GPS has provided the world with an unprecedented utility to accurately navigate regardless of whether it is by foot, car, boat or aircraft. And while it is a phenomenal technology, GPS does have the necessary qualities of accuracy, integrity, availability and continuity to act as a replacement for the venerable Instrument Landing System (ILS).
ILS is the gold standard approach and landing navigation aid. This precision approach aid is used at larger airport to provide landing guidance to approach minimums of 200 ft but in some special cases, completely blind landings can be performed.
Air navigation service providers such as NAV CANADA and the Federal Aviation Administration (FAA) face the prospect of replacing thousands of ILS’s in the future. At a price tag of over $1 million each, this represents a huge capital outlay. But what if satellite navigation can be improved to provide the same qualities of accuracy, integrity, availability and continuity as the ILS? What about using satellite navigation to provide precision approach capability rather than ground based infrastructure? Both NAV CANADA and the FAA concluded that this would be desirable outcome and as a result an enhancement to GPS was developed to provide precision approach and landing capability. This initiative is know as Wide Area Augmentation System (WAAS).
Before describing how WAAS works it is important to know that WAAS is not a local area device, such as one where equipment must be installed at an airport. That is known as Local Area Augmentation System (LAAS) – this type of augmentation was the early favourite as satellite landing navigation but has since fallen off the radar with the implementation of WAAS.
How does WAAS work? A series of monitoring stations are located throughout Canada, the US and Mexico. These stations are placed at highly surveyed locations. Errors for each of the satellites in view can be computed and forwarded to two master stations. At these stations, a correction for each satellite is calculated – these corrections resolve errors such as timing, satellite orbit, and ionospheric. These corrections are sent to geostationary satellites (stationary over a fixed point on the earth&rsquot;s surface) which, in turn, relay this information to a WAAS capable receiver. The receiver then decodes this information and applies the correction to each GPS satellite that is being used in the navigation solution. Fortunately, all of this occurs automatically within the receiver.
With WAAS, the accuracy improves to less than one meter, horizontally and vertically. The integrity – the confidence that the position provided by device is truly accurate – improves to ILS-like levels. As an example, non-augmented GPS can take hours to provide an alert that there is an error in the network. With WAAS this value is six seconds, identical to Category 1 ILS. With these qualities WAAS is now an ILS replacement or, in the case of most airports, an opportunity to have ILS approach and landing capability without the enormous capital and operational outlay of an ILS.
Virtually every airport in North America is now eligible for WAAS instrument approaches. A real-time display of WAAS coverage is available at the following website: http://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm. This website is updated every two minutes. Within the yellow boundary, the WAAS coverage is adequate for precision approaches to as low as 200 ft; within the red boundary, WAAS coverage is adequate for approaches to as low as 250 ft. As a general rule, WAAS coverage in Canada is from the Arctic Ocean to the US border and from coast-to-coast. In other words, every airport in Canada, except those in the most northerly regions, has WAAS coverage and is eligible for ILS-like instrument approaches without the capital or operating cost of an ILS. In the US, over 4000 airports that couldn’t possibly afford this capability will achieve it with WAAS; in Canada, over 400 airports with no precision approach capability today will be eligible to enjoy the airport access and safety benefits of WAAS.
With a free navigation signal now available compliments of GPS and WAAS, how is an instrument approach procedure created to produce a means for aircraft to safely approach and land at an airport in poor weather or at night. In the next article, the process of instrument procedure design is presented.