SAM Electronics GmbH NG3028X25KW Manual Do Utilizador
RADARPILOT / CHARTRADAR
ED 3038 G 232 / 01 (2002-06)
Operating Instructions
15 Evaluation of the Radar Video
15.2 Distortions of the Radar Video
b_r1_e33.fm / 21.06.02
108
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The reflections of radar waves from ice depend largely on the surface structure of the areas illumi-
nated by the radar. Rough and jagged surfaces produce good echo displays, whereas smoothly
frozen-over surfaces, rounded surfaces and surfaces at an oblique angle generally reflect the trans-
mitted pulses away from the ship. Thus, for example, the echo display of large icebergs can fluctuate
extremely severely, depending on the existing aspect.
nated by the radar. Rough and jagged surfaces produce good echo displays, whereas smoothly
frozen-over surfaces, rounded surfaces and surfaces at an oblique angle generally reflect the trans-
mitted pulses away from the ship. Thus, for example, the echo display of large icebergs can fluctuate
extremely severely, depending on the existing aspect.
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The detection of low-lying blocks of ice (growlers) probably presents the greatest uncertainty:
WARNING:
Even if the anticlutter facilities are operated carefully, the echoes from
growlers can no longer be definitely identified if moderate disturbances
from the sea surface are present.
Even if the anticlutter facilities are operated carefully, the echoes from
growlers can no longer be definitely identified if moderate disturbances
from the sea surface are present.
15.2
Distortions of the Radar Video
Because of the physical characteristics of the radar principle used, the video displayed does not corre-
spond exactly to the position and form of the reflecting target. A distinction must be drawn between radial
distortions (in the direction of the target) and azimuthal distortions (perpendicular to the target direction)
- see figure on next page.
spond exactly to the position and form of the reflecting target. A distinction must be drawn between radial
distortions (in the direction of the target) and azimuthal distortions (perpendicular to the target direction)
- see figure on next page.
Radial Distortions
Radial distortions in the direction of transmission are caused by the fact that a target reflects the trans-
mitted pulse for as long as the pulse is moving past the target
mitted pulse for as long as the pulse is moving past the target
1)
. Thus, radial lengthening depending on
the pulse duration occurs on the screen. In the case of range measurements, it must be noted that the
echo edge facing own ship corresponds to the true front edge of the target.
echo edge facing own ship corresponds to the true front edge of the target.
If there are several targets behind one another with small (radial) distances between them, they are
shown separately on the screen if, and only if, their distance apart is so large that the radial lengthened
echoes do not overlap.
shown separately on the screen if, and only if, their distance apart is so large that the radial lengthened
echoes do not overlap.
Azimuthal Distortions
Azimuthal distortions are caused by the width of the beam radiated by the antenna. A point target
appears wider the further away it is. At a distance of 10 NM with a horizontal antenna-beamwidth of 1.5°,
an individual target is displayed as an arc with a length of 480 m. Because of this effect, individual targets
situated beside one another can merge to form a single target.
appears wider the further away it is. At a distance of 10 NM with a horizontal antenna-beamwidth of 1.5°,
an individual target is displayed as an arc with a length of 480 m. Because of this effect, individual targets
situated beside one another can merge to form a single target.
Not only individual targets but also small spits of land aligned in the radial direction appear wider than
they really are, and radially oriented entrances appear narrower than in reality.
they really are, and radially oriented entrances appear narrower than in reality.
☞
As a result of azimuthal distortions and/or changes in the target position, the radar centroid of a
target can change, especially at short range. In the case of a target being tracked, this can lead to
a sudden change in the displayed vector.
target can change, especially at short range. In the case of a target being tracked, this can lead to
a sudden change in the displayed vector.
In addition to these undesirable azimuthal distortions, targets can also be deliberately widened with
increasing distance, so that narrow, close-range targets too can be seen clearly. This function is used in
the Clean Sweep Modes MEDIUM and OPEN SEA. It ensures that the azimuthal distortion corresponds
to the radial distortion over a wide range of distances. Because the radial distortion depends on the pulse
length, when Target Enhancement is switched on the targets are displayed particularly large by the selec-
tion of Long Pulse.
increasing distance, so that narrow, close-range targets too can be seen clearly. This function is used in
the Clean Sweep Modes MEDIUM and OPEN SEA. It ensures that the azimuthal distortion corresponds
to the radial distortion over a wide range of distances. Because the radial distortion depends on the pulse
length, when Target Enhancement is switched on the targets are displayed particularly large by the selec-
tion of Long Pulse.
1)
The pulse length setting LP thus generates larger radial distortions than SP.