Storm Cell
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Side Lobe Overscanned TT
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Resolution
There are several factors which affect the resolution of the radar system.
Range
Increasing range will decrease the radar return. The system compensates for this by varying the system gain with range; therefore giving as accurate a return as possible, at varying ranges.
Attenuation
Intervening precipitation and increasing range tend to attenuate the beam. The radar compensates for precipitation or range attenuation up to 80 NM, so that the correct color is displayed on the indicator. This feature is called path attenuation correction (PAC) and is intended for weather modes only. Using weather mode for ground mapping may result in an inaccurate picture due to the PAC circuit.
The PAC Alert annunciation identifies the areas of severe attenuation within 80 NM. Should the intervening precipitation be so intense that the signal is attenuated below the lowest signal level (green), a yellow arc (PAC alert bar) is painted at the outermost range mark to indicate the direction where the heavy precipitation is present. Targets beyond the intervening storm cell in this direction may not be accurately displayed or indeed displayed at all.
Caution: Although this special circuitry compensates for areas of
precipitation, weather radar should not be used for penetration of
thunderstorm areas where the precipitation between aircraft and
target is moderate to heavy.
The storm behind the storm may not be displayed under extreme attenuation conditions. Do not penetrate strong targets assuming there is nothing behind it. If the ground cannot be painted behind the storm, then the attenuation compensation is not effective due to extremely high attenuation.
Nature Of Target
Storm targets differ in their ability to return a signal. Precipitation tends to absorb part of the transmitted signal which “masks” targets behind heavy precipitation areas.
As the tilt control is used to sweep a storm target, the return may change color, not due to a change in precipitation rate, but to the type of precipitation target encountered.
Gain Control and Turbulence
Manual gain control is available in all modes. The fully CW position, marked AUTO, is the normal position for gain control. As the gain control knob is moved CCW out of the AUTO position and toward the MIN setting, gain is adjusted from maximum to minimum. The variable gain control should only be used to reduce the receiver’s sensitivity to aid in determining the relative intensity of multiple thunderstorms and embedded cells.
Caution: Manual gain settings of maximum, 9, and 8 will somewhat
enhance radar receiver sensitivity, but as the gain is reduced
there is a chance that all radar displays will be eliminated.
Selecting the TURB position causes all weather targets (precipitation and turbulence) to be displayed. Turbulence detection is limited to the first 50 NM regardless of the range selected. It will be displayed in magenta on the indicator superimposed over the weather information. This feature allows the detection of storm-related turbulence by measuring the Doppler shift of detected particles. Precipitation must be present for this mode to operate. Clear air turbulence (CAT) will not be detected. Manual gain is available in TURB mode, but should have no effect on turbulence targets.
Selecting the WX position will cause all precipitation targets to be displayed. Detectable weather will be displayed in three colors; red, yellow, and green.
Inflight Operations
Antenna Tilt Operations
Takeoff and Landings
Operations below 10,000 feet usually require a tilt setting of 2-3 degrees upward tilt. This will provide target detection up to 40 NM, without excessive ground returns and eliminate frequent tilt adjustment. The tilt setting should be adjusted as necessary to optimize target display. A solid ground return between 35 to 40 NM ensures targets within 35 NM will be detected. If tilt settings below 4 degrees are used for takeoff, some ground return will be detected until passing 5,000 feet AGL. This is due to minor sidelobes. Set 7 degrees up initially for takeoff.
Middle Altitudes (near 20,000 feet):
Antenna tilt settings should be roughly 0 degrees or slightly down. For over land operation, adjust tilt control until a small arc of ground return appears at the outer edge of the display. Storm cells displayed between half scale and the outer edge of the display should be monitored; tilting the antenna down and alternating range setting as necessary to avoid overscanning as you approach these cells.
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本文鏈接地址：MD-80 Flight Manual 麥道80飛行手冊 2(41)
