Furuno FR-8251 manual Suppressing Rain Clutter, The Heading and North Markers, Measuring the Range

A common mistake is too over-adjust the A/C SEA control so that the surface clutter is com- pletely removed. By rotating the control fully clockwise, you will see how dangerous this can be; a dark zone will be created near the center of the screen, causing a loss of close-in targets. This dark zone is even more dangerous if the gain has not been properly adjusted. Always leave a little surface clutter visible on the screen. If no clutter is observed (on very calm waters), set the control at the fully counterclockwise position.

1.12 Suppressing Rain Clutter

In adverse weather conditions, clouds, rain or snow produce a lot of spray-like spurious echoes and impairs target detection over a long distance. This situation can be improved by using a Func- tion key that is so programmed. If the Function key fails to effectively suppress rain clutter, use the A/C RAIN control.

The A/C RAIN control adjusts the receiver sen- sitivity 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.

Appearance of rain	A/C RAIN control adjusted;
clutter on the display	rain clutter suppressed

Figure 1-10 Effect of A/C RAIN control

1.13The Heading and North Markers

The heading marker indicates the ship's heading in all presentation modes. It appears at zero de- grees on the bearing scale in the Head-up mode, in any direction depending on the ship orienta- tion in North-up and True Motion modes. The north marker appears as a short dashed line. In the Head-up mode, the north marker moves around the bearing scale in accordance with the compass signal.

To temporarily extinguish the heading marker to look at targets existing dead ahead of own ship, press the HM OFF (PUSH) control. The heading marker reappears when the key is released.

1.14 Measuring the Range

There are three ways to measure the range to a target: by the range rings, by the cursor and by the VRM (Variable Range Marker).

Measuring range by the range rings

Use the range rings to obtain a rough estimate of the range to a target. They are concentric circles around own ship, or the sweep origin. The num- ber of rings is automatically determined by the selected range scale and their interval is displayed at the upper-left position of the screen. Press the RINGS (PUSH) control to show the range rings if they are not displayed. Successive presses of the RINGS (PUSH) control gradually increase the brightness of the rings in four steps and a fifth press erases the rings.

Measuring range by the cursor

Rotate the trackball to place the cursor on the in- side edge of the target. The range to the cursor appears at the bottom of the display.

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