1) What is WAAS?
Wide Area Augmentation System (WAAS) is a way for you to receive a more accurate GPS signal onboard your plane for all types of navigation: departure, en route, and arrival.
So how does WAAS work? Signals from the GPS satellite constellation are collected by ground stations called Wide Area Reference Stations (WRS). These ground stations check GPS signals for precise timing and positioning.
Next, WAAS Master Stations (WMS) in the United States collect the data from the WRS. The WAAS Master Stations then create a correction message, which is uplinked to geostationary WAAS satellites through a ground-uplink station.
Finally, the correction message is sent from the WAAS satellites to the receiver in your plane, giving you an accurate, precise, and reliable position signal.
2) How accurate is WAAS?
Basic GPS has an accuracy of about 7 meters (~23 feet). WAAS accuracy is less than 2 meters (~6.5 feet).
3) Instrument approaches and WAAS
With a WAAS receiver, you can fly LP and LPV approaches.
4) Alternate airport requirements with WAAS
First, when you have WAAS, neither your destination nor your alternate is required to have a ground-based instrument approach (this differs from basic GPS).
Second, FAR Part 91 non-precision weather requirements must be used for your planning.
And third, when you’re using WAAS at an alternate airport, your alternate planning needs to be based on flying the RNAV (GPS) LNAV or circling minimums line, or minimums on a GPS approach procedure, or conventional approach procedure with “or GPS” in the title.
However, if you arrive at an alternate and the WAAS navigation system indicates that LNAV/VNAV or LPV service is available, then vertical guidance can be used to fly the approach. (AIM 1-1-18 c. 9 (a))
5) What do you need to receive WAAS?
WAAS is free and available for all types of operators; airlines, commercial, and private. All you need is the right equipment installed in your plane. There are three classes of WAAS GPS sensors:
- Class 1: Provides lateral navigation (LNAV) for approaches, but no vertical guidance.
- Class 2: Provides lateral and vertical navigation (LNAV/VNAV) guidance for approaches.
- Class 3: Provides the highest standard of position, allowing for LPV approaches.
Most avionic panels built today are delivered with Class 3 WAAS receivers.
Do you have WAAS equipment in your plane? Tell us what you’re flying in the comments below!