The ellipsoid is already tangent to the ground in the best case, and therefore the ellipsoid between the VOR and this middle point is slightly obstructed. For an aircraft at a distance of 20 NM from a VOR located on the airfield, flying at 1000 ft QFE, the radius of FE1 at the middle point of the link is 500 ft. The ratio between the obstructed areas in FE1 and FE2 determines the ratio of energy with constructive interferences, thus when siting radio stations, at least 60% of FE1 is kept clear of obstacles. Therefore when planning a radio link, only FE1 is taken into account. However 80% of energy is in FE1, and the rest is mostly in FE2. In the second Fresnel ellipsoid where the difference is between $\small \lambda / 2$ and $\small \lambda$, interferences are destructive, amplitude is decreased, and this continues with other multiples of $\small \lambda / 2$. In a volume called the first Fresnel ellipsoid (FE1), centered on the LoS, where the length difference cannot exceed $\small \lambda / 2$, $\small \lambda$ being the VOR carrier wavelength, interferences are constructive, amplitude is increased. Interferences occur, and they change the signal amplitude. This means their phase is different on the receiver antenna. But waves travel different distances before reaching the receiver: it extends considerably the range beyond LoS. The corresponding rays can be reflected, diffracted and refracted by obstacles and reach the receiver antenna as indirect waves. In practical most VOR stations are well below this limit, possibly using Doppler VORs in most cluttered sites.Ī word about these multipath errors: The direct wave propagates along the line of sight (LoS), but the transmitting antenna radiates in other directions. ICAO limits this error to 6.5° in the volume of service.
When direct and reflected waves interfere, an error can be introduced. However errors are mostly created by the environment, by reflection and other phenomena changing the signal phase, similarly to a mirror inverting left and right on its reflected image. A perfectly sited VOR could deliver this precision to the receivers. Most stations are able transmit a signal with an accuracy better than 0.5°. The transmitting elements must be accurate to 2°.
That said both types have the same tolerances from an ICAO standpoint: They are commonly used for runway approach.īearing is more accurate with a Doppler VOR unless the conventional VOR site neighborhood is exempt of obstructions.
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They are less sensitive to multipath, and are easier to install at airfields.They are less efficient, inactive antennas from the array absorb a significant part of the signal emitted from the active antennas, reducing the effective range.Bearing is determined from the Doppler effect created by scanning the array at 1,300 m/s.They requires a large 14 m diameter circular array of antennas.
There are also several types of Doppler VORs, single or double sideband: They are often used for airway beaconing. They are very sensitive to multipath interference created by reflection on nearby obstacles, and must be located at isolated places.Bearing is determined from a HF signal rotating 30 rounds per second.There are several types of conventional VORs using either rotating or static antennas.