Errors in Position Fixing Using Radio Aids

Types of Position Fixing Errors

VOR Position Errors

• Scalloping: Rapid bearing fluctuations due to terrain reflections, especially at the edge of coverage, can cause the Course Deviation Indicator (CDI) to oscillate unpredictably.
• Site Error: Reflections near the beacon site itself can introduce consistent bearing errors.
• Coastal Refraction: When VOR signals cross coastlines, the change in propagation speed between land and water bends the wavefront, distorting bearings—this is most pronounced when the station is far from the coast.
• Multipath Effects: Signals reflected off terrain or buildings combine with direct signals, causing bearing inaccuracies.

ADF/NDB Position Errors

• Night Effect: At night, sky-wave propagation allows signals from distant NDBs to interfere with the desired signal, leading to bearing errors.
• Coastal Refraction: Similar to VOR, NDB signals refract at coastlines, distorting bearings.
• Quadrantal and Mountain Effect: Aircraft structure and terrain can deflect the signal, causing further inaccuracies.

DME Position Errors

• Slant Range Error: DME measures the straight-line distance to the station, not the horizontal distance, which can be significant when overhead the station at altitude.
• Multipath and Signal Reflections: Reflected signals can cause the receiver to interpret incorrect distances.

ILS Errors

• Multipath: Reflections from terrain or structures near the runway can distort localizer and glideslope signals, leading to false indications.
• Course Deviation: Exceeding half-scale deflection on the localizer or glideslope means obstacle clearance is no longer guaranteed—an immediate go-around is required.

Satellite Navigation Errors

• Ionospheric Propagation Delay: GPS signals are delayed by the ionosphere, especially at low elevation angles. Dual-frequency receivers can correct this almost entirely; single-frequency receivers rely on models, reducing but not eliminating the error.
• Multipath: Reflected GPS signals degrade positional accuracy, especially near the ground or in mountainous terrain.

Synchronous Transmission Errors

When two stations transmit synchronously, their signals can interfere, causing ambiguous or incorrect position fixes.

Reducing Errors

• Use dual-frequency receivers for satellite navigation to minimize ionospheric delay.
• Be aware of environmental factors (terrain, coastlines, night conditions) that can affect signal propagation.
• Monitor navigation indications for unexpected deviations, especially near obstacles or during critical phases of flight.