How WAAS Works
In the article on this website on “How GPS Works” it was explained that the GPS position solution suffers from clock errors, erroneous orbit data, and time delays through the ionosphere. The Wide Area Augmentation System (WAAS) system was designed to improve on your position accuracy, and is the US version of a Satellite Based Augmentation System (SBAS).
There are future plans for a Ground Based Augmentation System (GBAS), which in the US is called a LAAS system (L for Local).The LAAS systems will be installed at airports, and serve all runways there with Cat I, and eventually Cat II and III precision. The LAAS systems are still in development, so this article briefly describes the WAAS system and the pilot benefits in using it.
The WAAS System
The WAAS system includes 38 ground based reference stations (including the lower 48 states, Alaska, Mexico, Hawaii, Canada, and Puerto Rico), 3 Wide-area Master stations, and 3 geosynchronous WAAS satellites. The ground stations are shown here (picture from FAA presentation by Larry Oliver; FAA Flight Standards).
Each reference station is precisely located by survey, and it continuously monitors signals from the GPS satellite network. It compares its position as computed from satellites with its know position. WAAS satellite signals are also monitored to provide integrity information about them. Each ground station then sends its information to the Master stations.
The data collected by the Master stations is used to calculate correction messages every 5 seconds and send them to the geosynchronous WAAS satellites. Those satellites broadcast the WAAS messages to aircraft in the WAAS coverage area. Your WAAS receiver can tune in any 12 of the regular GPS satellites, and the 3 WAAS satellites.
The surface areas covered by the 3 geosynchronous WAAS satellites are shown below. On these boundaries they are only visible on the horizon (those boundaries are actually circles on the globe, but are distorted in this Mercator projection). The satellites are directly overhead in the center of the areas.
Two types of messages are created, for data changes that are fast and for those that are slow. The fast changing data includes where the satellites actually are and their current clock error. The orbits drift from their positions as expected from orbit date stored in your receiver, so you need to know this error as well as its clock error, to compute your position accurately. Your receiver uses this data immediately, since those errors are independent of your location.
The slowly changing data does depend on your location. The data has new ephemeris data (projected orbits), estimates of the clock error, and most importantly the ionospheric delay data. Your receiver first determines your position from the regular GPS satellites, and then uses the WAAS slow message to improve on it. This slow data is refreshed every 2 min.
You need to account for the extra time delay of the signal through the ionosphere (compared with the delay through the same distance in a vacuum), which is the major contributor to your position error. The ionosphere is created by UV solar radiation, ionizing some of the particles in the region from 100 to 600 km above the earth. The electrons in the ionosphere (produced by the ionizing radiation) slow the GPS signals from their speed-of-light value in vacuum, introducing an extra time delay that must be estimated for an accurate position calculation. Since the UV source goes away at night, the density of the electrons (up to a million per cc) varies significantly between night and day, as does the thickness of the layer. Since the delay depends on the total electron content along the signal path, signals coming through the ionosphere on an oblique angle increases the delay.
The slow data message includes ionospheric data on a large-scale grid, so your receiver uses delay data nearest your location and partially corrects it for the actual signal paths through it on a line between you and the satellite. Parenthetically, this delay is frequency dependent, so if you use two different frequencies (L1 and L5 satellites, for example) you can eliminate this error using the difference in total time delay from them, even without WAAS. With the WAAS message corrections, your 95% position accuracies are about 1 m horizontal and 1.5 m vertical.
Not only does WAAS improve you position solution, but it can afford other advantages to the pilot as well. Using a WAAS receiver permits use of GPS as your primary navigation system. Otherwise, even though you use your GPS for navigation, it is considered a backup (secondary navigation) to your primary system, which must be based on ground systems like VOR or NDB facilities. If there is a RAIM failure of a secondary GPS system you must revert to your primary navigation system.
With WAAS you no longer need to do RAIM checks, since the system continuously checks GPS errors (HPL, VPL) and must give you an integrity warning within about 6 seconds. Here, HPL is your horizontal protection level, and is the "5-nines probability" (99.99999%) that your position error is bounded by that value. This is a probability of 1 in 10 million. VPL is the vertical protection level. [See the article on Integrity and RAIM for the integrity requirements for various approaches.]
While you can purchase a Class 1 WAAS receiver like the FreeFlight 1201 used with the Chelton EFIS system (certified only for LNAV operations), most IFR panel mount receivers today are certified for Class 3 operations. These are required in order to fly GPS vertical courses (see article on the home page). A Class 3 receiver must compute position 5 times per second, compared to once per second for Class 1.