Aviation Safety Letter 1/2004

Global Positioning System/Wide Area Augmentation System
(GPS/WAAS) - Satellite Navigation (SatNav) Phase 2

by Ross Bowie, Director, ANS Service Design, NAV CANADA

The diagram illustrates WAAS system elements and data flow.
The above diagram illustrates WAAS system elements and data flow.

Canadian pilots have been using GPS since the early 1990s as an aid to visual flight rules (VFR) navigation and for IFR en route, terminal and non-precision approach operations. For the IFR pilot, the ability to go direct saves time and fuel, and RNAV (GPS) approaches often mean lower minima. These approaches also have safety benefits, because they can be aligned with the runway, eliminating the need to fly circling procedures in low visibility. The accuracy of GPS also means that the runway will be straight ahead, reducing the need for visual manoeuvring to line up and land.

With the advent of the WAAS, we are entering a new phase that promises even better approaches. The U.S. Federal Aviation Administration (FAA) commissioned WAAS on July 10, 2003, and WAAS signal coverage extends into Canada. NAV CANADA is working with the FAA to expand WAAS coverage, and is working with Transport Canada on the regulatory aspects of WAAS operations in Canada.

WAAS uses a network of reference stations that monitor GPS satellite signals and send data to a master station, which creates a WAAS message containing corrections and integrity data. The WAAS message is uplinked to geostationary (GEO) satellites orbiting over the equator, which broadcast the message over a hemisphere.

Aircraft WAAS receivers apply the WAAS message to the data from GPS satellites, resulting in horizontal and vertical accuracy that is usually better than 2 m. Even more importantly, the integrity portion of the message provides assurance that the aircraft will not be misled by a faulty satellite signal. The end result is a high availability of en route, terminal and approach guidance.

Like GPS, WAAS supports non-precision approaches, but it also supports approaches with vertical guidance to decision altitudes as low as 250 ft above ground. This new level of service is termed "LPV" by the FAA because the lateral guidance is as accurate as an instrument landing system (ILS)-based precision approach, and because it provides vertical guidance (Lateral Precision, Vertical guidance). LPV approaches will mean lower decision altitudes, therefore higher airport usability at many sites. Accident records and safety analysis, based on many years of experience with ILS, show that approaches with vertical guidance have a significantly better safety record than non-precision approaches. Why? Vertical guidance translates into a stabilized descent to a decision altitude. The decision to land or start a missed approach is therefore made at a specific point. There have been numerous non-precision approach accidents associated with late decisions to attempt a landing, resulting in excessive descent rates and often excessive airspeeds. If the pilot has the runway environment in sight at the decision altitude, descent can continue without any change in airspeed, flap or landing gear position, and this reduces the probability of striking obstacles or terrain before reaching the runway, landing short or landing long or fast and running off the end (or the side).

The strategy with SatNav has always been to move ahead in stages, providing more capability with each advance in technology. The use of GPS has expanded since it was first approved for IFR flight in Canada in 1993, and now we are moving into the WAAS era, with new operational and safety benefits.

Delivering WAAS benefits requires NAV CANADA to: validate WAAS coverage and performance in Canada against international performance standards; develop a NOTAM system for WAAS-based operations, based on continuous monitoring of system status and modelling of the resulting performance levels; arrange for the precise airport surveys required to support approach procedures; adopt FAA LPV approach design standards, training staff and producing approach charts; continue working with the FAA on fielding WAAS stations in Canada; and, develop flight check requirements for WAAS approaches. This will all be co-ordinated with Transport Canada to ensure that the appropriate regulations are in place. It is expected that the first WAAS approach procedures could be published in 2005. Readers of the ASL will be kept up to date on progress and on the safe use of GPS and WAAS.

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