Houston- we have a system! More accurately- America has a system – a Wide-Area Augmentation System (WAAS). In Europe- the parallel system goes by the acronym EGNOS. WAAS (on-line and operationally complete for nearly eight years) needed the industry and regulators to catch up with its potential. With accuracy levels to within a few feet- GPS enhanced – or “augmented” – by the more-precise position data WAAS provides underpins virtually every element of the much-debated Next Generation Air Traffic

Dave Higdon  |  01st April 2011
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Dave Higdon
Dave Higdon

Dave Higdon is a highly respected, NBAA Gold Wing award-winning aviation journalist who has covered all...

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Air navigation has never been more accurate.

Houston- we have a system! More accurately- America has a system – a Wide-Area Augmentation System (WAAS). In Europe- the parallel system goes by the acronym EGNOS. WAAS (on-line and operationally complete for nearly eight years) needed the industry and regulators to catch up with its potential. With accuracy levels to within a few feet- GPS enhanced – or “augmented” – by the more-precise position data WAAS provides underpins virtually every element of the much-debated Next Generation Air Traffic Management system (NextGen).

With a growing list of approved receivers available- more operators embracing the technology- and a regulatory agency enabling WAAS GPS-based approaches at a rapid rate- it’s quickly growing in use- and increasing its value to operations from personal to corporate- commercial and military. Better still- there’s more to come.

People who need accurate position information long ago embraced the wonders of the Global Positioning System – GPS. Thanks to an orbital network of 28 satellites- users enjoy access to anywhere-on-the-planet navigation capability. Whether civil- commercial or military- GPS has revolutionized how we keep track of our movements on and over the planet.

Aviators- in particular- quickly embraced the potential- spending millions on compact hand-held navigators with rudimentary moving-map displays to augment the more-expensive- equally sophisticated navigation systems in their aircraft – up to (and including) inertial and Loran C. Long before the first panel-mounted GPS navigators appeared- the pocket-size units helped free pilots from the rigid structure of airways and jet-ways – once- that is- air traffic controllers learned and embraced the benefits of the GPS.

Of significant potential- and even greater interest: using GPS alone to define all-new- ground-equipment-independent Instrument Approach Procedures (IAPs). In the early 1990s the first GPS navigators approved for use under Instrument Flight Rules (IFR) reached the cockpits of General Aviation.

Initial units supported IFR use only en route; approach-capable units followed quickly- and the flying community signed on in huge numbers. But as good as the satellite network and the receivers became- issues inherent to the technology prevented GPS navigators from determining their position to an accuracy level needed for instrument approach use on anything more than a non-precision level.

The pilot community- the avionics makers and the FAA embraced GPS for what it was- leading to the creation of thousands of non-precision instrument approaches. Many of those new GPS approaches were all-new- stand-alone procedures; most- though- merely overlaid existing non-precision approaches that used ground navigation aids (VORs- the intersection of multiple VOR radials and Non-Directional Beacons (NDB)).

Nevertheless- those overlay approaches often improved the pilot’s options with minimums 100 to 200 feet lower than the original. But- these gains left aviation far short of the full promise of GPS: namely stand-alone- point-in-space approaches to every runway end to minimums equal to the Gold Standard Instrument Landing System (ILS) – with nothing more than suitable approach lighting fulfilling ground-equipment requirements.

As good as IFR GPS became- its accuracy levels remained well short of what regulatory authorities wanted to support precision approaches. The problem: mostly- signal interference and data lags – or delays – occurring in the milliseconds it takes for the satellite signals to travel 22-000 miles from space to the receivers in the cockpits; and timing problems from the time the receiver took to process the signal and deliver a position.

The lag amounts to only fractions of a second- but the compounding of the errors in the system result in errors in the position reports- sometimes by as much as 50 meters. Fifty meters doesn’t mean a lot to an aircraft flying the en route segment- but it’s grossly inadequate as a reference for a zero-visibility approach to a 15-meter-wide runway under an 80-meter ceiling.

