11/30/2021

Radar Diffraction

X – band Radar wave length 3cm frequency 9 GHz. S – band Radar wave length 10cm, frequency (2.9-3.1)GHz. When electromagnetic pulse passes through atmosphere it. Station de radar. Radar cross section defined in simple terms for even novices. Shows how the RCS of targets can be predicted and measured. Describes the design, operation and characteristics of indoor and outdoor. Propagation of Radar Waves: Forward Scattering. Forward scattering (reflection) from the surface of the earth enhances the energy at some elevations and decreases it at other. Refraction (bending). Ducting (trapping), a severe form of refraction that can extend radar range. Diffraction. Attenuation. External Noise. A surveillance radar that develops tracks on targets is sometimes called a track-while-scan (TWS) radar. In a modern radar, the target detection and tracking can be automatically processed by a data processor called automatic detection and tracking. The parasitic diagram produced by diffraction results from the slender parts of the fuselage.

  1. Radar Diffraction Training
  2. Radar Diffraction Meaning
  3. Radar Diffraction System
  4. Radar Edge Diffraction
Radar diffraction meaning
RCS (m2)RCS (dB)
automobile10020
B-52100
B-1(A/B)10
F-1525
Su-2715
cabin cruiser1010
Su-MKI4
Mig-213
F-165
F-16C1.2
man10
F-181
Rafale1
B-20.75 ?
Typhoon0.5
Tomahawk SLCM0.5
B-20.1 ?
A-12/SR-710.01 (22 in2)
bird0.01-20
F-35 / JSF0.005-30
F-1170.003
insect0.001-30
F-220.0001-40
B-20.0001-40
The radar cross section (RCS) of a target is defined as the effective area intercepting an amount of incident power which, when scattered isotropically, produces a level of reflected power at the radar equal to that from the target. RCS calculations require broad and extensive technical knowledge, thus many scientists and scholars find the subject challenging and intellectually motivating. This is a very complex field that defies simple explanation, and any short treatment is only a very rough approximation.

The units of radar cross section are square meters; however, the radar cross section is NOT the same as the area of the target. Because of the wide range of amplitudes typically encountered on a target, RCS is frequently expressed in dBsm, or decibels relative to one square meter. The RCS is the projected area of a metal sphere that is large compared with the wavelength and that, if substituted for the object, would scatter identically the same power back to the radar. However, the RCS of all but the simplest scatterers fluctuates greatly with the orientation of the object, so the notion of an equivalent sphere is not very useful.

Different structures will exhibit different RCS dependence on frequency than a sphere. However, three frequency regimes are identifiable for most structures. In the Rayleigh region at low frequencies, target dimensions are much less than the radar wavelength. In this region RCS is proportional with the fourth power of the frequency. In the Resonance or Mie Region at medium frequencies, target dimensions and the radar wavelength are in the same order. The RCS oscillates in the resonance region. In the Optical Region of high frequencies, target dimensions are very large compared to the radar wavelength. In this region RCS is roughly the same size as the real area of target. The RCS behaves more simply in the high-frequency region. In this region, the RCS of a sphere is constant.

In general, codes based on the methods-of-moments (MOM) solution to the electrical field integral equation (EFIE) are used to calculate scattering in the Rayleigh and resonance regions. Codes based on physical optics (PO) and the physical theory of diffraction (PTD) are used in the optical or high-frequency region. The target's electrical size (which is proportional to frequency and inversely proportional to the radar wavelength) that determines the appropriate algorithm to calculate the scattering. When the target length is less than 5 to 10 wavelengths, the EFIE-MOM algorithm is used. Alternatively, if the target wavelength is above 5 to 10 wavelengths, the PO-PTD algorithm is used.

The RCS of a stealth aircraft is typically multiple orders of magnitude lower than a conventional plane and is often comparable to that of a small bird or large insect. 'From the front, the F/A-22's signature is -40dBm2 (the size of a marble) while the F-35's is -30 dBm2 (the size of a golf ball). The F-35 is said to have a small area of vulnerability from the rear because engineers reduced cost by not designing a radar blocker for the engine exhaust.' [Aviation Week & Space Technology; 11/14/2005, page 27] The F-35 stealthiness is a bit better than the B-2 bomber, which, in turn, was twice as good as that on the even older F-117. B-2 stealth bomber has a very small cross section. The RCS of a B-26 bomber exceeds 35 dBm2 (3100m2 ) from certain angles. In contrast, the RCS of the B-2 stealth bomber is widely reported to be about -40dBm2 .

