Global Positioning System (GPS) Errors: Types, Causes

Global Positioning System measurements can be degraded by several independent error sources that may combine to displace the calculated position by several metres. Satellite clock error, arising from inaccuracies in both the satellite and receiver clocks together with relativistic effects, routinely contributes up to two metres of deviation. Timing discrepancies therefore form a baseline level of uncertainty that affects every fix.

Additional disturbances originate in the radio-frequency path between spacecraft and aircraft. Natural interference can be caused by malfunctioning or incorrectly configured transmitters, while intentional jamming devices emit energy within the GPS band and overwhelm the desired signal. A more insidious threat is GPS spoofing, which rebroadcasts a look-alike signal that convinces receivers the aircraft is at an incorrect position or time. These hazards intensify during instrument approaches, when the greatest potential for GPS errors coincides with the phase of flight that demands the highest accuracy.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What are the common GPS errors in aviation?

The common GPS errors in aviation are listed below.

• Programming errors cause input of wrong latitude and longitude for intersections, leading to GPS errors in aviation
• Outdated databases can cause the moving map display to offer erroneous information around pivotal airspace areas, leading to GPS errors in aviation
• RAIM messages indicate an inconsistency in the navigation solution for the given phase of flight, indicating GPS errors in aviation
• Mode error occurs when selecting GPS mode rather than localizer for HSI
• Multipath error results from reflection and refraction of the satellite signal by objects and ground near the receiver
• Satellite clock error can result in position errors of up to two metres
• Ionospheric propagation delay causes pseudo-range measurement errors
• Greatest potential for GPS errors in aviation comes during an instrument approach

Without RAIM there is no assurance of GPS position integrity. Fault detection and exclusion (FDE) excludes a failed satellite from the position solution, but GPS receivers capable of FDE require six satellites or five satellites with baro-aiding, and the current altimeter setting must be entered into the receiver to guarantee baro-aiding is available. Interference and outages stem from outdated databases, programming mistakes, and mode error. Issues and failures are amplified by large GPS vertical errors that make the integrity monitoring function invalid and by receiver installation error. GPS-derived altitude has large vertical errors, and PDOP, a subset of GDOP that affects latitude, longitude, and altitude, translates into heading and track errors. Many GPS receivers are able to provide an estimate of PDOP, but applicants still fail to set up the CDI correctly so that it is full-scale deflected while descending to minimums on the magenta line. Misinterpretation includes pilots not understanding moving maps or text display and becoming lost on final, perceiving the aircraft above controlled airspace. Systematic cross-check with other navigation techniques identifies a RAIM failure, and pilots are responsible to verify waypoint location referencing an official current source like Chart Supplement U.S. Satellite status is available through the Notice to Air Missions (NOTAM) system.

I encountered a sudden difference between my aircraft's indicated location and the anticipated path. Aboard, the GPS set was giving legitimate information one moment, then location data came differently the next, creating a 10-second loss. Cross-referencing with navigational assistance showed failure when the receiver displayed failure, forcing me to revert to radio guidance.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What is GPS jamming in aviation?

GPS jamming is an intentional radio frequency interference (RFI) with GNSS signals, which involves saturating GPS receivers with unknown signals. This jamming prevents receivers from locking onto satellite signals and degrades everyone's ability to effectively use GPS for navigational purposes.

GPS jamming is intentional interference with GPS signals to disrupt navigation and tracking in aviation. GNSS jamming, including GPS jamming, is an act of overwhelming global navigation satellite systems receivers with radio signals that drown out signals from the GPS, GLONASS, BeiDou, or Galileo satellite constellations. This interference happens accidentally if a GPS receiver providing false information is located close to a higher power transmitter, or it is used deliberately to obscure movements, protect sensitive operations, or cause disruption to both military and civilian navigation systems.

