RAdio Detection And Ranging. Send a radio pulse out into the world, listen for the echo, and time how long the round trip took. Multiply the delay by half the speed of light and you have a number: the distance to whatever bounced the wave back. Do it once a millisecond. Spin a dish, scan a phased array, sweep a continuous chirp; suddenly you have a moving picture of every airplane within 200 nautical miles, every car within 200 meters, every raindrop in a thunderstorm cell, and every fighter trying not to be seen.
Radar is a particularly demanding application of everything we have built in earlier chapters. The wave physics from Chapter 0 tells us a moving target shifts the reflected frequency. The Fourier and matched-filter machinery from Chapter 3 lets us pull a faint echo out of receiver noise. The modulation tricks of Chapter 7 reappear as FMCW chirps. The antennas of Chapter 13 are weaponized into 70-meter parabolas and active electronically scanned arrays. The microwave devices of Chapter 18, the magnetrons and klystrons and traveling-wave tubes, are how we generate the multi-megawatt pulses in the first place. Radar is where every chapter in this curriculum cashes in at once.
The pure physics is a one-liner: light goes out, light comes back, time the trip. The engineering, on the other hand, has spawned entire industries, entire branches of mathematics, and an entire field of electronic warfare. Three things conspire to make radar hard. First, the round-trip path-loss. The transmitted pulse spreads out on the way to the target, the target reflects only a tiny patch of it, and that reflection spreads out again on the way back. By the time the echo reaches the receiver, you are listening for a signal sometimes a hundred trillion times weaker than the transmitted one. Second, the world is full of unwanted reflections, called clutter, that are usually larger than the targets you want. Third, your adversary, if you have one, will deliberately try to deny, deceive, or confuse your radar. The history of radar from 1935 to today is the history of getting better and better at all three problems.
This chapter walks through the entire system. We start with the canyon-echo intuition, derive the radar equation step by step, work out why the round-trip dependence is to the fourth power of range and what that implies for transmitter design, then dig into pulse radar, CW and FMCW radar, MTI and pulse Doppler radar, the matched-filter receiver, mechanical and phased-array scanning, tracking radars, and finally the applications and the security battle. By the end you should be able to look at any radar, from a $30 police gun to an Aegis destroyer, and understand in your bones what it is doing.