A conventional fighter aircraft such as an F-4 has an RCS of about six square meters (m2), and the much larger but low-observable B-2 bomber, which incorporates advanced stealth technologies into its design, by some accounts has an RCS of approximately 0.75 m2 [this is four orders of magintude greater than the widely reported -40dBm2 ]. Some reports give the B-2 a head-on radar cross section no larger than a bird, 0.01 m2 or -20dBm2. A typical cruise missile with UAV-like characteristics has an RCS in the range of 1 m2; the Tomahawk ALCM, designed in the 1970s and utilizing the fairly simple low-observable technologies then available, has an RCS of less than 0.05 m2.

The impact of lowered observability can be dramatic because it reduces the maximum detection range from missile defenses, resulting in minimal time for intercept. The US airborne warning and control system (AWACS) radar system was designed to detect aircraft with an RCS of 7 m2 at a range of at least 370 km and typical nonstealthy cruise missiles at a range of at least 227 km; stealthy cruise missiles, however, could approach air defenses to within 108 km before being detected. If such missiles traveled at a speed of 805 km per hour (500 miles per hour), air defenses would have only eight minutes to engage and destroy the stealthy missile and 17 minutes for the nonstealthy missile. Furthermore, a low-observable LACM can be difficult to engage and destroy, even if detected. Cruise missiles with an RCS of 0.1 m2 or smaller are difficult for surface-to-air missile (SAM) fire-control radars to track. Consequently, even if a SAM battery detects the missile, it may not acquire a sufficient lock on the target to complete the intercept.

Radar scattering from any realistic target is a function of the body's material properties as well as its geometry. Once the specular reflections have been eliminated by radar absorbing materials, only nonspecular or diffractive sources are left. Non-specular scatterers are edges, creeping waves, and traveling waves. They often dominate backscattering patterns of realistic targets in the aspect ranges of most interest. The traveling wave is a high frequency phenomenon. Surface traveling waves are launched for horizontal polarization and grazing angles of incidence on targets with longs mooth surfaces. There is little attenuation from the flat smooth surface, so the wave builds up as it travels along the target. Upon reaching a surface discontinuity, for example an edge, the traveling wave is scattered and part of it propagates back toward the radar. The sum of the traveling waves propagating from the far end of the target toward the near end is the dominant source to the target radar cross section.

The radar cross section (RCS) of a target not only depends on the physical shape and its composite materials, but also on its subcomponents such as antennas and other sensors. These components on the platforms may be designed to meet low RCS requirements as well as their sensor system requirements. In some cases, the onboard sensors can be the predominant factor in determining a platform's total RCS. A typical example is a reciprocal high gain antenna on a low RCS platform. If the antenna beam is pointed toward the radarand the radar frequency is in the antenna operating band, theantenna scattering can be significant.

The traditional measure of an object's scattering behavior is the RCS pattern which plots the scattered field magnitude as a function of aspect angle for a particular frequency and polarization. Although suitable to calculate the power received by a radar operating with those particular parameters, the RCS pattern is an incomplete descriptor of the object's scattering behavior. While the RCS pattern indicates the effect of the scattering mechanism, it does not reveal the physical processes which cause the observed effect. In contrast, imaging techniques, which exploit frequency and angle diversity to spatially resolve the reflectivity distribution of complex objects, allow the association of physical features with scattering mechanisms. These processes, therefore, indicate the causal components of the overall signature level observed in RCS patterns.

Target RCS (m2)References
Navy cruiser (length 200m)14000[5]
B-52 Stratofortress100 –125[1], [5], [6]
C-130 Hercules80[5]
F-15 Eagle10–25[7], [8]
Su-27 Flanker10–15[7], [8]
F-4 Phantom6–10[1], [5]
Mig-29 Fulcrum3–5[1], [7], [8]
F-16A5
F-18 C/D Hornet1–3[6], [7]
M-20001–2[7], [8]
F-16 C (with reduced RCS)1.2[6]
T-38 Talon1[5]
B-1B Lancer0.75–1[1], [5]
Sukhoi FGFA prototype0.5[9]
Tomahawk TLAM0.5
Exocet, Harpoon0.1[6]
Eurofighter Typhoon0.1 class[6], [7], [8]
F-18 E/F Super Hornet0.1 class[6], [7], [8]
F-16 IN Super Viper0.1 class [2]
Rafale0.1 class[1], [2], [6], [7], [8]
B-2 Spirit0.1 or less[1], [6]
F-117A Nighthawk0.025 or less[1], [6]
bird0.01[3]
F-35 Lightning II0.0015 –0,005[6], [7], [8]
F-22 Raptor0,0001–0.0005[6], [7], [8]
insect0.00001 [3]
  • [1] D.Richardson: Stealth Warplanes, Zenith Press, 2001
  • [2] Ashley J. Tellis : Dogfight! India's Medium Multi-Role Combat Aircraft decision, Carnegie Endowment for International Peace, 2011
  • [3] M.I.Skolnik:Introduction to Radar Systems (2nd edition),McGraw Hill Book Company, 1981
  • [4] E.Knott, J.F.Schaeffer and M.T.Tuley: Radar Cross Sections, SciTech Publishing Inc, 2nd revised edition, 2004.
  • [5] D.K.Barton and S.A.Leonov, Eds.: Radar Technology Encyclopedia (Electronic Edition), Artech House, 199
  • [6] http://www.users.globalnet.co.uk/~dheb/2300/Articles/PG/PGSA.htm
  • [7] http://www.f-16.net/f-16_forum_viewtopic-t-3018-start-15.html
  • [8] Serkan Ozgen: Radar Signature, Dec. 12 http://www.ae.metu.edu.tr/~ae451/signature_SO.pdf
  • [9] http://www.rusembassy.in/index.php?option=com_content&view=article&id=249%3Aindia-russia-close-to-pact-on-next-generation-fighter&catid=16%3Apress-on-bilateral-relations&directory=1&lang=en'>india russia close to pact on next generation fighter
    SOURCE : Low Observable Principles, Stealth Aircraft and Anti-Stealth Technologies Konstantinos Zikidis, Alexios Skondras, Charisios Tokas, December 2013

