WAAS up?

October 2, 2008

In July 2003 a wonderful new era began for aviation. That was when WAAS capability was added to GPS navigation. And that meant that GPS navigation accuracy improved by five times over plain-Jane GPS navigators. Today, someone with a WAAS-enabled GPS receiver can shoot instrument approaches to altitudes as low as 200 feet above the runway’s elevation, thanks to the extreme accuracy of the WAAS GPS signal. This accuracy comes via correcting a network of 25 ground stations that monitor GPS satellites, plus two additional master stations—one on each coast—that create correction messages and send them on to two other satellites in geostationary orbit. Finally, these satellites send the final signals to your WAAS-equipped GPS receiver. The result: in en route operations, WAAS GPS accuracy is within three meters.

With this kind of accuracy, instrument approaches using WAAS have been commissioned over the past few years, and now they’re becoming commonplace. You’ll know you’ll be shooting a WAAS-enabled approach by the header on the approach plate, of course, but also by the various WAAS annunciations on your WAAS-GPS display. Here’s where some new terminology enters the picture, because there are four different types of WAAS GPS approaches.

The most basic is the LNAV (lateral navigation) approach. This is a nonprecision approach that resembles the kind of pre-WAAS GPS approaches you may have been flying all along.

An extra feature comes with LNAV+V approaches. You’ll have a “glideslope” needle with this sort of approach, but it comes with a big caveat. The vertical guidance is there to help you make stabilized approaches, but this guidance gives you NO obstacle clearance. It is best used in VFR conditions—or conditions where ceilings are high enough to allow a safe visual approach to the runway.

Next up the ladder of accuracy is the LNAV/VNAV—or L/VNAV—approach. This gives you both lateral and vertical guidance down the final approach course. This is not a precision approach like an ILS, but you will have a needle issuing vertical guidance. Keep the VNAV needle centered, and you are assured of clearance above terrain and obstacles.

The most accurate WAAS GPS approach is the LPV approach. Unlike the other WAAS GPS approaches, this is a precision approach that gives you lateral and vertical guidance to decision altitudes as low as 200 feet agl. On an LPV approach, accuracy is 16 meters laterally, and four meters vertically. Currently, there are more than 1,100 LPV approaches, many of those into airports that otherwise would not have a precision approach.

As always, you’ll need to check your approach plates for the minimums that correlate with the type of approach you’ll be flying. You’ll also want to keep an eye out for any trouble with the GPS signal’s integrity. If there are any signal problems, the WAAS GPS navigator should warn you within 5.2 seconds of its occurring. If that happens, a missed approach is probably in order.

But for the most part, flying a WAAS GPS approach is very similar to flying any other kind of approach. Simply keep the needles centered, descend when and where you should, and fly no lower then the published minimums. Just know that thanks to WAAS, many of those minimums have been lowered, opening more airports up to better accessibility.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.