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Display unit or PPI
Analogue type (older) radars consist of a cathode ray tube (CRT), the face or screen of which is commonly referred to as the scope, and various timing circuits and controls. In the scope a stream of electrons is directed towards a fluorescent screen. The phosphorus glows when illuminated by the electrons, while internal circuitry forms the trace or sweep. A beam begins at the centre and sweeps out again and again, each sweep corresponding to the progress of a microwave pulse going out and back, and each successive sweep moving a little further around the screen in time with the rotating scanner. A returning echo is added to the sweep signal so that the screen is more brightly illuminated (painted) at a point corresponding to the bearing and range of the targets echo. This produces a very clear but very dim picture and the scope is fitted with a cowl to allow daylight viewing. This presentation is called a Plan Position Indicator (PPI).
The diagram above shows a cathode ray tube (CRT) using magnetic deflection obtained from coils placed around the neck of the CRT-other CRT’s use electrostatic deflection, by use of deflection plates. Modern displays units, called raster scan radars (colour radar) use microprocessors to analyse echo range and bearing and present like a TV. The raster scan picture (paint) is rectangular rather than a circular. In raster scans there is no rotating trace, instead the echo is digitalised and painted line by line across the screen (called interlacing). As the sweep moves across the screen the pixels (picture cells) are illuminated and can be further electronically enhanced in colour or duration of illumination. A small rasterscan radar is shown below.
To obtain a sharp image a high resolution is required. Resolution is measured by the density of the pixels, the more pixels per square inch the higher the resolution. A common radar display resolution of 640480 means that the screen consists of 640 columns by 480 rows of dots, that is, 640 x 480 = 307,200 pixels. Pixels are electronically refreshed or updated so they will remain bright, at a rate of about 30 or more times each second. Displays may be interlaced or non-interlaced. Interlaced displays do not draw the entire picture in one pass. On the first pass, it draws every other line and draws the remaining lines on the second pass. Non interlaced displays will therefore be more stable. On early model raster scan radars piccolos were either on or off, which meant that there was no visible difference between a weak echo and a strong one. This could result in weak echoes like rain clouds completely blocking out the display and masking targets within the blocked out display. Later models have multi-level quantisation displays, in which each pixel can operate at several levels of monochrome, which allows the operator to distinguish between a weak and a strong echo. On modern rasterscan radars remote displays or full dual displays units are available. Most radars produced for small craft today have a green and black monochrome quantised display. However, some of the larger more expensive radars feature full colour displays.
