The basic output of  a radar module is the doppler frequency, it is a measure for the speed of the moving object.

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The output signals from the mixer have amplitudes as low as a few microvolts, careful amplification and filtering is needed before a microcontroller can read the doppler signals.

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For a 24 GHz module the relation is :

44 Hz / km /h 

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A car driving at 100 km/h will show a doppler frequency of 4400 Hz. 

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Radars with 2 channels can measure the direction of motion ( I/Q detection )

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For this mode of operation the transmit frequency is not modulated. It is called 

CW mode ( continous wave )

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​This is how door openers or security alarms are working, sensing motion by analyzing the doppler frequency. Analysing the doppler spectrum is the key for object classification.