Radar Principle

The Doppler radar is used to measure velocities and a kind of continuous wave (CW) radar, where a constant frequency is transmitted by the radar. The transmitted signal is reflected from the targets and received by the radar. The reflected signals from unmoved objects are not changed by frequency, while moving targets change the received frequency due to the Doppler effect. By mixing the transmitted with the received signal, the Doppler frequency can be extracted. Due to the proportional relation between Doppler frequency and velocity, the velocity can be calculated from the measured Doppler frequency.

The frequency modulated continuous wave (FMCW) radar is based on a periodically linear frequency variation of transmitted signal. For distance measurement, sawtooth or triangular frequency modulations are used. While the transmitted frequency is linearly changed during the runtime to the target and back to the radar, the received signal is shifted by a time delay to the initial signal. By mixing the current transmitted signal with the reflected signal, the frequency difference caused by the runtime can be defined. Due to the known modulation parameters of the transmitter, the runtime of the signal can be calculated, which is proportional to the distance of the object. For a precise distance measurement, an extremely highly linear modulation of the transmitted frequency is necessary, because each nonlinearity of the modulation will decrease the accuracy of the radar.

Due to this indirect measurement of the runtime and by choosing adequate modulation parameters, even very nearby object can be measured precisely and cost-efficiently. The accuracy of distance measurement ist related to the bandwidth of the radar. By using transmit frequencies in the range of 100 GHz, bandwidths of several GHz are possible.