Plan position indicator

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Image of a thunderstorm line (in dBZ) seen on a 0.7 degree elevation PPI (NOAA)

The plan position indicator (PPI), is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles. As the radar antenna rotates, a radial trace on the PPI sweeps in unison with it about the center point.

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[edit] Description

Diagram showing the evolution of the height above ground, in kilometers, with the distance to the radar for the 24 PPI angles used on the Canadian weather radars (curved lines)

The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed angle. It can then change angle or repeat at the same angle according to the need. Return echoes from targets are received by the antenna and processed by the receiver and the most direct display of those data is the PPI.

It is to be noted that the height of the echoes increases with the distance to the radar, as represented in the image to the right. This change is not a straight line but a curve as the surface of the Earth is curved and sinks below the radar horizon. For fixed-site installations, north is usually represented at the top of the image. For moving installations, such as ship and aircraft radars, the top represents the front part of the ship or aircraft, i.e., its heading (direction of travel) and this is usually represented by a lubber line. The signal represented is the reflectivity at only one elevation of the antenna, so it is possible to have many PPIs at one time, one for each antenna elevation.

[edit] History

A photograph of an H2S PPI display taken during an attack on Cologne - the annotations were added later for post-attack analysis

The PPI display was first used prior to the start of the Second World War in a Jagdschloss experimental radar system outside Berlin. The first production PPI was devised at the Telecommunications Research Establishment, UK and was first introduced in the H2S radar blind-bombing system of World War II.

Originally, data was displayed in real time on a cathode ray tube, and thus the only way to store the information received was by taking a photograph of the screen. With the development of more sophisticated radar systems, it became possible to digitize data and store it in memory, allowing access at a later date.

Philo Taylor Farnsworth, the American inventor of all-electronic television in September 1927, has contributed to this in an important way. Farnsworth refined a version of his picture tube (cathode ray tube, or CRT) and called it an "Iatron." It could store an image for milliseconds to minutes (even hours). One version that kept an image alive about a second before fading, proved to be a useful addition to the evolution of radar. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar.

[edit] Uses

Simplified animation of a Plan Position Indicator radar display

The PPI is used in many domains involving radars, including air traffic control, meteorology, on board ships and aircraft etc. In meteorology, a competing display system is the CAPPI (Constant Altitude Plan Position Indicator) when a multi-angles scan is available.

Using computers to process data, modern sonar and lidar installations can mimic radar PPI displays too. (block diagram)

[edit] Bibliography

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