Garmin GMR 21/41 - Manual (Page 11)

GMR 21/41 Owner’s Manual 
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Gain
Gain controls the sensitivity of the Radar receiver. Auto (default) is
the recommended setting. It automatically adjusts the gain for each
range scale to provide optimal performance. You can also set gain
manually. To see more detail, increase the receiver sensitivity by
selecting a higher gain. If there is too much detail, or the screen is
cluttered, lower the sensitivity (lower the gain) to increase the clarity
of the screen.
Sea Clutter
Sea Clutter controls unwanted screen clutter caused by choppy sea
conditions. The slider has three preset positions: Calm, Moderate,
and Rough.
Rain Clutter
Rain Clutter controls unwanted screen clutter caused by rain at close
ranges (use the slider to select 1-100%).
FTC (Fast Time Constant)
FTC controls unwanted screen clutter caused by rain at a distance.
The Fast Time Constant settings are: Off, Low, Medium, and High.
Zoom
Zoom allows you to zoom in on a selected area of the radar screen.
When you select a zoom range, a window showing a zoomed portion
of the screen appears in the lower-left corner and a selection box
appears. To view different areas at the selected zoom range, open the
Radar adjustment menu, highlight Zoom, and press the Move soft
key. Use the ROCKER to move the selection box. Zoom has four
settings: Off, 2x, 4x, and Move.
Zoom vew Selecton box
Target Expansion
Use Target Expansion to increase the size of targets. Select On or
Off.
NOTE: The Target Expansion setting has no effect at range scales
of 4 nm or greater.