approaches for most of the world.

In simple terms, WAAS eventually will bring instrument approaches to any facility for which a procedure can be developed. That won’t include every airport. Full-on WAAS approaches won’t work everywhere, as terrain anomalies and existing real-estate development may restrict capability, but it should extend precision approaches to a wide variety of airports, not just the few hundred terminals served by commercial airlines. From an operator’s point of view, WAAS approaches won’t require any transmitters on the ground and should be ridiculously inexpensive for airports with limited budgets.

Like everything GPS, WAAS is extremely simple in execution and virtually transparent to the operator. For pilots flying behind Garmin boxes, the WAAS upgrade includes a significantly faster processor, a revised database (including WAAS approaches), a modified antenna and quicker, more accurate terrain resolution.

The result for those with a WAAS-enabled GPS is the most accurate position above the planet. And as I mentioned earlier, position is everything.



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