TM 11-5840-360-14-1-1
When the interference reject feature is selected
3.10 RANGE AND BEARING MEASUREMENTS
ghost images caused by other radars operating in the
vicinity are eliminated.
3.10.1 Plotter
3.9.8 Radar Interference
Each indicator (IP-1282 and IP-1283) is supplied
with a plotter assembly to aid in recording target range
Whenever two or more radar equipped vessels
and azimuth information. Target position is periodically
are operating within reception range of each other,
marked using a grease pencil, resulting in a temporary
mutual interference is likely. This will usually appear on
record of the targets position relative to that of own ship.
the screen as a series of small dots, which move to and
This record is used to determine the target course and
from the PPI center, sometimes in a straight line, but
speed.
more often in a long, sweeping curve. This should not,
as a rule, impair the effectiveness of the radar as a
The brightness of the grease pencil reflections
navigation aid. Radar interference can be completely
on the plotter is determined by the setting of the PNL
eliminated by setting the INTRF REJECT switch to ON.
LIGHTS control on the front panel.
3.9.9 Side Lobes
3.10.2 Range and Bearing Measurements
A very small part of the RF energy from each
3.10.2.1 VRM Discontinuities. - Range and bearing
transmitted pulse is radiated outside the single narrow
measurements at Indicators IP-1282 and IP-1283 are
beam, producing side lobe patterns. Side lobes have no
performed using the Electronic Bearing Line (EBL) and
effect on distant or small surface objects, but the echo
Variable Range Marker (VRM) features.
from a large object at short range may produce an arc
on the PPI similar to a range ring, or appear as a series
NOTE:
of echoes forming a broken arc. Side-lobe echoes
normally occurs at range below 3 miles and can be
When the indicator is turned on, the
reduced by adjustment of the ANTI-CLUTTER SEA
VRM will not be displayed. The VRM
control.
handwheel must be turned slightly
clockwise to display the VRM spot
3.9.10 Blind Sectors
on the CRT. When the VRM readout
is zero, the VRM spot is not
Funnels, masts, and samson posts (when
displayed on the CRT.
located near the antenna array) may cause shadows. In
the shadow area beyond the obstruction there will be a
The VRM nautical mile is equal to 2025 yards.
reduction of the beam intensity, although not necessarily
At distances greater than 10,000 yards when using
a complete cut-off. However, if the subtended angle is
range scales less than 24 miles, the VRM readout
more than a few degrees there will be a blind sector.
increments in 100-yard steps. In the 24-, 48-, and 64-
mile ranges the VRM updates internally by the standard
In some shadow sectors the beam intensity may
increment and the display will be rounded accordingly.
not be sufficient to obtain an echo from a very small
On the 24-mile range, where the standard increment is
object even at close range, despite the fact that a large
160 yards, the display will increment by either 100 or
vessel can be detected at a much greater range. For
200 yards. The standard increments for each of the
this reason the angular width and relative bearing of any
ranges is given below.
shadow sector must be determined at installation. This
information should be posted near the indicator, and
Range
Standard Increment
operators must be alert for objects in blind sectors.
(miles)
(yards)
3.9.11 Navigation and Surface Surveillance
0.25, 0.5, 0.75
5
1.5
10
The radar system is designed for navigation and
3
20
surface surveillance. Harbor and coastal navigation
6
40
should be practiced in daylight under clear weather
12
80
conditions so that observations can be taken to compare
24
160
the PPI presentation with visible surroundings.
48
320
Navigational information shown on the chart of a
64
320
particular operating area will be easily recognizable on
12 RT
80
the PPI, within the limits of the range in use at that time
and the shielding effect of land masses.
3-21
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