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RADAR
Errors
Basic radar parameters
Frequency
bands
Marine radar operating frequencies are allocated
within the following bands of frequencies:
The S band from 2000 to 4000 MHz
The X band from 8000 to 12500 MHz
These correspond to a range of wavelengths of:
S band 7.5 cm to 15 cm – 10 cm Radar
X band 2.4 cm to 3.75 cm. – 3 cm Radar
The International Telecommunications Union allocates
the two bands.
For example, a vessel might transmit on:
9445 ± 35 MHz in the X band and
3050 ± 10 MHz in the S band.
Use of these high frequencies has an added advantage
of reducing the size and thus weight of the antennas.
Three-centimetre radar is used for relatively
high-bearing definition and good echo return.
In the atmosphere gas molecules absorb some energy, but
three-centimetre waves can experience severe attenuation and reflection in rain
and dense fog.
Wavelengths of ten centimetres
are less affected by rain, fog and snow and also permit returns from large
targets beyond the normal radar horizon of the shorter wavelength.
Choice of PRF is influenced by several design factors.
Successive pulses must be separated by a period of time which permits the echo
from a transmitted pulse to return and be displayed before the onset of the
next transmitter pulse.
Therefore, the PRP must have a length that is equal to
twice the one-way trip time from antenna to target.
Practically the PRP is greater than the calculated
minimum.
Values are between about 500 PPS to 4000 PPS and a radar will have two or three PRFs
giving a choice of PRP to suit the ranging conditions.
Additionally, the pulse length will be switched to
suit those conditions.
The following values of pulse length and PRF’s are automatically selected when the radar range is
changed. Also the facility exists for manually changing the pulse length from
short to long and vice versa. Selecting a longer pulse instead of a shorter one
will always downgrade range resolution.
Radar 1
Range ¼ ¾ 1.5 3 6 12 24 60
nm
Pulse
length (μs) 0.05 0.05 0.05 0.05 1.0 1.0 1.0 1.0
PRF 3200 3200 3200 3200 800 800 800 800
Radar 2
Range ¼ 11/2 3 6 12 24 48 64
nm
Pulse
length (μs) 0.06 0.06 0.06 0.5 0.5 1.0 1.0 1.0
PRF 3600 3600 3600 1800 1800 900 900 900
Relating PRF and pulse
length:
Range PRF pulse
length PRF pulse
length
(N.M.) (Short) (Long)
0.25 2000 0.05 2000 0.05 only 1 PRF – 1 pulse length
0.5 2000 0.05 1000 0.25 2
PRF – 2 pulse length
0.75 2000 0.05 1000 0.25 2 PRF – 2 pulse length
1.5 2000 0.05 1000 0.25 2
PRF – 2 pulse length
3.0 1000 0.25 500 1.0 2 PRF – 2 pulse length
6.0 1000 0.25 500 1.0 2
PRF – 2 pulse length
12 1000 0.25 500 1.0
2 PRF – 2 pulse length
24 500 1.0 500 1.0 only
1 PRF – 1 pulse length
48 500 1.0 500 1.0 only
1 PRF – 1 pulse length
Comparison
between X- band and S- band:
Target
Response: For a target of a given
size, the response at X – band is greater than at S – band.
Bearing discrimination: For a given aerial width the horizontal beamwidth
effect in an S – band system will be approx. 3.3 times that of an X – band
system.
Vertical Beam structure: The vertical lobe pattern produced by an S – band aerial
is about 3.3 times as coarse as that from an X – band aerial located at the
same heights.
Radar Horizon: The radar horizon with S – band is
slightly more distant than with X – band.
Sea clutter response: The
unwanted response from sea waves is less at S – band than at X – band, thus the
probability of targets being masked due to saturation is less.
Precipitation response: The probability of detection of targets, which lie within an area
of precipitation, is higher with S – band transmission than with X – band
transmission.
Attenuation in precipitation: In any given set of precipitation conditions, S – band
transmissions will suffer less attenuation than those at X – band.
