VOR Errors and Limitations

Types of VOR Errors

• Site Effect: Reflections from nearby terrain or structures can distort the VOR signal at the transmitter, leading to errors known as site error. This can cause the course deviation indicator (CDI) to behave erratically.
• Scalloping: This is a rapid, unpredictable oscillation of the CDI or bearing needle, making it difficult for pilots to maintain a steady course. Scalloping is a direct result of multipath reflections, often from terrain or buildings near the VOR station.
• Beam Bends: Unlike scalloping, beam bends are smoother, more predictable deviations of the VOR radial, still caused by signal reflections but within system tolerances.
• Interference and Coverage: VORs operate on VHF, which is line-of-sight and immune to sky-wave propagation, coastal refraction, and night effect. However, outside the designated operational coverage (DOC), interference from other stations or terrain shielding can reduce accuracy.

VOR Accuracy and Limitations

• Accuracy: ICAO standards specify a VOR bearing accuracy of ±5° for 95% of indications. For approaches, pilots must remain within half-scale CDI deflection (±5°) to be considered established on the required track.
• Types of VORs: There are several types: Conventional VOR (CVOR), Doppler VOR (DVOR, which reduces site error), Terminal VOR (TVOR, with limited range), and Test VOR (VOT, for ground checks).
• Automatic Monitoring: VOR stations have ground monitoring systems. If the signal drifts outside specified limits, the station will automatically remove its identification or cease transmissions to prevent navigation errors.

Operational Considerations

• Pilots should be aware of published VOR limitations and any known site or scalloping effects, which are often noted in aeronautical information publications and on charts.
• Doppler VORs are less susceptible to site error due to their antenna design, offering improved reliability in challenging environments.
• Always confirm you are within the VOR's service volume for accurate navigation, and be alert for any erratic CDI behavior, especially near terrain or at the limits of coverage.