Suppressing Precipitation Clutter; Interference Rejector - Furuno FR1500 Mk3 Operator's Manual

1.16 Suppressing Precipitation Clutter

The vertical beamwidth of the scanner is
designed to see surface targets even when the
ship is rolling. However, by this design the unit
will also detect rain clutter (rain, snow, or hail)
in the same manner as normal targets. Figure
at right shows the appearance of rain clutter on
the display.
The A/C RAIN control adjusts the receiver
sensitivity as the A/C SEA control does but
rather in a longer time period (longer range).
Clockwise rotation of this control increases the
anti-clutter effect.

1.17 Interference Rejector

Mutual radar interference may occur in the vicinity of another shipborne
radar operating in the same frequency band (9 GHz). It is seen on the
screen as a number of bright spikes either in irregular patterns or in the
form of usually curved spoke-like dotted lines extending from the center
to the edge of the picture. Activating the interference rejector circuit can
reduce this type of interference.
The interference rejector is a kind of signal correlation circuit. It
compares the received signals over successive transmissions and
suppresses randomly occurring signals. There are three levels of
interference rejection depending on the number of transmissions that are
correlated. These are indicated by the legends IR1, IR2 and IR3 at the
upper-left position of the screen.
To activate the interference rejector;
1. Press the [F1] key. The following display appears.
1. VECTOR
2. PULSE
3. INT REJ
4. STRETCH
5. ECHO AVG
6. CONTRAST
7. N REJ
8. DISP SEL
9. PM
0. SART
2. Press the [3] key to select interference rejection level (OFF, 1, 2, or 3) from the INT REJ field.
Selected level is shown as IR1, IR2 or IR3 at the bottom left-hand corner on the display.
T/R
A/C RAIN OFF
-1.12-
A/C RAIN ON – Thin clouds
can be cleared