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RADAR
Controls
Anti-clutter
rain control (FTC)
The echo energy due to an individual raindrop is very
small but the combined effect of returns from rainstorm cause rain clutter, which is displayed, on the indicator as a bright
indistinct area of paint.
In addition, the video signal can be passed through a differentiating or fast time-constant (FTC) circuit.
The FTC circuit is generally a combination of
capacitance and resistance having a time-constant much shorter than the
returned video pulse length.
Such pulses will therefore be differentiated when
passed through the filter.
Rain clutter builds up a Root Mean Square (RMS)
voltage due to the volume of the rainstorm and behaves as if the rain was
returning a much longer pulse than that transmitted.
The act of passing all video signals through such a
circuit is to considerably reduce the effect of the rain on the display whilst
having little effect on a discrete target return which echoes a consistent
pulse.
The degree of control may be fixed or variable;
variable controls must be used with care since loss of signal can occur due to
maladjustment.
The video signal after amplification is used to
intensity modulate the cathode ray tube.
Range ring generator and control
Range ring generation is a mandatory requirement.
The range scales provided for such sets have as a
minimum requirement range scales of 1.5, 3, 6, 12 and 24 nm, and one range
scale of not less than 0.5 and not greater than 0.8 nm.
On the 0.5 to 0.8-nm scale at
least two range rings are specified and on the other mandatory ranges six range
rings must be provided.
Rings are generated from the output of a stable
oscillator, which is in some cases crystal controlled. The waveform generated is shaped to give
short duration pulses at the generator frequency and synchronize to the timebase sweep.
These pulses are superimposed on the video information
in the combining circuit and appear on the screen at equal intervals of time
and therefore range, along the timebase sweep as
bright dots of light.
Rotation of the sweep produces a series of concentric
rings from which the range of a target may be interpolated.
The range ring generator switched to the appropriate
frequency by the action of the range selector switch which also controls
receiver bandwidth, pulse length and PRF selection for the particular range in
use.
Variable range marker
A circuit is provided which generates a marker
synchronized to the timebase sweep.
A manual control varies the delay between timebase initiation and the time at which the VRM pulse is
produced.
Because the sweep is rotated the range marker pip will
produce a bright circle on the PPI having a variable radius.
The range scale of the VRM should coincide with the
calibrated range rings value when the VRM is overlaid with each ring on the
PPI. Fixed range rings and variable
range markers found in type tested equipment’s must enable the range of a
target to be measured with an error not greater than 1.5 per cent of the maximum
range of the scale in use or 70 metres, whichever is greater. Variable ring and calibrated range rings must
be capable of removal from the PPI, usually achieved by turning the appropriate
brilliance control fully anticlockwise.
Electronic bearing indicator
The electronic bearing indicator circuit generates a
brightened radial line, sometimes broken into dashes on the PPI and emanating
from the sweep origin
Many EBL circuits generate the bearing line only once
per antenna revolution which can make the application of the indicator a slow
process, since in moving the line the operator has to wait one antenna
revolution to ascertain its new position.
Modern radar sets generate the bearing line during the
interscan period, which is the time between the
end of one timebase sweep and the beginning of the
next sweep.
In this method the EBL is continuously displayed on
the screen at all periods of an antenna rotation, facilitating more rapid
application of the line for bearing measurement.
There are certain mandatory requirements for the EBL in
type tested equipment.
It must have controllable brilliance,
be free to
rotate clockwise and anticlockwise continuously through 360 degrees,
the direction
of turning being the same as that of the control,
have a maximum
thickness not exceeding 0.5º measured at the edge of the display,
be updated at
least once per antenna revolution and be clearly distinguishable from the
ship’s heading marker.
An accuracy of ± 1º in bearing measurement is required
for a target having an echo lying at the edge of the display.
Off-centring
Off centring is achieved by
adjustment of current passing through a set of fixed deflection coils on the
PPI tube neck.
Timebase generator
It is the circuit where a sawtooth current waveform is
generated which when passed through the PPI deflection coils causes linear
deflection of the electron beam and linear movement of the luminous spot on the
PPI.
When the range switch is changed, the timebase is also changed to provide the necessary beam
deflection.