The solution: a second- corrected signal- delivered by separate satellites for use in specific areas defined by a network of ground stations.

It works via a network of 38 ground stations that measure those variations in the GPS satellites' signals. Twenty of those stations are in the Lower 48 states- seven in Alaska- one in Hawaii- one in Puerto Rico- plus four in Canada and five in Mexico. Each station knows its exact location thanks to precision surveying to precisely pinpoint each one.

Measurements from these reference stations routed through master stations which develop and relay Deviation Correction (DC) to two- soon three- geostationary WAAS satellites at intervals of under five. The WAAS satellites then broadcast the correction messages for the use of WAAS-enabled GPS receivers- which apply the corrections to the positions they compute from the main GPS constellation.

The result is accuracy improved to as good as three meters in some instances. That’s more than accurate enough to use for RNP 1.0 to RNP 0.1 navigation (Required Navigation Performance). That RNP 0.1 authorization requires demonstrated accuracy to one-tenth of a nautical mile; that’s 600 feet – during arrivals and departures using often-complex curving routes. The standard for accuracy of WAAS GPS over a 24-hour period is 6 meters – barely 20 feet.

According to the latest update from the FAA- nearly 2-400 new WAAS-enabled Lateral Precision with Vertical Guidance (LPV) approaches were operational and available as of February 11. This approach is just one of the benefits of WAAS.

Similar to an ILS in form and function- the LPV approach exists independent of any specific equipment at the effected runway – save appropriate lighting. LPV approaches generally employ ceilings of less than 100 meters while looking and working like an ILS on the cockpit display. LPV 200 is available with the appropriate ground infrastructure of lights and markings – similar in appearance and function to the same equipment for an ILS- but without the ILS.

One FAA insider reports that an LPV approach costs only about $50-000 per runway end- “less if we do both ends of the same runway- since we can use much of the same survey and profile data twice.” In the case of an ILS- you can factor on around $1.5 million- with $2 million nominal after site preparation and installation.

WAAS has also opened the way to other approaches- with nearly 5-000 LNAV procedures- more than 2-300 LNAV/VNAV approaches in addition to the nearly 2-400 LPV and more than 400 LPV200 approaches. Meanwhile- RNAV procedures are contributing to the decline in stand-alone GPS approaches - which is down to about 400. Nor can we forget the headlining role WAAS GPS plays in NextGen. Under rules enacted by the FAA last year- operators who want to fly in the instrument system in most places- or access the busiest terminal areas must install ADS-B in their aircraft. Automatic Dependent Surveillance-Broadcast Out makes each individual airplane the reporting source controllers use to monitor and route air traffic.

ADS-B Out does this by linking a WAAS GPS navigator to a transmitter which automatically broadcasts the navigator-derived position- speed- altitude and flight direction; a network of more than 330 ground stations receives the ADS-B broadcasts and routes the information to the controller- as well as any aircraft equipped with ADS-B In capability (the ability to receive ADS-B Out directly or relayed).

If the FAA’s plan holds- by 2020 the entire U.S. will be linked by the ground stations providing seamless- affirmative coverage – even in areas that defied Radar’s ability to track aircraft.

As you can see- GPS- WAAS and the entire system face natural challenges to system integrity and utility. WAAS helps GPS perform at a higher level – as long as WAAS is consistent and available. But recently the WAAS satellite constellation suffered with the temporary loss of one of the two satellites needed.

Solar activity brought down the signal of the westernmost satellite and temporarily made it unresponsive to ground control. As such- it not only shut down WAAS error-correction transmissions- but it drifted freely more than 15 degrees east of its needed location. As a result- some communities in Alaska faced periodic- mostly predictable WAAS-service outages.

Fortunately- controllers re-established contact and control of that satellite; it resumed broadcasting useable corrections earlier this year and was en route to its optimal location as of the end of February.

