Range gate pull-off (RGPO) is an
electronic warfare
Electronic warfare (EW) is any action involving the use of the electromagnetic spectrum (EM spectrum) or directed energy to control the spectrum, attack an enemy, or impede enemy assaults. The purpose of electronic warfare is to deny the opponen ...
technique used to break
radar lock-on
Lock-on is a feature of many radar systems that allow it to automatically follow a selected target. Lock-on was first designed for the AI Mk. IX radar in the UK, where it was known as lock-follow or auto-follow. Its first operational use was in t ...
. The basic concept is to produce a pulse of radio signal similar to the one that the target radar would produce when it reflects off the aircraft. This second pulse is then increasingly delayed in time so that the radar's
range gate A range gate is an electronic circuit that selects signals within a given time period; the "gate" allows signals to pass through only within the selected time. The term is mostly used in radar, where range gates are used to select certain targets fo ...
begins to follow the false pulse instead of the real reflection, pulling it off the target.
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
s may not use range gates and instead select a single target by narrowly filtering frequencies on either side of the target's initial return. Against these radars, the related velocity gate pull-off (VGPO) can be used. These send a return signal that slowly changes in frequency, rather than time, hoping the radar's velocity gate will be pulled off the target in the same general fashion.
Pull-off belongs to the wider family of "deceptive jamming" concepts that use details of the target radar to their advantage, rather than attempting to simply overpower the radar's signal. Alternate names for "pull-off" include "stealing" and "walk-off". A related technique is
angle deception jamming.
Description
Range gates and strobing
Even the earliest
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
systems included a system to highlight a single selected target for further analysis. For instance, 1939's
Gun-Laying Mark I, the
British Army
The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurk ...
's first operational radar, used an on-screen cursor known as the ''strobe'' to highlight a single target. This worked by filtering out, or ''gating'', signals that were not within the strobe's short time period, typically a few microseconds, corresponding to a range of a few hundred meters. The signal within the strobe's window was then sent to secondary displays where two operators would determine the azimuth and elevation of that single target, by keeping its ''blip'' centered in their displays. Similar systems were used by many radars by the mid-war period.
By the end of the war, many experiments were being carried out on automatic target following, or
radar lock-on
Lock-on is a feature of many radar systems that allow it to automatically follow a selected target. Lock-on was first designed for the AI Mk. IX radar in the UK, where it was known as lock-follow or auto-follow. Its first operational use was in t ...
. In these systems, the operator would select a target using the strobe, and then circuits in the radar would automatically track the target in azimuth and elevation. This eliminated the need for the additional operators. Since the target's range would continue to change as it moved, the circuitry also attempted to keep the strobe centered in range. Some systems automated even the strobing; the
AI Mark V was designed for single-seat
fighter aircraft
Fighter aircraft are fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the airspace above a battlefield ...
where the pilot would be too busy to adjust the strobe, and instead had a second system to sweep the strobe through a wide range and then lock onto the first signal it saw.
In the post-war era the circuitry that produced the strobe and filtered out other returns became more widely known as a
range gate A range gate is an electronic circuit that selects signals within a given time period; the "gate" allows signals to pass through only within the selected time. The term is mostly used in radar, where range gates are used to select certain targets fo ...
.
Range Pull-off
While testing a late-war radar design, the
AI Mk. IX, a serious problem with the auto-follow system was found.
While this system was being developed,
Bomber Command
Bomber Command is an organisational military unit, generally subordinate to the air force of a country. The best known were in Britain and the United States. A Bomber Command is generally used for strategic bombing (although at times, e.g. during t ...
was pressing the
Air Ministry
The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State ...
to use "window", better known today as
chaff
Chaff (; ) is the dry, scaly protective casing of the seeds of cereal grains or similar fine, dry, scaly plant material (such as scaly parts of flowers or finely chopped straw). Chaff is indigestible by humans, but livestock can eat it. In agri ...
, as a radar countermeasure.
Fighter Command
RAF Fighter Command was one of the commands of the Royal Air Force. It was formed in 1936 to allow more specialised control of fighter aircraft. It served throughout the Second World War. It earned near-immortal fame during the Battle of Britai ...
pointed out that the Germans could easily copy the system and use it against England, potentially re-opening
The Blitz
The Blitz was a German bombing campaign against the United Kingdom in 1940 and 1941, during the Second World War. The term was first used by the British press and originated from the term , the German word meaning 'lightning war'.
The Germa ...
. It was suggested that the AI Mk. IX would ignore window because it decelerated rapidly after it was dropped, and would thus quickly pass out of the range gates and not be tracked. But exactly the opposite occurred in testing; the radar unerringly locked onto the window and the target disappeared from the display.
Range gate pull-off is essentially an electronic version of window. Instead of producing the secondary return by dropping a packet of foil reflectors, the second return is created by a
transponder
In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend word, blend of ''transmitter'' and ''responder''.
In air navigation or radio frequency identification, a T ...
in the target aircraft. The transponder initially responds as rapidly as possible to the radar's signal, producing a second blip that overlaps the original. Over a period of time, it increasingly delays the return so that it falls "behind" the radar signal in time. The goal is to delay the signal so it counters the aircraft's motion, leaving a signal at what appears to be a (nearly) fixed location in space. If the radar was locked on to the aircraft, it will hopefully remain locked to this second pulse as the aircraft moves away from the original location. Eventually, the aircraft will fall outside the range gate and disappear, while the radar continues tracking the false signal. Thus, the false signal is said to "pull the range gate off the target".
One way to reject the signal from the RGPO jammer is to note that the transponder always takes some non-zero time to respond. This means the signal will always have some component that represents the original "skin reflection" before the transponder signal is superimposed. On a
plan-position indicator
A plan position indicator (PPI) is a type of radar display that represents the radar antenna in the center of the display, with the distance from it and height above ground drawn as concentric circles. As the radar antenna rotates, a radial tra ...
, the false signal will appear as a second dot at increasing distances from the first, which the operator can then manually strobe to regain lock. Alternately, if the operator is aware there is a jammer operating, they can look for the closest signal, representing the "skin reflection", and mute down any following signals. This is easily accomplished in simple electronics, and often referred to as a "leading-edge tracker".
Such systems can be defeated by tracking the original radar signal and extracting its
pulse repetition frequency
The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar.
In radar, a radio signal of a particular carrier frequency is tu ...
(PRF). With even a basic measure of the PRF, the jammer can broadcast noise across the time frame of the skin reflection in order to obscure it. This can be particularly effective against leading-edge trackers, which will no longer have a sharp signal to gate on. Since these systems generate two signals, one to blank the leading-edge and another to perform pull-off, these are sometimes known as "dual-mode jammers".
A more complex solution requires extremely accurate tracking of the PRF. If this can be achieved, the RGPO can then broadcast its deception signal on either side of the skin reflection and walk-off in either direction. This technique easily defeats leading-edge tracking, and also makes it difficult for a manual operator to tell which of the returns is the "real" signal.
Velocity pull-off
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
s directly measure the target's velocity via the
Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
. In typical early implementations, the received signal was amplified and then sent into a bank of narrow-band filters, each one corresponding to a particular target velocity. A simpler system is used in some
semi-active radar homing
Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range Air-to-air missile, air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is ...
missiles, which are pre-programmed with a measured target velocity which is used to calculate the expected Doppler shift of the signal, and then filter out signals outside a narrow band around that frequency.
If an RGPO jammer responds to such a signal by sending out the same frequency it received, this additional signal will be sent into the same filter, adding to the original signal and making it stronger. If the transponder instead responds at a fixed frequency, it will fall into a different filter and can be easily distinguished. In either case, the original target return remains locked-on.
Modifying a transponder to deal with Doppler radars is easy, it simply requires it to be able to adjust its frequency. In this case, the system initially responds at the same frequency as the original signal, and then increasingly shifts the frequency over time in a manner similar to the RGPO case. This will cause a second signal to appear in adjacent filters, with no way to know which is the original. Since the frequency can be easily adjusted up or down, it does not have the added complication seen in RGPOs that want to pull-off in either direction.
Pulse-Doppler radar
A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars an ...
s use both pulse timing and Doppler shifting to track targets, so by varying both the frequency and return timing (through
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
), these can be pulled off as well. Such a transponder will continue to work against non-Doppler radars as well, as these generally have wide frequency response and continue to see the signal as long as its frequency shift does not become significant.
Countermeasures
The effectiveness of the pull-off can be reduced if the radar changes its pulse repetition frequency, thereby making it difficult for the transponder to continue smoothly delaying the fake signal.
Frequency agility Frequency agility is the ability of a radar system to quickly shift its operating frequency to account for atmospheric effects, jamming, mutual interference with friendly sources, or to make it more difficult to locate the radar broadcaster through ...
has the same effect, as the transponder cannot guess what frequency to send out the fake signals on until it hears the one from the radar.
Denying this capability means the signal from the transponder can only respond to signals after hearing them on its receiver. These signals will always represent returns from greater distances than the jammer aircraft. Pulse-to-pulse comparison techniques, like
moving target indication
Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees ...
, can be used to filter out these sorts of returns as they appear on the radar to be slower-moving targets.
Notes
References
Citations
Bibliography
*
*
* {{Cite book
, publisher = Pen & Sword
, isbn = 9781844155323
, first = Ian , last = White
, title = The History of Air Intercept (AI) Radar and the British Night-Fighter 1935-1959
, year = 2007
::Excerpts are available i
Part One; 1936 – 1945an
Part Two; 1945 – 1959
Electronic warfare