Blip-to-scan ratio
Encyclopedia
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. 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.
"Blip" refers to the dots drawn on early warning radar
s 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.
s 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 (it spins from side to side). The antenna is shaped to produce a narrow beam to allow the radar 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 a radar also has a minimum range, the time it takes for reflections to travel back to the antenna while the pulse is being broadcast. A radar with a 30 km minimum range, for instance, can have pulses no longer than 0.1 ms in duration. For an early warning radar the minimum range is generally not important, so longer pulses are used to maximize the returns.
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. 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.
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. 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.
of manned interceptors was the only practical anti-bomber technique. Before the U-2 became operational in June 1956, CIA officials had estimated that its life expectancy for flying safely over the Soviet Union
would be between 18 months and two years. After overflights began and the Soviets demonstrated the capability of tracking and attempting to intercept the U-2, this estimate was adjusted downward; in August 1956, Richard Bissell reduced the number to six months.
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.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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. 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.
"Blip" refers to the dots drawn on early warning radar
Early warning radar
An early warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as early as possible before the intruder reaches its target, giving the defences the maximum time in which to operate...
s based on plan position indicator
Plan position indicator
The plan position indicator , is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles...
(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
Lockheed U-2
The Lockheed U-2, nicknamed "Dragon Lady", is a single-engine, very high-altitude reconnaissance aircraft operated by the United States Air Force and previously flown by the Central Intelligence Agency . It provides day and night, very high-altitude , all-weather intelligence gathering...
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 radarEarly warning radar
An early warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as early as possible before the intruder reaches its target, giving the defences the maximum time in which to operate...
s of the 1950s were little changed from the earliest examples operated by Germany and England during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. A radar antenna rotated around its vertical axis to allow it to scan the sky in azimuth (it spins from side to side). The antenna is shaped to produce a narrow beam to allow the radar 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
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
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
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
(PRF). Since the radio pulse travels at a finite speed, the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, 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
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
. 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 a radar also has a minimum range, the time it takes for reflections to travel back to the antenna while the pulse is being broadcast. A radar with a 30 km minimum range, for instance, can have pulses no longer than 0.1 ms in duration. For an early warning radar the minimum range is generally not important, so longer pulses are used to maximize the returns.
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. 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. 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 interceptionGround-controlled interception
Ground-controlled interception an air defense tactic whereby one or more radar stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic was pioneered during World War II by the Royal Air Force with the Luftwaffe to follow closely...
of manned interceptors was the only practical anti-bomber technique. Before the U-2 became operational in June 1956, CIA officials had estimated that its life expectancy for flying safely over the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
would be between 18 months and two years. After overflights began and the Soviets demonstrated the capability of tracking and attempting to intercept the U-2, this estimate was adjusted downward; in August 1956, Richard Bissell reduced the number to six months.
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
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
, 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
Convair KINGFISH
The Kingfish reconnaissance aircraft design was the ultimate result of a series of proposals designed at Convair as a replacement for the Lockheed U-2...
.
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
U-2 Crisis of 1960
The 1960 U-2 incident occurred during the Cold War on May 1, 1960, during the presidency of Dwight D. Eisenhower and during the leadership of Soviet Premier Nikita Khrushchev, when a United States U-2 spy plane was shot down over the airspace of the Soviet Union.The United States government at...
.
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
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
. 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.
Further reading
- Queen, F. D.; Maine, E. E., Jr., A blip-scan ratio scoring system, 1974, Naval Research Lab, Washington, DC.