. False Wind Shear Alerts
The weather radar may wrongly interpret strong returns due to large side lobes as wind shears. This explains why the first generation of weather radars is not able to detect wind shears.
6.1.1.5.3. Antenna Stabilization
The antenna stabilization maintains the antenna scanning parallel to the horizon regardless of the aircraft attitude. The antenna is stabilized in roll and pitch. Thanks to the antenna stabilization, the display of ground returns on ND does not change when the aircraft attitude change.
If the antenna stabilization fails, the radar antenna scans parallel to the wings per default. Consequently, if the aircraft turns, ground returns may cover half of the display (on the side of the turn).
6.1.1.6. RADAR BEAM
6.1.1.6.1. Diameter and Resolution
. Diameter
The aperture of the radar beam is approximately 3.5°.
The beam diameter at a given distance may be approximately determined by the following formula:
Beam diameter [ft] =
3.5 x (Distance [NM] + “00”)
Example:
Distance = 50 NM Distance + “00” = 5 000 Beam diameter = 17 500 ft
. Range Resolution
The radar beam emits pulses to detect weather. For long-range detection (more than approximately 80 NM), high-energy pulses (i.e. wide pulses) are transmitted to compensate the beam attenuation. Therefore, pulses for long-range detection are wider than pulses for short-range detection.
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When the pulse width is shorter than the distance between two precipitations, the weather radar detects two precipitations.
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When the pulse width is longer than the distance between two precipitations, the weather radar detects one precipitation.
Shorter the pulse (or shorter the range), higher the range resolution.
Consequently, the weather radar may display two distinct precipitations detected at long range as a single block. And when the aircraft gets closer to the precipitations, the weather radar may display the precipitations as two distinct blocks.
. Azimuth Resolution
With an aperture of 3.5°, the width of radar beam significantly increases with the range. An analysis similar to the range resolution may be made with the width of the radar beam.
Thinner the beam width (or shorter the range), higher the azimuth
resolution.
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6.1.1.6.2. Attenuation
. Beam Attenuation
The energy of the radar pulse attenuates when the radar pulse goes away and returns back to the radar antenna (i.e. absorption and refraction). If the weather radar does not compensate the beam attenuation, the weather radar may display a precipitation:
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As weak (i.e. green) when the aircraft is far from the precipitation
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As strong (i.e. red) when the aircraft is approaching the precipitation.
. Path Attenuation
Some precipitations may be so strong that the radar beam is not able to penetrate them. Consequently, they mask the weather behind them. This is called the path attenuation or radar shadow.
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