Radar cross-section
The radar cross section (german radar cross section, RCS) ( in some sources as engl. " Equivalent echoing area " means ) indicates how large the reflection of an object back towards the source of a radio wave (radar ) is. It indicates the size of an isotropic reflecting surface, which delivers an equivalent level of radar echo as the object.
The radar cross-section is dependent on the shape of the article, the material composition, as well as of wavelength, and angle of incidence of the radiation.
The radar cross-section, in some publications also called reflecting surface or effective reflection surface, is an object-specific size by a radar device has significant influence on the probability of detection of an object. In mathematical formulas ( for example, in the radar equation ), the reflecting surface is the Greek letter σ ( sigma ) refers to and has the unit square.
A high radar cross section, for example, for ships, buoys ( navaids ), bridge crossings or weather balloons desirable to be able to pick them out better and is often achieved with radar reflectors ( retro-reflectors for microwave ).
A small radar cross section is now one of the main quality characteristics of a military missile, as for example an aircraft with a low radar cross-section for enemy radar systems difficult to detect. Therefore, so-called stealth techniques have been developed to reduce the radar cross section. The radar cross section of a military missile is usually considered a military secret and shall not be disclosed publicly.
The diagram is a publication by M. I. Skolnik modeled, which shows a diagram with the experimentally determined relative reflecting surface σ/σ0 of the B-26 bomber " Invader" at a frequency of 3 GHz in the original.
- 4.1 Passive methods
- 4.2 Active Process
Measurement
The measurement of the radar cross section of an object is typically done with radars. It can be done outdoors or in an anechoic chamber, the electromagnetic waves of the frequency in question completely absorbed.
Typical values for a centimeter -wave radar are:
- Insect: 0.00001 m²
- Vogel: 0.01 m²
- Aircraft with stealth technology: < 0.1 m
- Anti-aircraft missiles: ≈ 0.1 m
- Man: 1 m²
- Small combat aircraft: 2-3 m²
- Large combat aircraft: 5-6 m²
- Transport aircraft: up to 100 m²
- Corner reflector with 1.5 m edge length: ≈ 20,000 m²
- Coaster ( 55 m length): 300-4000 sqm
- Frigate (103 m length): 5000-100000 m²
- Container ship (212 m length): 10000-80000 m²
Calculation
The reflecting surface is dependent on many factors. An analytical determination of the reflecting surface is only possible with simple bodies. It depends on the body shape, and the wavelength, or rather, the ratio of the dimensions of the body structure to the wavelength. Quantitatively is the radar cross section of an effective area that captures the incoming wave and radiates isotropically in the room. This formula applies only to the optical ( frequency-independent ) area that is, for objects whose dimensions are ten times larger than the wavelength. Do the objects approximate the size of the wavelength of the radar beam anstrahlenden it comes to resonance phenomena that increase the RCS significantly. In three dimensions, the radar cross-section is defined as:
Here, the power density on the radar target, and the scattered power density at a distance from the radar target.
Alternatively, can be written:
With - radiated field strength - scattered field strength
Have simple forms when they are large compared to the wavelength λ, the following theoretical radar cross sections σ:
Sphere of radius r:
Transversely to the beam direction plate ( aligned plane mirror ) with the area A:
Corner reflector ( retro-reflector ) consists of three square areas with sides of length a:
Instead of measurements, it is now common practice to calculate the radar cross section by computer simulations. So it is possible already in the design phase of the development of military aircraft or other radar targets to calculate the radar cross section at relatively low cost and optimize accordingly.
Reduction of the radar cross-section
Passive methods
The reduction of the radar cross section can in principle by
- The shaping
- The use of absorbent material ( resin bound in ferrite or graphite particles)
- Permeable material ( plastic materials )
Be achieved.
The ( small ) reflection at dielectric materials can analogous to optical methods (see anti-reflective layer ) can be reduced by appropriate layer thicknesses and different materials on, but the effect is wavelength dependent.
A military aircraft such as the Lockheed F- 117 has not only a microwave-absorbing surface, but also a modern design that prevents radar radiation is reflected back to the transmitter: you just used space - the likelihood that they are perpendicular to the source is low. It avoids inner edges / inside corners and any metal parts on the outside skin. Especially right angle inside corners of metal lead in a wide range to an almost complete reflection of the incident radiation to the transmitter.
Active methods
An active reduction of the radar cross-section is based on destructive interference. The radar signal is transmitted and received phase-shifted with nearly equal amplitude, but at about 180 ° again. The amplifier to operate with very low gain to avoid self-excitation, so they resemble only the losses of the antennas and turn the phase. This method is used primarily for VHF radar.
Increase the radar cross-section
Passive methods
Among others in the civilian seafaring, in civilian and military ports and airports, navaids and buoys, to bridge crossings and also to weather balloons measures to increase the radar echoes are used. The goal is to safely navigate to locate and coordinate the sea lanes. To this end, corner reflectors are used which increase the radar cross section of the many thousands of times their geometric area.
In general, metal objects have a higher radar return than non-metals. Even relatively small metal parts can therefore cause an increase of the radar cross-section to make, for example, a boat with a plastic hull visible on the radar. Often be of such small boats small radar targets carried with dimensions in the centimeter range, which in often used by radars X-band (about 6 ... 12 GHz ) and in the I-band (Europe, 8 ... 10 GHz) as a resonant secondary radiator already provide increased response and act as a corner reflector only at shorter wavelengths due to their shape.
Active methods
It offered commercial radar reflectors, which operate on the principle transponder or as a repeater jammer. The radar signal is received and sent back on the same frequency amplified. These devices have the advantage that a warning signal, the detection signals by a radar device.
The through the receipt and processing of the radar echo -imposed time delay may only be a few microseconds to not display the target on the radar screen in too wrong distance. Modern devices analyze the signal and send the response synchronously a pulse train period later again.