Radar reflector

The triple mirror consists of three orthogonal metal plates and is the archetype of the radar reflector

Principle of the triple mirror: The signal is always reflected back in the direction of the source, regardless of the direction of incidence.

Radar reflectors are retro-reflectors for radar applications and mostly designed as a triple mirror ( english corner reflector ). They produce for radars a particularly strong echo signal and thus a safe target characters ( english target ) on the radar screen for objects that would otherwise only have a very low or unstable effective reflection surface feature. Radar reflectors are constructed according to quasi-optical principles and mostly use mirror reflection ( reflection on smooth surfaces), but also refraction and diffraction to create an effective reflection surface . As a reference for the effectiveness of a radar reflector, this area is usually given in square meters.

Reference value

Of the reference (spherical reflector), only a very small proportion of the surface can be effective for retroreflection. The majority is steered in directions that cannot be used by the radar device.

A spherical reflector with an ideally conductive surface is given as a reference, whose parallel projection on a plane perpendicular to the projection direction (i.e. its shadow on this plane) has an area of one square meter. Of this reference reflector, however, only a very small area is effective as a reflector: there are only a few centimeters exactly in the middle, which can reflect the incoming transmission energy back in the direction of the radar device. All other surfaces of this sphere distribute the incoming energy in space. You are not involved in the retroreflection. However, this reference has the advantage that it is direction-independent: it reflects equally well in all directions.

A radar reflector makes a much larger proportion of its surface effective for retroreflection. Even geometrically very small areas of a corner reflector with a surface area of a few square centimeters can reflect a comparable amount - sometimes even more - of energy back to the radar precisely than this sphere with a diameter of around 1.13 m. Despite the small geometric size, you have an effective reflection surface of one - or more - square meters.

In practice, calculations of this effective reflection area can only be carried out approximately. It is not only simply reflected by these corner reflectors, but there are even more physical effects, for example diffraction at the outer edges. This leads to a so-called circulating wave , the energy part of which then interferes with the reflected part in either constructive or destructive interference . Depending on the size of the corner reflector and thus depending on the length of the detour of the circulating wave, the energy reflected back can in the best case increase by more than double or in the worst case be less than half. The effect is also dependent on the wavelength of the radar device. This behavior can be specifically exploited with corner reflectors used in seafaring, since the radar devices used there all work in a very narrow frequency range (around 9.3 to 10.5 GHz, which corresponds to a wavelength of around 3 cm).

Corner reflectors

Corner reflector with two surfaces

In the basic element, corner reflectors consist of two or three electrically conductive surfaces at an angle of exactly 90 ° to each other. For this basic element, various shapes of corner reflectors are constructed. Corner reflectors with three surfaces reflect radio waves and microwave radiation in exactly the direction from which the source radiation originates, without having to be aligned perpendicular to it like a mirror .

Mode of action

In-phase reflection through the same path lengths: a + b + c = a '+ b' + c '.
A parallel wave front is thus formed again in the direction of the transmitter.

If the wavelength is small compared to the geometric dimensions of the corner reflector, there is retroreflection according to purely optical laws. Incoming electromagnetic waves are reflected back in exactly the direction from which they come by double or triple reflection. Even small objects with a small reflective surface receive a strong reflection back in the direction of the radiation source, they deliver a much stronger radar echo than other reflective bodies and appear much brighter on an analog radar screen . The double reflection on mutually perpendicular electrically conductive plates occurs phase-synchronously, since the distances of the individual phases are the same (a + b + c = a '+ b' + c '). At any angle of incidence, the corner reflector thus acts like a plate at right angles to the direction of incidence. This phase congruence is only given in the direction from which the source radiation occurs. In the main direction, the effective area corresponds approximately to the projection of the corner reflector onto the plane of this plate.

Corner reflectors with two surfaces

These corner reflectors only consist of two surfaces that are exactly at right angles to each other. In order to function properly, the partial surfaces must be mounted exactly perpendicular to the plane of the incident waves. Contrary to the declared aim of the construction to reflect approximately equally well in every direction, these corner reflectors are still strongly direction-dependent. The maximum energy reflected back can only be effective in one main direction - in the direction of the axis of symmetry. This main direction is given when the parallel projection of both individual reflection surfaces is approximately the same. The surfaces are thus approximately at a 45 ° angle to the axis of symmetry. The energy reflected back in the direction of the radar device drops to half of the maximum from a deviation of 15 ° from the axis of symmetry.

The following formula applies to the theoretical calculation of the effective reflection surface of a corner reflector with two square surfaces:

{\ displaystyle \ sigma = {\ frac {8 \ pi \ cdot s ^ {2}} {\ lambda ^ {2}}}}

(1)

{\ displaystyle \ sigma}

= effective reflection area

{\ displaystyle {\ text {s}}}

= Area of one side of the reflector.

{\ displaystyle \ lambda}

= Wavelength of the reconnaissance radar device

When using the formulas, it must be noted that if the corner reflector is less than approximately ten times the wavelength of the interrogating radar device, the practical value of the effective reflection area can deviate by up to four times from the theoretically calculated value due to local resonances. Since the frequencies of frequently used navigation radar devices are known (around 9.3 to 10.5 GHz), standard sizes have established themselves in practice as approximate multiples of the wavelengths used, the geometric size of which is just about a positive deviation from the theoretical calculated value comes into play. The larger a corner reflector, the less the influence of resonance. From ten times the wavelength, this influence is negligibly small. This then applies in the above-mentioned frequency band from an edge length of 30 cm - so it does not apply to most of the corner reflectors offered in stores.

Corner reflectors with three surfaces

Triangular corner reflector as a section of a cube

Corner reflectors with three reflecting surfaces at an angle of 90 ° to each other are used where retroreflection in three-dimensional space is necessary. These triangular reflectors work like the optical analog of the retroreflector, sometimes referred to as the cat's eye . The maximum effective reflecting surface occurs in the direction of the axis of symmetry. The effective reflecting area of the triangular corner reflector made of triangular areas shown in the figure is calculated according to:

{\ displaystyle \ sigma = {\ frac {4 \ pi \ cdot a ^ {4}} {3 \ cdot \ lambda ^ {2}}}}

(2)

{\ displaystyle \ sigma}

= Effective reflective surface

{\ displaystyle {\ text {a}}}

= Length of the touching edges of the three isosceles triangles of a triangular corner reflector

{\ displaystyle \ lambda}

= Wavelength of the reconnaissance radar device

The amount and phase of the reflected energy remain relatively constant in the frequencies used by radar devices, regardless of the angle of incidence, up to the range in which the reflected wavelength comes in the order of magnitude of the dimensions of the reflector surfaces. The individual surfaces of the corner reflector should therefore be large compared to the wavelength. The larger a corner reflector, the more energy it reflects. The half width of the main direction is about 20 ° to 40 °, depending on the geometry of the three surfaces.

If at least eight or more pieces of this calculated corner reflector are joined together in such a way that it reflects back in every direction, then one of the corner reflectors is always more or less effectively facing the observing radar device. This type of corner reflectors has a very complicated diagram of the reflection in which the respective size of the effective reflection surface can be entered as a function of the direction of the incoming radar radiation. The main direction is again the position in which all three visible partial areas have an equally large parallel projection.

The advantage of the corner reflector with three surfaces is its position-independent function (it can also be slightly inclined vertically) and the greater mechanical stability. This design is therefore used when the angles can change in two directions. The disadvantage is that it has to be built larger than a two-surface corner reflector with the same effective reflecting surface, since the pointed ends of the corner reflector do not participate in the reflection in the original direction. Depending on the position, these ends may only be reflected twice, so that this energy is directed in a different direction. Of the total area of the parallel projection, only an approximately equilateral hexagon is effective in the direction of the axis of symmetry (main direction). Some manufacturers therefore do without these corners, so that the corner reflector has a polygonal shape like in this picture . Often, designs made from circular areas are also used. They offer a good compromise between radar cross section and wind load. They are often housed in a plastic ball to protect them from ice accumulation and to further reduce the wind load. Such plastic balls with corner reflectors can often be seen on high-voltage lines near highways, since the thin lines pose a danger to helicopter pilots in the air rescue service or the police when visibility is poor .

The highest back reflection in relation to the side length has shapes made up of three squares (cube with three missing sides), it amounts to in the direction of the missing cube corner:

{\ displaystyle \ sigma = {\ frac {12 \ pi \ cdot a ^ {4}} {\ lambda ^ {2}}}}

(3)

A corner reflector made of square aluminum plates with an edge length of 20 cm each is sufficient as a radar target for navigation radar devices with a wavelength of around 3 cm, which mostly work in the I / J band in inland shipping .

