AWR is a Weather Advisory system that gives a constant picture of the Weather lying ahead of the A/c in flight and warns the Pilot of any significant present in the clouds.
(Precipitation = Amounts of H2O droplets concentrated). It displays the representation of Precipitation on a Weather Radar Screen, made of Cathode Ray Tube.
It must be understood that the AWR only gives representation of H2O droplets concentration, any turbulence not associated with any significant warning remains invisible to the AWR.
AWR works on Primary Radar Principle.
Frequency is 9375 MHz.
A high Frequency of 9375 is used since it gives Strong return of Echoes from big H2O droplets and clouds of high H2O contents, usually Thunderstorm movements, but do not reflect the clouds with lower contents of H2O or Fog.
The type of beams primarily used are-
- Pencil Beam- is used for the Weather Mapping purposes. It is a narrow beam therefore gives a better resolution over a particular area, resulting in a better resolution in bearings and direction. It is used for Longer Ranges.
- Fan Beam- is used for the Ground Mapping purposes. It is a relatively wider beam, which lights up a large area in front of the A/c. It is used for Shorter Ranges up to Approx 50 N.M.
- When A/c is transmitting in Fan Beam; progressively large energy is transmitted to the relatively farther targets than the closer targets. This causes the Radar representation to be unequal display due to difference in the strength of returning Echoes. A “Cosecant Beam” is used to eliminate this error; it causes the equal representation of the Echoes to be displayed on the Radar Screen.
Target Alert System- A cloud with very high H2O concentrations is displayed as a warning signal by a Red Square with Yellow “T” in it, which starts flashing intermittently.
Display of Intensity of Precipitation is as follows
Black and White Radar Screen
Black- Area of no Precipitation/Clear Area
Yellow- Area of Precipitation/Turbulence
In a Black and White Radar, due to the lack of Colors to denote Precipitation level, an ISO ECHO CONTOUR CIRCUIT is installed, which is used when there is a region of high Turbulence in front of the A/c, and it wishes to pass through it.
ISO ECHO CONTOUR CIRCUIT- represents Maximum Turbulence on the Radar Screen, when a Black Area is surrounded by a Yellow area.
Color Radar Screen
Black- Area of no Precipitation/Clear Area
Green- Area of Light Precipitation/Light Turbulence
Yellow- Area of High Precipitation/Moderate Turbulence
Red/Magenta- Area of Extremely High Precipitation/Heavy Turbulence
Hold system- is used to freeze the previous return of the Echoes, and imposes new Echoes for the sharp detection of Thunderstorm movements
To find the Height of the Cloud, Aerial Tilt can be used, for manually calculating the formulae can be used is as follows-
Height of Cloud =Resultant Beam ×Distance to go (in feet) ÷ 60
Example-
Aerial tilted to 5° up and the Echo goes off.
Beam Width is 4° and Distance from the cloud is 20 N.M.
Resultant Beam is calculated by dividing the Beam Width into 2, and then subtracting it from the Tilted Angle.
If total width of the Beam is 4°, Beam should be divided into two 4÷2= 2
The Echo goes off at 5°-2° = 3° is called Resultant Beam
Therefore to calculate the cloud height
We use-
Height of Cloud =Resultant Beam ×Distance to go (in feet) ÷ 60
On using the formulae we get
3 × (20×6080) ÷ 60= 6080 feet or 1 N.M.
This shows that the top of the cloud is at 6080 feet or 1 N.M above the height of the A/c.