Selection of PRF and pulselength and their
relationship with Range scale.
For any range scale the period between two pulses
should be at least as long as the time base for that range scale. This because the trace spot should be able to go up to the edge of
the screen and then return to the centre – thereby tracing a single echo line.
If however the time between the two pulses (pulse
repetition period or p.r.p.) is equal to the time
base then the problem would be to resolve any echoes, generated by the first
pulse but outside the range scale in use arriving in the time base of the
second pulse (second trace echoes).
Therefore a time delay (receiver rest time) must be
introduced between the completion of one time base and the commencement of the
second time base – so that the above confusion is reduced as much as possible.
Thus the combined length of the time base and the
period of rest of the receiver must be long enough so that echoes due to any
given pulse are all returned before the transmission of the second pulse.
The above thus defines the minimum acceptable pulse
repetition period (p.r.p.) and thus the maximum PRF
for any given range scale.
The next criteria is to
assess how far a pulse would go, since that would determine the time duration
delay that we have to allow for the echo to come back.
The range of this EM wave is among others dependent on
the transmitted pulse length. Longer the pulse length the more distant the
pulse would travel and thus the echo from the furthest point would take an
appreciable time.
This would entail a longer pulse repletion period and
consequently a lower PRF.
Thus we see that long pulses are used on the
longer-range scales and are associated with low PRF. The short pulses are used
on the shorter-range scales and are associated with high PRF.
It should be remembered that selection of a short
pulse would reduce the probability of detection of any one return. However this
is compensated with the fact that when on short pulse mode, the PRF is higher
so more shots are being fired at the target.
The Radar
Horizon:
Although the Radar transmitter emits EM waves and that
these waves are supposed to move in straight lines (line of sight), it does so
happen that the radar waves are bent slightly downward due to the atmospheric
conditions.
Therefore the radar horizon is given by the equation:
Distance in NM = 2.21 x square root of Height of Own
antenna in metres
To get a return from a target the target is supposed
to be above the sea level, and then the detectable range using the above
equation would be as follows:
Distance in NM = 2.21 x square root of Height of Own
antenna in metres + 2.21 x square root of Height of target in metres
Thus if we take that a target is of a height of 1 m
above the sea, and that our antenna is placed at a height of 25 m above the
sea, then from the above it follows:
Detectable range of target = (2.21 √01) + (2.21
√25)
=
(2.21 x 1) + (2.21 x 5)
=
2.21 + 11.05
=
13.26 NM
Of course the above is purely theoretical since the
echo to be received by the radar would depend on a number of causes:
Standard atmospheric conditions
Pulses are powerful enough
The target has a good reflector surface and would
return the echoes
The water vapour/ rain in the atmosphere does not attenuate the pulse on its journey to and from the
target.
When we talk of standard atmospheric conditions, it is
taken to be as follows:
Pressure: 1013mb and decreasing at 36mb/1000ft height
Temperature: 15°C decreasing at 2°C/1000ft height
Relative humidity: 60%
The above conditions give a refractive index (RI) of
1.00325 decreasing at 0.00013 / 1000ft of height.
Second
Trace Echoes
If the PRF is too high then a target echo may be
displayed on the time base of the next pulse.
1 μs = 300 metres
or 1 nm =
12.35 μs as displayed in the CRT
In the above, pulse
1 has enough energy to return a echo from targets
A and B.
The selected maximum range is 48 nm, producing a timebase sweep of 12.35 x 48 = 595.0 μs.
Target A, returns an echo
446.0 μs after timebase
waveform initialization, indicating that the target lies at 36 nm range on the
first transmission trace.
A PRF of 1500 pps is chosen
giving a PRP of 666 μs.
1 second = 1500 pulse’s
1000,000 μs = 1500
pulse’s
PRP = 1000,000 / 1500
PRP = 666.67 μs
Target B returns its echo due to pulse 1 after 800 μs and is due to
the target lying at range 64 nm. The echo due to target B therefore returns 134
μs after the start of the timebase
trace due to pulse 2.
