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A pilot is tracking inbound to a VOR on a designated radial using the HSI. The VOR ground station is located in an area with varying terrain and man-made obstructions. The pilot notices that the CDI is fluctuating slightly, even though he/she is constantly and accurately tracking inbound to the station and there is no apparent wind effect.

What is the term used to describe these fluctuations?

  • A
    Night effect
  • B
    Quadrantal error
  • C
    Scalloping
  • D
    Coastal refraction

Site error is caused by uneven terrain, for example, hills, trees, buildings and long grass near the transmitter. Although the propagation has left the VOR with ±1° accuracy, propagation error still exists.. Propagation error is caused by the terrain and distance. At a certain range from the VOR so called ‘bends’ or ‘scalloping’ can occur.

VOR scalloping is described as an imperfection or deviation in the received VOR signal. Scalloping causes the signal to bend as a result of reflections from buildings and terrain. Scalloping also causes the Course Deviation Indicator (CDI) to slowly or rapidly shift from side to side. Scalloping has a negative effect on the accuracy of the navigation aids used.

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