A corner reflector that reflects all around in all directions consists of 12 isosceles triangles. Practical construction (see construction instructions ) is based on three square metal sheets of the same size, one of which is halved diagonally. Then incisions with a cutting thickness equal to the sheet thickness are made in the sheets and the four parts are pushed into one another. With an edge length of the square sheets of 0.3 m, such a corner reflector in the X-band (approx. 9.4 GHz) common for sport and professional shipping has an effective reflecting area of about 8 m² and thus about as much as a simple sailing boat . Such a reflector provides a target mark on the screen that cannot be overlooked on radar devices even at great distances. To avoid the risk of injury, the corners can be rounded off without significantly impairing the radar cross-section.

Lüneburg lentils

Principle of a Lüneburg lens

Lüneburg lenses as radar reflectors are available in different designs. You use a material with an inwardly increasing gradient index , for example a foamed material, which has a greater density towards the inside. Half of this sphere has a metallic mirror. The main direction of reflection is the axis of symmetry in the non-mirrored direction. This construction also reflects incident electromagnetic waves in exactly the direction from which they come.

For practical use on boats and ships, three of these spheres are cast into an assembly unit, the main directions of which are offset by 120 °, so that an all-round reflection of 360 ° is achieved. This construction is known as a tri-lens radar reflector . Tests commissioned by the British authorities showed that this design is the only passive reflector available on the market that achieves a satisfactory echo effect even when heeled .

In a similar way to a Lüneburg lens, all-round radar reflectors can also be manufactured, which instead of being mirrored on the back only have a narrow, horizontal belt made of a conductive layer. Compared to corner reflectors, it is geometrically larger with the same effective reflection surface. They are extremely incline-dependent, as the radio waves entering the sphere must be precisely focused on the reflective metal strip on the back. They are therefore well suited as reference or laboratory devices, but not for practical use on boats and ships.

Active radar reflectors

Transponders and Search and Rescue Radar transponders are also used in recreational shipping and by rescue workers . If their electronics receive a radar pulse, a strong radar pulse is sent back. This appears on the radar as a larger echo than the ship itself. In addition, the active radar reflectors indicate with a light or a warning tone that a radar pulse has been received and thus indicate other ships in the vicinity. A big advantage compared to passive reflectors is the drastically increased range, since with transponders, according to the radar equation, the power received in the radar device is not subject to two-way attenuation (on the way there and back), but only to simple free space attenuation , since instead of passive reflection an active response with a fixed Transmission power takes place. Their disadvantage is that they can only respond to constructively predetermined radar frequencies. Transponders can also send back coded signals that enable automatic identification. However, their use is controversial today, as new radar technologies sometimes use other frequency bands on which an active radar reflector is ineffective.

application

The radar reflector on the Inachabkuppe in Namibia was used for the national survey (2017).
The radar reflector is on the middle peak.

Corner reflectors are used as targets for radar devices :

as a calibration standard for a free space calibration of antennas (e.g. in air traffic control to check the direction of a precision approach radar )
as a navigation aid for marking shipping routes in difficult water (on floating nautical signs , on bridges)
for marking the runway at airfields
Radar reflectors on watercraft to make them easier to recognize by all-round radar from other ships and on land
to simulate a large missile using so-called decoy targets
on a weather balloon for radar tracking and measurement of wind speeds at high altitudes
as a defined target for distance measurement in radar devices in industrial applications
for marking dangerous objects for aviation
for national surveying (radar triangulation)

Octahedron- shaped corner reflector as a frequently used radar reflector in the top of a motor yacht

Installation of radar reflectors for marine applications

In order for a radar reflector to achieve its theoretically possible effectiveness, certain conditions must be taken into account when setting up:

Install as high as possible, for example on a sailing ship at the top of the mast or the top spreader . This ensures that the sailing ship also reflects a sufficient radar echo when essential parts such as the hull or the rig are already behind the radar horizon.
Fixed attachment, for example by screwing the radar reflector so tightly that it cannot slip or turn. This avoids non-stationary, so-called pumping echoes, as would be the case, for example, with a "flying" attachment to traps , the backstay or the toppnant . In the case of pumping echoes, the reflected signal is alternately displayed and not displayed on the receiver's radar screen. However, raster scan systems for radar in sport and commercial shipping may suppress such pumping echoes so that the sailing ship is not perceived.
Correct alignment of the radar reflector.

