Simrad ARGUS FMCW 3G, User Manual

Page: 69 / 150

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Radar presentation and video processing | Argus Radar Operator Manual
Sea
The function of this control is to reduce sea returned signals. The sea waves return a spot simi-
lar to the one of rain, but with a different behaviour. The spot drawn on the screen is normally
around the PPI origin and its intensity decreases with increasing range. The function of the sea
control is to cut the video detected under the anti sea clutter curve, its effect is stronger close
to the origin (antenna) and weaker going farther in range, following the sea clutter behaviour.
Sea clutter is composed of a very large number of echoes, originated by the random wave
reflections from scan to scan. The clutter from these reflections will mak
e it difficult to detect
real targets. Such sea clutter usually appears as multiple echoes on the display at short range
scales, and the echoes are not repetitive or consistent in position. Since the sea condition
depends on the wind, it is not always easy to determine the most important factor affecting
the sea radar echo. Generally at a higher frequency (3 cm), wind is a sig
nificant parameter to
which sea echo can be compared. If the sea is calm and not disturbed by wind, sea echoes
are very small, but targets even as small as seagulls can easily be seen on the PPI.
Due to the electrical characteristics, the sea clutter and land clutter reflections are compa-
rable. The surface of large waves is sometimes bigger than the surface of a boat or small ship,
in that case there is no way to discriminate between the two echoes. The only option is to ap-
ply scan to scan correlation with the highest setting (3), but only if the target dimension and
speed follow the table (see table 3.3.1 Scan to Scan correlation).
Reflections from sea waves and targets are “competing” among themselves, so also automatic
processing like auto anti-sea/rain clutter tend to make targets intensity lower and size smaller
than the real values. Sometimes sea waves are presented in a coherent and stable way on the
PPI and it is possible to recognize the linear shape of the wave front.
It is very hard for any automatic radar video processor to discern this type of wave from a
target echo. They have almost the same amplitude, they have a shape that is consistent with
time. The operator can increase the gain threshold, but sometimes has to accept that a clear
scan is almost impossible without cancelling interesting echoes
The sea clutter can be suppressed with the manual sea control. This one reduces the signal
received following an attenuation curve that is calculated at the installation, according to the
antenna height transceiver and antenna types and especially the sea state selection.
The attenuation curve changes considerably from sea state 1 to sea state 4, after this last one
the difference between the sea state selections becomes less important.
The clutter residuals are then reduced acting on the manual sea bar. The main drawback of
manual sea processing is that it acts in the same way for the entire scan.
When the sea clutter return varies heavily with the bearing angle, because wind condition are
different, or when the ship enters in closed areas protected from winds by land, the manual
sea control will be optimized only for the highest sea clutter direction.
Target echoes in lower sea clutter directions, will be dimmed unnecessary. In these conditions
it is preferable to use auto sea clutter control. This processing evaluates the peaks of sea clut-
ter around the ship and tries to follow the sea clutter signal envelope.
The biggest challenge is that sea clutter returns have a clear amplitude average, but around
this value they are peaking a lot and sometimes it is possible to discern wave structures.
These peaks can be higher also from small target reflections, but they change position in
every PPI scan. Scan to scan correlation is useful for reducing them because real echoes can
be found in almost the same place from scan to scan (see limitations in Table of Scan to Scan
correlation “Video preset” on page 69).
Field experiences demonstrate that:
•
As wind blows up and exceeds approximately 5 Knots, sea echoes quickly increase from the
previous small value.
•
The amplitude of the sea clutter decreases with increasing range from the PPI centre.
Sea return is especially bothersome in heavy seas, since buoys or other small objects are pro-
ducing echoes of a level lower than the waves and cannot be detected even by means of the
Auto-adaptive Anti Sea Clutter function. The knowledge of approximate position of buoys or
light ships is of considerable assistance, as it is then possible to use the Manual Anti Sea Clut-
ter and to concentrate on the appropriate section of the radar screen. Land masses should be
clearly visible despite of heavy sea conditions.
Care should be exercised using the Anti Clutter Control. If the control is set too low, targets