At short ranges the deflecting current amplitude
necessary to deflect the spot from the scan origin to the edge of the screen
has to be reached in a very much shorter time than on one of the longer
ranges.
Assuming a centred display
for the 0.5-nm range it will be 6.2 μs and for
the 48-nm range it will be
595.2 μs.
If the tube radius is eight inches, the luminous spot
on the screen will travel at a velocity 96 times greater on the shorter range
than on the longer range and would produce a very faint trace if the same
brightness level were used in both cases.
A brightening waveform known as the brightening pulse is generated and
applied to the grid of the CRT. This pulse is rectangular waveform acting in
synchronism with the timebase waveform and serves to
lift the trace to the threshold of visibility.
IF gain control - contrast and brilliance
Receiver gain control acts on the IF amplifier
allowing the radar user to adjust the overall signal amplification.
Excessive gain will cause background noise to intrude
upon the target paint.
Acceptable adjustment is obtained by turning the gain
up to a point where only a faint speckled background is visible.
The brilliance control should be adjusted to suit the
ambient light conditions in the vicinity of the PPI.
Excessive brightness in addition to affecting the eyes
and the paint of the targets, can cause defocusing of
the screen image, an effect known as blooming
where the target paint is saturated by electron bombardment causing a
bright halo to form around the paint.
Excess brilliance demands higher beam current than
normal levels; it will ultimately reduce the tube life and can result in the
tube phosphor being burned.
To some extent tube brilliance control is interactive
with the contrast control. The contrast
control adjusts the video level at the cathode, and careful adjustment selects
the best level to provide a sharp contrast between the paint and the darker
unpainted background. Maladjustment of
brilliance can destroy the contrast and negate the effect of the contrast control.
Swept gain
A manually controlled swept gain circuit operates on
the head amplifier.
The purpose of the circuit is to suppress signal
return from the sea (sea clutter), which tends to obscure the centre of the
display.
The swept gain control reduces the gain of the head
amplifier at short range on each transmitted pulse and as the timebase generator sweeps the electron beam across the tube
face the gain is progressively increased with range.
Excessive anti-sea clutter control can cause loss of
small short-range targets at the PPI.
The manual anti-sea clutter control acts on each timebase sweep, treating the received signal information as
though sea clutter was equal through 360 degrees of azimuth.
In practice, the sea returns vary as the scanner rotates
and an automatic swept gain circuit can be employed which dynamically adjusts
the correct level of sea clutter control by rapidly sampling signal returns
from short ranges.
It is a mandatory requirement in type tested radars
that automatic anti-sea clutter control can be switched off by the user.
Local oscillator tuning
Although the L.O. is structured to operate at one
particular frequency, it can be ‘tuned’ by placing it in a resonant cavity and
then tuning the cavity, which is an integral part of the local oscillator.
Cavities are coarse tuned to a particular resonant
value by means of a mechanical screw projecting into the cavity.
Fine electronic tuning is done by use of a variable
capacitance diode placed in the cavity and having its capacitance varied by
means of bias applied from the manual tuning control at the display position.
The fine tuning facility enables the user to produce
the best possible display of target returns.
Heading mark switch
Different Radars have different arrangements for the
heading marker switch.
In a magnetically operated reed switch, the contacts
are operated whenever a small permanent magnet mounted on the antenna drive
passes over the switch.
The switch is invariably mounted on a small adjustable
base plate carrying a scale graduated in degrees of azimuth.
An overall possible adjustment of ±5º is adequate, to a required accuracy of
at least 0.5º. Switch action and magnet are mutually arranged to coincide when
the main lobe of the antenna beam is pointing along the ship’s heading. This action produces a bright radial line at
the display with own ship at the scan origin by brightening a few consecutive
traces, to show the ship’s heading on the indicator.
Type tested radars display heading lines whose
thickness is 0.5º or less at the tube periphery. The maximum error permitted of the heading
marker line is ±1º.
Adjustment of heading mark
The display is unstabilized
(relative motion ship’s head up). A small target is selected on which visual
bearings may be taken, its range being such that the radar will display the
target echo as a separate paint lying near the edge of the PPI on one of the
shorter displayed ranges (say 1.5 nm).
