What's Up With WAAS?

Flying the new WAAS approaches

December 1, 2004

"So this is all I have to do? Follow the needles?" She sounds disbelieving.

While giving an hour of instrument dual in a Piper Archer, I introduce the new WAAS-enabled approach to a pilot accustomed to entering the various frequencies and cross radials, activating marker beacons, and identifying navaids during the ILS approach to this very runway. After activating the approach on the GPS navigator, the course lines appear on the unit's moving-map display, and we follow prompts to the initial approach fix, final approach fix, and on down the slide to the runway.

"Well, that was easy."

While the nuts and bolts of WAAS (Wide Area Augmentation System) remain complex, the execution is simple. The highly corrected GPS signal provided by WAAS gives any capable GPS navigator accurate enough position information to concoct a glideslopelike course and guide you down it.

While the WAAS signal has been available to pilots for use in flight — it was turned for IFR on July 10, 2003 — the greater accuracy it brings has been playing in the background. The hardware to fly the WAAS signal to lower approach minimums, complete with vertical guidance, underwent several phases of tweaking before the first WAAS-approach-capable navigator was ready to go live. With the latest software update, Garmin's GNS 480 (which used to be known as the CNX80, previously built by UPS Aviation Technologies before Garmin acquired that company) is ready to make expanded use of the WAAS signal.

AOPA President Phil Boyer flew his first WAAS-enabled GPS approach, the RNAV (GPS) Z Runway 23 approach on October 5 into Frederick Municipal Airport in Maryland. This particular approach has three sets of minimums, from lowest to highest: LPV, LNAV/VNAV, and LNAV. We've been flying the LNAV (lateral navigation) minimums for about 10 years now — they're the standard GPS nonprecision approach minimums. What's new is the highly accurate vertical navigation (VNAV, and the V in LPV) that allows for the LNAV/VNAV and LPV minimums — in the case of the RNAV 23 approach, to 394 feet agl and 1.5 miles (current ILS minimums to that runway are 388 feet agl and 1.5 miles).

The benefit is clear, even if the terminology sounds like a different language. As Boyer reported at AOPA Expo in October, flying the approach with vertical guidance feels just like flying an ILS.

The RNAV 23 approach into Frederick is an example of the first round of WAAS-enabled approaches delivering vertical guidance that don't require the use of baro VNAV equipment or flight management systems using inertial navigation systems (INS) — found in airline and large business jet cockpits — to calculate the vertical descent profile. WAAS also avoids the temperature and altimetry errors possible with baro-nav use (see " Proficient Pilot: Look Out Below," October Pilot).

In the future, with constant improvements to satellite navigation technologies, the FAA and user groups intend to have WAAS GPS approaches (GPS landing system — or GLS approaches) with performance akin to that of an ILS, with similar minimums. But more on that in a bit.

What is WAAS?

This part might get a little painful, so grab a cup of joe and put on your ground-school cap. We'll help you to understand the background.

The basic GPS signal has been available to civilian pilots for more than a decade. The system of 24 satellites sends signals to airborne receivers to triangulate your position (and take a fourth satellite to eliminate a duplicate location possibility, correct timing errors in your receiver, and determine a rough altitude). But despite its relative accuracy, basic GPS still results in position errors of 20 to 50 meters — good enough for a nonprecision approach with higher minimums but not for precision approach minimums and vertical guidance to lower altitudes.

WAAS is the fix for accuracy and integrity issues with the basic GPS signal. Remember the name: Wide Area Augmentation System. It is a system for augmenting the accuracy and integrity of GPS signals over almost the entire United States. WAAS incorporates 25 reference stations on the ground that listen to the satellite transmissions from the 24 satellites in the primary GPS constellation, and two master stations whose computers fix any errors. Then three ground stations deliver a signal corrected of the normal atmospheric anomalies that show up in GPS emissions to two geostationary satellites, which send the signal to any WAAS-capable receiver, enhancing the GPS position accuracy to 1 meter and to 1.5 meters (horizontal and vertical, respectively), according to results from system testing over the past couple of years. In other words, if you're centered on the approach using the WAAS signal, you're positioned as accurately as if you had the needles perfectly crossed on an ILS as you reach decision height.

