Lowrance Radar Operation Instruction Manual page 61

Time Base: line that rotates around the radar display, updating im-
ages on the display.
Transmit Off Zone: an area set up in the radar transmission path
where the radar will not transmit a microwave pulse.
Tuning: intermediate frequency adjustment that enhances the per-
formance of the radar.
VRM (Variable Range Marker): a range ring used to measure the
precise distance to a target, the variable range marker may be adjusted
to measure any distance within the radar's range.
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