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P-12 Sichtgeräte

Figure 1: Display racks of the “Spoon Rest B” radar with the goniometer


A Goniometer (from ancient Greek γωνία and μέτρον) is an angle measuring device. In a radar set, a goniometer is a special device for measuring the elevation angle of an incoming echo signal. The elevation angle and the measured distance can then be used to calculate the altitude of the aircraft from which the echo signal originated.

Altitude determination with a goniometer requires a radar antenna with two antenna planes mounted one above the other and whose received signals are fed separately to the goniometer. The goniometer measures the phase difference between the two signals received by the antenna planes. The goniometer contained two slip rings on which the received RF pulses from each antenna plane produced a standing wave. A slider, constantly moved back and forth by a motor, sampled the signals on these line sections and formed a difference between the two signals. In a given position, this RF voltage difference was zero. This mechanical position was associated with an elevation angle and displayed on an E-scope The conversion from elevation angle to an altitude was distance-dependent and could be made with a nomogram attached to the screen.

In the Russian radars in the VHF range, this elevation angle measurement was already performed on the simplest radar P-3 (NATO-Designator: “Knife Rest”). In the radar P-12 (NATO-Designator: “Spoon Rest B”), the goniometer is located between the two display racks with the control panels: easily recognizable by the illuminated green circular scale.

Tactically, this elevation angle measurement had little value: the accuracy of this measuring was about 1000 meters. However, the antenna had to be moved very slowly over the target, even stopped if the target was weak. This procedure was no longer possible with a more complicated dynamic air situation. Therefore, this measurement option was abandoned again with the P-18.