Airborne Weather Radar — Page 220, Lesson 203

We're starting a fresh topic now: the Airborne Weather Radar, or AWR. This is the radar you'll use to see weather ahead of the aircraft, and it's a critical tool for avoiding thunderstorms and turbulence. Let's look at the function switch on the control unit, because that's the heart of how you operate this system. The function switch has four positions: MAP, MAN, WEA, and CONT. Each one changes the radar's beam shape and how it processes the return signal, so let's go through them one by one. First, the MAP position. In MAP, the radar produces a mapping beam. The purpose here is to give you a picture of the surface features below—the terrain, coastlines, and so on. To get an even presentation of those surface features, the transmitted power is progressively reduced as distance decreases. That means the power directed to the closest object is minimum. This reduction in power with decreasing range is a function of the cosecant of the depression angle—hence the name "cosecant² beam." Another description for it is a "fan-shaped" beam. Its dimensions are 85° deep in the vertical plane and 3.5° in azimuth. So it's a wide, flat fan of energy. The signal amplification is adjustable via the adjacent MANUAL GAIN knob. Now, the mapping ranges. The minimum mapping range is 15 NM, and the maximum is 60 to 70 NM. These depend upon the aircraft's height and the type of terrain. If you need to map beyond 70 NM, you should use the conical pencil beam by selecting the MANUAL position—that enables the gain to be adjusted for ground mapping. Next, the MAN position. MAN is used for cloud detection and mapping between about 70 and 150 NM. It selects the conical pencil shaped beam. And here, the MANUAL GAIN for signal amplification is operative with this selection. So you have manual control over the gain. Then we have WEA. WEA also selects the conical pencil beam, and it's the usual position for observing cloud formations. But here's the key difference: the MANUAL GAIN control is now INOPERATIVE. Instead, a facility called Swept Gain, Sensitive Time Control, or Automatic Gain Control—AGC—is automatically available. This system of circuits decreases the gain for echoes received from the ever decreasing ranges of clouds. It operates up to about 25 NM and ensures that the intensity, or brilliance, of the display of a particular cloud is independent of range. So a small cloud at 5 NM does not give an increasingly stronger return than a larger and more dangerous cloud at 20 NM. All clouds up to about 25 NM are thus compared on equal terms. That's the whole point of Swept Gain—it levels the playing field so you can judge cloud severity fairly. Finally, CONT. CONT stands for CONTOUR. Figure 13.15 shows a cloud formation presentation with CONT selected for a colour display. The darker colours indicate dangerous areas of concentrated rainfall and potential turbulence. The degree of danger depends upon the steepness of the rainfall gradient. Therefore, the narrower the paint surrounding a red area, the greater the danger from turbulence. Hooks, scalloped edges, finger protrusions, and U-shapes are also indicators of potential areas of severe turbulence. The Swept Gain facility, or automatic gain control, is also in operation in the CONT position and ensures that a display's intensity does not vary as range decreases. So to tie it together: MAP gives you a fan-shaped beam for terrain mapping with manual gain, MAN gives you a pencil beam for longer-range mapping with manual gain, WEA gives you the same pencil beam but with automatic gain control for fair cloud comparison, and CONT adds the contour feature to highlight dangerous rainfall gradients and turbulence indicators. Each position is a tool for a specific job, and knowing which one to select—and what the display is telling you—is part of flying safely in weather.