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You know the old philosophical question: Which came first- the chicken or the egg? For many a community airport- that question takes the form of this variation: Which came first: The ILS or the traffic?

That dilemma stops cold many a growing community savvy enough to know that an ILS at their hometown airport would help the community compete for new business- business increasingly in search of locations off the beaten path. However- without the traffic to qualify- no federal support for an ILS; without the ILS- little chance of attracting the traffic.

What’s the big deal? Well- the big deal is- in an increasingly global- mobile business environment- airport access attracts companies with corporate aircraft. Those companies seek the relative reliability of airports equipped with an ILS.

Thanks- though- to the expanding abilities of a new GPS-enhancement system known as WAAS- the day is quickly approaching when ILS-level approaches will be available to airports that could never hope to land an ILS.

The Wide Area Augmentation System already in use offers a degree of accuracy and reliability that translates into ILS-comparable approach capabilities – without the investment and recurring expense of the ground-based ILS we know and love today.

Today versus Tomorrow:

For decades the venerable Instrument Landing System served as Gold Standard for airport access when ceiling and visibility levels were the worst.

At some airports the ILS allows Category I approaches to ceilings as low as 200 feet and runway visibility as short as half a mile. No wonder savvy airport operators put an ILS at the top of their wish lists – and corporation planners seek out business locations near airports with plenty of runway and an ILS to maintain access for the greatest percentage of the year. But two major roadblocks keep an ILS as little more than a dream for most general aviation airports- even ones with runways long enough for most business jets to use.

Obstacle one is money. The costs of an ILS- about $1.5 million installed and commissioned- is out of reach to many airport owners- particularly without federal funding paying 80 percent or more.

Obstacle two: FAA support. The FAA poses strict traffic-based requirements for an airport to qualify for getting on the list.

In addition to the above hurdles- the community generally also faces the recurring costs of ILS maintenance and calibration – upwards of $80-000 per year.

Barring a cash surplus unheard of by most airport operators or intervention by a friendly member of Congress- an ILS remains an illusive dream for many airports that with an ILS could compete for new businesses to move into their communities.

The ability to clear the financial and traffic hurdles is still no guarantee that an ILS can be in an airport’s future. Physical site considerations can- at best- add another six figures to installation costs or- at worst- preclude installation regardless of funds and traffic.

So for some communities- no amount of money or demand can make up for the geography challenge – just ask the folks at Aspen. Enter the wonderful world of WAAS.

Start with 24 GPS Satellites…

For more than a decade aviators across the flight spectrum have come to rely on the high accuracy of the Global Positioning System for navigating en route. Thanks to a constellation of 24 GPS satellites- accuracy of 20 to 50 meters is possible in conditions and at locations where ground-based systems can’t compete – and at costs for the airborne systems far below Inertial Navigation gear.

Yet at 20 meters – about 66 feet – the accuracy of GPS is good enough only for non-precision approaches. That means the best conditions for some NPA procedures may be under ceilings of about 400 feet (or higher) with visibility requirements of a mile to three miles – a long way from 200 and a half miles.

The errors come through timing differences caused by atmospheric conditions and those errors can be detected and corrected with the use of a receiver that knows its exact location on the Earth’s surface.

What the Wide Area Augmentation System does is add 25 of those ground-installed reference stations to correct errors in the GPS signals from the 24 orbiting satellites. Those corrections are sent skyward by three of the ground stations to one of two additional GPS satellites in geostationary orbits above North America.

In turn- those two geostationary satellites broadcast the signal correction on a special channel that allows a WAAS-enabled GPS receiver to know its location to within 1.5 meters of vertical accuracy and a tiny one-meter laterally. Now we’re within the realm of precision necessary for approaches with ILS-level accuracy.

The airborne receiver needs no additional input from any other ground or orbiting source- no airport-based unit- no nothing. A fully functional WAAS-certified navigator doesn’t even need an independent barometric pressure input. Using only the GPS and WAAS correction input- the receiver knows its altitude above mean sea level to within 1.5 meters – less than five feet.

The lateral accuracy of about 39 inches means accuracy greater than the width of the typical business jet cockpit!

Practical limitations shrinking

Think of the potential for a moment. Thousands of general aviation airports with runway lengths suitable for use by a light jet – or larger – with instrument-approach capabilities to both runway ends equal to a Category I ILS approach.

The FAA turned on WAAS for en route IFR use in July 2003 and many aircraft operators were immediately able to obtain upgrades to their IFR GPS navigators to use the higher-accuracy signal for cross-country flights.

However- it wasn’t until last fall that the first WAAS-enabled Instrument Approaches became available. Currently- the FAA is hoping to add as many as 500 WAAS-based approaches per year- many of them designed to replace existing non-precision GPS and ground-oriented approaches. Don’t expect one of these approaches to show up everywhere fast. But over the next five to 10 years- hundreds of new runways will receive the benefits of a WAAS approach.

These approaches come in three categories- of which only two are currently being served. The LNAV approach is what’s been in use for about a decade and serves as the basis for all those non-precision GPS approaches already certified.

LNAV/VNAV approaches are new approaches with accuracy capabilities approaching the ILS. Currently- most are being written for this category to minimums lower than non-precision approaches.

LPV Approaches promise the ILS-degree of accuracy- complete with a funnelling effect that improves pilots situational awareness and receiver accuracy as the aircraft draws closer to the runway. But even the middle-level LNAV/VNAV approaches largely match the ILS minimums at some airports – and are expected to earn lower minimums as experience and time are gained.

So if approaches are one limitation- what else stands in the way?

Receiver availability is the next limitation. Currently- only Garmin’s GNS 480 is approved for stand-alone WAAS approaches in all existing categories. Chelton Flight System’s FlightLogic EFIS system is close- but still requires an independent baro-pressure input – though the company expects to correct that around mid-year. Honeywell’s upcoming APEX EFIS panel- due out this summer- is set to have full WAAS capability too.

Best of all for the pilot- using a WAAS approach is as simple as selecting the approach from a menu in the navigator- activating the approach and then following the airplane icon on the navigator screen and the same two needles used by an ILS receiver.

Better still- that icon will track the aircraft through the entire approach – something no ILS needles alone could ever do. So follow the moving airplane- keep the needles centered on runway centerline and the glide path and the runway will emerge exactly where it’s supposed to be.

Furthermore- what could be better for the busy corporate aircraft crowd than the ability to get into more airports in a wider variety of conditions than would ever be possible by ILS alone?

For a community that values expanded utility- GPS/WAAS promises more than any existing navigation aid to more places for more aircraft. We only have to be patient enough for the FAA to make enough approaches – and for the right opportunity to tell the boss it’s time for a new box in the panel.


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