Airborne Weather Radar — Page 220, Lesson 205

We're starting a new topic now: the mapping operation of the airborne weather radar, and then the pre-flight checks. Let's get into it. For the basic monochrome AWR — that's the airborne weather radar, the single-colour display type — the maximum range is 150 nautical miles. Now, the radar uses two different beam shapes, and which one you use depends on how far you want to map. Up to about 70 nautical miles, you select the MAP position, and that uses the cosecant² beam. That's the fan-shaped beam. The name comes from the mathematical function — cosecant squared — which shapes the beam so it's broad and flat, spreading energy out over the ground close to the aircraft. That's ideal for mapping terrain directly beneath and ahead of you. Beyond 70 nautical miles, you switch to the MAN position, and that uses the pencil beam — a narrow, concentrated beam that reaches out much further. Both of these modes have manual gain control, so you can adjust the receiver sensitivity yourself to improve the quality of the radar picture you get on the display. Now, the key adjustment for mapping is the downward tilt. You adjust the tilt downward to get the best target presentation. But here's the important part about what reflects and what doesn't. Very little energy reflects from a calm sea, from fine sand, and from flat terrain. So those surfaces give you weak returns. On the other hand, coastlines, built-up areas, skyscrapers, bridges, and power stations — those give very bright returns, because they have hard, angular surfaces that reflect strongly. Ice is interesting. Ice has jagged edges, and those jagged edges do reflect the radar energy. But snow is a poor reflector, and it actually masks ground features — it hides what's underneath it. So if you're mapping snow-covered ground, you lose the detail. There's also a classic false image to watch for. When you fly over high ground, the mountains or hills block the radar beam beyond them, creating what's called a radar shadow. That shadowed area behind the high ground shows up on the display as a dark region, and it can give you a false impression of a series of lakes. So if you see what looks like water on the display, remember it might just be hill shadow — the radar shadow cast by high terrain. That's Figure 13.16 in your materials. Now, before you ever switch the radar on, you have to think about pre-flight checks. Electromagnetic radiation is a serious hazard — both to personnel and to electronic equipment. So great care is required before you check the radar on the ground. There are two precautions to take. First, ensure the aircraft is clear of personnel, other aircraft, vehicles, and buildings — you don't want to be radiating energy at anyone or anything. Second, select the conical beam with maximum uptilt, then switch the radar on, check that you have a picture, and then go back to standby. That way, the beam is pointed up and away from people and equipment while you verify the radar is working, and you don't leave it radiating. So to summarise the whole picture: mapping uses either the fan-shaped cosecant² beam up to 70 NM, or the pencil beam beyond that, with manual gain in both. You tilt down for best presentation, you know what reflects brightly versus what reflects poorly, you watch for hill shadow false lakes, and you follow strict precautions on the ground before radiating.