Was Ist WAAS?

WAAS: No, it’s not a name from the Netherlands; it’s an acronym — it stands for Wide Area Augmentation System and, simply put, it’s GPS on steroids. At this point the general aviation community has long since become acquainted with ADFs, VOR receivers, and DME. Most have at least seen, if not used, LORAN equipment. Most pilots, (even non-instrument rated pilots) know what an ILS precision approach is, and what it can do for the pilot. In addition during the last few years, the vast majority of GA pilots have been introduced to GPS. Just a decade ago, the accuracy afforded by these now very affordable receivers would have been considered nothing short of astounding.

What You’re Not Using
All the equipment is in place, and operational (and has been for three years, though not fully). The promises to the civil aviation community are nothing short of revolutionary. Some are already using it, but according to a recent iPilot poll, those who are represent about 14 percent of us. In addition to providing a much more accurate en route signal, more direct flights, simplified on-board equipment, and eliminating many costs associated with maintaining a large and diverse system of ground-based navaids, it will provide a tremendous increase in the number of precision (or near-precision) approaches, and represents another big step towards a more seamless ‘free flight’ NAS environment. Here’s the lowdown:

How it works: Unlike most ground-based navaids, WAAS equipment covers a much wider service area. About two dozen ‘ground reference stations’ cover the entire US. (A few are in eastern Canada.) These precisely surveyed stations (each one has triple-redundant and separate equipment strings) work to:

Receive GPS signals and send correction signals to one of two master stations with information on ionospheric activity and time signal corrections particular to their area. (One of these is at the FAA’s Hughes Technical Center in Atlantic City; the backup is at Stanford University.)

  • The master station uplinks correction information and an assessment of system integrity to a geostationary satellite (via an uplink station in Connecticut),
  • Which is then broadcast to airborne WAAS receivers on the same GPS ‘L1’ frequency presently in use (1575.42 MHz). An added bonus is that these ‘GEO’ satellites also become additional navigation satellites — further improving signal accuracy.

Keeping Track Of The Bugs In The Works: If you like acronyms, here’s another: Receiver Autonomous Integrity Monitoring (RAIM). Any navigation system has to have some minimum accuracy, of course. It has to be available for a very high percentage of the time. But it also has to have integrity: a way to warn the user against any losses of accuracy, whether due to solar activity or intentional signal interference.

Integrity At Work
An ILS installation
continually monitors its signal. If the localizer signal exceeds tolerances, it shuts down within six seconds (one second for a Cat III), although of course it does stay up if the problem is just the glideslope.

GPS satellite receivers without RAIM might not know something’s wrong for 30 minutes. (That’s a key feature of the ‘C129’ TSO’d GPS installations.) If atmospheric interference or poor satellite geometry (for RAIM, a minimum of five birds in good view) degrades accuracy, WAAS is designed to provide this information directly to the user … but there were some initial problems with that. On the other hand, these same initial tests found the navigation signal itself to be four times as accurate as had been expected. Raytheon, a WAAS contractor, measured a horizontal accuracy of about three meters, and a vertical accuracy about four meters. In comparison, an ILS needs to have 7.6!

Bettering GPS… What Dreams May Come
So, if you thought things got a lot better when the government turned off Selective Availability, WAAS will be better still. There’s also a plan to provide the GPS signal on a second civil frequency on the GPS ‘follow-on’ birds, which will allow significant corrections for ionospheric delays, but that satellite constellation is not expected to be operational until about 2010. (WAAS will be fully functional much sooner.) In addition, Japan and Europe are working with ICAO to develop similar compatible ‘en route through precision approach’ systems.

Even further down the pike, there’s the Local Area Augmentation System (LAAS), which will boost accuracy to sub-meter levels. With LAAS, a ground reference station at a primary airport will broadcast range corrections to all aircraft within about 25 nm, and will even allow Cat III (autoland) accuracy, as well as surface movement guidance.

What We’ve Got Now
Right now, only about 12% of the nation’s 5,300 or so public-use airports have an ILS. While it may take a few months — or a couple of years — to iron out the bugs in the ‘integrity’ part of the WAAS system, for those that meet certain lighting and obstruction clearance standards, the number of airports that will effectively gain precision approach capability will soar. And for those that don’t quite make it, 350 and 3/4 sure beats whatever VOR approach — or none — they might have, right now. (Another nice thing about WAAS, compared to an ILS, is that the CDI/HSI needles are reportedly much less wobbly than those for an ILS.)

What We’re Getting
The FAA Technical Standard Orders for WAAS antennas and receivers (C144, C145, and C146) have been worked out, and manufacturers will surely be in hurry-up mode to meet the expected demand. But just how much industry collaboration there will be, and how intuitive each manufacturer’s GPS box will be to use, remains to be seen. Today’s GPS avionics have been notorious for their variety of often non-intuitive, high-workload user interfaces. First generation C146 panel-mount WAAS receivers will probably be more expensive than their descendants, but word is that their user interfaces will be much improved, and much more consistent. At least that’s what the WAAS ‘Special Committee Working Group’ is aiming for. One can only hope.

Of Ground-based Navaids and Dinosaurs
WAAS will not be a wholesale replacement for ground-based systems anytime soon. Assuming nominal performance, implementation, and user acceptance, the latest Federal Radionavigation Plan calls for a 30% reduction in the 1,000+ VOR-type navaids, 1,060 or so ILSs and about 750 NDBs between 2008 and 2010. More would follow within the next two years, until a basic skeletal ‘backdrop’ system of a few hundred VOR/DMEs (and a few ILSs at the busiest airports) would be what our kids will inherit, beginning about a dozen years from now. At least that’s the plan. Better hope for few sunspots and zero radio-wave-terrorism.

After all, we all know what happens to the best-laid plans. Stay tuned.