ARPA Displays
From the time radar was first introduced to the present day the radar picture has been presented on the screen of a cathode ray tube. Although the cathode ray tube has retained its function over the years, the way in which the picture is presented has changed considerably. From about the mid-1980s the first raster-scan displays appeared. The radial-scan PPI was replaced by a raster-scan PPI generated on a television type of display. The integral ARPA and conventional radar units with a raster-scan display will gradually replace the radial-scan radar sets.
The development of commercial marine radar entered a new phase in the 1980s when raster-scan displays that were compliant with the IMO Performance Standards were introduced.
The radar picture of a raster-scan synthetic display is produced on a television screen and is made up of a large number of horizontal lines which form a pattern known as a raster. This type of display is much more complex than the radial-scan synthetic display and requires a large amount of memory. There are a number of advantages for the operator of a raster-scan display and concurrently there are some deficiencies too. The most obvious advantage of a raster-scan display is the brightness of the picture. This allows the observer to view the screen in almost all conditions of ambient light. Out of all the benefits offered by a raster-scan radar it is this ability which has assured its success. Another difference between the radial-scan and raster-scan displays is that the latter has a rectangular screen. The screen size is specified by the length of the diagonal and the width and height of the screen with an approximate ratio of 4:3. The raster-scan television tubes have a much longer life than a traditional radar cathode ray tube (CRT). Although the tubes are cheaper over their counterpart, the complexity of the signal processing makes it more expensive overall.
Read more about this topic: Automatic Radar Plotting Aid
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