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Doppler Radar

Oscillogramm of an outputsignal of a phase-detector: The echo signal of a moving target at the output of the phase-detector changes it's value and also the polarity in every pulse period. A fixed cluttersignal will keep it's value and polarity in every pulse period.
fixed target
„Läuschen“
fixed target

Figure 1: Bipolar pulse: in German the so called „Läuschen“ (a small louse).

What is a Doppler-Radar?

Doppler-Radar

A Doppler radar is generally a radar which uses the Doppler effect upon reception of the echo signal. In modern radar this is always and without exception the case. For this reason, this additional attribute is generally dispensed with. If, nevertheless, the term “Doppler pulse radar” is used, this is only an explicit indication that this radar is not an FMCW radar.

History

However, this was not always the case in the history of radar. The first radars that could use a Doppler frequency for fixed-target suppression were the Würzburg giant, built during the Second World War. Especially those sets which were equipped with the accessory “Würzlaus”. This was a special filter for the intermediate frequency, that filtered out the interferences of chaff. This radar was thus the first Doppler radar in the world.

During the calibration, a coherent oscillator had to be tuned exactly to the intermediate frequency of the receiver (see Figure 1). Fixed targets generated a stable pulse shape. Moving targets with Doppler frequency generated a bipolar alternating pulse. In line with the camouflage name “Würzlaus” this bipolar impulse in German was called “Läuschen” until the late 80s.
 

The term “Doppler radar” was later reused with slightly modified meaning:

Use of the Doppler frequency

In modern radar sets, the Doppler frequency is used for a variety of purposes: