Plane & Pilot
Thursday, June 11, 2009


GPS Approaches for Every Airport?

waasThe benefits of transitioning from pilotage to dead reckoning, four course ranges, ADF, VORs, Loran and then to GPS have been nothing short of spectacular.
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What Is WAAS?
The WAAS infrastructure consists of 38 ground reference stations spread throughout the United States, two master stations that provide control for the other stations, two geosynchronous satellites, four uplink stations, two operational control centers and the WAAS terrestrial communications network. The ground stations operate from precisely measured locations and send data that adjusts the normal GPS accuracy to the higher level of accuracy that WAAS receivers require. Additionally, the aircraft’s WAAS computer typically operates at a much higher speed than previous GPS devices, thus ensuring continuous and accurate monitoring.

WAAS Approaches
Like ILS approaches, LPV approaches are easier to fly than conventional nonprecision approaches because they have a glideslope. But there’s a feature of the LPV approach that makes it even easier to fly than an ILS, especially as the aircraft gets closer to the runway. ILS approaches get increasingly more accurate—read “twitchy”—as the aircraft descends. In other words, the aircraft requires increasing precision to stay on the localizer and glideslope as it gets closer to the runway threshold. An LPV’s glidepath scaling changes to linear, as opposed to an ILS’s angular scaling. In other words, it remains at the same scaling on the approach. The result is the likelihood of a more stable approach.

In addition to the LPV approach, there are two other types of WAAS approaches. If you include them in the mix, practically every airport in the country could have a WAAS-based GPS instrument approach.

LNAV approaches are analogous to normal GPS approaches, but at a higher level of accuracy. LNAV/VNAV provides a “pseudo” glideslope that allows a constant rate of descent rather than the normal step-downs of the LNAV or standard nonprecision approach. Although there’s a glidepath indication, an LNAV/VNAV approach also isn’t considered a precision approach and doesn’t have the lower LPV approach minimums. But otherwise, it’s as easy to fly as an LPV and easier than an LNAV.

As of January 2009, there were 3,915 LNAV approaches, 1,577 LNAV/VNAV approaches and, as mentioned earlier, 1,445 LPV approaches. That’s a total of 6,937 WAAS approaches in the United States. For a sense of the future, there are only 783 stand-alone GPS approaches remaining.

What WAAS means to the instrument pilot is the potential availability of precision-like and nonprecision approaches to an increasingly large number of airports, including some airports that previously couldn’t have any instrument approaches. It also means that approaches like the LPV and LNAV/VNAV approaches are easier to fly because of constant-rate-of-descent glideslope indications. Finally, it means that many (maybe most) of these approaches are to lower minimums than existing approaches at a given airport.

In an interesting dichotomy, some conventional nonprecision approaches have lower MDAs than the newer WAAS approaches, even though they’re over exactly the same routing. It pays to review all approaches to an airport before choosing one.

There’s an interesting safety issue for aircraft with GPS devices, but especially for those with WAAS navigators. Because of their more precise course and altitude accuracy, these aircraft have an increased likelihood of midair collisions when flying on airways. As more aircraft are equipped with WAAS GPS units, looking out the window is going to become even more important.

The Future Is Now
The availability of WAAS has and will continue to open up airports to instrument approaches and enable new approaches to lower minimums. Consider, if you will, that just a scant few years ago, the FAA implemented the first “overlay GPS approaches” over existing nonprecision approaches. These overlay approaches required that the aircraft have, in addition to IFR-approved GPS devices, appropriate equipment to fly the approach using the underlying VOR or ADF technologies. Those days are long gone. Today’s TSO-146a receivers operate independently of any other navigation technologies. A WAAS GPS unit can be the sole source of navigation on a flight. That’s comforting since the FAA is phasing out NDBs and the first major shutdowns of VORs is scheduled for 2011.

If you haven’t considered upgrading to WAAS technology, now may be a good time to think about WAAS. If you’d like to read more about WAAS, read the AIM, section 1-1-20, where you can find out everything you might want to know about it.

WAAS-Enabled Units For GA
Avidyne Entegra • Bendix/King KSN 770 • Garmin 430 • Garmin 530 • Garmin G1000


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