Blip-to-scan Ratio - Aircraft Projects

Aircraft Projects

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. This led to a miniature arms race in itself, albeit a brief and abortive one.

The Lockheed U-2 flew at high altitude but not at particularly high speed. Even before the U-2 became operational in June 1956, CIA officials estimated that its life expectancy for flying safely over the Soviet Union before the Soviets developed countermeasures would be between 18 months and two years. 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. (In practice, this window proved slightly longer; but the general point was alarmingly demonstrated in the U-2 Crisis of 1960.)

In consideration of this expectation, a replacement for the U-2 had been under consideration even before its operational missions began. Originally these studies focused entirely on the reduction of the radar cross section (RCS), but after the idea of spoofing the blip/scan was introduced in 1957, the plans were changed to research high-speed, high-altitude designs instead. Lockheed calculated that in order to be effective against known Soviet radars, an aircraft would have to travel between Mach 2 and Mach 3 at 90,000 ft and have an RCS of about 10 square meters. This led to a number of proposals which were down-selected to the Lockheed A-12 and Convair Kingfish (neither of which could operate at that altitude).

It was during the development of these aircraft that problems with blip/scan avoidance became clear. 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. It would be possible for the Soviets to modify their radars to use these frequencies, and thereby track the targets indirectly but reliably.

It was also realized that since blip/scan avoidance relied more heavily on a problem in Soviet displays rather than in the principles of radar, changing these displays could render the technique moot. A system that recorded the radar returns in a computer and then drew the targets on the display as an icon whose brightness was independent of the physical return (a system in which returns did not have to "add up" in order to appear on the display) eliminated the potential for operator confusion. 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.

Finally, the introduction of the first effective anti-aircraft missiles dramatically changed the game. Radars for plotting an air intercept were generally made as long-range as possible in order to give the operators ample time to guide intercept aircraft onto the target as it moved across the display slowly (from the operator’s perspective). This led to low blip/scan ratios and inaccurate prediction of aircraft trajectories. This had been compounded by the difficulty of quickly scrambling intercept aircraft. Missiles solved both of these problems. Missiles stations guided their missiles with their own radar systems, which had maximum ranges only slightly longer than the missile's own flight range, about 40 km in the case of the SA-2 Guideline; therefore they had much higher PRFs, and as a result the blip/scan problems were greatly reduced for the missiles themselves. In other words, a radar station could initially identify an enemy aircraft and its general location, at which point a forward-based missile could launch quickly and target it independently. 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.

By the time the A-12 was operational in the early 1960s the blip/scan technique was no longer considered useful. The A-12 never flew over the Soviet Union (although it came close to doing so) and was limited to missions against other countries, like Vietnam. 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 case.