GPS Goes Low

The navaids, avionics, and procedures of the future — today

September 1, 2003

Ever since 1994, when the first GPS receiver was certified for IFR operations, the general aviation community has been waiting for the day when GPS navigation would reach its highest expression — providing approach capabilities with the precision to rival those of an instrument landing system (ILS). That day came on July 10, when the FAA's wide area augmentation system (WAAS) entered its initial operational capability (IOC) phase.

The IOC kicks off a timetable that's set to certify some 300 WAAS approaches per year. By the time WAAS is fully operational — as late as 2015 — some 5,000 airports could have as many as 11,000 new approaches.

The big advantage of WAAS is vertical guidance. GPS approaches are nonprecision. In other words, they provide lateral guidance only, leaving the pilot on his own to manage the descent. Because of this lack of vertical guidance and the relative coarseness of raw GPS signals, the minimums for such approaches are about on par with VOR approaches.

On an approach with WAAS, you'll have lateral, or lateral and vertical guidance cues — rather like those of a localizer or ILS. This will allow lower minimums. Initially, approaches published with lateral navigation (LNAV) and lateral and vertical navigation (LNAV/ VNAV) minimums will typically let you descend as low as 400 feet above the airport elevation, in visibilities as low as 1.5 statute miles. Later in the development cycle, a more exact approach with tighter vertical guidance standards will permit approaches to 250 feet in three-quarter-mile visibilities. These are what have come to be known as localizer performance approaches with vertical guidance (LPV, for short) WAAS approaches, though they have never been officially defined as such. Ten-plus years from now there's the opportunity for WAAS approaches with the potential for ILS minimums (200 feet and one-half mile) — GPS landing system, or GLS, approaches. At this juncture, however, it's important to remember that the FAA is focused on the IOC phase, and that some longer-range goals are still undefined.

But first things first. Let's get a handle on what WAAS is all about, and what it means to general aviation pilots.

WAAS basics

Standard GPS receivers use signals from a constellation of 24 satellites. Hardware aboard the satellites ensures that navigation accuracy can fix a position within 100 meters, or about 330 feet. The signals are beamed from the satellites to a master control station, which detects any signal errors and broadcasts corrections to our GPS receivers via a network of ground antennas.

While 100-meter accuracy is more than adequate for en route navigation and today's nonprecision GPS approaches, additional equipment is needed to make the signals accurate and reliable enough to safely be used for instrument approaches with vertical guidance. That's where WAAS comes in.

WAAS uses its own terrestrial network of wide area reference stations and a special master station to further remove errors in GPS signals. Correction messages are then sent to two WAAS satellites in geostationary orbit. From there, the signals are beamed to WAAS-enabled GPS receivers in our aircraft.

The result is routine 7.6-meter lateral and vertical position fixing accuracy. FAA testing, however, has demonstrated that WAAS position fixing accuracy is often on the order of 1.6 meters laterally and 2 meters vertically. That's the kind of accuracy that can be used to descend to near-ILS ceiling and visibility minimums.


WAAS approaches present several exciting opportunities. One is the potential to open up thousands of airports — previously inaccessible when instrument meteorological conditions prevailed — to extremely accurate instrument approaches. Another is the ability to standardize GPS navigation for all phases of flight, which translates into needing just one black box to make full use of the National Airspace System. A reduction in maintenance costs associated with a huge network of ground-based navaids like VORs, NDBs, DMEs, and even ILSs is another advantage. But the biggest draw has to do with safety. With LNAV/VNAV WAAS approaches, gone are the step-down fixes and steep descents of traditional nonprecision approaches. Instead, we have glideslope-style indications that permit stabilized descents.

WAAS approaches are published as RNAV (GPS) approaches; they don't have their own, named approach. So you won't see any "WAAS Rwy 5" descriptors at the top of any approach plates. The giveaway that it's a WAAS approach is in the minimums section. Prototype approach plates list the type of approaches down the left side of the page, with the minimum descent altitudes (MDAs) and visibilities off to the right, according to the airplane's approach category.

LNAV/VNAV approaches will typically be published with 400-foot (and sometimes 350-foot) HATs (heights above touchdown zone) and 1.5-mile visibility minimums. On these approaches you'll have vertical guidance via a glideslope-style display on your course deviation indicator (CDI), horizontal situation indicator (HSI), or other navigation display.

