John Sheridan

If ever there was a Comeback Kid in avionics, it would have to be the FAA’s wide area augmentation system (Waas). Heralded by the agency in 1994 as the future Swiss Army knife of navigation, Waas was going to bring greater accuracy and enhanced reliability to the sometimes unpredictable GPS and, in so doing, promised a new era where satellites would replace not only the nation’s NDBs and VORs, but also the more than 600 Category 1 ILS installations in the National Airspace System at the time. Development would cost more than $300 million, and take about four years.


But by 2000, Waas was hanging by a thin thread. The basic concept may have been great, but achievement was abysmal. By then on its third contractor, the program had run into unexpected problems, costs were nearing $1 billion, delivery was expected to be five years late, and Congress was threatening cancellation, with one member describing the whole program as a boondoggle.


Fast forward to mid-2013. By dint of major engineering efforts–and unspecified added investment–by prime contractor Raytheon, FAA Technical Center engineers and industry specialists, Waas has fought its way back to the top. Now vindicated, the system is providing high-accuracy approach guidance to aircraft equipped only with a GPS/Waas receiver, at airports lacking any landing aids whatsoever. Currently, the system provides more than 5,700 instrument approach procedures across the NAS, including 800 in support of ILS Category 1-like 200-foot decision heights, and with a reported 65,000 users from small general aviation aircraft to large airline and corporate jets.


Today, Waas is bringing business aviation a level of safety and reliability far superior to those of the old days of NDB and VOR step-down approaches to less well equipped airports. A not-yet-released ICAO list shows Waas certifications in virtually all models of business aircraft from Bombardier, Cessna, Dassault, de Havilland, Embraer, Gulfstream, Hawker, Israel, Piaggio and Saab. The same “all models” Waas certification is also true of Boeing and Airbus, with the exceptions of Boeing’s 787 and Airbus’s A380 (although the new Airbus A350 and the company’s earlier Beluga mother ship, which transports fuselage sections and wings between factories, are Waas equipped). We should also include McDonnell Douglas too, since Delta Air Lines–with a large number of MD-88s and MD-90s–<a href="http://www.ainonline.com/aviation-news/2013-04-10/delta-retrofit-new-cockpits-md-88s-md-90s">ordered Waas for its whole MD fleet</a> last month. And to complete the record, we should note that CMC Electronics, Garmin, Honeywell, Rockwell Collins and Universal all produce certified Waas avionics units.


Ensuring Accurate Data


So what has made the big difference between the earlier “raw” GPS and Waas, and caused the market swing toward Waas? It can be attributed primarily to the continuing engineering efforts in refining the algorithms and accuracy of the basic signals transmitted from the 30 moving satellites of the main GPS constellation. To pilots, these delicate but essential improvements present themselves mainly when a non-Waas receiver shows you somewhere on the runway, while a Waas receiver would show you within a couple of meters of the centerline.


An important step here lies in the precise monitoring of the GPS signals. Across the U.S., Canada and Mexico, high-precision GPS monitor receivers at fixed, accurately surveyed geographic locations continuously receive positions derived from the “raw” GPS signals the satellites transmit. The difference between those positions and the known surveyed location shows the local GPS position error. It’s then necessary to get those error corrections into user receivers on earth, at sea or in the air.


This is the task of the Waas satellites, three of which are in geostationary orbit above North America and which appear to us to be fixed in space, far above the regular GPS constellation orbiting the earth. A terrestrial Waas master station continuously gathers these local errors, converts them to local error corrections and beams them up to the Waas satellite, which then retransmits them down over a very wide area (hence the system’s name) to all GPS receivers in view. However, the errors measured by a monitor station in, say, Texas, won’t be the same as those measured in Alaska, due to the constantly changing geometry between individual satellites, and with them and the user, so there’s some pretty clever space geometry involved to ensure that the right error correction goes to the right area on earth. But when these corrections arrive at the user’s Waas receiver, they are automatically applied to update the user’s position, and the operator never sees it happen, since Waas is a never-ceasing process.


Inevitably, of course, Waas has brought with it terminology changes. The FAA still uses the term “Waas,” but interchangeably with SBAS, for satellite-based augmentation system. That’s because SBAS is ICAO’s generic term for similar Waas-like systems being implemented in Europe (Egnos), Russia (SCDM), India (Gagan), China (BeiDou) and Japan (M-SAS.