Along with accuracy, WAAS brings integrity to the proverbial table. When WAAS or GPS signals are broken, a WAAS-enabled receiver will tell you they are broken. A traditional GPS receiver cannot do this — it's why we have RAIM (receiver autonomous integrity monitoring) — and the WAAS signal provides similar integrity as much as accuracy. The needle sensitivity of the WAAS receiver during the final approach segment improves from 556 meters to 70 meters at the approach end of the runway for an LPV approach. LNAV (standard GPS) and LNAV/VNAV nonprecision approaches don't "funnel" to the runway in the same manner that an LPV approach does, but their course accuracy is improved as well.

And the accuracy of the signal remains the same whether you stay on the final approach course or not — whether or not you're even in the vicinity of the airport. If you fly outside the "cone" on an ILS, all you know is which side of the course you're on — not how far out of bounds you are. With your WAAS-derived position constant on a moving map, you always know exactly how right — or wrong — your track is.

And what do I do with it?

Use of the WAAS-corrected signal is like using any other navaid, such as a localizer, in which you follow guidance delivered via a course deviation indicator (CDI) — also known as "the needles." Under certain conditions, and with the necessary equipment and charted minimums, you can use a localizer signal to fly either a nonprecision localizer or a precision ILS approach if a glideslope is also available. As we stated above in describing the RNAV 23 approach, there are essentially three kinds of straight-in approach minimums you'll see with GPS: those for an LNAV approach, which is the nonprecision GPS approach you've been flying; those for an LNAV/VNAV (vertical navigation) approach, which is WAAS approach with vertical guidance to as low as 350 feet agl and 1.5-mile visibility; and those for a GLS or LPV approach. GLS is a term defined by the International Civil Aviation Organization that requires more system performance than WAAS can provide at this point. It is a placeholder for the "precision" approach executed using WAAS, and at some point the GLS term will denote an approach that meets or exceeds traditional ILS minimums. As the FAA's experience with the WAAS signal improves, we should see minimums start to improve as well for LPVs, until true GLSs come into existence.

So, LPV is the current WAAS precision approach in use (named LPV to perhaps avoid confusion that it, as GLS will, provides Category I ILS minimums, which it does not do at this time). You may have wondered if LPV is an abbreviation. Not exactly. The term identifies a precision approach with localizer-like lateral guidance and glideslope-like vertical guidance. The associated minimums appear within the minimums table at the bottom of the approach chart, and go as low as 250 feet and three-quarters or one-half mile visibility (depending on whether approach lights are available). To the FAA, an LPV is simply another APV (approach with vertical guidance). To describe the term, Randy Kenagy, AOPA's senior director of advanced technology and our point man on WAAS, uses the phrase "localizer performance with vertical guidance."

So how reliable is it?

A standard GPS signal is available 99.99 percent of the time to a high level of accuracy in the continental United States. The FAA has also tracked the availability of the WAAS signal usable for precision approaches since it was turned on in 2003, and has found that 95.95 percent of the Lower 48 has access to the signal 95 percent of the time. Depending on how you look at it, this is fantastic coverage — but not if you live in the northeastern half of Maine, in the extreme southwestern United States, or in northern Minnesota, where signal availability isn't as great. In these areas, lack of signal availability adds up to 72 minutes cumulative time each day — though most of the continental United States enjoys availability 99 percent of the time, for all but 14 minutes of the day when the signal is unavailable.

However, think of the number of times you've found a navaid notamed out of service, or you've intercepted a localizer course only to have the glideslope remain flagged as you cross the final approach fix. It doesn't happen often, but it happens. As with any kind of flying, you need a backup plan when flying with WAAS. And as with a glideslope crumping on you, your backup to a lost WAAS signal is to simply fly the RNAV approach using LNAV-only minimums. Or, you can dust off your VOR receiver.

And WAAS is maturing. The FAA expects to see 99-percent availability by the time the GLS goes live. Just having the WAAS signal "on" and a WAAS-enabled panel-mount GPS in your airplane already allows you to file IFR to alternate airports only served by GPS approaches — without WAAS you need to flight-plan an alternate to an airport with an approach other than GPS available, if your original destination only has GPS approaches. And there is no need for a separate barometric input to the panel-mount receiver — WAAS corrects the GPS altitude already — and no longer will the unit need to calculate RAIM when the WAAS signal is available. A new algorithm used by the network leads to consistent availability of the lateral WAAS signal. If you've got the signal, you're good to go.

