getting to grips with surveillance(93)

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The weather radar is optimized to detect rains. Therefore, the frequency of the radar pulse is approximately 9333 MHz. Consequently, the weather radar is not able to detect fog, light rains, or dry clouds. In addition, the weather radar may not detect weather behind a heavy rain.
Figure 6-3 gives an idea of the rain reflectivity per frequency band. It has to be noted that cloudburst (rain more than heavy) may block pulses of ground radars.
Therefore, each time an ATC controller announces a heavy rain from his radarscope, the flight crew should consider this heavy rain as extremely severe.

6.1.1.4. GAIN
The weather radar measures the precipitation rate (millimeters per hour). For the calibrated gain, the precipitation rates are color-coded as follows:
Color  Precipitation  Precipitation rate (mm/h)  Reflectivity factor2 (dBZ) 
Black  PR < 1  Z < 20 
Green  Weak  1 < PR < 4  20 < Z < 30 
Yellow  Moderate  4 < PR < 12  30 < Z < 40 
Red  Strong and higher  12 < PR  40 < Z 
Magenta  Turbulence  5 m/s < ΔPR  N/A 

2 The reflectivity factor Z measures the strength of the radar return in a volume of precipitation. Z depends on the number and the size of raindrops in a volume, and is expressed in decibels (dBZ).
- 6-5 -

The weather radar displays turbulence in magenta each time the variation of precipitation rates (ΔPR) is greater than 5 m/s. It should be noted that Clear Air Turbulence (CAT) is not detected.
A variation of the gain by 10dBZ implies a variation of display by one color level (e.g. green becomes yellow with a variation of +10dBZ, red becomes yellow with a variation of –10dBZ).
Note: Turbulence remains magenta regardless of the gain setting.
6.1.1.5. ANTENNA
6.1.1.5.1. Parabolic and Flat Antennas
There are two types of weather radar antenna: parabolic for the first generation of weather radar (on A300/A310 and former A320 aircraft) and flat for the recent generation (on A320/A330/A340 aircraft).
. Parabolic Antenna
The parabolic antenna produces a wide main beam and large side lobes. Consequently, the large side lobes present the advantage of scanning below the aircraft, but also drawbacks like ground returns on display and the inability to reliably detect wind shears.
. Flat Antenna
The main beam from a flat antenna is more contained than the one from a parabolic antenna. Moreover, side lobes are smaller. With a flat antenna, the display is more accurate, ground returns are significantly reduced, but the capability to scan below the aircraft is lost.

6.1.1.5.2. Side Lobes
Significant side lobes may be produced by parabolic antennas or may be the result of damages/degradations on the antenna or the radome.

. Cat’s Eyes Phenomenon
The Cat’s eyes (also known as Altitude rings or Ghost targets) appear at +/-45° between 4 and 8 NM when the aircraft is at approximately 3000 ft above the ground. They are the results of ground returns due to side lobes. They are not visible when the aircraft is on ground.
　
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