1. Plane Plane Go
GBAS receives microwave signals from 4 to 13 GPS satellites. "The most difficult thing isn't determining the plane's GPS position, but ensuring SmartPath doesn't cause any errors," says Honeywell's T.K. Kallenbach, who has refined the system so that the chance of receiving erroneous data from even one satellite is essentially less than one in 10 million.
2. Meter Maids
The heart of GBAS is the four GPS antennae mounted around the airport. Unlike radar, they aren't prone to interference and can be placed almost anywhere. The raw feed is accurate within 10 meters. The finished product -- data for every plane in a 23-mile radius are collected, compared, contrasted, and harmonized by proprietary algorithms -- is precise within 1 meter.
3. The Big Broadcast
Using these corrected coordinates, a VHF antenna broadcasts approach paths to the planes. These are passed to an onboard multimode receiver.
4. In the black box
The onboard multimode receiver, a black box, parses ILS radar, GBAS data, and the plane's own GPS. "Imagine that it knows where you are and it's talking to the ground, which explains where it wants you to go," Kallenbach says. "One continually updates the other." The raw data are rendered in a pilot-friendly interface, and the pilot isn't likely to notice many differences between GBAS and the instrument landings she is used to. Training to land with GBAS doesn't require much time in the simulator.
5. A Different Approach
The approach path is logged by the captain upon filing her flight plan. GBAS recognizes this and digitally broadcasts 26 separate paths to incoming planes and guides them in (ILS radios one). Some save fuel, while others reduce noise by weaving around neighborhoods. Smaller planes will be able to coast over the "wake turbulence" caused by engines ahead of them and land on shorter stretches of runway. The savings add up quickly.
6. Extending The Runway
The advantages of GBAS become more visible when the runway is not. With ILS, air-traffic control spaces planes more widely to account for its imprecision. Taxiing planes must also scoot back from the end of the runway to avoid interfering with radio beams. Honeywell estimates airports lose up to 25% of capacity during bad weather. GBAS renders moot all these causes for delays.