How the early radars could know the angle of a basic radar rotating antenna?
How the direction of the time base on radar screen is synchronized with the direction of the radar rotating antenna?
The terminals of the potentiometer were driven with complementary copies of the sawtooth waveform generated by the radar timebase, and the taps of the potentiometer fed the horizontal and vertical deflection amplifiers of the display tube, creating a radial scan at the correct angle that's synchronized to the receiver output.
Note that this system even allows the operator to interrupt the normal periodic scan in order to sweep the radar beam across a specific target. The display remains synchronized to the antenna at all times. Note also that "pan & zoom" of the display is possible by varying the gain and DC offsets of the deflection amplifiers.
Modern digital systems simply use a shaft encoder on the antenna.