GNSS jamming has the main effect of rendering the GNSS system ineffective or degraded for users in the jammed area. GPS jamming causes navigational failures, positioning errors, and loss of tracking, which directly impacts the flight management system (FMS), GPS/IRS hybrid systems, and the Traffic Collision Avoidance System (TCAS). It disrupts the Automatic Dependent Surveillance-Broadcast (ADS-B) transmission, the terrain avoidance warning system (TAWS), enhanced ground proximity warning systems, and satellite communication (SATCOM). These disruptions lead to non-optimal tracks, larger separation, and increased pilot workload.

Air Traffic Control (ATC) is impacted by GPS jamming, affecting communications and increasing safety risks. GPS jamming incidents potentially lead to diversions, go-arounds, and higher safety risks. Multilateration (MLAT) is used to track flights experiencing GPS jamming, providing a fallback when GNSS signals are unreliable. While flights have fallback systems, these are less reliable than GPS, making jamming a concern for aviation safety.

GPS jamming is illegal in the US and UK, and its use - whether intentional or accidental - disrupts not only aviation but also mobile phone networks and precision agriculture. To counter these threats, GPS anti-jamming and anti-spoofing solutions actively counter GPS jamming and spoofing attempts. These solutions integrate with existing avionics and flight deck procedures, offering a complete modification kit that includes all parts and engineering bulletins. Such measures complement existing mitigation measures implemented by aviation authorities and state agencies, adding a vital layer of protection to guarantee continued operational safety and peace of mind for pilots, crews, and airlines.

What is GPS spoofing in aviation?

GPS spoofing manipulates signals transmitted by the Global Positioning System (GPS) and involves broadcasting counterfeit satellite signals, which are stronger than genuine satellite signals, to send false GPS data to an aircraft.

GPS spoofing in aviation is a deliberate action in which a ground-based spoofing platform broadcasts counterfeit satellite signals that the aircraft's GPS receiver treats as valid. During spoofing, a ground-based transmitter causes GPS receivers to calculate incorrect position, altitude and time, leading to GPS position jumps of 50 to several hundred miles; decoded look-alike signals place the aircraft at incorrect coordinates and incorrect time. The plane's Inertial Reference System often fails to detect the manipulation and tries to verify its position using the spoofed GPS data, with the result that spoofing disables the entire navigation system. In spoofing situations, the spoofed-to position is false GPS coordinates received by the GPS receiver. Consequently, the aircraft follows incorrect routes and appears on the controller's situation display in places it is not, causing trajectory deviations or unexpected turns that indicate spoofing.

Pilots and controllers can recognize spoofing by trajectory deviations, unexpected turns on the controller's situation display, unavailability of GNSS navigation, GNSS degradation messages, altitude anomalies, sudden changes to position data, and suspicious time indications. Signal-strength monitoring can detect spoofing attempts by identifying unusually high signal-strength levels that exceed those of genuine satellite signals. Analysing received GPS signals for gross discrepancies between shown and expected position provides further evidence. Typical GPS spoofing equipment - moveable units installed at fixed locations or portable on a vehicle, high-grade military equipment, or purpose-built devices originating from individuals, criminals, researchers, private organisations or state actors - can thus be revealed by concurrent onboard system indications.

I think spoofing’s threat exists in its misleading disposition, and I think it is inadequate to depend exclusively on GPS. Therefore, a multi-layered way incorporating inertial guidance arrangements and secondary location applications is necessary, and the industry industrialization must rank the advancement and execution of strong countermeasures against this danger.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

Does aircraft GPS interference occur near conflict zones?

GPS jamming typically happens when planes fly over conflict zones, near military areas, or around sensitive government installations. Since the start of 2018, GNSS disruption and EMI have been reported in airspace areas neighbouring Syria, and interference now extends well beyond the front lines, impacting aircraft operating hundreds of nautical miles away. Approximately 80% of GPS interference events occurred in southeastern Europe and the Middle East, with the eastern Mediterranean, Black Sea, Baltic Sea, and Arctic regions also affected.

