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 "could" have been seen. [ [ blip-scan ratio] ] Alternately it can be defined as the ratio of the number of scans in which a return is received to the total number of scans. [ [ United States Patent 5535303] , see "Description of Related Art"]

"Blip" refers to the dots drawn on early warning radars based on plan position indicator (PPI) displays. "Scan" is a single search of the entire sky made by rotating the antenna. Radars with a low blip-to-scan ratio draw only a few reflections from the aircraft, making them more difficult to detect. By flying high and fast the ratio can be further reduced, rendering the aircraft almost invisible. This fact was the primary reason the Lockheed U-2 was replaced by the much faster Lockheed A-12, although upgrades to Soviet radar systems rendered the A-12 vulnerable as soon as it was available.

Radar basics

Early warning radars of the 1950s were little changed from the earliest examples operated by Germany and England during World War II. A radar antenna rotated around its vertical axis to allow it to scan the sky in azimuth (side to side). The antenna is shaped to produce a beam that is narrow from side-to-side to allow it to locate objects accurately in angle, but quite broad vertically, 30 to 40 degrees, in order to scan the entire sky from the horizon up to high altitudes.

The radar electronics produce a series of pulses of radio energy. These are sent out of the antenna, which then listens for a short period for any reflections before sending out the next pulse. Reflections are amplified and sent to an oscilloscope for display, causing a "blip" on the screen. An encoder in the antenna mount sends the current direction of the antenna to the display, rotating the blips around the face of the display. Distances, determined by the time between sending and receiving the pulses, were displayed with longer ranges being further from the center of the scope. The result is a 2D top-down image of the airspace around the radar.

One key characteristic of any radar is the "pulse repetition frequency" (PRF). Since the radio pulse travels at a finite speed, the speed of light, the time you have to wait for a reflection to return is a function of the range to the target. For instance, a radar designed to have a range of 300 km needs to wait 2 milliseconds (300 km / 300,000 km/s times 2 for there and back) in order to see a reflection at its maximum range. This implies that such a radar can send out at most 500 pulses per second, the PRF. For instance, if it sent out 1000 pulses, it would be impossible to determine if a particular reflection was a target at 150 km from the pulse just sent out, or 300 km from one pulse ago.

Intertwined with the PRF is the length of the pulse, or duty cycle. Longer pulses mean that more energy will be reflected from the target, making it easier to amplify and display. However, the radar system cannot listen for reflections while the pulse is being sent. This means that the radar has a certain minimum range, the time it takes for reflections to travel back to the antenna during the time while the pulse is being broadcast. For an early warning radar the minimum range is generally not important, so longer pulses are used to maximize the returns. A radar with a 30 km minimum range, for instance, can have pulses no longer than 0.1 ms in duration.

Further assume that the horizontal beamwidth is one degree, and the antenna rotates once every ten seconds, or 36 degrees a second. An aircraft will be "painted" by the beam for only 1/36 th of a second, as the one degree beam sweeps over it. With a PRF of 500, that means the aircraft will be hit with less than 14 pulses. In order to become visible on the "slow" displays of the era, a number of these pulses will have to be returned and drawn on the screen. If a number of these pulses are "lost", due to electronics noise or other reasons, the blip may never become visible. This is the blip-to-scan ratio.

Avoiding detection

Consider the target aircraft after the antenna has completed one rotation and returned to the same area of the sky ten seconds later. An aircraft traveling at 1000 km/h will have moved almost three kilometers in that time (1000 km/h = 278 m/s). On a display showing the example radar's entire 300 km radius this represents movement of only 0.5% across the display's face (600 km diameter), producing a tiny line segment between the two dots.

The small movement can aid the operator in interpreting the display. Radars are often filled with dots of random noise known as "clutter", but rarely do they produce the same slowly-moving line as an aircraft. Additionally, the phosphor coatings on the displays are deliberately chosen to have a half life on the order of a few scans, allowing the returns from any one target to "add up" and make them much more obvious on the display.

But if the target speed is increased its movement becomes more pronounced. At Mach 3 (3500 km/h at 25,000 m) the same ten seconds of movement represents over 1.5% of the display's face. At this point the slowly moving dot turns into a series of individual spots, which can easily be mistaken for clutter. Additionally, since the spots are separated by a distance on the tube, the returns no longer "add up" on the display, making them as dim as the other noise.

Of course an operator seeing a straight line of small dots across their screen might eventually "see" the target. In order to frustrate even this, aircraft were designed to fly as high as possible. Recall that the radar's scanning beam is fan-shaped and spread vertically across an angle. The beam only scans high altitudes at long ranges, and a large volume of airspace above the radar is out of sight. This means that there is only a ring-shaped area at long range where a high-altitude aircraft would be visible. Crossing this area quickly would result in only a few dots, hopefully not enough to become obvious.

And thus the concept of using the blip/scan to avoid detection. A high-speed, high-altitude aircraft could fly over early warning radars and never be seen. Even if it became visible, the small number of returns and fast movement across the operator's display would make manual calculation of an intercept extremely difficult.

Aircraft projects

Blip/scan spoofing was discovered during the late 1950s at a time when ground-controlled interception of manned interceptors was the only practical anti-bomber technique. In 1956 the CIA started flying the Lockheed U-2 over the USSR in order to gather intelligence (disproving the bomber gap in the process) although they were concerned when they saw that the Soviets were able to track the U-2s with ease. Although interception proved difficult, it was only a matter of time before the Soviets were able to get one of their aircraft into the right place at the right time and a U-2 would be shot down.

A replacement for the U-2 had been under consideration even before their operational missions began. Originally these studies focused entirely on the reduction of the radar cross section, but after the idea of spoofing the blip/scan was introduced in 1957, the plans were changed to study high-speed 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.

It was during the development of these aircraft that it was realized that using blip/scan avoidance was problematic. 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.

It was also realized that since blip/scan avoidance relied on a problem in the "displays", changing these displays could render the technique moot. This was particularly worrying, because the USAF was in the process of introducing precisely this sort of display as part of their SAGE project. SAGE recorded the radar returns in a computer, which then drew the targets on the display as an icon, whose brightness was independent of the physical return.

Finally, the introduction of the first effective anti-aircraft missiles dramatically changed the entire concept. Radars for plotting an air intercept were generally made as long-range as possible, in order to give the operators ample time to guide their aircraft as the targets slowly moved across the display. This led to low blip/scan ratios. Missiles, on the other hand, had radars with maximum ranges only slightly longer than the missile's range, about 40 km in the case of the SA-2 Guideline. They had much higher PRF's, and as a result the blip/scan problems were greatly reduced. They still had the problem of finding the target in time to prepare for an attack and launch, but this was by no means as difficult as guiding a manned aircraft onto the same target. This point was alarmingly demonstrated in the U-2 Crisis of 1960.

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 USSR (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.


* [ The U-2's Intended Successor:Project Oxcart, 1956-1968]
* [ Radartutorial]

Further reading

* Queen, F. D.; Maine, E. E., Jr., " [ A blip-scan ratio scoring system] ", 1974, Naval Research Lab, Washington, DC.

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