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Frequency-Modulated Continuous-Wave Radar
Figure 1: Ranging with an FMCW system
Figure 1: Ranging with an FMCW system
CW radars have the disadvantage that they cannot measure distance, because there are no pulses to time. In order to correct for this problem, frequency shifting methods can be used. In the frequency shifting method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects. When a reflection is received the frequencies can be examined, and by knowing when in the past that particular frequency was sent out, you can do a range calculation similar to using a pulse. It is generally not easy to make a broadcaster that can send out random frequencies cleanly, so instead these Frequency-Modulated Continuous Wave radars (FMCW), use a smoothly varying “ramp” of frequencies up and down.
| R = | c0 · Δt | c0 = speed of light = 3·108 m/s Δt = measured time-difference [s] R = distance altimeter to terrain [m] |
(1) |
| 2 |
This kind of radar is used as „radar altimeter” often. The radar altimeter is used to measure the exact height during the landing procedure of aircraft. Radar altimeters are also a component of terrain avoidance warning systems, telling the pilot that the aircraft is flying too low or that terrain is rising to meet the aircraft.
