doppler: radar speed trap

The received frequency of the radar is of very high frequency (about 10^10 Hz). If you measure the received frequency of receiver directly, there will be a great uncertainty in measuring it and hence deducing a great discrepancy.

For the beat frequency, it is of much lower order of frequency. Hence, the frequency can be measured more accurately and hence decreasing the uncertainty in deducing the speed of the car.

This is the reason to use the beat frequency.