Airspace users and operators have reported an increase in GPS/GNSS interference over the past year, and the FAA has issued Notices to Air Missions warning of GPS interference over the Black Sea, eastern Mediterranean, Baltic Sea, and Arctic region. Jamming and spoofing have increased substantially since Russia's invasion of Ukraine, and Osprey analysis indicates that civil aviation flight operations face a nascent credible risk of being exposed to GNSS disruption and EMI while operating in airspace near and/or over conflict zones.

Aircraft operators must therefore be aware of conflict regions where GPS/GNSS interference has been commonly reported, navigate at heightened awareness, and check NOTAMs for their intended route of flight. Crews are trained to spot and react to potential interference, and flights have fallback systems so that the crew can continue the flight with other radio navigation devices when GPS malfunctions occur.

Why does an airplane experience a GPS outage?

An airplane may experience a GPS outage due to different reasons. Natural phenomena like solar storms temporarily interrupt or degrade GPS signals, and GPS signals are also blocked by objects. Jamming attacks drown out legitimate satellite signals, and an event occurred due to a transmitter errantly broadcasting in the GPS frequency. There are frequently large-scale interference events on the East Coast and in the Southern US. The VOR MON considers areas prone to GPS outages to be primarily the Western US.

When the GPS signal is briefly lost, timing systems lose synchronization and pilots are forced to switch to older tools. Because multiple receivers will not lose reception at the same time, the first two causes are generally ruled out if symptoms are experienced by more than one aircraft. The number of airplanes that have experienced the problem has risen to the thousands, and some flights were canceled until an alternative to GPS was established. Thirty-five percent of pilots indicated they had experienced a GPS outage or unavailability in flight. Pilots experiencing a GPS anomaly must report the time, location, speed, magnetic heading altitude, and duration of the outage.

During one transatlantic flight, the unexpected ‘Signal Lost’ statement appeared on my small display, the screen that normally showed our place was empty. I found out the reason was not a malfunction of the GPS but an intentional experiment by an armed forces association congesting signals in that distant region. They were jamming communications to assess their personal gear's elasticity, and this deliberate noise totally drowned the navigation radio. The episode was a message of my full dependence on a technology I did not really master, and an interesting example of the vulnerability of the network.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the disadvantages of GPS in aviation?

The disadvantages of GPS in aviation are listed below.

• Lack of accuracy (for pivotal phases of flight) in GPS in aviation is a limitation of existing systems.
• Lack of availability and continuity of service in GPS in aviation is a limitation of existing systems.
• GPS in aviation can cause crashes or flying away if the signal is lost or delayed.
• GPS drones in aviation have faster power consumption as a disadvantage.
• GPS in aviation alone does not meet accuracy and reliability standards for precision approaches.
• GPS-based navigation equipment in aviation can cause missed runway approaches if GPS is unavailable.
• No control by an international civil body in GPS in aviation is a reason for inaccuracies in existing systems.
• GPS dependence in aviation can cause pilots to skip monitoring other systems.
• You cannot perceive real-time position in aviation if GPS signal is lost or delayed.
• GPS signal loss in aviation may cause inability to use GNSS for navigation.
• GPS signal loss in aviation may cause navigation displays to show on-course incorrectly.
• GPS signal loss in aviation can require aircraft to wait longer to land.
• GPS signal loss in aviation may cause loss of position/navigation alignment on ground.
• GPSS in aviation can only show where your antenna was located when the last set of signals was received.
• Overreliance on GPS in aviation leads to loss of situational awareness.
• Aircraft in aviation may have to go around and re-attempt a landing if GPS is unavailable.
• GPS signal loss in aviation may cause pilots to rely on the Inertial Reference System.
• GPS signals in aviation are affected by electromagnetic interference in the ionosphere.
• GPS power consumption in aviation drains the battery.
• GPS signal loss in aviation may cause loss of TAWS functionality.
• Reduced ability to transmit location in aviation increases risk of straying into no-fly zones.
• GPS signals in aviation do not penetrate solid walls.
• Pilot loss of situational awareness in aviation can cause inability to transition safely to another acceptable means of navigation without GNSS.
• ADS-B in aviation may report false positions.
• GPS signals in aviation can be spoofed.