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    Warning! Radar Operating

    By Jim Sparks
    May-June 1998

    The primary purpose of weather radar is to detect storms along the flight path and give the pilot a visual indication of rainfall intensity, and with doppler radar, possible turbulence. This enables the crew to navigate around potentially hazardous areas. Another capability is to 'ground map.' In this mode, the flight crew receives a topographical picture which can be of assistance for navigation and position reference. In some cases, weather radar systems will detect aircraft in their path; however, these systems are not intended as collision avoidance.

    A typical radar system includes a flight deck display, receiver transmitter, and an antenna. The cockpit indicator is referred to as a plan position indicator (PPI) as it indicates both distance and direction to the target. This can be a stand alone cathode ray tube (CRT), or in aircraft using electronic flight instruments (EFIS), a multi-function display (MFD) can be used to paint the radar image.

    Control panels for weather radar contain a function switch, which in addition to selecting the unit 'OFF,' can be positioned to 'standby' (STBY or SBY). This allows warm up with the antenna not scanning and the transmitter inhibited. Most recently developed systems require about 90 seconds to become operational, while older units require several minutes. Some installations have what is known as a 'forced standby.' This is a situation where the radar will automatically stop transmitting and the antenna will stop sweeping. One common means of introducing this forced stand by is to have the aircraft in a 'weight on wheels' configuration or by activating the approach mode in a flight guidance system.

    It's important to investigate possibilities of forced standby prior to performing maintenance. Jacking some aircraft with avionic equipment powered up may result in activation of the radar unit. With Electronic Flight Instruments (EFIS), separate radar controls may be available to both pilot and copilot. In this case, both will need to be selected to STBY or OFF. When in doubt, pull all radar circuit breakers prior to turning on electrical power.

    After the warm-up is complete, selection to the 'weather' (WX) mode causes the transmitter portion of the receiver transmitter (RT) to deliver high power pulses to the antenna. Between two transmissions, the RT serves as a receiver processor and configures the return signal into a format for useful display to the crew. Most of today's weather radars are 'X'-Band and operate on a frequency of around 9345 Mhz, and power outputs of 8 to 10 kilowatts. This enables a viewing range of up to 300 nautical miles. The length of the pulses may vary depending on the requirements of the system, with one to two microseconds being nominal values. Pulse repetition frequency (PRF) will vary depending on the range being monitored. For long distances, the rate must be slow enough to allow the return to be processed before the next transmission. This is determined by the range selector position on the radar controller panel.

    The antenna is supplied by the transmitter and radiates the electromagnetic energy into the air. It then becomes the receptor of the echoes from the targets. These devices come in sizes from 10 to 24 inches and the larger the diameter the narrower the beam width.

    Radar antenna platforms are rather complex as they have to rotate and pitch to accurately scan the area in front of the aircraft.

    'Scan' or 'sweep' control will determine the total area in front of the aircraft to be observed. In most systems, the rotation of the antenna will encompass a 120 degree arc. This 'azimuth' is displayed in either 15 or 30 degree increments. When an area of weather is detected and the crew wishes to observe it in more detail, the scan can be reduced to a 60 degree sweep. In most cases when the area observed is reduced, the 'looks per minute' will increase. Normally, in an 120-degree arc there are 12 looks per minute. With a 60- degree sector being observed, the scan rate goes to 24 looks per minute. This provides the crew with the most up to date and accurate view of the target.

