Simrad ARGUS FMCW 3G, User Manual

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Radar presentation and video processing | Argus Radar Operator Manual
Rain
The rain progress bar indicates the intervention of the Anti Rain Clutter processing. In case of
automatic Anti Rain Clutter processing, the inscription “AUTO” will precede “RAIN” inside the
progress bar.
Marine radars are sensitive to rain drops, snow flakes and fog, the returning signal is some-
thing similar to a big hazy spot of video, which can saturate the receiver and mask all targets
covered by the humidity.
The purpose of the rain control is to reduce this kind of clutter, so that targets masked by the
clutter can be seen and distinguished on the radar picture.
To obtain good results, the rain echo spots must be weaker than the real echoes, otherwise
the rain control cannot distinguish their shapes and they cannot be drawn on the screen.
The operator should turn the control down, even to zero, when its use is not required (i.e.
sunny condition). Since storm cells are usually visible at long distances due to their extreme
altitude, the operator may wish to leave them displayed so that it is possible to observe the
location of bad weather.
In order to reduce the effects of rain drops, place the trackball cursor over the rain progress
bar, the Hep line will change accordingly. To increase the rain control, the right SK must be
pressed, and to decrease the rain control, use the left SK.
Inside the rain progress bar, the function of the centre SK is to switch between manual and
automatic rain clutter control, according to the suggestion of the Help line.
Manual and Auto rain
Rain clutter shows as an echo that has almost constant intensity across the area seen on the
PPI. It can mask all target echoes that have same or less intensity.
The gain processing will try to equalize the detected video, so usually this means that higher
amplitude target over rain are presented with the same brilliance on the screen and they are
not recognizable. The only way to discriminate between target echoes and rain clutter is not
to look at signal amplitude, but at their rate of change in range.
That is exactly what the manual rain control is doing, it works as an FTC (fast time constant)
filter whose output shows only echoes that are well defined in range.
The main drawback is for the same reason, land echoes are also filtered and target size in
range is drastically altered and only the leading edge is visible.
The rain clutter residuals can be suppressed by adjusting the gain control, but it is also pos-
sible that small false echoes are presented as result of rain processing. Usually scan to scan
correlation is the best choice to clean up the PPI from these clutter residuals.
The manual rain control should be adjusted in function of the TXRX pulse length that is de-
pending of the range scale selected on ARGUS radar displays.
Higher rain control is necessary for shorter pulses.
Long/medium pulses should not be used under medium/heavy rain conditions, the signal
received will be high in the rain area and will mask all target echoes in between, and will at-
tenuate the transmitted signal, so echoes farther than the rain clutter area will be very dim or
completely invisible.
The auto rain control takes care of different pulse lengths and is also able to show the leading
edge of land returns. This control implements a CFAR (Constant False Alarm Ratio) filter and it
is the optimum choice in almost every weather condition.
As for the manual rain, clutter residuals are suppressed with the gain control and scan to scan
correlation.
Loss of detection caused by rain clutter
Radar performance is measured for two rates of rain, 4 mm/h (moderate rain) and 16 mm/h
(heavy rain) and it considers the losses caused by the attenuation of the transmitted pulse.
Long pulses should not be used in heavy rain as the range will decrease significantly.
Maximum range detection of target is slightly affected with moderate rain and short pulse,
but becomes only 30% with heavy rain.
On long pulses the range decreases to unacceptable 30% with moderate rain to 10% with
heavy rain. So this underlines that long pulse should never be used on X-Band in rain condi-
tions.