Radar Principles — Page 192, Lesson 180

Let’s pick up right where the radar picture gets practical. We’ve already seen how a radar sends out a pulse and times the echo to find range. Now I want to walk you through the factors that decide whether that pulse actually comes back cleanly — and what limits how close and how accurate your radar can be. First, the weather. Rain absorbs and scatters the radar energy, and the total effect depends on two things: the size of the water droplets and the wavelength you’re transmitting. Here’s the rule of thumb. At wavelengths longer than 10 centimetres, attenuation is negligible — the rain barely touches your signal. Between 10 and 4 centimetres, attenuation is significant only in tropical rain. But once you drop below 4 centimetres, attenuation becomes significant even in temperate-latitude rain. And there’s a hard conclusion drawn from this: wavelengths less than 3 centimetres should not be used for long-range systems. Now, different radar types pick different wavelengths for exactly this reason. The Airfield Surface Movement Indicator — that’s ASMI — operates at 1.75 to 2 centimetres. Airborne Weather Radars, AWR, and Precision Approach Radars, PAR, both use 3 centimetres. Ground-based surveillance radars use 10, 23, or 50 centimetres. Notice the pattern — the longer wavelengths are for the ground surveillance radars that need range, and the shorter ones are for the close-in, high-resolution jobs. Next, atmospheric conditions. This is a fascinating one because the atmosphere can actually bend the radar waves. Normally the waves travel in straight lines, but certain conditions refract them. When the waves bend downwards toward the earth’s surface, that’s called super-refraction. It gives you ranges beyond normal line of sight — over-the-horizon capability. That happens when there’s a temperature inversion and a decrease in humidity with height. On the flip side, you can get sub-refraction, where the waves bend upwards away from the surface, and that reduces the theoretical range of the radar. Now let’s talk about the pulse itself, because pulse width decides your minimum range. Look at Figure 11.7. Radio waves travel at 300,000,000 metres per second. In 1 microsecond — that’s one millionth of a second — they travel 300 metres. So a pulse 1 microsecond wide physically extends 300 metres in space. Think about what that means. If an object is at 150 metres, it reflects that pulse, and the echo arrives back at the receiver just as the tail of the pulse is leaving the transmitter. Any object closer than 150 metres would reflect a pulse that couldn’t be received, because the transmitter would still be transmitting — the receiver simply can’t hear while the transmitter is on. There’s also a second consequence. Two objects in line, 150 metres or less apart, would appear as a single return — you couldn’t separate them. So if you need short-range operation for target resolution and accuracy, you use short pulses — for example, 0.1 microseconds. And there’s one more design factor I want to mention: restoration time. That’s the time it takes for the receiver to recover to normal after transmission has occurred. It’s a design factor that affects how quickly the receiver is ready to hear again after the transmitter has fired. So the whole picture is this: pulse width sets your minimum range and your ability to resolve two close targets, restoration time sets how fast the receiver recovers, and the wavelength you choose trades off rain attenuation against range and resolution.