Future approaches to usher in an entire new set of acronyms

 - May 29, 2008, 9:31 AM

Before GPS, approach classifications were cut and dried–they were either precision (ILS) or nonprecision approaches. But as pilots move into the future, they will need, before considering an approach into an “obstacle-rich environment,” to first navigate through an acronym-rich environment of new terminology to decide how to reach the threshold. Attendees at the FAA’s twice-yearly Satellite Operational Implementation Team (SOIT) meeting last month were briefed on a new series of approach classifications planned for use when the GPS wide-area augmentation system (WAAS) enters operational service.

For a precision approach, there will be three supporting technologies: ILS, WAAS and LAAS. WAAS is intended to be the prime, continental U.S. (Conus)-wide GPS Category I precision approach aid, with LAAS Cat I planned for around 46 locations, such as in mountainous areas, where the accuracy correction signals from the WAAS geostationary satellites may not be fully reliable. But since WAAS won’t be able to support better than Cat I approaches, the FAA also plans to colocate 114 LAAS Cat II and III systems with present ILS Cat II and III installations.

In the precision-approach application, WAAS and LAAS are described as GNSS (global navigation satellite system) landing systems (GLS). But while GLS will become a worldwide term, U.S. pilots who fly internationally will have to remember that outside North America, those WAAS and LAAS tags are translated into ICAO terminology as SBAS and GBAS, respectively, for satellite- and ground-based augmentation systems.

The FAA currently forecasts WAAS initial operational capability (IOC) for late next year, but this trouble-plagued program may slide into the following year. Today’s Lnav and Lnav/Vnav (using a baro altimeter) nonprecision approaches will continue, with the latter renamed as an approach with vertical guidance Type I (APV-I).

During the two years following IOC, extensive testing should allow the addition of Cat I GLS in locations where WAAS horizontal and vertical approach accuracy equals that of ILS. But full-time, Conus-wide WAAS Cat I GLS availability is not expected before the 2009 to 2015 timeframe, although most of the LAAS Cat I and some of the LAAS Cat II and III installations are forecast to be available well before then.

However, due to various technical factors, the WAAS vertical accuracy at certain locations may not always meet the GLS and ILS vertical alarm limits (VALs) of ±12.5 meters, but could be within a ±20-meter VAL. But WAAS is expected to provide fairly consistent ±40-meter horizontal accuracy, which means that the earlier approach obstacle clearance surfaces can now be confined to a much narrower, and parallel-sided, lateral inbound “corridor,” which can in turn reduce the number of critical obstacles on the approach path, resulting in decision heights down to around 250 ft vs the typical 200 ft for ILS/GLS. (The corridor is parallel because unlike ILS, the WAAS lateral accuracy would not worsen with distance from the threshold.) ICAO has approved this “near-Cat I” concept, which is now designated as an approach with vertical guidance Type 2 (APV-2), and it, along with APV-1, went into ICAO’s Annex 10 in November.

Unlike APV-2, which will be based on WAAS, APV-1 will remain a non-WAAS GPS procedure using current receivers with RAIM (receiver autonomous integrity monitoring). Consequently, the former Lnav/Vnav criteria will apply, with the basic GPS-only obstacle clearance “containment” limit extending to 556 meters either side of the centerline and vertical guidance provided by a baro altimeter, typically accurate to 50 ft.

But having set out the criteria for the two new APVs, FAA approach procedure specialists then noted that there was a fairly wide performance gap between them, and this has now given birth to APV-1.5. This WAAS-based approach will combine baro vertical guidance with the much narrower horizontal limits provided by WAAS, and give an intermediate, but basically better controlled and safer, general aviation approach. Detailed analyses presented at the SOIT meeting indicated that useful cumulative benefits in ceiling and visibility limits became available in the progressive steps of APV-1, APV-1.5, APV-2 and GLS.

So when can we expect to start flying APV-1.5 and APV-2? That’s the $4,000 question, to use a number sometimes mentioned as the rough price of a plain vanilla general aviation WAAS receiver. It’s a rough estimate, because avionics manufacturers seem a little shy about what such a unit is likely to cost. As well, a full blown, FMS-compatible unit is expected to cost more, and a Cat I box would be more expensive again. But their availability date looks like 2005, or maybe even later.

Finally, how long will ILS remain with us? That, too, is an interesting question. Originally, ILS was scheduled to be completely superseded by WAAS/SBAS and LAAS/GBAS both here and overseas by around 2012. But recent revelations about GPS vulnerability to various forms of interference, and the associated need for backups, has caused some observers to suggest that ILS could continue operating in North America until at least 2015 or later.

This would allow the incorporation of signals from Europe’s GPS-like Galileo satnav system, which would significantly improve GPS integrity and robustness. ILS termination in 2015 would also provide a more than adequate overlap period for holdout operators to install equipment and train their pilots, and to accommodate program slippage.

Some experts also believe ILS could remain in operation overseas until 2025 or longer, depending on the worldwide availability of WAAS equivalents such as Europe’s EGNOS and Japan’s MSAS.