Airborne Weather Radar: Operation and Limitations

The Airborne Weather Radar (AWR) is a primary radar system designed to detect precipitation and allow the crew to avoid adverse weather conditions.

Operating Principles

• Frequency: Operates in the SHF band (Super High Frequency), typically at 9.375 GHz (X-Band).
• Wavelength: Approximately 3.2 cm. This wavelength is ideal for reflecting off large water droplets and wet hail, while ignoring very small particles like fog or drizzle.
• Reflectivity:
  • Wet hail and large drops: High reflectivity (Strong echoes).
  • Dry snow and ice crystals: Low reflectivity (Weak or no echoes).
  • Display Colors:
    • Green: Light precipitation.
    • Yellow/Amber: Moderate precipitation.
    • Red: Heavy precipitation.
    • Magenta: Extreme precipitation or severe turbulence.

Functions and Modes

Weather Mode

Uses a narrow conical beam (Pencil Beam, approx 3-5° wide) to scan ahead of the aircraft.

• Turbulence: Modern radars (Doppler) can detect turbulence by measuring the frequency shift (Doppler shift) of moving raindrops. Only works if there is precipitation (wet turbulence); it does not detect Clear Air Turbulence (CAT).
• Windshear: Uses the Doppler effect to detect wind shear (microbursts) ahead of the aircraft, generally at low altitudes.

Mapping Mode

Uses a fan-shaped beam (Cosecant Squared Beam) or tilts the conical beam downwards to obtain terrain echoes.

• Useful for basic navigation (coastlines, islands, mountains) when other systems fail.
• Distinguishes land (echoes) from water (no echoes).

Tilt Management

The pilot must manually adjust the Tilt to scan different levels of the atmosphere, although modern systems have automatic functions (Auto-tilt).

• Takeoff/Climb: High tilt (+4° to +15°) to see weather in the climb path and avoid nearby ground echoes.
• Cruise: Low tilt (0° to -1°) to scan the most reflective part of storms (middle/lower levels where liquid water exists).
  • Danger: At high altitude, if the tilt is too high, the radar may scan only the storm top (ice crystals) and show nothing, hiding severe activity below (Over-scanning).
• Descent: Tilt slightly up (+4° to +5°) to avoid ground clutter as the aircraft descends.

Limitations and Hazards

Attenuation (Shadowing)

Very intense precipitation can absorb or reflect all radar energy, preventing detection of what lies behind.

• Appears as a black "shadow" behind an intense red echo.
• Rule: Never fly into a radar shadow behind a storm; it is unknown what is there (possibly another severe storm).

Radiation Hazard

The radar emits high-energy microwave radiation. It should not be operated on the ground when personnel are near the radome or during refueling.

Storm Avoidance

• Deviate at least 20 NM laterally from intense echoes.
• Preferably deviate to the upwind side to avoid hail and turbulence that the wind may carry outside the cloud.
• Do not attempt to fly under a storm (danger of microbursts and hail).