Airborne weather radar

Navigation · Chapter 42 · DGCA CPL and ATPL

Why 3 cm, reading the colours and hail shapes, beams and modes, cloud tops by tilt, and flying round storms.

Remember

9 375 MHz (3.2 cm): big drops and wet hail; not fog, not CAT.
Stabilised antenna; pencil beam for weather, fan (cosecant²) for short-range mapping.
Green 12 mm/h, magenta turbulence; steep gradient and hooks = danger.
Cloud top above you ≈ (tilt − ½ beam) × range × 100 ft. Beware shadow areas.
STANDBY on the ground until the area is clear; WEA has swept gain, MAP/MAN manual gain.

Eyes for storms

What it is for • AWR (airborne weather radar) shows where the heavy rain is ahead, and so where the turbulence is • Also: cloud-top heights (using the tilt), a map of the ground below, and a range and bearing fix off a landmark
The parts • A transmitter-receiver, an antenna in the nose, a display and a control box • The antenna is stabilised in pitch and roll: it stays level with the horizon while the aircraft climbs or banks
Primary radar • It is a primary radar: the echo time gives the range • The sweep gives the bearing relative to the nose; range and bearing lines are drawn on the screen
What the beam touches • Drag the tilt. The bright ring is where the beam meets the storm • On the wet core: red. On the dry ice top: almost nothing. Above it: nothing at all, though the storm is still there

Why 3 cm, why a narrow beam

Match the drops • Echoes are strongest from things about as big as the wave. The drops that mean turbulence, big rain and wet hail, are about 3 cm • So the radar works on 9 375 MHz (X band): 300 ÷ 9 375 = 3.2 cm
Not shorter, not longer • Move the wavelength. Shorter: every small droplet answers (clutter) · longer: the wave rolls past even the hail • Fog, stratus and clear-air turbulence (CAT) have no big drops: not shown
Beam width • The beam is a 3–5° pencil, so it gets wider the further it goes • Far away it is wider than the gap between two storms: they paint as one. Closer, they split • A narrower beam needs a bigger dish: no room in the nose
Water and ice on the radome • Water in the radome (the nose cover) or ice on it soaks up the energy, like food in a microwave oven • Echoes come back weaker, so storms look milder than they are