LNAV-only approaches won't have the luxury of vertical guidance. LPVs, when they begin their promised phase-in at the end of 2003, will typically let you descend to 250-foot and three-quarter-mile HATs, close to the typical ILS' 200 feet and half-mile minimums.

WAAS-enabled GPS receivers have fault-detection and exclusion (FDE) features that warn pilots of degraded signals or bad GPS channels. This works by sampling position information at tremendous rates — a one-time-per-second update rate for LNAV and LNAV/VNAV approaches; a five-times-per-second rate for LPV and GLS approaches. This ensures signal integrity in the cockpit. Then, for example, if your VNAV capability conks out during an approach, an annunciator lets you know. This functionality works like the receiver autonomous integrity monitoring (RAIM) alerting we've always had in IFR-certified GPS boxes. It's all part of the technical standard order (TSO) requirements for WAAS boxes, which come under TSO C-145.

The first 500 LNAV/VNAV approaches — to 200 airports — were commissioned July 10. On September 10, the Frederick (Maryland) Municipal Airport (AOPA's home base) and six other airports are scheduled to be able to use LNAV, LNAV/VNAV, and the first LPV approaches. The others include Wittman Regional Airport in Oshkosh, Wisconsin; Leesburg Executive Airport in Leesburg, Virginia; Will Rogers World Airport in Oklahoma City, Oklahoma (two approaches); and Atlantic City International Airport in Atlantic City, New Jersey.


A program as ambitious and complex as WAAS is bound to have complications. The issues surrounding the program continue to garner skeptics and naysayers. Here are a few:

Schedule delays. Originally, the FAA said that WAAS would be up and running by 1997. It missed the schedule by six years. Now the biggest challenge to full WAAS implementation will be the move to GLS approaches. For that to occur, the entire GPS system must be modernized. GPS receivers need two GPS frequencies to shoot GLS approaches, and the current equipment operates on a single frequency. This may complicate the 2014 GLS target date.

Airport infrastructure limitations. WAAS may not serve up low altitude approaches at as many airports as originally advertised in 1995. Just because WAAS signals can reliably serve an airport with near-precision navigation information doesn't mean that the airport is precision approach-ready. For approaches to provide the lowest possible minimums, TERPS (terminal instrument procedures) standards require obstruction-free approach and departure clearways, parallel taxiways, runway markings, and lighting systems. Plus, it can take a year or so for the FAA to survey, test-fly, and publish a candidate approach. Some approach minimums will be quite low, some won't.

Not-so-low visibility minimums. LNAV/VNAV approaches may not always grant you permission to descend to 350 and 1.5. LNAV approaches may be published with lower visibility minima than those with vertical guidance. With an approach conducted with vertical guidance, your decision height (DH) may be reached a mile and a half or more from the threshold — the theory being that with a good lighting system and other approach aids you'll see the runway at — or very shortly after — you break out of the overcast. Depending on the airport and the results of its specific procedural test flights, you may have a better chance of seeing the runway environment under low-visibility conditions using an LNAV approach.

Signal availability. Right now, GPS signals are accurate and reliable 95 percent of the time over a one-year period. The FAA is committed to WAAS achieving a 99.999-percent reliability score by December 2006, and it expects full coverage of the contiguous United States and most of Alaska by that time. Problems crop up when the system detects an error while you happen to be on an approach and have to fly to an alternate airport. Since the signals for vertical navigation are more likely to drop off line, pilots on instrument flight plans must currently make their alternate-airport decisions based on the higher ceiling minimums of an LNAV approach, even if LNAV/VNAV or LPV approaches serve an alternate airport.

Satellites. The two satellites now used for WAAS don't belong to the government. They're owned by Inmarsat, and the FAA leases transponders on them for broadcasting WAAS signals. For reliability, redundancy, better signals, and more LPV and GLS approaches, a third satellite will be necessary. That satellite is now scheduled for launch in 2006, and is the product of an industry (Lockheed-Martin, Boeing, and Raytheon) partnership with the FAA.

