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 |
| Tracing of an octahedral's 'butterfly'
response |
ORTHOGONAL CORNER (optimum
aspect for radar 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 5 n.miles 0.160 sq.m
 4 n.miles
0.391
3 n.miles
1.235
2 n.miles
6.250
1.5 n.miles
19.753
1.25
n.miles 40.960
Corner 8.76” – 0.225 metres 10 sq. m
RCS at 5 n.miles 0.016 sq.m
4 n.miles
0.03910
3 n.miles
0.1235
2 n.
miles 0.625
1.5 n.
miles 1.9753
1.25
n. miles 4.0960
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
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