You will be able to use WAAS approaches at an alternate, as well as at the destination (instead of having to restrict your alternate airport selection to those airports with a VOR or ILS approach available). Your flight planning, however, should be based on flying the approach to LNAV minimums. If you get to the alternate and find that the WAAS GPS receiver indicates that LNAV/VNAV or LPV service is available, you can complete the approach using vertical guidance to those minimums if necessary. However, you still won't be able to use certain airports as alternates, because of other factors — for example, would you really want to choose Telluride Regional Airport, located in a tight valley in Colorado's San Juan Mountains, as your alternate? Check for the alternate minimums "NA" symbol on the chart to ensure you can file the airport as an alternate before you launch.

If you plan an instrument flight to an airport in the fringe area of WAAS coverage (in the extreme northeast, southwest, and north central parts of the continental United States), you need to flight-plan to plain-vanilla LNAV minimums, rather than to LPV or LNAV/VNAV minimums — at least until additional reference stations are put in place to serve these areas. You'll get a heads-up from the approach chart: A white W in a black square in the upper-left corner of the chart, alongside icons for takeoff and alternate minimums, denotes possible daily outages of WAAS vertical guidance at the airport — and that no WAAS notams for vertical outages will be provided. In practice, you can treat it like you would an ILS approach — if the box tells you that you're able to fly an LPV approach, you can complete the approach using LPV minimums. But if you lose LPV — like getting a glideslope flag — you need to fly the approach to the higher LNAV minimums.

If you note on recently issued RNAV (GPS) approach charts, there's a box within the briefing section (on the top of both Jeppesen and NACO charts) that lists, for example, "WAAS Ch 56203, W06A." This denotes the WAAS channel on which you would find the first approach to Runway 6 at a given airport. The same box lists a localizer, VOR, or ADF frequency on corresponding charts, which implies that you must have this channel dialed in somehow in order to execute the approach. No worries — the box does it for you. In reality, it's like a DME channel paired with a VOR frequency — transparent to the user unless you ferret it out. However, the way the GNS 480 is set up currently, you can select a WAAS-enabled approach by its discrete channel number, if you want. We're guessing that most pilots will still find a procedure by overall type (GPS) and runway (23).

To fly the RNAV (GPS) Z (the Z denotes there are multiple approaches, just like an A or B) Runway 23 approach at Frederick using the GNS 480, you would load the RNAV 23 approach with the LPV notation (or the same approach also comes up in the roster of RNAV approaches as "WAAS channel 81802"). Then, as you establish the airplane on the approach, note that the LPV indication shows on the screen of the 480. Ensure you're on the inbound course, and from there on in, you simply keep the needles in place. The lateral needle should remain centered as long as you follow the approach segments inbound, and you will capture the vertical needle (the "glideslope") as it comes alive when you intercept the descent angle.

Where do I get it?

There is currently just a handful of RNAV LPV approaches active, but several hundred with LNAV/VNAV minimums. (You can download these and other approaches from AOPA Online.) In order to qualify for an LPV approach, an airport needs to have certain approach and airport lighting, obstacle clearance surfaces — or zones — and runway infrastructure — and the FAA needs to design and flight-test the approach before it's added to the database. Consider also that a typical ILS localizer and glideslope costs roughly $1.5 million (plus annual costs of roughly $87,000 to maintain) for each installation to achieve the same precise path — and there needs to be a separate one for each front-course ILS approach at a given airport. So there's some cost associated with getting an LPV near you, but it doesn't involve the headaches of an ILS — a primary reason AOPA has fought so hard for WAAS over the past nine years.

To fly an LPV approach, your GPS must be either a Class 3 or 4 TSO-c146 WAAS unit. To execute LNAV/VNAV approaches, Class 2, 3, or 4 TSO-c146 WAAS equipment is required. Garmin refers to these levels as "gamma-1, -2, and -3," with gamma-3 receivers (such as the GNS 480) capable of giving LPV guidance. At this point, Garmin is planning an upgrade path for its GNS 400- and 500-series navigators for roughly $1,500, available in mid-2005. Chelton Flight Systems' FlightLogic EFIS (electronic flight information system) is currently capable of flying approaches to LNAV/VNAV minimums, using baro VNAV with the corresponding very cold temperature limitations. Company President Gordon Pratt says the FreeFlight GPS used by the EFIS will be LPV-capable by summer 2005.

Honeywell has developed a WAAS "engine" and plans its first delivery of WAAS-capable equipment in the Apex EFIS next year. "We're on board with it," says Dan Barks, director of marketing for Honeywell's business, regional, and GA avionics. Barks adds that the WAAS engine will be integrated with other Honeywell Bendix/King products shortly thereafter, and an upgrade or exchange path will be provided for existing customers.


E-mail the author at julie.boatman@aopa.org.