For the correct alignment of the radar reflector, it must be noted whether it is an object with a reflective surface that is almost the same in all directions, e.g. B. in a motor ship, or whether certain directions already have a sufficiently good reflective surface, such. B. on a sailing ship. In the case of a sailing ship, if radar beams strike from the side, the rig or the standing rigging already leads to sufficient reflections, whereas the ahead and aft is insufficient. In this case, a radar reflector should preferably reflect ahead and aft, that is, have a particularly large effective reflective surface.

An octahedron radar reflector with eight triangular corner reflectors, the so-called corners, can basically be installed in three different positions:

In the four-way position, in which one point is aligned exactly upwards and one point is exactly downwards. However, this position has poor reflection characteristics in the horizontal direction, since effectively only four corner reflectors contribute to the reflection.
In the six-position position, in which a corner reflector is oriented exactly upwards and one corner reflector is oriented exactly downwards, so that this position is also known as the rain catching position . This position has a balanced reflection characteristic that is almost equally good and balanced horizontally in all directions. It is therefore particularly suitable for motor ships.
In the yacht position, a special form of the six-position, in which a corner reflector is aligned exactly to the front and a corner reflector is aligned exactly to the rear. This position has a very good reflection ahead and aft and less to the sides, so that it is particularly suitable for sailing ships.

	Quadruple	Six formation	Yacht position
arrangement
Reflection diagram (qualitative)

According to the SOLAS Convention , Chapter V, all ships, including leisure yachts, must be equipped with radar reflectors "if practicable". ISO 8729 currently applies to the equipment , which is available in two parts (ISO 8729-1 for passive, ISO 8729-2 for active reflectors). Passive reflectors must have an effective area of at least 2.5 m², which, however, results in a reflector size that is not practical for small yachts. There are also hardly any products available that meet this standard. Accordingly, the recommendation is to use the largest possible reflector that can be attached to the ship.

Web links

Commons : Corner reflectors - collection of images, videos and audio files

Individual evidence

↑ Derivation of the formulas on radartutorial.eu
↑ ^a ^b ussailing.org ( Memento from September 28, 2007 in the Internet Archive )
↑ aerospaceweb.org
↑ ^a ^b microwaves101.com
^ Müller, Krauss: Handbook for the ship's command. Volume 1C, 8th edition, Springer-Verlag, 1986, ISBN 3-540-13484-0 , page 124
↑ ^a ^b ^c ^d Performance of marine radar reflectors on the market . Marine Accident Investigation Branch of Great Britain. 2007. Accessed December 23, 2015.
↑ ^a ^b ^c ^d Investigation report 56/09, p. 23 ff . Federal Bureau for Marine Casualty Investigation. 2010. Accessed in 2019.
↑ See English description of the radar triangulation
↑ Egon Ohlrogge: Applied radar science - practice for professional and recreational shipping. Delius Klasing 2001, ISBN 3-88412-353-X
^ SOLAS V Regulations . Royal Yachting Organization. Retrieved December 24, 2015.
^ Marine Rules and Regulations . Retrieved December 24, 2015.

[1] Derivation of the formulas on radartutorial.eu

[radtarg-jpg-2] ussailing.org ( Memento from September 28, 2007 in the Internet Archive )

[3] rospaceweb.org

[cornerrefl-4] rowaves101.com

[5] Müller, Krauss: Handbook for the ship's command. Volume 1C, 8th edition, Springer-Verlag, 1986, ISBN 3-540-13484-0 , page 124

[maib-6] Performance of marine radar reflectors on the market . Marine Accident Investigation Branch of Great Britain. 2007. Accessed December 23, 2015.

[bsu-7] Investigation report 56/09, p. 23 ff . Federal Bureau for Marine Casualty Investigation. 2010. Accessed in 2019.

[8] See English description of the radar triangulation

[9] Egon Ohlrogge: Applied radar science - practice for professional and recreational shipping. Delius Klasing 2001, ISBN 3-88412-353-X

[10] SOLAS V Regulations . Royal Yachting Organization. Retrieved December 24, 2015.

[11] Marine Rules and Regulations . Retrieved December 24, 2015.