The avionics angle

You'll need a WAAS-capable GPS receiver to play in the system of the future, so GPS manufacturers are vital elements. While all manufacturers sign on with the WAAS idea, at this writing only two manufacturers have built WAAS-ready panel-mount GPSs: UPS Aviation Technologies (UPSAT) of Salem, Oregon; and Chelton Flight Systems of Boise, Idaho.

This past June 27, UPSAT received LNAV certification of its new CNX80 GPS/WAAS nav/com. LNAV/VNAV certification was to have followed in August, and LPV approval is set for sometime this fall. Buy this $12,000 box and you're ready to shoot WAAS approaches as they're published. As this article goes to press, Free Flight — the GPS system behind Chelton Flight Systems' FlightLogic 3D synthetic vision system — had only received certification for LNAV approaches.

Garmin, Honeywell Bendix/King, and Rockwell-Collins are taking more of a wait-and-see attitude. However, as this issue went to press, Garmin announced its agreement to purchase UPSAT, giving Garmin access to UPSAT's WAAS technology inroads. The general opinion among the remaining companies is that the initial operating phase promises little benefit to customers because so few approaches will be certified, and that the really big benefits — LPV and GLS approaches — will be longer in coming. By this time next year, however, all of these companies plan to offer their own WAAS receiver variants or provide WAAS hardware and software upgrades to existing products.

To buy or not to buy

Let there be no doubt, WAAS is the future standard of air navigation. Refinements and schedules may be unclear, but sooner or later, the system will be implemented and the phase-out of many (but not all) VORs, VOR-DMEs, and NDBs will begin. As for the upgrade decision, that's strictly up to you. Those with the need to be at the cutting edge of technology — or the need to fly instrument approaches to airports previously off-limits in bad weather — should seriously think about buying into WAAS.

For the more conservative, it's reassuring to know that terrain-based navaids will be around for quite a while, and that ILS will continue to serve as the gold standard of instrument arrivals for some time to come. So, yes, you will be able to make full use of the airspace system without WAAS, probably for years.

On the other hand, technology continues to sprint forward. In a few months, today's cutting edge may devolve into pitiable anachronism. Displays will grow in size; sprout more features like synthetic vision, highway-in-the-sky navigation cues, and terrain-and traffic-avoidance; and incorporate primary flight and engine instrumentation. How long can you wait until the right nexus of cost and benefit transforms you into a buyer?

Remember, 15 short years ago GPS was the sole province of the military and used for secret maneuvers. Now GPSs are sold to the masses at Wal-Mart, and automobile GPSs maneuver us to the nearest restaurant.

E-mail the author at [email protected].


AOPA is 100 percent behind the WAAS initiative, has participated in the decision-making processes surrounding the program, and views it as one of the most important safety advances in recent years. In order to gain more first-hand experience with WAAS, AOPA is scheduled to fly its Beechcraft A36 Bonanza and Piper Archer with UPSAT's CNX80s. Our time spent flying in the system — and shooting LNAV/VNAV approaches — will help us understand WAAS better, and help us pass information along to you, our members. With the approval of LNAV, LNAV/VNAV, and LPV approaches to Frederick Municipal Airport, our familiarity with the system will extend to nearly all our staff pilots.

AOPA's main concerns have to do with increasing WAAS signal availability and reliability, and with ensuring that the transition to WAAS will be smooth and gradual enough to allow owners to upgrade at their own pace. In the meantime, our experts in AOPA's Government and Technical Affairs Division are pushing to improve the GPS nonprecision approaches that already exist — by urging more step-down fixes and by urging the FAA to approve more offset (i.e. non-straight-in) GPS approaches to increase capacity at more airports.

"To see the WAAS system turned on at 12:01 a.m. on July 10 was quite a once-in-a-lifetime thrill, worth the hour or two of lost sleep!" said Randy Kenagy, AOPA's senior director of advanced technology. "AOPA appreciates the work of the FAA, Raytheon, MITRE/CAASD, and an impressive group of satellite navigation industry experts on a WAAS performance panel and an independent review board. They persisted through the most trying of times to ensure that the safety benefits of satellite navigation are enabled for general aviation. To sleep through that minute in time would have been a slap in their faces. The dream of vertical guidance on nearly every instrument approach we fly is one giant step closer to reality."

We also solicit your input. You can e-mail us with your WAAS observations, opinions, and experience ( [email protected]). — TAH

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.