i think it has something to do with the frequency the initial carrier wave is emitted from. since it works on the doppler effect, there has to be a start frequency. as the wave hits the object, the frequency shifts on its way to the return point antenna. maybe the calibration is assuring it continuously sends and receives at various distances via doppler shift to correlate actual speeds...... n stuff.

i really dont know what i am talking about. i totally just bullshitted that.