A third WAAS satellite joined the network and- as of this writing- was completing trails before going on-line. But other threats do exist to the integrity of GPS in general- and the use of WAAS GPS in particular for its precision mission.

A new company in the U.S. has applied for approval to install 40-000 ground sites to service the new- so-called 4G broadband access system- and the 4G spectrum’s position (adjacent to the GPS frequencies) has generated a storm of opposition from WAAS GPS users across the board. According to the GPS industry- the power of these signals can overwhelm aGPS navigator’s ability to “hear” the satellite signals. Such interference – possible at distances as far as 15 miles from the 4G ground stations – would render the GPS accuracy unusable.

“Imagine”- one avionics company engineer explained- “that you’re on final to an approach with a ceiling reported at 300 feet and visibility of a half-mile- when the GPS suddenly alerts you that your very expensive navigation box is unable to guide you in.” He noted that this is not necessarily the end of the world if it happens before you’re down to 350 feet- “but what if you’re near the break-out point and you lose even the ability to make the missed-approach point with the GPS?”

Of course- the smart pilot breaks off the approach- calls ATC- climbs and takes some time to remedy the situation. “If that airport’s only viable approach is an LPV - if there’s no ILS - at 300 and a half- you’re going to the alternative destination.” (That’s presuming you successfully terminated the approach.)

“Now they’re using WAAS to underpin ADS-B and NextGen-” the engineer continued. “How is that going to work with 40-000 ground stations screwing-up your signal every few miles- and controllers wondering about the drop-outs?”

This is only one issue – and benign on the part of the potential offenders. Between so-called “spoofing” and “jamming” of GPS signals- serious concerns exist about the wisdom of depending solely on GPS for the nation’s air navigation and air-traffic surveillance.

Calls remain for a second- alternative- such as E-Loran- or Enhanced Loran C. Not only is the system capable of delivering comparable accuracy- the long-wave signal of Loran broadcasts is largely immune to jamming and spoofing.

As this report came together news came from Europe that the European Geostationary Navigation Overlay Service (EGNOS) was available for “safety-of-life” use. EGNOS employs three geostationary satellites working with an interconnected network of 40 ground stations and four control centers in Europe to produce accuracy levels on a par with WAAS.

And like WAAS- EGNOS supports precision GPS-only approaches and highly accurate direct routing across Europe. To qualify for safety-of-life service using EGNOS- aircraft must be equipped with an EGNOS-enabled GPS receiver- while the airport needs EGNOS-specific IAPs.

The first EGNOS-enabled approach was recently published for France’s Pau Pyrénées Airport – an approach successfully flown by a WAAS-equipped Falcon 900LX to mark the occasion. Thanks to a great deal of transatlantic cooperation and collaboration- U.S.- Canadian and Mexican operators sporting WAAS GPS in their aircraft are already equipped to use Europe’s EGNOS system and they are interoperable- so that EGNOS receivers will work with WAAS.

Consider how far air navigation has come since World War II: The birth of Loran helped sea-faring forces to navigate the North Atlantic even when no Sun- Moon or stars were visible for taxing a fix using a sextant- a watch- geometry and some charts.

Loran didn’t originally gain a foothold in aviation because the boxes were large and unwieldy- and the process of deciphering its signals didn’t lend itself to the speed of an aircraft. Than came the 1980s- when the microprocessor revolution brought the development of the first compact- aircraft-size units; no sooner did Loran C catch on- though- GPS came along to steal its thunder.

As marvelous as the GPS revolution was- it failed to meet aviation’s precision standard. Nevertheless- in the 20 years since GPS has become ubiquitous- in our airplanes- as well as our cars and our cell phones – and supremely accurate thanks to WAAS. Now with WAAS GPS available- more airports have more bad-weather options than ever before. Pilots can fly hundreds- even thousands of miles direct and never waver more than a few hundred feet from their optimal course- saving time- fuel and frustration.

WAAS- at last- is delivering on its potential. Now- we need to assure its stability- integrity and availability- by looking for a back-up.


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