The FMCW radar calculates this by evaluating the Doppler effect.
You probably know the Doppler effect from everyday life? When a vehicle with a blaring siren approaches, the frequency of the sound increases slightly. As soon as it passes, the sound lowers a little and the frequency drops. The fundamental tone, however, always stays constant. This phenomenon is the Doppler effect, and radar can measure this in the electromagnetic range.
The radar beam spreads forward in a cone shape. In order to be able to detect several lanes on motorways, many manufacturers use a three-beam radar. This results in three radar cones that extend the horizontal field of view. Nowadays, the radar control units are often FlexRay participants (note/node) in the data bus system of the vehicles. Sensor and control unit form the radar for this purpose.
When taking turns, the control unit still needs supplementary data from other data bus participants. Additional sensors, such as the rate-of-turn indicator, steering angle sensor, wheel speed sensors, camera control unit, and many more, are necessary so that the distance can also be maintained correctly in bends. Information from the GPS system is important as well, since topographical data is also included.
The radar, just like the camera, needs to be calibrated. The radar is calibrated according to the manufacturer's specifications and not necessarily after every change of the windscreen. The radar is always calibrated when a wheel alignment, with changes to the front or rear axle, has been carried out. It is calibrated on the basis of the geometric driving axle, as is the camera.
In the static calibration procedure, a distinction is made between "passive" and "active" radar. The "passive" radar must be aligned according to the manufacturer's specifications using an alignment laser. An "active" radar performs a test measurement when requested and transmits the direction of rotation in which the adjusting screws on the radar head should be set. The test measurement determines the misalignment angle. In this process, the radar beam is projected against a solid steel plate that has been placed according to the manufacturer's specifications. In this way, the horizontal and vertical axes are adjusted in both radar systems (active and passive).
In addition to these, there are radar systems that are calibrated dynamically, that is, while driving.
Images
Direct Radio Waves: Charly Whisky - Eigenes Werk, CC BY-SA 3.0, https://commons.wikimedia.orgwindex.phpcurid=19663736.png
Indirect FMCW: Georg Wiora (Dr. Schorsch) - Self drawn with Inkscape, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=353385
Doppler Effect: Charly Whisky 18:20, 27 January 2007 - Eigenes Werk, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1606823