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. Radar basics Classic radars measure range by timing the delay between sending and receiving pulses of radio signals, and determine the angular location by the mechanical position of the antenna at the instant the signal is received. To scan the entire sky, the antenna is rotated around its vertical axis. 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, the phosphor coatings on the CRTs were mixed so that they would have a half-life on the order of the rotational speed of the antenna. This meant that the display would show the latest returns