The paint due to echo B then appears on timebase trace 2, producing an ambiguous echo at an
apparent range of 11 nm. All unambiguous
echoes will, of course, be displayed at normal ranges.
On board ship’s it may be possible for second trace
echoes to be displayed only when the pulse transmitted has sufficient energy to
cause a echo to return from a large target area such as a land mass beyond the
visible horizon but within the radar horizon.
Or sometimes due to a
phenomenon known as super refraction.
A displayed coastal outline may be shown near the
central area of the CRT screen, and the navigator should be suspicious of such
unexpected targets and aware of the possibility of super refraction in the
atmosphere.
Changing PRF, done by just changing to a lower range
can eliminate second trace echoes.
Since Radars have at least two PRF’s,
these second trace echoes thus pose no hazard if understood by the navigator.
Minimum
detectable range
RF radiation for 1.0 μs
pulse duration will occupy a length in air of 300m.
During the transmission of the 1 μs
pulse the receiver is switched off by T/R cell.
The forward edge of this pulse is at A and the after
edge is at B. As soon as the transmission stops the after edge B, of the pulse
has just left the antenna. The T/R cell switches on the Receiver circuit.
However,
during that 1.0 μs of transmission there may be
a target, at for example 100m range, returning an echo.
Since the pulse length is 1 μs
(=300m), therefore the forward edge of the transmitted pulse traveled 100m to
the target and the echo so generated again traveled 100m back to the scanner,
total distance traveled 200m. This is less than the pulse length of 300m (1 μs), so the T/R cell does not open the receiver
circuit for the echo to be processed and displayed.
But if the target is at a distance of 150m, and the
forward edge of the pulse hits it, and there is an echo then this echo will be
received. This will be so, because now the forward edge of the pulse traveled
150m, generated an echo of the target which traveled another 150m back to the
scanner, a total of 300m (1 μs).
Thus, 150m is the minimum range at which a target is
detectable and the value of that minimum range depends on the pulse
length.
In reality however the Radar manual states more
distance (nearly double).
If a Radar has a minimum pulse length of 0.05 μs for the lower ranges then the minimum theoretical
range is given by 0.05 x 300/2 = 7.5m.
As the Radar set becomes old the efficiency of the T/R
cell goes down and the minimum detectable range increases from that stated in
the manual.
Radar’s operating under short pulse will have a better
minimum detectable range.
Range discrimination
Range discrimination means the ability of a radar to detect two or more targets lying close together
on the same bearing and at differing ranges and display those targets as
separate echoes.
In the following figure there are two targets A and B,
each on the same bearing and separated by 150m range. The transmitted pulse
length is 1.0.
In
this case also it is nearly the same as the minimum detection range.
If
the pulse length is more then the minimum distance between two targets for each
to be shown separately, the pulse length has to be less than the minimum range
discrimination stated.
It is seen that range discrimination will be degraded
as pulse length increases.
In a set having three pulse lengths of 0.06 Rs, 0.5 Rs
and 1.0 Rs, the range discrimination cannot be better than 9m, 75m and 150m
respectively.
Range Discrimination:
Range discrimination means the ability of the radar to
differentiate between two targets at the same bearing but separated by a
minimum distance.
The discrimination is usually expressed in metres,
which must separate the two targets as mentioned above so that they may be
observed separately.
IMO performance standard for radar specifies condition
for two targets, on the same bearing but separated by 50 metres in range.
The
specification states that on a range of 2 NM or less be
capable of displaying the echoes of the two targets as separate targets when
the two lie at a range of between 50% and 100% of the range scale in use.
In general half the pulse length can be said to be
theoretically the distance between the two targets required to paint them
separately. However another factor comes into play is the spot size of the
paint.
A sheet of metal of unit width will also send in an
echo and that will be painted on the screen with a certain predetermined
dimension. This dimension will again change with the range scale in use.
