What is WAAS and How it Works What is it? The Wide Area Augmentation System (WAAS) system was developed to correct and minimize possible GPS position errors by using satellite and fixed ground position information to lesson GPS position and vertical errors. Many Aviation GPS units are now capable of WAAS activities including VNAV (non-precision) and LPV (precision) GPS approaches. Even many portable, handheld, backpacking units are WAAS enabled. Check your specific GPS unit to see if it is WAAS equipped and approved before using.

How does it work? WAAS uses a series of 38 receiver sites throughout North America. Each site receives signals from all GPS satellites above the horizon. Each site transmits this information to a WAAS master site, where the major sources of GPS errors are analyzed. The master site then develops a correction message, which is transmitted to two geostationary satellites.

The geostationary satellites re-transmit the correction message to your WAAS-enabled aircraft GPS unit, which applies the correction. While basic GPS typically has an error of around 75 feet horizontally, the corrected WAAS position calculation is usually within 10 feet. This precise position and altitude location information is what allows WAAS units to be certified for use on the new precision GPS approaches we see being added every day. In addition to the correction message, the geostationary satellites broadcast a position message that is used by your WAAS-enabled receiver. This means you typically have two additional satellites always in view over North America. While basic GPS requires you to perform a check to ensure that you have sufficient positioning information (RAIM), WAAS-enabled equipment has no requirement because of the additional Geostationary Satellies and the number of regular GPS satellites assured of being in view. Back to the newsletter main page.