Airborne Weather Radar — Page 208, Lesson 196

Let’s start at the very beginning of the Airborne Weather Radar chapter. I want you to think of the AWR, the Airborne Weather Radar, as a primary radar. That’s the first big label. A primary radar works by sending out a pulse of energy, and then listening for the echo that comes back off a target. The AWR has two jobs, and both of them use that same echo principle. The first job is weather detection, and the second is ground mapping. Now, the echo principle gives you the range to the target, and the searchlight principle gives you the relative bearing. Let me unpack that. The searchlight principle just means the antenna sweeps a narrow beam around, like a searchlight, and the direction the beam is pointing when it gets an echo tells you which way the target is. So range comes from the echo timing, and bearing comes from the beam direction. To help you read that picture, the display has range lines and azimuth marker lines. Azimuth is just the fancy word for bearing, the direction in degrees. So the range lines are the concentric circles showing distance, and the azimuth markers show direction. One thing I really want you to note, because it catches people out: the range you see on the display for ground targets is the slant range, not the true ground range. Slant range is the straight-line distance from the aircraft down to the target, measured along the beam. Because the aircraft is high up, that line is angled. To get the actual ground range, the horizontal distance along the surface, you use the Pythagoras formula. It’s a right-angled triangle: the aircraft’s height is one side, the ground range is the other, and the slant range is the hypotenuse. So ground range squared equals slant range squared minus height squared. Now let’s talk about the antenna. The radar beam is produced by an antenna mounted in the nose of the aircraft. The antenna shape can be parabolic, which is the dish shape, or a flat plate. And the shape of the antenna determines the shape of the beam it produces. You get two types of beam. One is the conical, or pencil-shaped, beam. The other is the fan-shaped, or cosecant squared, beam. Cosecant squared is just the mathematical name for that fan pattern. The type of radiation pattern you use depends on what you’re doing. The pencil beam is used for weather detection and for longer-range mapping, and I want you to remember that threshold: longer range means greater than 60 nautical miles. The fan-shaped beam is used for short-range mapping, so that’s the 60 NM and below case. When you use the radar in the mapping mode, it’s usually necessary to tilt the antenna down. That makes sense, because you’re trying to paint the ground ahead of you, so you point the beam down toward it. And here’s a really important detail about the antenna: it is attitude-stabilized in relation to the horizontal plane. That means it uses the aircraft’s attitude reference system to keep the beam level with the horizon. Why does that matter? Because if the aircraft is banking or pitching during manoeuvres, without that stabilisation the presentation on the display would become lopsided. So the antenna is held steady relative to the horizon, not relative to the aircraft’s nose. 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 width of the beam. And the beamwidth must be as narrow as possible for efficient target resolution. Resolution here means the ability to see two separate targets as two separate returns. Let me give you the example from the book. Imagine two clouds at, say, 100 nautical miles. With a wide beam, they might appear as one large return, merged together. But as you get closer, at a shorter range, they are shown correctly as separate entities. So a wide beam blurs them together at long range. Now here’s the trade-off, and it’s a classic one in radar. A narrower beam would give you better definition, better resolution. But a narrower beam requires a larger antenna. And a larger antenna becomes impractical in an aircraft, because you can’t fit an enormous dish in the nose. So how do you get a narrow beam without a huge antenna? The answer is to use shorter wavelengths. That’s the key relationship: to produce the narrower beams, it is essential to use shorter wavelengths. So wavelength and beamwidth are linked through the antenna size. Let me just pull that together. You’ve got a primary radar with two functions, weather and mapping. Range from echo timing, bearing from the searchlight sweep. Slant range on the display, Pythagoras for ground range. A nose antenna, parabolic or flat plate, giving either a pencil beam for weather and long-range mapping beyond 60 NM, or a fan-shaped cosecant squared beam for short-range mapping. Tilt down for mapping, attitude-stabilised to keep the picture level. And a 3° to 5° pencil beam, kept narrow for resolution, with shorter wavelengths as the way to achieve that narrowness in a practical aircraft antenna.