Radar angels are an effect seen on radar displays when there is a periodic structure in the view of the radar that is roughly the same length as the signal's wavelength. The angel appears to be a physically huge object on the display, often miles across, that can obscure real targets. These were first noticed in the 1940s and were a topic of considerable study in the 1950s. The underlying mechanism is due to Bragg's law. [1] [2]
Early radars were subject to strong returns from the ground and their plan position indicator displays often featured many permanent echos that blanked out portions of the screen. Angels appeared on these systems, but were difficult to distinguish from these ground returns and generally not noticed. Development of the COHO concept in the UK eliminated these permanent echos, at which point angels were clearly seen for the first time on a continual basis. One of the earliest examples was seen in 1953 on the Radar, Anti-Aircraft No. 4 Mk. 7, one of the first COHO systems. Some of these were identified as flocks of birds, which led one ornithologist to purchase a surplus Radar, Anti-Aircraft No. 3 Mk. 7 to perform bird tracking. [3]
When they were first seen, there was widespread agreement that many such angels were being caused by meteorological effects, but no one was able to explain their behavior based on this theory. It was known that birds could cause radar returns, as this had been noticed very early on Chain Home systems even before World War II. Experiments were performed by the Radar Research and Development Establishment that demonstrated the radar cross section of a dead bird was about 0.01 square meters, about the same as a bag with 1 pound (0.45 kg) water. [3] This is much smaller than the normal detection limit of the radars, and there were certain aspects of the motion that seemed to be at odds with the conclusion these were caused by birds. [3]
In one such example, the experimental COHO MEW radar at Great Baddow noted repeated ring-shaped angels that appeared to be slowly radiating outward from a point and drifting in the wind, but only in the morning. They were convinced this was due to a local factory starting up its steam plant and the resulting hot air was causing the display due to thermals. When they went to the location they found open parkland with a stand of trees. [4]
The mystery was solved when they checked in the morning and found huge flocks of starlings leaving the trees in a curious wave-like pattern. At night, the birds were clustered in trees in the center of the grove and at dawn began tree-hopping towards the outermost trees. Then, based on some invisible signal, all the birds at the outside of the grove would leave at once and begin flying off, radiating outward. As soon as one group left, more birds would, over a period of minutes, individually tree-hop outward to fill up the outer trees and repeat the process. At night, the birds arrived in small groups and did not cause any display to appear. [4]
It was not until the later 1950s that it was widely accepted that birds were the primary cause of angels. This conclusion was eventually put forth in 1957 by no less than the Royal Society:
...they have been widely believed to be of meteorological origin, by reflexion or refraction of energy from atmospheric discontinuities, but no meteorological theory so far proposed has managed to explain all their observed properties. It is shown that these properties can be satisfactorily explained on the assumption that the echoes are received from birds on migration. [5]
With pulse radars, a solution was soon found, known as swept gain in UK parlance and sensitivity time control (STC) in the US. According to the radar equation, the energy of a return signal varies with the fourth power, so nearby objects have much stronger returns and can swamp more distant objects. The idea of STC is to lower the sensitivity of the receiver for nearby targets before reaching maximum gain at longer range, perhaps 50 miles (80 km). By adjusting the magnitude of the gain suppression, the returns from birds can be eliminated while still allowing aircraft to be seen. [4]
Although angels were a problem for all radars of the era, they rendered the Canadian Mid-Canada Line almost unusable in the spring and fall when millions of large birds migrated by the stations. [6] This was made worse by the birds landing near the warm Diesel generators at the stations. [7] Typical radars send out short pulses of signal, and the STC can be triggered by that pulse. The Mid-Canada Line was a continuous wave radar (CW) that had no inherent timing to its signals. The effect was so overpowering that a significant feature of the similar AN/FPS-23 radars used on the DEW Line, then under construction, was the addition of Doppler filtering to remove objects traveling slower than 125 miles per hour (201 km/h) from the display. [8]
Although birds are the most widespread cause of these effects, any periodic structure in view of the radar can cause similar effects. This is particularly notable in sea-scanning radars in aircraft and satellites when the pattern of waves matches some multiple of the wavelength of the radar. [1] This effect has been exploited in radars that measure the sea state offshore, or wind-measuring radars that create the required patterns using acoustic waves generated by large loudspeakers. [9]
Radar is a detection system that uses radio waves to determine the distance (range), angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.
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 frequency of the returned signal. This variation gives direct and highly accurate measurements of the radial component of a target's velocity relative to the radar.
Anomalous propagation includes different forms of radio propagation due to an unusual distribution of temperature and humidity with height in the atmosphere. While this includes propagation with larger losses than in a standard atmosphere, in practical applications it is most often meant to refer to cases when signal propagates beyond normal radio horizon.
Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer spatial resolution than conventional beam-scanning radars. SAR is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar (SLAR). The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large synthetic antenna aperture. Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical or synthetic – this allows SAR to create high-resolution images with comparatively small physical antennas. Additionally, SAR has the property of having larger apertures for more distant objects, allowing consistent spatial resolution over a range of viewing distances.
The Mid-Canada Line (MCL), also known as the McGill Fence, was a line of radar stations running east–west across the middle of Canada, used to provide early warning of a Soviet bomber attack on North America. It was built to supplement the Pinetree Line, which was located further south. The majority of Mid-Canada Line stations were used only briefly from the late 1950s to the mid-1960s, as the attack threat changed from bombers to ICBMs. As the MCL was closed down, the early warning role passed almost entirely to the newer and more capable DEW Line further north.
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.
Imaging radar is an application of radar which is used to create two-dimensional images, typically of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths. It uses an antenna and digital computer storage to record its images. In a radar image, one can see only the energy that was reflected back towards the radar antenna. The radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so.
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 and continuous-wave radars, which were formerly separate due to the complexity of the electronics.
Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which causes the received signal to have a different frequency from the transmitted signal, allowing it to be detected by filtering out the transmitted frequency.
Inverse synthetic-aperture radar (ISAR) is a radar technique using radar imaging to generate a two-dimensional high resolution image of a target. It is analogous to conventional SAR, except that ISAR technology uses the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes. In situations where other radars display only a single unidentifiable bright moving pixel, the ISAR image is often adequate to discriminate between various missiles, military aircraft, and civilian aircraft.
Fluctuation loss is an effect seen in radar systems as the target object moves or changes its orientation relative to the radar system. It was extensively studied during the 1950s by Peter Swerling, who introduced the Swerling models to allow the effect to be simulated. For this reason, it is sometimes known as Swerling loss or similar names.
A radar system uses a radio-frequency electromagnetic signal reflected from a target to determine information about that target. In any radar system, the signal transmitted and received will exhibit many of the characteristics described below.
A radio acoustic sounding system (RASS) is a system for measuring the atmospheric lapse rate using backscattering of radio waves from an acoustic wave front to measure the speed of sound at various heights above the ground. This is possible because the compression and rarefaction of air by an acoustic wave changes the dielectric properties, producing partial reflection of the transmitted radar signal. From the speed of sound, the temperature of the air in the planetary boundary layer can be computed. The maximum altitude range of RASS systems is typically 750 meters, although observations have been reported up to 1.2 km in moist air.
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 theoretically could be displayed. Alternately it can be defined as the ratio of the number of scans in which an accurate return is received to the total number of scans.
Wave radar is a type of radar for measuring wind waves. Several instruments based on a variety of different concepts and techniques are available, and these are all often called. This article, gives a brief description of the most common ground-based radar remote sensing techniques.
COHO, short for Coherent Oscillator, is a technique used with radar systems based on the cavity magnetron to allow them to implement a moving target indicator display. Because the signals are only coherent when received, not transmitted, the concept is also sometimes known as coherent on receive. Due to the way the signal is processed, radars using this technique are known as pseudo-coherent radar.
The Motorola AN/FPS-23 was a short-range early warning radar deployed on the Distant Early Warning Line. It was used as a "gap filler", looking for aircraft attempting to sneak by the DEW line by flying between the main AN/FPS-19 stations at low altitude. It could detect aircraft flying at 200 feet over land or 50 feet over water. The system was known as Fluttar during its development at the Lincoln Laboratory, and this name was widely used for the production units as well. It was also sometimes known as "Type F".
AIRPASS was a British airborne interception radar and fire-control radar system developed by Ferranti. It was the world's first airborne monopulse radar system and fed data to the world's first head-up display. The name is an acronym for "Airborne Interception Radar and Pilot's Attack Sight System". In the Royal Air Force (RAF) it was given the official name Radar, Airborne Interception, Mark 23, normally shortened to AI.23. AIRPASS was used on the English Electric Lightning throughout its lifetime.
The AMES Type 84, also known as the Microwave Early Warning or MEW, was a 23 cm wavelength early warning radar used by the Royal Air Force (RAF) as part of the Linesman/Mediator radar network. Operating in the L-band gave it improved performance in rain and hail, where the primary AMES Type 85 radar's performance dropped off. It operated beside the Type 85 and RX12874 in Linesman, and moved to the UKADGE system in the 1980s before being replaced during UKADGE upgrades in the early 1990s.
The AR-3D was a military air traffic control and early warning radar developed by Plessey and first produced in 1975. It used a pencil beam and simple frequency scanning system known as "squint scan" to produce a low-cost 3D radar system that was also relatively mobile. About 23 were produced in total and found sales around the world into the early 1980s.