Lowrance electronic LRA-1000 - Manual (Page 55)

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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.