By Adam Hunt
Jeff Cochrane, Nav Canada's manager CNS Service Design, was our guest speaker for the March 26, 2008, COPA Flight 8 meeting. Cochrane is a commercial pilot and former flight instructor who flies the new CRJ200 for Nav Canada in their flight evaluation program.
He is also one of the key company people in the implementation of WAAS, the Wide Area Augmentation System for GPS in Canada.
As part of that role he is the Canadian representative to the ICAO Navigation Systems Panel. Nav Canada's involvement in GPS and WAAS at ICAO ensures that fielded systems comply with international requirements and are compatible with other systems similar to WAAS around the world, including the European Geostationary Navigation Overlay Service (EGNOS), Japanese Multi-functional Satellite Augmentation System (MSAS) and the Indian GPS Aided Geo Augmented Navigation (GAGAN) systems under development.
Cochrane started his presentation to the Flight with background on what WAAS is. Essentially WAAS is a system that monitors the GPS signals to improve accuracy to user aircraft. Most of the errors in GPS data are caused by fluctuations in the earth's ionosphere which slows down radio signal propagation and introduces time-of-arrival (TOA) errors.
This error is measured by the receivers in the WAAS ground-based reference stations, through comparing their own surveyed location to the received location calculated from the TOA signal from the GPS satellites. A master station then compiles the corrections for all of North America and then broadcasts corrections and ranging data to receiving aircraft, through a pair of geostationary satellites
At present the operational system comprises 38 reference stations, of which four are in Canada at Gander, Goose Bay, Iqaluit and Winnipeg. These Canadian stations consist of US loaned equipment, cited and maintained by Nav Canada.
The Canadian stations were all integrated into the network in September 2007, along with five stations in Mexico.
The system also has three master stations, four ground earth stations and broadcast packages on two geostationary satellites all controlled by two operational control stations.
Currently unaugmented GPS signals are capable of accuracies of two to three metres in the horizontal and three to four metres vertically, 95% of the time.
With WAAS augmentation the accuracy can be improved and the reliability of that accuracy is greatly enhanced.
WAAS provides many advantages to Nav Canada and also to user aircraft. It is capable of an en route accuracy of 0.02 nm with the required level of integrity. This will eventually allow a reduction in ground-based nav aids.
It permits publicly available Localizer Performance with Vertical Guidance (LPV) approaches to be conducted with a one-to-two-meter accuracy and the level of integrity that is expected with ILS, all to locations with no ground nav aids.
Cochrane assured the Flight 8 members that LPV is not intended to replace ILS at those locations which already have ILS approaches. Instead it is being used to bring ILS-like accuracy to new locations.
Nav Canada is in fact, replacing older ILS ground installations with newer units. These are solid state ILS units that provide only front course guidance and lack the back course that older ILS equipment provided.
Cochrane also reviewed the equipment needed to make use of WAAS for IFR flight. To fly LPV approaches this means having a panel-mounted WAAS-equipped GPS receiver or Flight Management System (FMS). Aircraft equipped with non-WAAS GPS receivers will be limited to LNAV approaches, which lack the vertical guidance of the LPV approach.
Cochrane also gave a view into the future of WAAS and GPS. High on the list is a broadcast payload on an additional geostationary satellite to improve Canadian east coast WAAS coverage.
Dual frequency GPS is also coming, starting in 2015. This will eventually negate the need for WAAS, as it will provide its own corrections, based on data from the two different frequencies.
Support for single frequency GPS will be preserved until 2028, but after that everyone will need a new dual frequency receiver to make use of the system.
It is also planned to integrate the European Galileo and next generation GPS III to improve performance and reliability.
Currently Nav Canada is hand-designing LPV approaches for airports in Canada. The company had hoped to have an automated tool available to do the design work, but approvals from Transport Canada for the standard upon which the tool is based have not been forthcoming and this has greatly slowed availability of these approaches and thus the greatest advantages of WAAS to Canadians.
Cochrane's briefing was well received by the members of COPA Flight 8 and prompted many thoughtful questions on GPS and WAAS equipment and procedures.