How To Measure The Range To A Target; How To Measure The Range By Using A Vrm - Furuno FCR-21x9-BB Operator's Manual

How to read the graph
Using the X-band graph as an example, a radar target detected in rain on the 8 nm
range scale can only be detected at the ranges shown below:
4 mm/h rain (short pulse): approx 7.5 nm
16 mm/h rain (short pulse): approx 5.6 nm
4 mm/h rain (long pulse): approx 2.6 nm
16 mm/h rain (long pulse): approx 0.9 nm
2.9

How to Measure the Range to a Target

The range to a target may be measured three ways: with the fixed range rings, with
the cursor, or with the VRM.
Use the range rings to obtain a rough estimate of the range to a target. They are the
concentric solid circles about own ship, or the sweep origin. The number of rings is
automatically determined by the selected range scale and their interval is displayed in
the Range/Presentation mode box at the top-left position on the screen. Count the
number of rings between the center of the display and the target. Check the range ring
interval and judge the distance of the echo from the inner edge of the nearest ring.
The range rings can be turned on/off with [RANGE RING] on the [NAV TOOL] menu.
2.9.1

How to measure the range by using a VRM

There are two VRMs, No. 1 and No. 2, which appear as dashed rings so that you can
distinguish them from the fixed range rings. The two VRMs can be distinguished from
each other by the different lengths of their dashes; the dashes on the No. 2 VRM are
longer.
VRM1
310
300
290
280
270
260
250
240
230
How to measure the range from the Control Unit
1. Press the VRM 1 or VRM 2 key to display desired VRM.
2. Rotate the VRM rotary control to align the active variable range marker with the
inner edge of the target of interest and read its distance at the bottom-right posi-
tion on the screen. Each VRM remains at the same geographical distance when
you operate the range control. This means that the apparent radius of the VRM
ring changes in proportion to the selected range scale.
To erase a VRM, press the VRM 1 or VRM 2 key to erase corresponding VRM.
000
350 010
020
340
330 030
320
040
050
130
220 140
210
150
200
160
190 170
180
VRM1
TTG 45:02
0.66
2. RADAR, CHART RADAR OPERATION
Target
060
070
080
090
100
VRM2
110
120
VRM2
TTG 99:59
1.18
NM
NM
2-11