Blip-to-scan ratio

[2] "Blip" refers to the dots drawn on early warning radars based on plan position indicator (PPI) displays.

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.

However, upgrades to Soviet radar systems increased their blip-to-scan ratios, rendering the A-12 obsolete before it could be deployed.

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.

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.

One key characteristic in all radars is the pulse repetition frequency (PRF), which determines the maximum effective range.

Using a shorter duty cycle allows for better minimum range, but also means that less radio energy is being sent into space over a given time, reducing the strength of the return signal.

A radar antenna is normally arranged to produce a very narrow beam, in order to improve angular resolution.

Intertwined with beam width is the speed of the antenna’s rotation, because it also determines the amount of time that a spinning radar will spend painting a given object on every scan.

But if the target speed is increased, its movement becomes more pronounced on the scope, making it less recognizable and more difficult to track.

At this point, the slowly moving dot turns into a series of dim individual spots, which can more easily be mistaken for clutter.

Blip/scan spoofing was discovered during the late 1950s, a time when ground-controlled interception of manned interceptors was the only practical anti-bomber tactic.

[4] After overflights began and the Soviets demonstrated the ability to track the U-2 and made credible attempts to intercept it, this estimate was adjusted downward; in August 1956, Richard Bissell reduced the number to six months more.

Originally these studies focused entirely on the reduction of the radar cross section (RCS), but after Franklin Rodgers introduced the idea of spoofing the blip/scan in 1957, the plans were changed to research high-speed, high-altitude designs instead.

It was discovered that the high-temperature exhaust of these aircraft engines reflected radar energy at certain wavelengths, and persisted in the atmosphere for some time.

This was particularly worrying, because the USAF was itself in the process of introducing precisely this sort of display as part of their SAGE project.

Defenders would still have the problem of finding the target in time to prepare for a missile counterattack, but this was by no means as difficult or as time consuming as scrambling manned aircraft and relying on the radar operator to guide them onto the target before the aircraft left radar range.

Even here the performance of the aircraft proved questionable, and A-12s were attacked by SA-2 missiles on several occasions, receiving minor damage in one flight on 30 October 1967.

Radars paint only a portion of the sky with their signal. In this image, the yellow surface represents the lowest angles the radar can aim, based on the need to avoid reflections from the local terrain. The purple surface is the maximum angle the antenna can reach, often around 30 to 45 degrees. The red surface is the maximum range of the radar, which is a factor of many design decisions. In this example, an aircraft flying at high altitude would only be detectable for a short time, while passing through the small annular ring between the red and purple surfaces. An aircraft can also approach the radar below the yellow surface.