 PPI displays are actually quite similar to A-scopes in operation, and appeared fairly quickly after the introduction of radar.As with most 2D radar displays, the output of the radio receiver was attached to the intensity channel to produce a bright dot indicating returns.In the A-scope a sawtooth voltage generator attached to the X-axis moves the spot across the screen, whereas in the PPI the output of two such generators is used to rotate the line around the screen.