Several Doppler radar antennas pointing downwards at an angle receive the radar echoes reflected from the ground, the frequency shifts of which depend on the airspeed and the radiation angle. From this, the precise ground speed and the drift can be calculated, as well as the exact flight distance from the integral of the speed.
In the first systems (developed in the 1950s) the antennas were swiveled sideways until their Doppler shift left and right had the same value, which resulted in the true course over ground . Newer systems analyze the frequency shifts mathematically and achieve accuracies of about 1 per thousand of the distance covered. This makes the process 5 to 10 times more precise than classic dead reckoning and also independent of the weather conditions.
In the 1970s, the Stuttgart Institute for Flight Navigation (Prof. Karl Ramsayer ) developed automatic coupling cards based on the Doppler radar with a display of the current course and current position. Today this measurement data is integrated into the displays for integrated navigation .