Airborne Weather Radar — Page 220, Lesson 203

We're starting a fresh topic now: the function switch on the airborne weather radar control unit. I want to walk you through the four positions of that switch — MAP, MAN, WEA, and CONT — because each one selects a different beam shape and a different gain control philosophy, and that's the heart of how the radar is used. Let's start with MAP. In the MAP position, the radar produces a mapping beam. The purpose here is to give you an even presentation of surface features — you want the ground returns to look uniform so you can actually interpret what's below you. To achieve that evenness, the transmitted power is progressively reduced as distance decreases, so that the power directed to the closest object is minimum. Think about that: without this reduction, the nearest objects would return a blindingly strong signal and wash out everything else. The reduction in power with decreasing range is a function of the cosecant of the depression angle — that's why it's called the cosecant² beam. Another description you'll see is "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. Signal amplification is adjustable via the adjacent MANUAL GAIN knob — you have direct control over the gain here. Now, the mapping ranges. The minimum is 15 NM and the maximum is 60 to 70 NM, and these depend upon the aircraft's height and the type of terrain. If you need to map beyond 70 NM, you should select the MANUAL position, which uses the conical pencil beam instead — that enables the gain to be adjusted for ground mapping. So MAP is your short-to-medium range fan beam; MANUAL is your long-range pencil beam. Let's move to MAN. This is used for cloud detection and mapping between about 70 and 150 NM, and it selects the conical pencil shaped beam. MANUAL GAIN for signal amplification is operative with this selection — so again, you're in control of the gain. This is the long-range workhorse. Now WEA. This also selects the conical pencil beam, and it's the usual position for observing cloud formations. But here's the key difference: MANUAL GAIN control is now INOPERATIVE. Instead, a facility called Swept Gain, Sensitive Time Control, or Automatic Gain Control — AGC — is automatically available. These are three names for the same system of circuits. What does it do? It decreases the gain for echoes received from the ever decreasing ranges of clouds. It operates up to about 25 NM, and it ensures that the intensity — the 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: you're judging clouds by their actual reflectivity, not by how close they are. Finally, CONT — that's 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. Now, the degree of danger depends upon the steepness of the rainfall gradient. So the narrower the paint surrounding a red area, the greater the danger from turbulence. And there are specific shape indicators: hooks, scalloped edges, finger protrusions, and U-shapes are all indicators of potential areas of severe turbulence. The Swept Gain facility — or automatic gain control — is also in operation in the CONT position, and it ensures that a display's intensity does not vary as range decreases. So to tie it together: MAP gives you the fan-shaped cosecant² beam for short-range ground mapping with manual gain. MAN gives you the pencil beam for 70 to 150 NM cloud detection and mapping, also with manual gain. WEA gives you the pencil beam for cloud observation, but with Swept Gain/STC/AGC automatic — manual gain is inoperative. And CONT keeps that automatic gain but adds contouring, where the colour and the shape of the paint tell you about rainfall gradient and turbulence danger. That's the full picture of the function switch.