Airborne Weather Radar — Page 208, Lesson 196

Let’s start with the big picture. The Airborne Weather Radar — we call it the AWR — is a primary radar. That’s a key phrase. A primary radar works by sending out a pulse of energy and listening for the echo that bounces back off a target. It doesn’t rely on the target transmitting anything back to us. So both of the AWR’s jobs — weather detection and ground mapping — use that same echo principle. Now, the echo principle gives us range, which is how far away the target is. But to know where the target is, we need a second idea: the searchlight principle. Think of the radar beam like a torch beam sweeping around. The direction the beam is pointing when the echo comes back tells us the relative bearing of the target — that is, its direction measured relative to the aircraft’s heading. So, echo gives range, searchlight gives bearing. To read that on the display, we have range lines and azimuth marker lines available. Range lines are the concentric circles showing distance from the aircraft. Azimuth marker lines are the radials showing bearing. Together they let you plot a target’s position. One important trap here: the range you read off the display for ground targets is slant range, not ground range. Slant range is the straight-line distance from the aircraft to the target, measured through the air — which includes the height difference. If you want the true distance along the ground, you use the Pythagoras formula. You’ve got a right-angled triangle: the slant range is the hypotenuse, the aircraft’s height is one side, and the ground range is the other. So ground range squared equals slant range squared minus height squared. Now let’s look at the antenna. The radar beam is produced by an antenna mounted in the nose of the aircraft. The antenna shape can be parabolic — that’s the classic dish shape — or a flat plate. These shapes can produce two different types of beam. One is a conical or pencil-shaped beam — a narrow, focused beam like a torch. The other is a fan-shaped or cosecant squared beam — a broad, flat beam that spreads out. Which one you use depends on the job. The pencil beam is used for weather and for longer range mapping — specifically beyond 60 NM, that’s 60 nautical miles. The fan-shaped beam is used for short range mapping. Here’s a practical point: when you use the radar in mapping mode, you usually need to tilt the antenna down. That’s because you’re trying to illuminate the ground ahead of you, and the ground is below the aircraft’s horizontal plane. There’s also a stabilisation system. The radar antenna is attitude-stabilized in relation to the horizontal plane, using the aircraft’s attitude reference system. Why does that matter? Because if the aircraft banks or pitches during manoeuvres, the antenna would tilt with the aircraft and the picture on the display would become lopsided. The stabilisation keeps the beam level with the true horizontal, so the display stays upright and usable even while you’re turning. Now, the radar beam itself. The pencil beam used for weather depiction has a width of between 3° and 5°. That’s the beamwidth — the angular spread of the beam. Why does beamwidth matter? Because it determines target resolution — how well you can separate two targets that are close together. The beamwidth must be as narrow as possible for efficient resolution. Here’s the classic example: two clouds at, say, 100 NM might appear as one large return on the display. They look merged into a single blob. But as you get closer, the beam narrows relative to the separation, and they show up correctly as separate entities. So a wide beam blurs close targets together; a narrow beam separates them. So why not just make the beam as narrow as we like? Because a narrower beam would give better definition, but it would require a larger antenna — and a larger antenna becomes impractical in an aircraft. There’s a physical limit to how big you can mount in the nose. That brings us to the final point: to produce the narrower beams, it is essential to use shorter wavelengths. The beamwidth depends on the ratio of wavelength to antenna size. If you can’t make the antenna bigger, you make the wavelength shorter — and that’s how you get the narrow beam you need for good weather resolution. So, to tie it all together: the AWR is a primary radar using echo for range and searchlight for bearing. The antenna in the nose produces either a pencil beam for weather and long-range mapping, or a fan-shaped beam for short-range mapping. You tilt down for mapping, and the antenna is attitude-stabilised to keep the display level. And the beamwidth — 3° to 5° for weather — is a trade-off between resolution and antenna practicality, resolved by using shorter wavelengths.