What is the Wide Area Augmentation System (WAAS) and what does it do?
Jim Sexton responds:
The FAA wanted the great benefits that GPS could bring to aviation, but they needed the accuracy of Differential GPS across the US. GPS is very reliable, but every once in a while a GPS satellite malfunctions and transmits inaccurate data. The DGPS monitoring stations detect this error and transmit a system status message that tells receivers to disregard the broken satellite until further notice. Unfortunately this process can take several minutes, which may be OK for boaters, but would be too late for an airplane in the middle of a landing.
So the FAA set up its own monitoring system that would respond much more quickly. It parked a geosynchronous satellite over the US that instantly alerts aircraft when there is a problem and may also be used to relay differential corrections. The FAA has 24 reference receivers (DGPS) scattered across the US to gather the correction data that makes aircraft GPS receivers accurate enough for "Category 1" landings (i.e., very close to the runway, but not for zero visibility). This system has been operational since 1997.
The ramifications of this go well beyond aviation, because the system guarantees that DGPS corrections will be available for everyone to use. This will greatly benefit automobile mapping, hikers, and boaters all across the US.
To complete the WAAS system, the FAA is establishing Local Area Augmentation Systems (LAAS) near runways. These work like the WAAS, but on a smaller scale. The reference receivers are near the runways and are able to give much more accurate correction data to the incoming planes. With LAAS, an aircraft will be able to use GPS to make Category 3 landings in zero visibility.
WAAS Up by Don Casey