To understand this note that
a point of 0.5mm of the screen will be of different dimensions (in NM)
depending on the range scale in use.
The range of each spot is a function of the range
scale in use as well as the spot diameter per radius.
Thus:
Number of Spot Diameter per screen radius (250mm
diameter screen) = Radius of the screen (125) / each spot diameter
Thus the range as represented by each spot would then
be found using the range scale used.
Range = Range scale in metres / spot diameters per
radius
Thus as an example for a Radar screen of 250 mm and
with a spot diameter of 0.5mm and at ranges of 12 NM and at 48 NM the spot
would represent as follows:
Number of spot diameters per screen radius = 125 / 0.5
= 250
Range = (12 x 1852) / 250 = 88.9 metres 12
NM
Range = (48 x 1852) / 250 = 355.6 metres 48 NM
Now noting the above we see that the minimum
separation between the targets would not be exactly half the pulse length but
this spot radius factor as well. However the spot radius would be different for
different makes of radar.
Thus to some practical value to range discrimination:
Range discrimination = Range (spot) + Range (pulse)
If we take our above example at 48 NM and use a pulse
length of 1μs, then
Range discrimination = 355.6 + 150 = 505.6 metres.
Bearing
discrimination:
Bearing discrimination means that ability of the radar
to differentiate between two targets on the same range and separated by a
minimum angular distance.
Shows two targets X and Y lying at range R metres.
This shows the antenna beam in just one direction,
assuming that the antenna is not rotating. It is seen that when the angle (φ)
subtended by the two targets at the antenna is less than the horizontal beamwidth (θ), the two targets will be hit
simultaneously by the pulse and they will appear as one displayed target.
This is generally expressed in metres as that distance
by which two targets lying on the same range are painted separately.
The IMO performance standard sets the specification
that when two targets on the same range of between 50% and 100% of the 1.5 NM
or 2 NM range scales, the two targets should be able to be displayed separately
by not more than 2.5° in azimuth (bearing).
Thus to comply with the above the trailing edge
(horizontal) of the rotating beam must leave one target before the leading edge
of the rotating beam strikes the second target.
Theoretically the Horizontal Beam Width (HBW) would be
the determining factor, such that the bearing discrimination would be at least
one HBW.
However again due to the
spot diameter of the screen resolution, in practice the minimum distance is
more.
Also this discrimination would be different at
different ranges since though the HBW angle would subtend the same angle the
arc in NM would be different.
Let us look at an example:
Screen diameter: 250 mm, spot size: 0.5 mm, HBW: 1.5°,
Range scale in use: 12 NM, Target at range of: 10 NM
Length of the arc at 10 NM:
= range x HBW in radians
= (10 x 1852) x 1.5 x (π / 180) metres
= 485.05 metres
Now the arc represented by one spot diameter is given
by:
= range scale in metres / spot diameters per radius
= 12 x (1852 / 250)
= 88.9 metres
Thus Bearing discrimination = 485 + 89 = 574 metres
For the same example but the targets lying at a range
of 1 NM would be:
Length of arc = 48.5 metres
Arc by spot diameter = 88.9 metres
Thus Bearing discrimination = 48.5 + 89 = 138.5 metres
Target glint
Statistics show that of the 25 pulses reaching the target approximately 10-15 detectable pulses may return.
Successive pulse transmissions will, however, build up
a bright area of paint on the display, which appears at the range of the centre
of echo returns.
Due to rolling and pitching of own/target vessel the
aspect of the target may change, this will change the echo signals polarization
as well as the strength.
The above will lead to the target appearing in one
scan and not appearing in some scans. This is known as target glint. Further
this also leads to ‘Lost Target’ when the target is acquired by the ARPA.
Summing up:
In order to produce a good signal return from a
distant target the energy in the transmitted pulse should be high. For a given peak power this suggests long
pulse duration.
However, lengthening the pulse will degrade range
discrimination.
Having a short pulse length ensures good minimum
detectable range.
