Plan-position indicator circuits

The Plan-Position Indicator (P.P.I.) is a form of radar display which gives a map-like presentation of the echoes from objects in the vicinity of the radar set. The antenna system of the radar set is highly directional and the beam is continuously rotated in order to give full angular cover. The display is generated by rotating about its starting point a time base which is intensity-modulated by the echo signals: the rotation is synchronous with the antenna rotation. The systems described in the paper were developed for use with airborne radar equipments in which the time-delay of the echoes is proportional to the distance from aircraft to reflecting object. This distance is called the slant range, whereas the range that it is required to display is the distance from the reflecting object to a point on the ground immediately below the aircraft, this distance being called the ground range. The correction to be applied to the delay time is a function of height and slant range and the paper describes several ¿height-correction¿ circuits. The display obtained in the aircraft will normally move across the face of the cathode-ray tube at a rate proportional to the velocity of the aircraft and some blurring of the presentation will therefore occur. Circuits are described which eliminate this motion of the display by shifting the centre of the display at a rate proportional to aircraft velocity, arrangements also being made, if desired, to maintain the North direction of the map-like presentation at a fixed angle on the tube for easy comparison with maps. The first part of this paper discusses methods of obtaining these displays using electrostatically-deflected tubes. The second part describes corresponding developments using electro-magnetically-deflected tubes. In each case several methods are described in outline and one in some detail.