Blip-to-scan ratio

In radar systems, the blip-to-scan ratio, or blip/scan, is the ratio of the number of times a target appears on a radar display to the number of times it theoretically could be displayed. Alternately it can be defined as the ratio of the number of scans in which an accurate return is received to the total number of scans.

"Blip" refers to the dots drawn on early warning radars based on plan position indicator (PPI) displays. A "scan" is a single search of the entire sky made by the rotating antenna. A radar with a low blip-to-scan ratio draws only a few reflections from an object (mainly aircraft), making it more difficult to detect.

For an aircraft flying at high speed and altitude the ratio is further reduced, rendering the aircraft almost invisible to radar. This change in radar signature is also known as the Rodgers Effect after its proponent in the US, Franklin Rodgers. The Lockheed U-2 was slated to be replaced by the much faster and stealthier Lockheed A-12 for this very reason. However, upgrades to Soviet radar systems increased their blip-to-scan ratios, rendering the A-12 obsolete before it could be deployed.

Classic radars—short for Radio Detection And Ranging—establish range by measuring the time delay between sending and receiving pulses of radio signals, and determine angular location by the mechanical angular position of the antenna at the instant the signal is received. To scan the entire sky, the antenna is rotated around its vertical axis by a motor and gear train. The returned signal is displayed on a circular cathode ray tube that produces dots at the same angle as the antenna and displaced from the center by the time delay. The result is a two-dimensional re-creation of the airspace around the antenna. Such a display is called a Plan Position Indicator, usually simply a "PPI".

These dots are known as "blips". Under optimal conditions, every pulse sent out by the radar will be returned and cause a blip to be displayed on the screen. Larger objects return stronger signals and therefore produce brighter blips. Slower aircraft also produce brighter blips because many returns are drawn at approximately the same location on the display, "adding up".

On Cold War era radar displays and before, CRT screen phosphor coatings were chosen because of their half life. This half life was on the order of more than ten seconds, or more than one complete rotation of the radar. This meant that the display would show the latest returns from a given object as a bright blip, and older ones as somewhat dimmer dots, as they faded. One might expect three or four such blips on the display, depending on the scanning speed of the antenna. These produced a series of dots that allowed the operator to easily determine the direction of travel, from the dimmest to brightest blip.

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