A plan position indicator (PPI) is a type of radar display that represents the radar antenna in the center of the display, with the distance from it and height above ground drawn as concentric circles. As the radar antenna rotates, a radial trace on the PPI sweeps in unison with it about the center point. It is the most common type of radar display.
The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed elevation 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.
The height of the echoes increases with the distance to the radar, as represented in the adjacent image. 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 small ship and aircraft radars, the top may represent the bow or nose of the ship or aircraft, i.e., its heading (direction of travel) and this is usually represented by a lubber line. Some systems may incorporate the input from a gyrocompass to rotate the display and once again display north as "up".
Also, 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.
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 (CRT), and thus the only way to store the information received was by taking a photograph of the screen.
Philo Taylor Farnsworth, the American inventor of all-electronic television in September 1927, contributed[ citation needed ] to this in an important way. Farnsworth refined a version of his picture tube (CRT) and called it an "Iatron;" generically known as a storage tube. It could store an image for milliseconds to minutes and even hours. One version that kept an image alive about a second before fading proved to be useful for radar. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar usage.
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.
The PPI is used in many domains involving display of range and positioning, especially in radars, including air traffic control, ship navigation, meteorology, on board ships and aircraft etc. PPI displays are also used to display sonar data, especially in underwater warfare. However, because the speed of sound in water is very slow compared to microwaves in air, a sonar PPI has an expanding circle that starts with each transmitted "ping" of sound. In meteorology, a competing display system is the CAPPI (Constant Altitude Plan Position Indicator) when a multi-angle scan is available.
Using computers to process data, modern sonar and lidar installations can mimic radar PPI displays too. [1]
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.
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A radar display is an electronic device that presents radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets and return to the radar system. The receiver converts all received electromagnetic radiation into a continuous electronic analog signal of varying voltage that can be converted then to a screen display.
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Green Satin, also known as ARI 5851, was a Doppler radar system developed by the Royal Air Force as an air navigation aid. The system provided direct measures of the drift speed and direction, and thereby allowed accurate calculation of the winds aloft. These values were then fed into the Navigation and Bombing System.
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Sutton tube was the name given to the first reflex klystron, developed in 1940 by Robert W. Sutton of Signal School group at the Bristol University. The Sutton tube was developed as a local oscillator for the receiver of 10cm microwave radar sets. Due to its geometry and long drift space, it suffered from mode jumping through the tuning range. For this reason, from late 1941 onward, it was replaced in many sets by the Western Electric 707A.
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Radar, Gun Laying, Mark I, or GL Mk. I for short, was an early radar system developed by the British Army to provide range information to associated anti-aircraft artillery. There were two upgrades to the same basic system, GL/EF and GL Mk. II, both of which added the ability to accurately determine bearing and elevation.
FuG 224 Berlin A was a German airborne radar of World War II. It used rotating antennae and a PPI display to allow its use for ground mapping.
Radar, Gun Laying, Mark III, or GL Mk. III for short, was a radar system used by the British Army to directly guide, or lay, anti-aircraft artillery (AA). The GL Mk. III was not a single radar, but a family of related designs that saw constant improvement during and after World War II. These were renamed shortly after their introduction in late 1942, becoming the Radar, AA, No. 3, and often paired with an early warning radar, the AA No. 4, which was also produced in several models.
The AN/APQ-7, or Eagle, was a radar bombsight system developed by the US Army Air Force. Early studies started in late 1941 under the direction of Luis Alvarez at the MIT Radiation Laboratory, but full-scale development did not begin until April 1943. By this time US-built, higher frequency systems promising better performance over the existing British H2S radar were entering production. Eagle's even higher resolution was considered important to Air Force planners who preferred precision bombing but were failing to deliver it, and high hopes were put on the system's abilities to directly attack small targets like docks and bridges.
The Type 277 was a surface search and secondary aircraft early warning radar used by the Royal Navy and allies during World War II and the post-war era. It was a major update of the earlier Type 271 radar, offering much more power, better signal processing, new displays, and new antennas with greatly improved performance and much simpler mounting requirements. It allowed a radar with performance formerly found only on cruisers and battleships to be fitted even to the smallest corvettes. It began to replace the 271 in 1943 and was widespread by the end of the year.
Radar, Air-to-Surface Vessel, Mark III, or ASV Mk. III for short, was a surface search radar system used by RAF Coastal Command during World War II. It was a slightly modified version of the H2S radar used by RAF Bomber Command, with minor changes to the antenna to make it more useful for the anti-submarine role. It was Coastal Command's primary radar from the spring of 1943 until the end of the war. Several improved versions were introduced, notably the ASV Mark VI, which replaced most Mk. IIIs from 1944 and ASV Mark VII radar, which saw only limited use until the post-war era.
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