Do you understand how WAAS works, and how it helps you fly highly-accurate GPS approaches?

Wide Area Augmentation System (WAAS), Defined

According the FAA's Instrument Flying Handbook, "WAAS is designed to improve the accuracy, integrity, and availability of GPS signals." WAAS service is available for all classes of aircraft in all phases of flight, including: en route navigation, airport departures, and airport arrivals. This includes vertically-guided instrument approaches in IMC at all qualified locations throughout the US national airspace system. More and more aircraft are WAAS-capable, allowing them to fly more precise GPS instrument approaches. In fact, WAAS-capable receivers can give you a position accuracy of better than 3 meters, 95 percent of the time.

So how exactly does WAAS work? Let's break it down...

WAAS Is A 4-Step Process

WAAS is the the U.S. implementation of SBAS (Satellite-Based Augmentation System). Here's how it works:

• Signals from the GPS satellite constellation are monitored by WAAS ground-based stations, to determine satellite clock and position corrections.
• Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message.
• The correction message is prepared and uplinked to a geostationary satellite via a ground uplink station. This correction accounts for GPS satellite orbit and clock drift, plus signal delays caused by the atmosphere and ionosphere.
• The corrected differential message is broadcast through 1 of 2 geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

All of this happens within a matter of seconds, ensuring the continuous accuracy of your GPS system.

What Is The Extra "Integrity" Provided By WAAS?

Integrity refers to usability of a satellite signal, and the ability of a system to provide timely warnings to users when WAAS should not be used for navigation as a result of errors or failures in the system. Beyond providing a correction signal, WAAS provides an additional measurement to your aircraft's receiver. The system effectively increases GPS integrity through real-time monitoring of GPS sources, whereas the accuracy is improved by provided differential corrections from these sources to reduce errors.

What About WAAS Outages?

WAAS outages are very rare, but the FAA provides a live feed of WAAS availability. Click here to view a live-feed of WAAS coverage in the United States.

WAAS And Instrument Approaches

While WAAS receivers support all basic GPS approach functions, they have the benefit of generating electronic glidepaths, which are independent of ground equipment or barometric aiding. This eliminates several problems, such as cold temperature effects, incorrect altimeter settings, or lack of a local altimeter source, and finally allows approach procedures to be built without the cost of installing ground-based navigation equipment.

LPV approaches are a WAAS/GPS based approach, and they're very similar to the ILS. But there is a difference. Even though LPV approaches have vertical guidance, they're not considered precision approaches. Instead, they're an approach with vertical guidance (APV). The extremely accurate WAAS system (7.6 meters or better accuracy) gives you lateral and vertical guidance down to a decision altitude (DA) like an ILS. And, just like an ILS, an LPV approach's angular guidance gets more sensitive the closer you get to the runway. Keep in mind though, to fly them, you need a WAAS receiver. A baro-aided GPS won't work.

There's definitely an advantage to LPV. Unlike an ILS, which gets more and more sensitive and difficult to fly near and below DA, the scaling on an LPV approach transitions to a linear scaling as you approach the runway. It has a total course width of 700' (usually) at the runway threshold. That 700' of width at the threshold is the same as an ILS localizer at the threshold, but it doesn't get any tighter than that as you continue to touchdown.

LPV approaches get you low as well. Like an ILS, most LPV approaches will get you down to 200' above touchdown, with 1/2 mile visibility.

But there is a downside. Since LPV approaches aren't considered precision approaches, you can't use precision alternate minimums for airports that only have LPV. According to the FAA, if you're using an airport with LPV only (no ILS or other ground-based navaid approach) as your alternate airport, you need weather minimums that meet the LNAV or circling MDA, or the LNVA/VNAV DA if you're equipped to fly it. (There are a few more details as well, which you can find in AIM 1-2-3, paragraph D.)

Click here to learn everything you'll need to know about LPV and LNAV/VNAV instrument approaches.

Makes Sense, Right?

As GPS becomes more widely used, ground-based navigation systems are slowly disappearing. Understanding how WAAS works is an important skill you'll need for the future of aviation. Simply put, WAAS uses an array of ground based-stations which are linked to satellites to provide your GPS receiver with highly accurate location information.

Is your aircraft WAAS-capable? Tell us in the comments below.