The low−strength data transmission signals from global navigation satellite systems (GNSS) satellites are vulnerable to intentional and unintentional interference. This inference can come from a wide variety of sources and can significantly reduce the reliability of the navigation signal.
Sources of interference include:
- radars
- microwave links
- ionosphere effects
- solar activity
- multi−path error
- satellite communications
- GNSS repeaters
- some systems onboard the aircraft.
In general, these types of unintentional interference are localised and intermittent.
Jammers or spoofers
Of greater and growing concern is the intentional and unauthorised interference of GNSS signals by people using 'jammers' or 'spoofers'. These can disrupt air navigation by interfering with the reception of valid satellite signals.
GNSS is a critical component of essential communication, navigation, and surveillance (CNS) in Australian airspace. Some satellite communications avionics use GNSS signals for operations in oceanic and remote airspaces. It is also the sole aircraft position−reporting source for Automatic Dependent Surveillance – Broadcast (ADS-B).
Some aircraft are also using GNSS as a reference source for aircraft flight control and stability systems. It is also a necessary component of the Aircraft Terrain Awareness and Warning System (TAWS) – an aircraft safety system that alerts pilots of upcoming terrain. There are examples of false 'terrain−pull up' warnings during GNSS anomalies.
There are many uses of GNSS in aviation including navigation, ADS−B, terrain awareness warning systems. This means operators of aircraft using GNSS need to be aware of these vulnerabilities, recognise when it happens, and adjust to degraded signals.
We recommend operators consider the likelihood of GNSS interference on their planned route and potential emergency diversion routes. You should also determine whether your aircraft can safely navigate and operate in the event of GNSS interference.
If flying in an area where GNSS interference is reasonably possible, we recommend to:
- not rely entirely on GNSS-reported position
- use several methods for crosschecking position during flight, including:
- reference to ground features,
- information from an aircraft's inertial navigation system (INS) or flight management system (FMS) – if so equipped
- information from ground-based navigation systems: non-directional beacon (NDB), very high frequency omni-range (VOR), distance measuring equipment (DME)
- surveillance position information from air traffic control (ATC)
- when planning performance-based navigation approaches, particularly in instrument meteorological conditions (IMC), have a backup plan in the event of GNSS anomalies.
Dealing with GNSS interference during flight
Although the appropriate response to GNSS interference will vary with the situation, in general pilots should:
- Maintain control of the aircraft.
- Use the last reliable navigation information as the basis for initial headings. If you are below a minimum safe altitude or minimum obstacle clearance altitude, climb above terrain.
- Change to another source of navigation, if available (for instance, VOR, DME, instrument landing system (ILS), ATC vectors).
- Notify air traffic control as soon as practical.