Airborne weather radars installed on aircrafts provides information on weather, clouds and terrain mapping
AWR can be used for fixing position in range and bearing from ground feature
Consists of a transmitter, receiver, antenna and display with control panel
Heating elements in the radar housing helps in reducing icing risk
Airborne Radars use Flat Plate Slotted Array Antenna on a Gyro Stabilised Platform to maintain antenna horizontal during Pitch and Roll
Advantage of Flat Plate antenna include Narrow Beams, Lesser Power and Side Lobe Elimination
AWR operates on weather and ground mapping modes using conical and fan beams respectively
Principle of Airborne Weather Radar
AWR also works on the principle of Primary Radar
Bearing by search light principle – direction of return provides bearing
Slant range by echo principle – time delay between transmitted and received pulses provide range information
Frequency and Wavelength of AWR
Wavelength of AWR has been selected as 3.2 CM and frequency of 9375 MHz
Since the size of large water droplets in thunderstorms are around 3 Cm wavelength of 3.2 cm is best for detection of dangerous clouds
Beam Width and Resolution oa AWR
Resolution of two separate objects depends on its Beam Width
The 3° Beam Width of the weather radar distinguishes two clouds
10,000 Feet apart at a distance of 30 NM
26,000 Feet apart at a distance of 80 NM
60,000 Feet apart at a distance of 180 NM
Weather Mode using Conical or Pencil Beam
Conical beam or Pencil Beam is used to detect weather and long range terrain mapping due to its narrow beam width that is capable of distinguishing between two nearby clouds
Calculation of Pencil Beam Width (Deg) = (70 X Wavelength) / Antenna Diameter
Weather mode used to detect turbulent clouds based on size of water droplets is the default mode of radar
Manual gain adjustment is not available and highly turbulent clouds are indicated by steep gradient or change in colour
Severity of turbulence in clouds is indicated by colour coding of returns
Severe turbulence can be expected in cloud returns in the shape of ‘U’, Finger, Scalloped Edge or Hooks
Automatic Gain Control Technique
Weather Radar has Automatic Gain Control (AGC) in the Weather Mode up to a range of 25 NM to produce same intensity returns irrespective of distance
For example, similar clouds at 5 and 25 NM would produce same returns irrespective of their distance from aircraft
Cloud Height Calculation
To calculate cloud top Radar stabilisation must be switched off and antenna tilt is increased or decreased until the cloud vanishes
Cloud height can be calculated using the formula – Relative Height = Tilt – (Beam Width / 2) X Range in NM X 100 Feet
AWR Controls in Weather Mode
Target Alert, Contour Setting and Manual Gain Control are available in Weather Mode of AWR
Manual Mode and Gain can be selected for ranges beyond 70 NM
Contour Setting button converts the colour radar to a black and white display
Target Alert facility warns the crew of dangerous clouds beyond selected range when the range selection is less than 50 NM
Flashing Red Square would indicate dangerous clouds must be between 60 to 160 NM within 15° either side of the aircraft
Map Mode using Cosecant Squared or Fan Beam
Cosecant Squared or a Fan Shaped beam is used for ground mapping at short ranges lesser than 60 NM
Transmission power is varied as cosecant squared of the depression angle in the cosecant squared beam
Equal intensity returns are generated at all distances since the farther part of the beam has higher power than the nearer part
Gain is adjusted manually by gain dial and Depression angle can be varied up to 85° vertically and 3.5° in azimuth
Airborne Radar returns form flat terrain, water, sand and snow produce weak returns while built-up areas and mountains produce bright returns
Mountains cause radar shadows which shows hills in the shadow of larger mountains to appear as a series of lakes
Control Panel of Weather Radar
Control Panel of AWR provides controls for Stabilisation, Range, Standby, Tilt, Pan and Manual Gain
Function Dial selects Map or Weather mode depending on the requirement
Stab On Position keeps antenna horizontal to earth surface while Stab Off Position locks antenna in the Fore and Aft axis
Range dial is used to select the maximum scan distance
Stand By button keeps radar in stand-by or non-operational mode
Tilt dial tilts antenna up to 15° up and down and move radar antenna by 90° either side of the nose of aircraft
Gain dial used for manual selection of signal strength in Map mode only
Aircraft Position using Airborne Radar
Aircraft’s position can be fixed using the airborne weather radar using the Fan Beam between 15 to 70 NM
Select a prominent ground feature like a bridge over a river on the map and find the relative bearing and distance of the feature using the radar
Convert the relative bearing to true bearing and slant range to plan range
Plot the reciprocal of true bearing and plan range from the selected point to obtain a position line
Electronic Flight Display System (EFIS)
Additional information is available in colour radar inbuilt into Electronic Flight Display System
EFIS can be selected to provide Weather Only, Weather and Turbulence, Target Alert facility and Wind Shear Alert
Variable Gain Control For Mapping, Test Button and Hold or Pause button is also available in EFIS