Landing Airplanes with GPS?

April 20, 2011  - By
Image: GPS World

Landing airplanes with GPS is not a new concept. People were demonstrating the feasibility back in the early 1990s. But landing transport airplanes in commercial airline service is something that’s only just getting to be a regular occurrence.

Before we go on, maybe we need a short side trip into Local Area Augmentation Systems (LAAS) and Wide Area Augmentation Systems (WAAS): both highly effective — and now both certified — means for getting aircraft “wheels on runways.” (Shucks, maybe we need even a further side trip into what constitutes “certified”…)

“Certified” for airborne things means that the aviation authorities have looked closely at how something was developed, tested, and qualified to meet the requirements set for people who make and sell systems, and the agency has decided that you did the job correctly and the system is good to use. The system could be an airplane, a switch on a panel in the galley, or an avionics system which lands the aircraft, and many other bits and pieces that help get you from the departure gate to luggage claim at your destination.

LAAS (also known as Ground-Based Augmentation System — GBAS) is a ground-based landing system which uses GPS to guide aircraft to a runway — there is a local ground installation with several antennas around the airfield, and a VHF radio uplink to approaching aircraft. It all works if the aircraft is also equipped with a VHF receiving radio and a LAAS-configured airborne GPS receiver. There’s also likely a Flight Management System (FMS) on the aircraft that enables navigation within today’s complex air-traffic-control systems. The LAAS system on the ground works high-integrity magic and provides precision GPS corrections to the aircraft GPS.

WAAS (also known as Satellite-Based Augmentation System — SBAS) also has parts which are ground based, in that there are a number of widely separated ground stations (hundreds of miles apart, across the whole country), each installed with a number of GPS receivers which, together with a ground data analysis system, assess the performance of the GPS constellation. A WAAS correction signal is then sent up to geostationary satellites which re-broadcast the correction signal to aircraft equipped with WAAS-configured GPS receivers flying within the geo signal footprint. The airborne receiver gets higher accuracy combined signals and an integrity message which regulates en-route and approach navigation.

OK, then….

Before GPS, everything landed using the Instrument Landing System (ILS) — an analog system developed as far back as World War II. GPS Localizer Performance with Vertical guidance approaches (GPS LPV) allow instrument approaches down to 200 ft height at ½ mile from the runway, which is what airborne receivers using the WAAS system are now able to achieve. As of March 10, 2011, FAA has published a total of 2,394 LPV procedures at 1,260 airports, which is more than twice the number of published ILS approaches.

It’s always been an objective for the FAA WAAS folks, that they improve the integrity of their system to allow high-precision approaches for all types of aircraft, but especially for commercial transport aircraft. For one reason or another the airframe manufacturers focused on making LAAS work on their airplanes first, so WAAS approaches and landings have been a rare and wonderful thing, and general aviation seems to have benefitted most from WAAS LPV. Until recently that is….
Canadian North, based in Yellowknife, Northern Canada, has a heck of job maintaining scheduled service in the severe weather they experience up there at the top of the world. The airline flies to a whole bunch of Northern outposts and connects remote communities with major population centers including Ottawa, Edmonton, Calgary, Winnipeg, and Halifax. During some of the extreme weather experienced from time to time, Canadian North has in the past canceled a significant portion of its scheduled service. The communities in the north absolutely rely on regular air service for survival, so anything which improves service reliability is also really good for the people living in those remote areas.

So when the team of CMC Electronics and Logic-Air Aviation Services — all in and around Montreal, Canada — came up with a solution using the CMC high-precision airborne GPS WAAS sensor, Canadian North became one happy operator. With a really simple ILS look-alike installation, Canadian North now has capability for GPS LPV guidance on one of its Boeing 737-300 aircraft, and can fly in pretty bad weather that previously held them on the ground. The installation is simple, was made without rework of other aircraft systems, and looks exactly like the ILS system the pilots flew with previously. It’s so much like the previous ILS that there was no requirement for any additional aircrew training to operate the system.

When I talked recently with Leo Eldredge, the FAA’s GNSS program manger, he told me, “We feel this is another great milestone for acceptance of WAAS by air carriers, particularly those operators with older aircraft that want an economical pathway to full RNAV capability with LPV approach procedures. We also see an opportunity with the Canadian North aircraft as a potential to demonstrate the feasibility of LPV autoland approaches.” So FAA sees this as good news, it would seem.

Fortunately, there are now several other manufacturers who also market receivers and systems approved for GPS LPV approaches for general aviation aircraft, commercial transports, and helicopters including Garmin, Universal, Free Flight, Avidyne, Rockwell Collins, and Honeywell, so the switch-over from ILS to GPS approach and landings is now well underway.

Aviation LightSquared. Except… the whole GPS infrastructure on which aviation depends in North America seems to be threatened by a decision to allow a high-powered out-of-band transmission adjacent to the protected ARNS frequencies. As anyone with basic radio frequency experience knows, high-powered transmissions tend to have side-bands which mix down or up to cross into adjacent frequency bands. In Europe, such high-power transmissions from TV towers spilling over into the ILS frequencies resulted in an international decision to pursue the Microwave Landing System, long before GPS was eventually selected as the system which aviation will use to navigate and land aircraft. For those of us who travel by air, let’s hope that common sense prevails and that the LIghtSquared transmissions are eventually allocated to alternate frequencies, where there will be no risk of impingement on such a safety-of-life critical aviation system as GPS.

Tony Murfin
GNSS Aerospace

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About the Author: Tony Murfin

Tony Murfin is managing consultant for GNSS Aerospace LLC, Florida. Murfin provides business development consulting services to companies involved in GNSS products and markets, and writes for GPS World as the OEM Professional contributing editor. Previously, Murfin worked for NovAtel Inc. in Calgary, Canada, as vice president of Business Development; for CMC Electronics in Montreal, Canada, as business development manager, product manager, software manger and software engineer; for CAE in Montreal as simulation software engineer; and for BAe in Warton, UK, as senior avionics engineer. Murfin has a B.Sc. from the University of Manchester Institute of Science and Technology in the UK, and is a UK Chartered Engineer (CEng MIET).