Remember that “Category-1-like” claim? Even though a Waas/SBAS procedure can take you down an approach slope essentially identical to the ILS, and usually to the same ceiling and visibility limits, Category 1, 2 and 3 are officially regarded as exclusive ILS descriptors, so the FAA calls the Waas/SBAS precision approaches LPVs, for localizer precision with vertical guidance. So, for example, LPV200 or LPV350 are the proper terms. And to be fair, that’s not FAA fussiness: ILS has some significant, though non safety-related, technical differences from SBAS, so precise terminology can be important.


Future Precision Approach Coming


The big terminology leap will occur when we come to GLS, the future GPS landing system. Yes, there is a future precision approach system on its way, because despite the remarkable advances in using Waas to bring GPS to ILS-equivalent Category 1 landing guidance standards virtually anywhere, Waas appears technically unlikely to be able to enhance GPS to support the more demanding accuracy and performance equivalent to ILS Categories 2 and 3, with one of its major handicaps being that Waas can’t provide an alert in less than six seconds following a failure; Categories 2 and 3 demand a one-second alert. So the agreed plan at ICAO, supported by the U.S., is to develop GPS to get to Categories 1, 2 and 3 by using a ground-based augmentation system (GBAS) instead of Waas/SBAS. (Note that there’s no unique FAA term for GBAS: for us, it’s that or nothing.)


GBAS is essentially a variant of Waas/SBAS. That is, instead of having GPS signal monitors spread SBAS-like across thousands of miles, GBAS employs three or four ground-monitor stations around each airport to measure the local “raw” GPS errors more precisely. Those errors are then converted to error corrections and passed to the approaching aircraft over a dedicated VHF datalink (currently planned to be in the ILS/VOR frequency band), and are then applied to the aircraft’s receiver. Recall that SBAS can only provide approach procedures down to LPV on specified runways. The increased precision from the local corrections will allow GBAS to provide corrections to all an airport’s runways, from Category 1 to Category IIIB, assuming a supporting runway environment, plus selectable glidepaths and, for smaller aircraft, displaced touchdown points further along the runway. This is because the GBAS datalink can carry a lot more airport-specific information than just the GPS errors, and ICAO is studying these additional data options.


Currently there are only a few GBAS installations around the world, operating primarily for system testing. However, systems at Newark and Houston are being flown in revenue service by United/former Continental 737s, as is a test installation flown by Air Berlin at Bremen, Germany, and by Qantas at Sydney and other airports in Australia, although the systems being used only support Category 1 service by approved operators. More correctly, flying GBAS to former ILS Category 1 limits is, in the new GBAS terminology, a GAST-C approach, while a Category 3 will be a GAST-D approach. GAST (for GBAS approach service type) is ICAO jargon for the next generation of landing guidance systems. And Category 2? Somewhat illogically, that’s undecided, and Category 2 could be either a GAST-E, or an advanced dual-frequency unit. And one day GAST-F, an advanced, multi-constellation, multi-frequency system having Category 3++ characteristics will arrive. GAST-A and -B, essentially less than Category 1 variants, are likely going to be pretty minor players in our future GAST-ly world.


Where does ILS stand in all this? The FAA’s policy has been to maintain Category 3 ILS installations for the foreseeable future, since there is no clear date when GBAS will be able to replace them. Consequently, there is then no clear date when Category 3 operators will be able to remove their Category 3 avionics. Category 1 ILS presents a different quandary. Should GAST-C (Category 1 GBAS) ground systems and avionics be fully approved and available in, say, 2016, would operators install it–for better Category 1 services, and for use on non-ILS runways–while retaining their Category 3 ILS receivers? And should traffic increase at the small Airport X to the point that a Category 1 or equivalent approach aid is needed, the choice becomes a Waas/SBAS LPV, or a Category 1 ILS, or a GAST-C. Even trickier is the choice of an airport’s precision approach system upgrade from Category 1 to a Category 2 or 3 service. Does an airport operator toss a coin to decide to install an expensive Category 3 ILS in the hope that most operators will continue to carry their Category 3 avionics for several more years, or does he bite the bullet and go for a much less costly GAST-D equivalent installation, on the assumption that if he builds it, they will come? And the opposite case is that of the aircraft operator’s quandary of reassuring himself that removing his ILS avionics won’t risk being unable to fly into some destination that doesn’t yet offer GBAS datalink service. ILS won’t disappear for quite a while.


Hanging over all this is the reality that even while the future GPS III will reportedly have improved interference resistance, the threat of inadvertent or deliberate interference to SBAS and GBAS appears likely to remain with us for the foreseeable future.

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