Australia Allowing GPS-Only Navigation

Australia's Civil Aviation Safety Authority (CASA) has approved the use of GPS as a primary means of navigation in Australian airspace.  For some types of aircraft and operations this allows GPS to be used as the only means of navigation.

According to a paper prepared by Airservices Australia's Greg Dunstone that will be presented at an ICAO ADS-B (automatic dependent surveillance-broadcast) meeting in New Delhi, India, in early April, CASA has published material in an Aeronautical Information Circular (AIC) that allows pilots of appropriately equipped aircraft, to plan and conduct IFR (instrument flight rule) flights without requiring any ground navigation aid, such as a NDB (non-directional radio beacon), VOR (VHF omnidirectional range) or ILS (instrument landing system).

Australia has been examining the comparative safety of GPS navigational equipment for non-precision approach purposes, compared to the use of a NDB, and concluded that the minimum navigation equipment required to fly IFR is now avionics that comply with TSO-(technical standard order) C145a/146a; there is no longer a requirement to carry a ground-based navigational system.

(TSO C-145 refers to airborne navigation sensors that use GPS augmented by WAAS (wide area augmentation system).  TSO C-146 refers to stand-alone airborne navigation equipment that uses GPS augmented by WAAS.)

The CASA approval, however, does require a prediction at the time of dispatch that RAIM (receiver autonomous integrity monitoring) FDE (fault detection and exclusion) will be available during the approach, taking into account GPS satellite availability and geometry.  Typically, RAIM outages for TSO146a receivers would occur infrequently and for a short duration, but if there is a predicated GPS service outage at the time of dispatch, the pilot can either delay departing, or carry extra fuel to hold during the no-service period.

Mr. Dunstone notes that since GPS supports both navigation and positional data for ADS-B, the availability of GPS-only navigation opens the possibility of equipping the general aviation fleet with GPS and ADS-B “out” in a single package, which could mean considerable acquisition cost savings.  And he notes that the availability of GPS-only navigation can mean that the scale of conventional navaid infrastructure also could be reduced.  In other words, one business case could be used to support both navigation and ADS-B because of the synergy between the two technologies.  03-31-2006.

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