    Weather on EFIS tube

    Radar Diffraction Training

    'Tilt' is another selection available to the crew and is used to set the 'up, down' position of the antenna beam relative to the horizon. Most systems provide a range of 15 degrees above zero to 15 degrees below zero. 'Altitude compensated tilt' (ACT) is a means where the antenna tilt is automatically adjusted as the aircraft changes altitude. This capability is available in many new technology systems and requires an input from an air data computer. Another automatic function is 'stabilization,' which uses a vertical gyro to provide aircraft turn and bank information so the radar antenna can remain in the same azimuth and elevation relative to the ground. The pilot has the option of disconnecting this stabilization by activating a switch on the radar control panel. Differences in some antenna installations may require stabilization trim adjustments. Airframe manufacturers' maintenance manuals, as well as radar manufacturers' procedures, should always be closely observed when making any adjustment.

    Receiver sensitivity needs to be properly calibrated to eliminate background noise, yet provide for reception of even the weakest reflected signal. The 'gain' control is a useful tool for weather analysis and ground mapping. In the mapping mode it is possible to reduce the level of the typically very strong returns from ground targets, while in the weather mode, calibrated sensitivity can be increased to allow very week targets to be observed.

    Radar display-indicator

    In some later technology weather radars, it can be difficult to distinguish where the receiver/transmitter (RT) stops and the antenna begins. This type of device is referred to as a receiver transmitter antenna (RTA).

    Radar Diffraction Meaning

    In earlier systems the antenna and the RT were connected by a radar wave guide. This is a hollow, usually rectangular metal conduit that would allow passage of the ultra high frequency (UHF) signal. The transmitted radio wave departing the RT could not escape through the walls of the wave guide, so it will then flow to the end where the antenna could radiate the signal through the air. Wave guides are sealed and frequently pressurized to prevent moisture ingress as any contamination has the potential to distort the radar signal.

    One of the numerous cautions in dealing with radar systems is to avoid an open end of a wave guide while the radar is operating. Severe eye damage can result. In fact, any technician who is involved in maintenance of radar equipped aircraft should obtain a copy of Advisory Circular 20-68B 'Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar,' and become thoroughly familiar with its contents.

    Weather radar antennas are located in some forward facing section of the aircraft. These areas may include a wing leading edge or most frequently the nose section, which incorporates a radar dome (radome). The radome is a covering whose primary purpose is to protect the radar antenna from the elements. This component has to be strong enough to withstand the aerodynamic loads of the aircraft, yet made of material that will allow free passage of the transmitted output as well as the return signal.

    The construction of a radome is usually in one of two methods. The 'thin wall' which is useful with low frequency systems and in areas where aerodynamic and structural loads will allow, and 'sandwich' style radomes that are constructed of two or more skins separated by a nonconductive core. This can include foam-filled or hollow chambers such as with honeycomb. Radome manufacturers frequently experiment with new exotic materials, including quartz, to deliver a unit with the ultimate in electrical and structural properties.

    Both physical and electrical thickness are important factors in the design of a radome. Electrical thickness is related to physical thickness and is based on mathematical equations that factor in operating frequencies, materials used, and the type of construction. A very small variation in physical thickness can have a major effect on electrical thickness. For this reason radomes are fabricated for specific types of radar. If a radome designed for a 'C' band radar was installed on an aircraft using 'X' band, the performance will be degraded. Paint or other coatings such as abrasion shields need to be of a material that will not interfere with the transmitted signal and cannot be an excessive thickness.

    Electrical bonding is another area of genuine concern. Airframe manufacturers typically have detailed procedures for maintenance and repair while frequent inspections are an excellent way to prevent radar discrepancies. When viewing, always check for cracks, erosion, delamination and condition of the 'static strips.' Anytime a radome is repaired or is a suspect in causing poorly operating radar, several tests should be conducted. These include 'transmissivity,' which is the ability of a radome to pass radar energy, 'reflection,' which is the amount of returned energy that did not pass through the radome back into the antenna, and 'diffraction,' which is the bending of the radar wave as it goes through the structure.

    RT and wave guide

    Radar Diffraction System

    When any of these electrical properties are not within prescribed tolerances, signal loss will occur. The targets may become cluttered and distorted.

    The most frequent damage to radomes are holes caused by electrostatic discharge. Regardless of size, these holes can cause significant damage by allowing moisture in. Structural damage will result when a quantity of trapped moisture freezes within the walls of the radome and causes delamination. The accumulation of water can also cause a reflection of radar energy that may result in a radar image displayed to the flight crew where nothing really exists. Electrostatic bonding tests on radomes are required by most aircraft manufactures. These checks should also be done anytime small discharge holes are revealed.

    Radar Edge Diffraction

    Many dangers can result from a radar operating at an inopportune time. With the high voltages and the escape of x-rays that could occur, most radar maintenance is best left to those who are well qualified. Avoid walking or working within the safety zone of an operating radar (usually seven feet). Anytime 'power-ON' maintenance is to be performed on a radar-equipped aircraft, take the extra time needed to make sure the system is OFF.