Continental Pioneers GPS Precision Approaches With Installations at Newark and at Minneapolis

Continental Airlines and Honeywell marked an important milestone on the route toward availability of public-use, augmented GPS precision approaches late last month when the airline conducted its first commercial operations using a GPS sanding system at Newark, NJ and at Minneapolis.

The approaches were the culmination of years of development by Honeywell and Pelorus Navigation Systems. In 1997, Honeywell received FAA type acceptance for the SLS-2000, as it calls its GPS landing system, and early this year earned FAA commissioning of the system at Newark and Minneapolis as well as receiving an STC for the airborne portion of the system.

The approaches in September marked the first full FAA operational approval for a precision satellite landing system.

The event was both a first and a last. Honeywell has no plans to certificate additional special category I (SCAT I) approaches like the one in Newark, and will now turn its attention to developing the Local Area Augmentation System (LAAS) that will offer public-use approaches. The FAA is in the process of finalizing all of the LAAS specifications, and Honeywell and Continental both are part of the Government Industry Partnership working on the LAAS. Honeywell says the SLS-2000 will be LAAS upgradable. LAAS is ultimately designed to provide GPS precision approaches down to Category IIIa minima.

Continental, which has helped spearhead development of GPS as an approach aid, said it wanted to be part of the development of the system so it better understands the technology and how the airline can use it when public-use GPS precision approaches become widely available.

"What we gained was to prove this will do what an ILS will do," said Jeff Ariens, a project director for Continental's flight operations department. "This gives us a leg up." Ariens said the airline already is working with Boeing to spec new aircraft so they will be LAAS-compatible.

The SLS-2000 system at Newark uses three remote satellite measuring units that monitor the network of GPS satellites. By comparing their known, fixed location to the location derived from the satellites, they compute a position correction that is sent from a ground reference station to the aircraft using a VL 500 VHF datalink unit. Honeywell pegs the value of the ground equipment at about $700,000.

The ground reference station transmits three pieces of data: a correction that results in horizontal position accuracy of one to 1.5 meters (compared to up to 100-meter accuracy of the raw GPS signal); the "health" of the GPS constellation; and the points that make up the approach.

The Continental MD-80 that flew the approaches is equipped with a Honeywell/Trimble HT9100 GPS FMS. Pilots simply select the appropriate approach-Runway 22L in the case of the flight into Newark-and fly just as though they were using a normal ILS. The only difference, according to pilots who have flown the system, is that the needles are rock-solid during the approach, and the signal is received at a range of 20 to 30 miles.

Four points along the approach are transmitted to the aircraft: the threshold position; threshold crossing height; glideslope angle, and a flight path alignment point. The HT9100 essentially connects those dots to derive the final approach path. As the aircraft flies down the approach, its actual position is compared to its desired position, sending corrections to the glideslope indicator and the autopilot. Just as with a conventional ILS, the SLS-2000 is designed to mimic increased sensitivity pilots experience with an ILS as the aircraft nears the runway threshold.

The approaches at Newark and Minneapolis overlay existing ILS systems, but are capable of providing precision guidance through a complex approach that involves curves or doglegs that could be used to avoid high terrain and other obstacles, or noise-sensitive areas. Honeywell expects the first LAAS to come on line sometime in 2000, and said it believes Category II and Category III approaches will be in place sometime in 2002.

By Perry Bradley