To improve VRM readout accuracy, the VRM
which can be identified on a chart. A position fix based
readout change step is intentionally caused to deviate
on two or more points will furnish a more accurate fix,
from normal incrementing at specific ranges. The listing
especially when the points are nearly 90apart from the
given below indicates where the VRM deviates from its
normal increment; this deviation is always 10 yards.
A highly accurate position fix should be plotted
by noting the ranges to three identifiable points and
Radar rangefinding is more accurate than visual,
rangefinding. The distances (ranges) are plotted by
using dividers to measure the distance on the chart
In order to compute the readout just prior to the
scale and transferring it to the chart. One end of the
VRM readout listed, take the standard increment for the
divider is placed on the point and the other end traces
selected range, add 10 yards and then subtract from the
the range arc.
applicable number in the listing.
Example: Range is 6 miles
only one point is available. The range is plotted as
Standard increment is 40
described above. The bearing is plotted from the point
Number from listing is 5830
back to intersect the range arc. The relative bearing
5830 - (40 + 10) = 5780
must be converted to a true bearing before plotting.
Therefore, the readout sequence will be 5780,
5830 and the display will increment or decrement by 40
position at the intersection of two plotted bearings. The
on either side of these numbers.
two points selected should be as near 90 apart as
possible. Greater accuracy is obtained if three bearing
to objects nearly 60apart are plotted. These bearings
18.104.22.168 Range/Bearing Measurement Procedure. -
should be converted to true bearings before plotting.
This method may be less accurate than plotting range
1. Adjust RANGE RINGS control for desired
arcs since radar bearings, due to the beamwidth of the
intensity of range rings.
radar, are less exact than radar range.
2. Turn the VRM handwheel 1/4-turn clockwise,
then adjust VRM intensity control for desired intensity of
3.11.2 Avoiding Collision
VRM spot on CRT.
As soon as an echo appears on the display, its
3. Identify target of interest on CRT and adjust
range and relative bearing should be noted on a plotting
EBL control to make displayed EBL pass through target.
sheet or chart. As in visual observation, "constant
4. Read target bearing from intersection
bearing with decreasing range indicates a collision
displayed EBL and fixed heading scale.
course." As soon as a series of plots indicates a closing
range and no significant change in relative bearing,
positive action should be considered mandatory. The
"Regulation for Preventing Collision at Sea" should
When HD UP operation is in effect,
always be observed.
the target bearing thus obtained is
relative bearing; when GYRO STAB
3.11.3 Determining Radar Line-of-Sight Range
is selected, target true bearing is
When searching for distant echoes, the radar
line-of-sight range to the echo can be a limiting factor.
5. Adjust the VRM handwheel to superimpose the
Radar waves behave like light waves but are refracted
VRM spot on the EBL over the target of interest; read
slightly more, increasing the distance to the radar
the target range directly from the VRM digital readout at
horizon to slightly beyond the optical horizon (displayed
the top of the CRT.
range is correct, however). As Figure 3-9 shows, the
radar line-of-sight range is a combination for the radar
3.11 RADAR NAVIGATION
horizon of the ship's radar antenna and the radar
horizon of the echoes. The nomograph shown in Figure
3.11.1 Obtaining a Position Fix
a convenient method for determining any
one of the three factors involved when the other two
factors are known.
methods of using radar echoes from prominent points