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Speaking of cars and radars, police have been using radar to trap speeders since just after WWII. Read a poem penned by the father of radar on the event of him getting caught!
Once considered a career killer, automotive radar systems had to wait for the silicon revolution in RF circuits to catch up to millimeterwaves. Now you can make a radar at price points that car companies can handle, at a few dollars for the chipset. This was not possible in the late 1990s when microwave companies first started developing radars for the masses.
There are two principle types of radars that are featured in cars today. Blind spot detectors are often used in mirrors and provide a visual indication that cars might be in your blind spot and you shouldn't change lanes on the highway. A better method for determining whether a car is in your blindspot is to turn your head before you change lanes and take a look. Blind spot detection is mostly done at 24 GHz.
The Holy Grail of car radar is adaptive (or autonomous) cruise control (ACC), to assist the driver in reducing speed when he is approaching a car or other obstacle in front of him. The cruise control can lock onto a slower moving car and merely follow it at a prescribed, non-tail-gating distance. ACC is mostly done at 77 GHz. Some of the problems that engineers have had to work out are significant clutter, sign posts and guard rails, and the possibility of two cars with ACC driving next to each other. ACC systems typically use FMCW (frequency modulated continuous wave) operation, so that range and range rate are provided. A minimum of four scanned beams are are necessary, to allow the little radar to see around turns. Indeed, highways in Japan have much smaller radii, and so ACC systems designed for Europe won't work there (more beams might be required in Japan). The beam former might be a Rotman lens (or it might not be, we are speculating...) Power out is on the order of 10 mW, now achievable in RF CMOS. Noise figure on receive might be 10 dB or more, which is adequate for the 100 meter range that is typically required.
Audi's ACC video
If you rely on either one of these radar assist devices, you might want to consider that you are actually a bad driver and should not be on the highway. However, these microwave devices are key features of autonomous cars that are now free to roam the highways, at least in Nevada (first state to allow this) and now California. Thanks to efforts of Google, Governor Brown signed SB1298 on September 25, 2012, which allows autonomous cars onto California roads so long as a licensed driver is behind the wheel ready to assist in an emergency (and get a ticket if the car is speeding). By 2017 it is expected that the assist driver will no longer be required by law.
Let's not forget that the overall driver of autonomous cars is the Global Positioning System, another microwave work of art that fellow microwave engineers should brag about, just like investment bankers brag about how much money they dipped out of the Wall Street money stream into their personal bank accounts. Which career would Jesus be more proud of?