As most of us know by now, WAAS is not a name from the Netherlands; its an acronym for the latest GPS enhancement, the Wide Area Augmentation System. The result is a more-accurate GPS signal-down to less than three meters. Think of it as GPS on steroids. 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, WAAS will provide a tremendous increase in the number of precision (or near-precision) approaches. At least thats the plan. First, lets discuss why its needed. 288 WHY WAAS IS NEEDED Normally, the Earths ionosphere has “waves” or “billows” which cause GPS signals to be refracted (slowed down) by varying amounts. Since GPS is all about timing, a slowed-down signal makes a given GPS satellite seem like it is further away than it actually is. The end result is degraded navigational accuracy for users. These billows propagate in relatively slow waves-with hours between their “crests”-and their movement can be described and predicted somewhat using signals from the ground or by deducing the information from the GPS signals themselves. Knowing this, a network of ground stations can provide the required supplementary data and transmit it up to the satellites. Except for one small problem: The constellation of GPS satellites didnt have additional channels that could be used to convey this extra set of information to the user community. Instead, the correction information is uploaded to geostationary communication satellites carrying a special WAAS package. By broadcasting this information to GPS receivers about once every two minutes, navigational accuracy is greatly improved. Unlike most ground-based navigational aids, WAAS equipment covers a much wider service area. About two dozen “ground reference stations” cover the entire U.S. These precisely surveyed stations, each of which has triple-redundant and separate equipment strings, receive GPS signals and send correction signals to one of three Wide-area Master Stations (WMS). What happens next is detailed in the sidebar “How WAAS Works.” 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, WAAS will provide a tremendous increase in the number of precision (or near-precision) approaches. At least thats the plan. First, lets discuss why its needed. A companion to WAAS, dubbed “LAAS,” for Local Area Augmentation System, is also under development. When fully operational, LAAS promises to 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. Thats the basic description of the technology behind WAAS and why the uncorrected GPS signal needs to be augmented. ILS Accuracy from gps But were not quite ready, yet, to use GPS down to ILS minima. According to the FAA, it will be next year before that can happen. The agency says that WAAS-enabled LPV procedures at precision instrument runways will be capable of supporting the same minima as provided by the Category I Instrument Landing System (ILS) in 2007. As the FAA notes, “since WAAS requires that no equipment be installed or maintained at the airport, it provides a cost-effective alternative to Category I ILS for airports.” But what will users need to benefit from the more-accurate signals and fly down to those lower minima? Two things: The aircraft needs to have an approved GPS/WAAS receiver and the airport must have an approved LPV approach to the airport. For the hardware part, the aircraft must have Technical Standard Order (TSO)-C145 or TSO-C146 Class 3 or 4 equipment installed as recommended in Advisory Circular AC 20-138A. For most of us, that means some kind of equipment upgrade, perhaps simply an add-on black box behind the panel. But the price of admission could be higher, including a complete panel overhaul depending on what GPS equipment is already installed. The first WAAS-enabled GPS approach procedures allowing Cat I minima will be published in 2007, according to the FAA. But, since the runways and airports must meet the FAAs Cat I approach infrastructure requirements-i.e., striping, lighting and other physical characteristics, plus the requirements to fly a missed approach-its likely that only runways already served by at least a straight-in localizer or a full ILS will be approved initially. The FAA notes that no specific locations for a Cat I GPS/WAAS approach have been determined yet. Meanwhile, WAAS procedures can be developed at airports not meeting the Cat I requirements. Those approaches will carry higher minima, but perhaps lower than would be the case without WAAS. Indeed, some LPV approach procedures already allow 250-foot minimum descent altitudes. These WAAS-based approaches are designed with a vertical alarm limit, or VAL, of 50 meters. The new minimums will be based on a tighter, 35-meter VAL. The need to delay until 2007 the lower, 200-foot minima is based on even more WAAS upgrades. When the latest upgrades are in place, a 35-meter VAL will be possible throughout WAAS coverage areas. Of course, once GPS/WAAS is up and running, what will happen to ILS? Again, according to the FAA, it will be relegated to “backup” status. As it has done so often in the past, the agency believes the “operational benefits of GPS/WAAS…will motivate most operators of aircraft used extensively for IFR operations to equip with GPS/WAAS in the five to six year period following the availability of services.” The agency says it believes the number of existing ground-based systems-VOR, DME and ILS-can be substantially reduced. Other Issues 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 ever be, and how intuitive each manufacturers new GPS box will be to use, we view with a jaundiced eye. So far, the records not too good in the consistency department. Todays GPS avionics have been notorious for their 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 thats what the WAAS “Special Committee Working Group” is aiming for. Changes Are Coming Presuming nominal performance, implementation and user acceptance, a recent Federal Radionavigation Plan called for a 30-percent reduction in the 1000-plus VOR type navaids, 1060 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 thats the plan. We know what happens to plans, though.