Own-ship is then aligned and a visual bearing on the
target is taken. When the ship’s head
and the visual bearing coincide, any error on the heading marker bearing at the
PPI is noted (should be reading zero).
Adjusting the heading mark contacts in the scanner
housing compensates error and the visual and radar bearings once more checked. In some equipment the heading marker can be
adjusted by rotating the stator housing of the synchro
transmitter.
Power monitor
The main purpose of this is to give at the display an
indication that power is being radiated in the main beam.
Sensitive and accurate methods employ crystal
detectors, which can be calibrated to indicate fall-off in relative performance
of the transmitter compared to some optimum level attained at installation of
the equipment.
A neon tube situated in the scanner housing if exposed
to the main beam is ionised. The ionisation of the
tube varies with power irradiating it and the subsequent change of tube
resistance can be used as a direct indication of the power in the beam.
The performance monitor
It comprises a resonant cavity, with dimensions, which
allow it to resonate within the marine band.
Radio frequency energy due to a transmitted pulse is fed into the cavity
via an aperture causing the cavity to resonate; the resonant oscillations will
persist in the cavity for some time after the pulse has terminated. During the resting period of the transmitter
(receive period) the cavity couples energy out
of its aperture forming a return echo signal for the duration of the cavity
oscillations.
This cavity signal is processed by the receiver and
display circuits and appears on the display, brightening the timebase traces from the scan origin and extending for a
measurable distance radially from that origin.
Since the energy radiated from the performance monitor
resonant cavity is directly proportional to the power injected into it (losses
being constant) the length of the brightened trace is measure of the overall
transmitted and received power.
Should the transmitter, the receiver or both be
operating at less than so measured optimum value, then the length of the
displayed trace indicates a fall-off in the overall
performance.
Plume and sunburst patterns
With the PM switch depressed, the cavity re-radiates
energy directly into the waveguide during the magnetron quiescent period and,
due to the sweep actions involved produces a sunburst pattern.
The length of the major ‘spokes’ in the pattern occur
when the echo box is resonating at magnetron frequency, measurement of this
length from the scan origin is a direct representation of the power received.
Some installations use separate echo boxes installed
above deck. Energy enters the box via an aperture whenever the main beam of the
antenna sweeps over it.
And the box, when switched on, re-radiates the energy.
In this case the returned signal displays a plume on the indicator
approximately twice the width of the horizontal antenna beam.
Whilst PM echoes are being displayed, some target
echoes may be obliterated, since the receiver received most of the echo from
the PM echo box. Use of the monitor is therefore limited to occasional
performance checks and for optimum tuning procedures.
Using the performance monitor
During use, controls such as the anti-sea clutter or
anti-rain clutter control are set to a minimum so that they have no effect on
the displayed PM plume.
Gain and brightness are adjusted as required.
The same pulse length is chosen each time the
performance monitor is used. A low range
is selected, such as the 1.5-nm range, and the VRM is adjusted to lie as close
as can be visually estimated to the extremities of the pattern. Correct tuning
of the radar receiver will produce optimum observed length.
When the radar is first commissioned the extent of the
PM pattern is entered in the radar log, and comparison can be made with this
initial value obtained when the set is considered to be at its best performance
level. The radar observer quickly
becomes familiar with particular radar set and any shrinkage of pattern in
subsequent checks indicates reduction in overall performance of the set.
This could be due to a variety of causes:
ageing of the
crystal, TR cell or magnetron,
receiver detuned,
water in the
waveguide, etc.
The PM is used when a radar set is first switched on
and is particularly useful for spot checks on overall performance and tuning in
conditions when there may be no visible returns on the display to immediately
indicate the set is correctly tuned.
Because the transmitted and the returned power are
proportional, the reduction of the performance monitor plumes to one half of
its length would indicate a fall off of 3dB in overall signal performance. Under these circumstances the possibility of
losing many of the weaker target returns is high.
In the case where there is good return of targets and
yet the PM plume is lesser than at installation then the PM itself may be
faulty.
The common causes of performance reduction are:
(1) incorrect receiver tuning
(2) defective receiver crystal(s)
(3) obscured antenna window
(4) ageing T/R cell (active types of cell)
(5) magnetron ageing
(6) water in waveguide
(7) damaged waveguide, loose choke joints - leaking.