Plane & Pilot
Tuesday, March 23, 2010

What’s Up With WAAS?


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


I was fortunate to discover GPS early on. I was on my way to the 1991 Paris Air Show in the one and only prototype Swearingen SJ30 business jet, and had stopped for fuel in Greenland. A Pressurized Navajo was refueling on the ramp as we taxied in, and I walked over to talk to the pilot as our airplane was taking on a full service of kerosene.

We had dual VLF Omegas on board the SJ30, but electrical problems with the little jet had caused both to burn up early in the trip, and we were relegated to flying the Atlantic on instruments by dead reckoning—not the best idea. The pilot of the P-Navajo had an experimental Garmin GPS 100 on top of the panel, a beta-test unit loaned by a then-new GPS company: Garmin of Lenexa, Kans. He was out of Cleveland for Bergen, Norway, and he had become a fan of GPS in the first 2,000 nm.

When I reached Le Bourget the following day, I went to the American pavilion and presented my problem to Garmin’s then–aviation product manager, Tim Casey. He arranged for the loan of another GPS 100, the company’s first portable GPS. It was everything the Navajo pilot had promised.

As many pilots know, all the exotic functions of today’s multitalented GPS receivers are related to a single electronic trick: the ability to pinpoint position. Satellite navigation, in one form or another, has become a fixture on practically everything that moves—from hikers, snowmobiles and off-road motorcycles to trucks, autos, boats, airplanes and military targets, which it was created for in the first place.

I tried an early, primitive form of satellite navigation in the late ’80s. It was a system called Transit that relied on a half-dozen satellites orbiting from pole to pole. I borrowed a Transit unit from another ferry pilot for a trip in a new 36 Bonanza from Dallas to Cape Town, South Africa.

Predictably, with so few satellites online, Transit’s biggest deficit was that fixes weren’t continuously available. Depending on your location, Transit provided a fix about once every hour. (In contrast, the current global positioning system can refresh position as fast as five times per second.)

As the name implied, Transit mostly was a tool for electronic surveying. It wasn’t intended to pinpoint the location of a moving target, but it could generate a reasonably accurate fix in an airplane if you were holding a constant track and speed.

Today’s GPS was an initiative of the Reagan administration, and the completed Navstar satellite system became fully operational in April 1995. GPS relies on a universe of 24 primary satellites plus eight alleged backups (they’re usually operating anyway), concentrating coverage between 60 degrees north and 60 degrees south latitude, and orbiting 10,900 nm out in space. (In contrast, the shuttle orbits at a 200- to 385-mile altitude.)




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