Radar Principles — Page 192, Lesson 180

Let’s pick this up right where the radar picture starts to get practical. We’ve already covered the basics of how a radar pulse is sent out and comes back. Now I want to walk you through the factors that decide whether that pulse actually reaches you, and how close together two targets can be before your radar can’t tell them apart. First, the weather. Look at Figure 11.6 in your mind’s eye — energy gets absorbed and scattered by raindrops. 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 here’s the hard conclusion the book draws: wavelengths less than 3 centimetres should not be used for long-range systems. Now, different radars pick different wavelengths for exactly this reason. The Airfield Surface Movement Indicator — that’s ASMI — operates at 1.75 to 2 centimetres. Airborne Weather Radar, AWR, and Precision Approach Radar, PAR, both use 3 centimetres. Ground surveillance radars use 10, 23, or 50 centimetres. So you can see the trade-off: shorter wavelengths give better resolution but suffer more in rain, so long-range ground systems go long. Next, atmospheric conditions. Normally radar waves travel in straight lines, but certain conditions can bend them. Super-refraction is when the atmosphere refracts the waves downwards toward the Earth’s surface. That gives you ranges beyond normal line of sight — over-the-horizon capability. It happens when there’s a temperature inversion and a decrease in humidity with height. On the flip side, sub-refraction is when the waves refract upwards, away from the surface, and that reduces the theoretical range. So the same atmosphere that can extend your reach can also cut it short. Now let’s talk about minimum range — this is the heart of Figure 11.7. Radio waves travel at 300,000,000 metres per second. In 1 microsecond — that’s 1 µs — they travel 300 metres. The pulse width, labelled W in the figure, decides your minimum range. Think about it this way. A pulse 1 µs wide extends 300 metres in space. If an object sits at 150 metres, it reflects that pulse back, and the return arrives at the receiver just as the tail of the pulse is leaving the transmitter. Anything closer than 150 metres reflects a pulse that can’t be received, because the transmitter is still transmitting — the receiver is blocked. And here’s the resolution consequence: two objects in line, 150 metres or less apart, would appear as a single return. So if you need short-range operation for target resolution and accuracy, you use short pulses — for example, 0.1 µs. One more design factor: 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 you can listen again after you’ve transmitted. So the whole picture is a balance. Pulse width sets your minimum range and your ability to separate two targets in line. Shorter pulses give you better resolution and shorter minimum range, but they carry less energy. Wavelength sets how the weather treats you and how far you can realistically go. And the atmosphere can bend your beam for better or worse. That’s the foundation of radar principles.