At longer displayed ranges, where range discrimination
is not so critical, the pulse energy is increased by
choosing a longer transmitted pulse.
The PRF and the pulse length determine the average
transmitter power for a given peak output power. Also, the PRF determines the maximum
displayed range and is preferably chosen so that second trace effect is
avoided.
Evidently reducing the antenna speed for a given PRF
will provide more paints per target; it is not feasible to increase antenna
horizontal beamwidth since this would reduce antenna
gain and degrade bearing discrimination.
The PRF and pulse widths are generally made variable
being switched to a desired value when the operator of the equipment changes
the displayed range.
Super and
sub-refraction
The atmosphere is not of uniform density throughout
its height above the earth. Also the density of the atmosphere changes during
the day as well as during summer and winter.
Early morning the sun appears red while rising due to
the above fact that the light from the sun has under gone refraction. This
phenomenon of bending of light rays is also applicable to other
electro-magnetic waves including radar beams.
Refraction causes the ‘radar horizon’ to appear at a
distance greater than if the radar waves travelled in
a straight line.
Radar waves are affected by above discrepancy in the
density and also due to the amount of water vapour in the atmosphere.
Extended ranges are caused by propagation through a
‘nonstandard’ atmosphere and are generally caused by a ducting phenomenon. Microwaves do propagate beyond the normal
horizon due to earth diffraction but this is a consistent propagation factor
for which an allowance is made in the application of the earth radius
factor.
Super
Refraction:
This occurs when the rate of decrease in refractive
index with height is greater than under standard conditions.
When super refraction occurs the RADAR’s
beam tends to be bent down slightly more and so targets may be detected at
greater ranges.
Conditions which are favourable would be:
A decrease in relative
humidity with height.
Temperature falling more
slowly than standard or even increasing with height.
These conditions are generally found in very good
weather/ visibility conditions. Especially in conditions of high pressure
weather systems.
Thus we would have – a cool sea – and a hot dry wind
blowing, reducing the relative humidity and inverting the temperature inversion
from standard conditions.
Atmospheric ducts are caused by rapid decrease of
refractive index with altitude which can itself occur due to an increase in
temperature and/or a decrease in humidity.
The refractive Index changes at a rate of about 4
times the standard rate.
It is the latter humidity gradients, which are
recorded as producing the most pronounced changes in refractive index. Enhanced propagation can occur by ducting if
the radar antenna or the target is near the water, which places the radar
antenna and the target within the duct.
When evaporation occurs from the sea into a still
atmosphere a layer of moist air is produced, extending perhaps 50 to 100 feet
and having a vapour content which decreases rapidly with height. The condition causes partial trapping of
radar energy and is further accentuated if there exists at the same time a
temperature inversion, i.e. temperature reduces less rapidly with increase of
height.
Such conditions can cause radar energy to reach and be
returned from targets many times the normal radar horizon and is known as
super-refraction.
Echo returns are generally from targets having a large
radar reflecting area such as high landmasses.
Due to their extended detection range such echoes return on the next
consecutive PPI trace causing second trace effect. In general warm dry air
settling over a relatively cold sea produces conditions ideal for super-refraction.
Sub-refraction
This would occur when the refractive index of the
atmosphere decreases less rapidly with height than under standard conditions.
Occurs where there is a very rapid reduction of
temperature with height and/or increase in humidity with height.
These conditions can arise when a very cold air layer
settles over a relatively warmer sea.
For conditions to arise formation of a cold front in
northern latitudes is often a cause. So this condition would arise when there
would a requireent for better radar capabilities and
visibility is poor (cold front).
The phenomenon of inferior mirage is almost the same,
when the air of differing densities refracts the visible light.
Generally experienced in or
toward the
Radar
ranges are reduced to visual eye sight distances or in some cases are reduced
below this distance also, wherein sometimes targets being visible to the eye
and the Radar has no echo on the display.
Large masses of ice drifting in relatively warm sea
currents can produce identical conditions for occurrence of sub-refraction.
