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Radar Reflectors explained

Radar Cross Section (RCS)

1. A sphere operates with a weak signal at all angles of incident radiation.

2. A flat plate is an extremely efficient reflector but has a very sharp angle of response.

3. RCS may, for practical purposes, be defined as the cross section area of a conducting sphere of such a size that it would return an echo equal in strength to that of an equivalent flat plate oriented so as to be perpendicular to the direction of the incident radiation.

4. One metre squared is the cross section of a sphere radius 0.565 metre (R2xPi - 1 metre 2)

Radar Interference by Sea State & Precipitation
Mariners Handbook Fourth Edition 1973

'Sea' is the name given to waves generated by wind blowing locally. A radar screen becomes cluttered when echoes from waves are received. Further clutter arises from precipitation (rain, snow, and fog).

Sea States:

Wave Clutter:

Precipitation:

Moderate: 1.25 to 2.5 metres

Rough: 2.5 to 4. metres

Very rough: 4 to 6 metres

Echoes from upwind are greater than those from seas running down wind. Wave clutter does not extend beyond five nautical miles but large echoes arise as beam grazing angles increase.

Rain

Light: 4mm/hour

Moderate: 10mm/hour

Heavy: 16mm/hour

Fog is caused by the cooling of air in contact with a surface at a temperature whereby it can no longer maintain, in an invisible state, the water vapour which is present in it. Condensation of the vapour produces minute, though visible, water droplets. Rain and snow are further examples of droplets which return radar clutter.

Transmitted power, to and from the target, is attenuated by precipitation on average by -5dB (-70% of reflected power). Visibility will be a guide to assessing power RCS lost.

Note:- Precipitation and wave clutter may or may not occur together.

The mandatory collision frequency is 'X' band. Precipitation is penetrated better on 'S' band but its target response is 1/10th 'X' band. S Band does not overcome sea clutter but may be used from beyond three nautical miles to penetrate fog, rain, snow etc. A 10m2 target will return 0.001 m2 from ten nautical miles. Doubling the distance requires target area to be16 times greater.

F. J. Wylie 'The use of Radar at Sea'

Sea Force 4 - Wave height one metre.
Sea Force 6 - Wave height Two metres.

Approximate clutter from waves:

Sea Force 4 - obscures 10m2 targets to 3 nautical miles

0.5 n.mile - 5M2

0.75 n.mile - 3M2

1 n.mile - 1M2

1.5 n.mile - 0.2M2

2 n.mile - 0.01M2

Sea Force 6 - obscures all targets to 3 nautical miles

0.5 n.mile - 100M2

0.75 n.mile - 32M2

1 n.mile - 10M2

1.5 n.mile - 1M2

2 n.mile - 0.1M2

In free space a 4M2 target's RCS at

0.5 n.mile - 4M2

1.5 n.mile - 0.8M2

2 n.mile - 0.25M2

In average precipitation at

0.5 n.mile - 2M2

1.5 n.mile - 0.3M2

2 n.mile - 0.08M2

ORTHOGONAL CORNER (optimum aspect for radar response)

Tracing of an octahedral's 'butterfly' response

Response from a 10M2 (23cm) calibration corner

Note. In the catchwater position an octahedrals lobes are directed 18 degrees above or below the horizon.

Add 15 degrees mast tilt and note the null area X

Z = dihedral side lobes

F. J. Wylie "The Use of Radar at Sea"

A = Wave height 3 feet (Force 4)
B = Wave height 6 feet (Force 6)
C = 2nd class buoy
D = 2nd. class buoy with 17" corner reflector
E = theoretical response from an ideal 10sq.m sphere reflector without sea surface reflections
F = statistical limit of effective response
'X' = dips due to sea surface reflections

One of the last faxes received from John Firth shortly before his death.

This has been reproduced from a poor quality fax and we hope we have managed to transcribe it correctly.

Orthogonal Corners (Type T)

Pi x C4 Lambda2 x 4/3 - RCS (corner-lambda in metres)
(‘X’ band lambda 0.032 metre)
Target area sq.m Distance4 – Response sq.m. (area sq.m. – distance in n. miles)
It is difficult to put the science of radar into simple terms but the following may help understanding by small vessel sailors who use radar reflectors

Small vessels radar

(Commercial shipping will have more powerful sets with scanners at great heights).
Power echoed is minute – eg 10 sq.m. at 10 n.miles = power 0.001 sq.m. (-40dB)
Maximum receiver gain is 80dB (x 1000,000,000)
Scanners rotate every 2.31 seconds
Ranges – short 1.5 n.m., medium 24 n.m. long 48 n.m.
Beam width azimuth at range limit is: - short 48 metres, medium 776m, long 44448m.
Transmission pulses on target each scan: - short 19, medium 9.5, long 4.75
When distance between scanner and target doubles echo power reduces by 1/16.

“Display paints of modern radars do not spread and may not change brilliance when the echo strengthens, nowhere is echo strength mentioned in the Collision Regulations”. The reason for yachting press ‘sea trials’ being subjective.

Anechoic chamber recordings accurately define power response over azimuth and vertical angles. Vessel yaw/roll ensures 10 degree nulls in performance are acceptable. The mandatory collision avoidance frequency is ‘X’ band 9Ghz – lambda 32mm. Large vessels use ‘S’ band 3 GHz – lambda 96mm – for its ability to penetrate precipitation (fog, snow, rain) at distances where ‘X’ band may not do so. Scanned at ‘S’ band the target power returned is one tenth that of ‘X’ band. When wave clutter is encountered in the absence of precipitation ‘S’ band offers little advantage over ‘X’ band.

Precipitation attenuation varies considerably decreasing as distance falls.
Conversely target power increases (see examples below)
For vessels such as yachts, 30 sq.m. RCS may be acceptable as a compromise to the 100 sq.m RCS necessary to meet all conditions.

F. J. Wylie’s sea state clutter graph

Target ‘D’ (2nd class buoy – 100 sq.m) at 1.5 n.miles and less is screened by line ‘B’
Target ‘C’ (2nd class buoy – 10 sq.m.) at 1.5n.miles and less is screened by line ‘A’
Due to ‘grazing angle’ increase sea state clutter increases rapidly within 5 n.miles
Within 3 n.miles a 2.5 sq.m. reflector is hidden by wave clutter.
Graph line ‘D’ 100 sq.m (ex sea surface reflection dips)

RCS at:

Corner 8.76” – 0.225 metres 10 sq. m

RCS at:

Note: Beware of claims overstating performance.
Use the largest reflector RCS available.
Beware of traffic and weather conditions. Your radar echo may save your life
Big ships speed leaves little time to react.

J. H Firth 22nd. April 2002