The principle advantage of a Doppler VOR (DVOR) over a conventional VOR (CVOR) is its increased accuracy due to its reduced susceptibility to interference from reflections (multipath propagation) caused by terrain and man-made structures.
Here's a more detailed explanation:
Understanding the Problem: Multipath Propagation
Conventional VOR (CVOR) signals are susceptible to reflections. These reflections occur when the VOR signal bounces off objects like hills, buildings, and even airplanes. These reflected signals arrive at the aircraft receiver slightly later than the direct signal, causing phase errors and consequently, inaccurate bearing information. This phenomenon is known as multipath propagation.
How Doppler VOR Overcomes this Issue
Doppler VORs are designed to mitigate the effects of multipath propagation. The key difference lies in how the 30 Hz variable signal is transmitted:
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CVOR: Transmits the 30 Hz variable signal as an amplitude-modulated (AM) signal. AM signals are more vulnerable to interference.
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DVOR: Transmits the 30 Hz variable signal as a frequency-modulated (FM) signal. FM signals are significantly less affected by reflections and interference.
The DVOR achieves this by using a rotating antenna array. This array transmits a signal that sweeps in a circle, effectively creating a Doppler shift that encodes the 30 Hz variable signal as a change in frequency. The receiver interprets this frequency shift to determine the aircraft's bearing. Since FM is more resistant to interference, the DVOR provides a more accurate bearing even in areas with significant terrain or obstructions.
Summary of Advantages:
| Feature | CVOR (Conventional VOR) | DVOR (Doppler VOR) |
|---|---|---|
| Signal Encoding | Amplitude Modulation (AM) | Frequency Modulation (FM) |
| Multipath Effect | High | Low |
| Accuracy | Lower | Higher |
In essence, the Doppler VOR's use of FM modulation makes it significantly more accurate and reliable, especially in environments prone to signal reflections.
