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.[1] 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.[2]
"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.[3]