An evolution in satellite navigation for our region is on its way. In the first of two informative articles, Andrew Andersen, a passionate advocate for general aviation and expert in Australian surveillance and navigation technology policy, discusses how GPS works for us now and what the introduction of SouthPAN will mean for General Aviation in the future.
The Next Big Leap in Satellite Navigation
By Andrew Andersen
It’s hard to believe, but 2024 is the 50th birthday of the USA’s Global Positioning System, or GPS. GPS is one of those things that crosses every aspect of modern life. From driving your car, to the timing of financial transactions, GPS has a role in so many things we do, yet its principles of operation are a mystery to most people.
A Matter of Trust
ICAO stipulates four principal criteria for the certification of aircraft navigation systems:
Accuracy: the difference between the real position, speed or time and what the system measures, reports and/or displays to the user
Integrity: the level of trust (“a threshold of confidence”) that can be placed in the navigation system and its ability to provide an alarm in the event of an anomaly
Continuity: a navigation system’s ability to operate without interruption
Availability: the percentage of time that the navigation system fulfils the above accuracy, integrity and continuity criteria.
Officially, the Global Navigation Satellite System, or GNSS, comprises the USA’s GPS, Russia’s GLONASS and other emerging systems. However, approved avionics in general aviation aircraft use only the GPS satellite constellation.
The accuracy of the GPS leaves most of us astounded, just about all the time. It’s what can happen if something goes wrong, even with just one, of more than 30 operational GPS satellites, that is the biggest reason for us to be wary.
At present, GNSS avionics in Australia and New Zealand use a technique called RAIM (Receiver Autonomous Integrity Monitoring) to catch errors and alert pilots. RAIM does its job by calculating the same position multiple times, using different combinations of satellites and comparing the results. If there are no significant differences, all is well and that’s what happens, just about all the time. But every so often, RAIM finds a problem and a pilot might see “Loss of Integrity” or LOI displayed, which means that the avionics cannot assure the pilot that its output is correct, and the pilot should use another means to verify their position. For VFR flying, that usually means taking a second look at the map and out the window, and for IFR, a ground-based navigation aid, such as a VOR, NDB or localizer will be used.
Happily, for lateral navigation (tracks and headings), sufficient tolerance can be built into safety-critical operations to allow enough time to determine that a problem has come up, so that an attentive pilot to respond. For example, separation standards applied by ATC to keep IFR aircraft apart include consideration of the required navigation performance along the route, or in the relevant airspace. There is still a risk that an errant aircraft could cross the path of another, but we are prepared to accept it, because we have multiple methods to avoid it occurring; for example, ATC surveillance and radio communications.
The problem is different, however, if GNSS is being used for vertical navigation. Put another way, if we want to use GNSS to determine an aircraft’s height above the ground, or its vertical path to the runway, then there is much less time for errors to be found and alerted. ICAO allows just 6 seconds for an alert to be displayed on an instrument approach to minima below 250 feet AGL, and the system must not permit misleading information to be displayed more than once or twice every 10,000,000 flight hours. On its own, RAIM just won’t cut it; a better method of detecting and alerting errors is needed.
The Answer is… Augmentation
Twenty years ago, the FAA introduced WAAS, the Wide Area Augmentation System for North America. WAAS is a Satellite-Based Augmentation System, or SBAS, which increases the accuracy and integrity of GPS positions by providing corrections for errors, especially those due to the ionosphere and drift in the satellites’ atomic clocks. The corrections are calculated on the ground broadcast to compatible navigation systems from additional satellites that serve a particular region.
For Australia and New Zealand, this system of corrections is called SouthPAN, and is being implemented by Geoscience Australia and Toitū Te Whenua Land Information New Zealand. SouthPAN will be an ICAO-compliant, certified SBAS, compatible with avionics certified to TSO C146, expected to be available for aviation use in 2028.
Next Time: Types of Approaches with Vertical Guidance