Squint angle
Slight non-uniformity’s in
emission along the slotted wave-guide introduce a shifting of the main beam
axis so that it is not truly perpendicular to the length of the antenna. The small angle so produced in the horizontal
plane is called the squint angle.
Magnetrons are manufactured tuned to a nominal
frequency within the radar band. Slight
changes in the transmission frequency which can, for example, occur when a
magnetron is replaced, can alter the squint angle of a slotted waveguide
antenna.
Speed error
Given that log input may contain some speed error the
true motion unit must operate on that input to produce a scaled motion of the
trace origin having an error not exceeding 5 per cent or 0.25 knot, whichever
is greater.
In the case of a speed of 20 knots the maximum error
permitted becomes one knot which proportionately diminishes with reduced
speed.
In digital form the speed is allocated to a single
byte (eight binary digits). This gives a
resolution of 1/256 or, at 20 knots, 0.078 knots error.
Course error; drift error
For type-tested systems, the course error generated by
the true motion unit (TMU) circuitry must not exceed 3 degrees.
A typical analogue true-motion unit specification will
quite likely quote zero speed drift in
terms of the observed scan origin motion when
there is zero speed input.
As an example, it might be that drift will occur over
not more than 5 per cent of one quarter of the tube diameter during a 30-minute
period or 0.25 knots, whichever is the greater.
For a 406-mm display this amounts to 5.0 mm.
On the 24-nm range the drift becomes equivalent to 1.2
knots, a drift of 0.6 nm in 30 minutes.
On the 1/4nm range the same distance represents
0.00625 nm or 0.0125 knots.
If the 0.25-knot tolerance is applied to this shorter
range the drift could be as great as one half the radius
in one half-hour period.
Drift can be minimized by careful adjustment of
parameters, but over a long period the integrator will drift to saturation. On a true motion PPI the effect is to
introduce apparent scan origin motion where none should exist. Since the integrator is usually reset at
frequent intervals on short ranges, this action discharges the capacitor to
produce zero integrator output voltage.
The problem is not then crucial.
Over longer tracking periods a track error will result due to drift
which, being on the longer ranges, is seen from the previous example not to be
critical for the observed errors.
Shadow sectors and blind sectors
Any part of the ship’s structure, which forms an
obstruction to the main beam of the radar antenna, can cause a shadow sector or shadow zone on the PPI.
However it should not be forgotten, particularly in
larger vessels that extended shadow sectors exist in the vertical plane also.
Typically
there is a core of at the shadow sector within which there exists a total blind sector.
False
echoes (due to ship obstruction)
False echoes can appear on the display due to
reflection of echo energy from an obstruction on the ship, which is causing a
shadow sector.
Usually such false echoes are found mainly in the
shadow sector and appear at virtually the same range as the true target
echo. They may also appear at slightly reduced brilliance in radar
sets, which display raw video signals.
Generally the echoes are produced by fairly large
echoing areas at close range; a change in course of own-ship causing the false
echo to disappear or a new false echo from a different target to appear in the
blind sector.
False echoes between own-ship and target
Such
echoes are produced due to the transmitted energy bouncing between a target at
close range and own-ship.
The effect is to produce from one transmitted pulse a
series of echoes which appear on the display equally spaced at multiples of the
target range and extending beyond the true target echo with gradually
diminishing paint.
False echoes appear on the same bearing as the true
echo and are usually easily due to their regular pattern and distinctive
characteristic. As own-ship changes
attitude relative to these targets the echoes tend to change or disappear.
Sidelobe echoes
Slotted waveguide antennas produce very low output
power in the sidelobes compared to the main
lobe. For targets, which are close to own-ship,
however, the energy can be transmitted and received not only via the main lobe
but also via the sidelobes.
The effect is to severely downgrade bearing
discrimination of adjacent targets and to produce on the display bright arcs of
paint at the target range and close to the scan origin.
Sidelobe echoes
diminish rapidly with range.