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Saturday, August 1, 2009

Technology Today: GBAS

By Chris Baur

Ground Based Augmentation Systems

What good is a technology if it only has one name? Ground based augmentation systems (GBAS) is one such technology that is part of the FAA’s NextGen roadmap. It resides within a category known as LAAS (local area augmentation system). Its purpose is to supply a differential correction or augmentation to the basic GPS or GNSS, while providing a significant improvement in accuracy. Instrument approaches flown with GBAS are called GLS approaches, otherwise known as global navigation satellite landing system. You may have also heard of WAAS (wide area augmentation system) and want to know what the distinction between them is. Are you wondering why we even need augmentation for GPS/GNSS?

A previous article discussed FAA’s Next-Gen roadmap relating to performance-based navigation (June 2009, page 60). We looked at required navigation performance (RNP) as a "game changer," as it provides greater access, increased safety, and reductions in both environmental impact and fuel consumption. RNP has certain accuracy limitations that are based on "raw" GPS/GNSS data from an aging constellation of satellites. Unlike a monitored ILS navigation aid, GPS approaches have no means of providing a real-time alarm, flag or alert for signal loss, interference, or satellite clock drift. This is where LAAS and WAAS come in to provide the alerting and correction for these potential sources of error, and providing lower instrument minimums than raw GPS alone.

Each sector of our industry incubates and shares different technologies, techniques and procedures. While RNP began with the airlines, it has quickly spread to corporate aviation and the military. Like RNP, GBAS also has its origins in the airlines, and now this technology is spreading. The military version of GBAS is called joint precision approach landing system (JPALS), which will allow the military to use GPS/GNSS augmentation to perform precision landings anywhere and at any time. The U.S. military requires the capability at airfields, aboard ships or temporary ad-hoc locations. Many former Army pilots can probably recall the "TAC NDB" approaches. Imagine possessing the all weather capability to operate Category I instrument approaches day or night, even when surrounded by terrain.

While both LAAS and WAAS provide signal augmentation, each system uses a different method to accomplish this task, and ultimately performs a different service for the user. WAAS utilizes a network of ground-based reference stations to detect minute variations in the GPS/GNSS signal. This information is routed to master stations and ultimately geostationary WAAS satellites where the correction is broadcast to WAAS-enabled receivers.

In contrast, LAAS is focused on providing signal augmentation at a single location. GBAS provides the correction with a sole VHF signal, broadcast on or near the local airport site. In comparison to WAAS, GBAS can provide an accuracy of one meter or better and provides auto-land capability for Category I/II/III. Combined with RNP, GBAS can support curved path multi mode approaches. These curved path approaches can be flown in a fusion of RNP and GBAS, with RNP supporting the curved path portion of the approach while GBAS provides the necessary accuracy to support Category I/II/III minimums.

Pilots flying a GBAS approach will require minimal training, since GLS approaches appear the same as legacy ILS approaches. Unlike a costly ILS system, one GBAS station can support 26 runway independent approaches, supplied in a digital broadcast on a single VHF frequency. Without RNP, GBAS can also provide curved path approaches and missed approaches as another feature called terminal area path, or TAPs. Since this technology is runway independent, it can be installed above ground, even on the roof of a building. GBAS stations are installed and operational in Sydney, Australia, Malaga, Spain, Frankfurt and Bremen, Germany, Guam, and later this year, Newark, N.J. In Australia, GBAS-equipped operators such as Capt. Alex Passerini are enthusiastic about the results in their Boeing 737 and Airbus A380 fleets, and see much potential for GBAS helicopter operations as well. A single GBAS station can provide a multitude of approaches to heliports, offshore oil platforms, and independent precision helicopter approaches at larger airports. This will provide unprecedented all-weather access in congested urban airspace, hospitals, conventional heliports and challenging locations with terrain or obstacle constraints. While the GBAS technology began with airlines, the potential for helicopters is huge.

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