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Northstar M121/M84 Installation and Operation Manual 107
B-5 Radar problems
5-1 Radar overlay doesn’t appear on the
chart screen
a Ensure that your boat is shown on the radar
window. If it is, but the radar overlay still
isn’t shown, try zooming in on the charted
area. (The charted area may be outside the
maximum range of the radar).
b The threshold or transparency settings for
the radar overlay are not correct (see 18-6).
5-2 Radar shows arcs and/or shadows
Side lobe patterns often appear as an arc or
a broken arc. Shadows are blind spots.
If the radar is installed at a suitable height
and in a suitable location on your boat,
side lobe patterns and shadows caused by
obstructions such as funnels or masts will be
minimized.
If you are having problems with these, you
may need to relocate your radar. Consult the
Installation Guide for your radar model for
advice.
5-3 Radar shows different patterns on
each sweep
Interference patterns always appear in a
different place with each sweep. They can
be randomly scattered bright dots or dotted
lines that extend from the centre to the
edge of the radar window, often producing
a spiral effect. Generally, the stronger the
interference, the wider the interference lines.
a Use the interference rejection feature to
reduce interference patterns, and the width
of the interference lines (see 19-4).
5-4 Radar shows false echoes and/or
background noise
False echoes and/or background noise can
be produced if the radar sensitivity is too
high.
a Try using the Auto setting for the radar gain
mode (see 19-4). If this doesn’t work, or if the
radar gain mode is already set to Auto, use
the radar gain setting to reduce the radar
sensitivity
5-6 Radar shows multiple echoes off the
same object
Multiple echoes off the same object are
most likely to occur when you are close
to a large target, and are usually only a
temporary nuisance.
5-7 Radar shows random or grouped dots
Rain clutter often appears as random dots
that can merge into badly-defined echoes
on the radar window. The random signals
are returned from rain, hail, sleet, or snow
and can result in a confusing display
5-8 Radar shows random signals in centre
Sea clutter usually appears as confusing,
random signals around your boat. High
waves will produce the maximum amount of
sea clutter.
You can optimize your radar window to
reduce the effects of sea clutter (see section
18-6-4).