The U.S. Wide Area Augmentation System (WAAS) has been approved for instrument approaches down to 200 feet above ground level (AGL), giving it equivalence to Cat I ILS.
In March, FAA’s GPS Wide Area Augmentation System (WAAS) reached a new level of performance. The agency announced that the system has been approved for instrument approaches down to 200 feet above ground level (AGL), equivalent to the guidance from a Cat I ILS.
Since WAAS was commissioned in 2003 as a nationwide means of reducing ionospheric accuracy errors in GPS, FAA has been actively working to develop the system’s landing guidance capabilities, by steadily incorporating improvements. Initially, lateral navigation (LNAV) procedures were introduced, followed by LNAV/VNAV–which incorporated vertical guidance–and, later, ILS-comparable localizer precision with vertical guidance (LPV) procedures. With each of these, the associated decision altitudes (DA)–the heights below which a pilot may not descend unless the runway is in sight–were progressively lowered, from 700 feet with LNAV to 400 feet with LNAV/VNAV and, subsequently, to 250 feet with LPV. The approval of Cat I-equivalent WAAS performance further lowers this to 200 feet.
The goal for WAAS engineers always has been to meet the runway approach standards of ILS Cat 1, a goal many felt was unreachable. For while WAAS-corrected GPS was always within International Civil Aviation Organization (ICAO) and FAA ILS localizer standards for lateral accuracy, the challenge has been to consistently meet ILS glideslope vertical accuracy, where WAAS GPS has previously been much less precise. But under a progressive series of system configuration releases over the past two years, WAAS software and hardware specialists have achieved significant performance improvements, using extensive simulations and data analysis, followed by actual flight tests. The simulations were accompanied by analysis of over 2.1 billion data points, recorded every second of each day since WAAS was commissioned in 2003. To confirm those results, FAA, U.S. Air Force and Eurocontrol aircraft flew 429 WAAS approaches at airports in the United States and Europe. These tests showed that, compared with the permissible ILS vertical error of 14.4 feet (+/- 4.4 m) on 95 percent of occasions, WAAS achieved less than 10.8 feet (+/- 3.3 m).
Interestingly, this was FAA’s third attempt to attain Cat I with WAAS. Upon the system’s initial launch in 1995, Cat I was promised by 1997. But this was postponed due to lengthy system teething problems, coupled with a change in contractors and, at one point, the threat of complete program cancellation. In 2004, the Cat I project was reintroduced, predicated on using accuracy-enhancing dual frequency transmissions from the Pentagon’s future GPS III. That would, however, have delayed its operational debut until 2012, and subsequent slippage of GPS III due to military priority realignments would have stretched it even further, possibly to 2015. Consequently, FAA investigated alternative WAAS enhancements, resulting in the agency’s March announcement.
Attainment of Cat I performance raises the question of the future of FAA’s Local Area Augmentation System (LAAS) program, formerly the planned satellite-based successor to ILS. Unfortunately, LAAS Cat I development has suffered from problems with signal integrity–essentially, guidance trustworthiness–and occasional transmissions of hazardously misleading information (HMI), resulting in its being relegated to research project status several years ago. LAAS would be a fairly costly system, requiring a ground station plus a monitoring and VHF station network at every airport. WAAS can now provide the same precision landing guidance, with no ground infrastructure. Nevertheless, WAAS specialists emphasize that Cat I is probably the limit of the system’s development, and that Cats II and III probably will remain the domain of ILS for the foreseeable future.
While FAA’s announcement was welcomed by general aviation and corporate users, it appeared to be ignored by the major airlines. These operators have long rejected WAAS in favor of the required navigation performance (RNP) technique, which integrates inertial navigation systems with non-WAAS GPS to achieve approach DAs of 250 feet. But since future GPS avionics will certainly include WAAS, the airlines’ views may change over time.
Approval of WAAS for Cat I guidance does not, however, mean that non-ILS runways, even those with previous GPS approaches, automatically will qualify as supporting 200-foot operations. ILS equivalence imposes ILS criteria, which are more demanding than previous GPS/WAAS safety standards keyed to higher minima. FAA specialists are now reviewing whether all ILS criteria will apply to WAAS Cat I, or whether some can be relaxed. Once this review is complete and promulgated, airport operators can assess their runways for compliance. Then qualifying runways must be prioritized and approach charts must be drawn up and flight checked. Consequently, public WAAS Cat I approaches are not expected before late 2007.
Currently Garmin’s TSO-146 panel mount GNS-480 remains alone in providing certified WAAS operations. The unit incorporates a 5-Hz "Gamma-3" signal update rate, compared with conventional Gamma-1 and -2 GPS receivers which meet the 1-Hz LNAV and LNAV/VNAV requirements. FAA has solicited shared cost development proposals from other companies.