How WAAS makes GPS Safe
WAAS is designed to provide augmentation information to GPS receivers, enhancing the accuracy and reliability of position estimates. It removes the errors that typically come through GPS signals, and unlike traditional groundbased navigation aids, WAAS covers most of the National Airspace.
“When GPS was in its earlier days we thought we could use it to navigate and land aircraft, but then we realized that was only true to a certain extent,” said Jonathan Auld, NovAtel director, safety critical systems. “How do you know you can trust the satellites to work to their required level? That’s kind of where WAAS came from. We needed a second system to monitor the first system so we could verify the data received is healthy and that pilots are able to make good decisions in the cockpit.”
So, how does the system improve positioning accuracy? By providing correction data to the receiver that’s inside the aircraft, Auld said. This includes correction data for GPS satellite orbits, GPS satellite clock corrections and ionosphere delays. The WAAS network calculates this data then provides it to the aircraft receiver and pilot over a geostationary satellite. This takes the achievable accuracy down to a level of a few metres rather than 10 metres.
The data WAAS provides also helps the aircraft receiver and pilot determine the protection level, Auld said, and if they can trust the data coming from a specific GPS constellation. If the receiver in the aircraft thinks the constellation is accurate to a metre but it is actually in error by 10 metres, for example, that will lead to problems.
Through the WAAS system, pilots know if a constellation isn’t healthy, minimizing and even eliminating the risk of receiving Hazardous Misleading Information (HMI). In fact, users are notified within six seconds of any issuance of HMI that would cause an error in the GPS position estimate, according to the FAA.
“WAAS makes it possible for the flight crew to make the best decisions based on the best data available,” Auld said. “It’s better the system says don’t trust me, I can’t be depended on right now, than for it to mislead you. Then you can fall back to other systems and take the necessary actions to do something differently or follow a different procedure. You don’t want to depend on data you can’t trust.”