Landings find common ground
Landings find common ground
- By William Jackson
- Nov 02, 2001
The Air Force's JPALS satellite landing system guided this Boeing 727 to six fully automatic landings at Hollomon Air Force Base, El Paso, Texas.
The Air Force and the Federal Aviation Administration are testing separate but interoperable satellite-based landing guidance systems based on the Global Positioning System.
The Air Force's Electronic Systems Center at Hansom Air Force Base, Mass., completed initial flight testing for its Joint Precision Approach and Landing System in August. The tests, conducted at Hollomon Air Force Base, N.M., used Raytheon Co. equipment and an aircraft fitted with both military and civil avionics packages.
The Air Force is several years away from deploying the system, said Col. Bud Vazquez, the program director.
'The plan now is about the end of the decade,' Vazquez said. 'We're not ready to go into production by any means.'
FAA plans to award a development and production contract for its Local Area Augmentation System in April, with initial implementation in 2003. The agency has been working with Raytheon, Honeywell International Inc. of Morristown, N.J., and Airsys Air Traffic Management Inc., a European company, to develop LAAS prototypes.Same blood type
JPALS and LAAS make possible instrument landings using GPS to replace old radar aids.
Both projects began in 1999 with interoperability as a goal. 'They are very similar at the heart,' Vazquez said. The main difference between them is that JPALS is designed for rapid deployment in remote areas and resistance to jamming.
'The military goes in harm's way,' said Eric Lekberg of the Air Force's Global Air Traffic Operations-Mobility Command and Control Program Office. 'We need a system that can provide safe landings in a GPS-jamming environment.'
Both systems correct GPS signals to guide aircraft to a touchdown in limited visibility, where signals straight from GPS satellites are not accurate enough.
JPAL and LAAS ground stations compare their known position against the GPS signal, figure the differential correction and send it to an approaching aircraft over a VHF data link.Doing the math
The aircraft applies the differential to correct its GPS position. Onboard equipment plots the landing approach. Each ground station can provide usable correction data to aircraft as far away as 20 miles.
'There is radar technology that does this,' said Bruce Solomon, Raytheon's program manager for air traffic management, but military and civilian radar systems are not interoperable. Each requires a ground station for each runway and can handle only straight flight paths.
Differential GPS needs only one ground station for an entire airport and can figure curving flight paths so that aircraft can avoid obstructions, restricted airspace and noise abatement areas. Satellite signals also work over difficult terrain where radar is not effective.
Both military and civilian differential GPS signals can work with existing autopilot equipment for hands-off landings.
FAA's LAAS will have a similar Wide Area Augmentation System for satellite navigation between airports. The agency formed a partnership with commercial developers in 1999 for prototype development.
Prototype Category I LAAS systems, designed for a ceiling no lower than 200 feet and visibility down to half a mile, are in use at Chicago's O'Hare and Midway airports and at airports in Cedar Rapids, Iowa; Memphis, Tenn.; and Salt Lake City.
They have handled more than 240 approaches by an FAA Boeing 727 and a Falcon 20 jet owned by the National Research Council of Canada.
FAA plans to deploy Category I LAAS in fiscal 2003. Category II and III systems for more restricted visibility are expected to go into production three years later.
Current JPALS tests have been restricted to Category I approaches, but accuracy is good enough for Category II and III landings, according to Raytheon.
In August, JPALS landed an Air Force C-12J aircraft with military instrumentation and a Federal Express Boeing 727-200 with a civilian package. The Boeing made 16 approaches and six fully automatic landings.
The Air Force is analyzing the results.
William Jackson is a Maryland-based freelance writer.