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Old 09-29-1999
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Radar Glossary

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Anti-Clutter Rain Control: An electronic circuit that suppresses the reflected radar energy from rain and other forms of precipitation to prevent masking of objects that might otherwise be lost in the clutter. Also called FTC (Fast Time Constant).

Anti-Clutter Sea Control: An electronic circuit that suppresses clutter caused by the reflection of echoes from waves in a seaway. Sea clutter could obscure the return from other objects in the area. Also called STC (Sensitivity Time Control).

Attenuation: The scattering and absorption of the energy in a radar beam as it passes through the atmosphere. Attenuation is greater at the higher frequencies or shorter wavelengths.

Beam Width: The horizontal and vertical angular measure of the main lobe of the transmitted radar pulse. (See Horizontal Beam Width and Vertical Beam Width.)

Brilliance Control: Adjusts the intensity or brightness of the screen.

Diffraction: The bending of a wave as it passes an obstruction. Because of diffraction, there is some illumination of the region behind a target by the radar beam.

EBL: Electronic Bearing Line. An EBL control is used to display the relative or true bearing of a target on the display. The EBL is moved with the cursor, and the bearing is usually read at the bottom of the screen in degrees. One end is always anchored at the center of the screen.

FTC. Fast Time Constant. With FTC in operation, only the leading edge of an echo of a long-time duration is displayed on the radarscope. FTC tends to reduce saturation of the scope by clutter. Also called Anti-Clutter or Differentiator.

Gain Control: Is used to adjust the sensitivity of the receiver and thereby regulating the intensity of the returns displayed on the screen.

Guard Zone: An adjustable zone around the vessel set by the guard control. Once a guard zone is set, any target that enters the guard zone will trigger an alarm. On some radar units, the alarm will also be triggered when a target leaves the guard zone and thus it can also be used as an anchor alarm.

Horizontal Beam Width The beam width of the radar pulse measured in the horizontal plane. The size of which determines the radar resolution. The smaller the horizontal beam width the better the resolution in bearing. Most small-boat radar has a beam width of 3 to 5 degrees.

Maximum Detectable Range: The maximum range at which a target can be detected is determined by transmitted power, scanner height, pulse length, receiver sensitivity, atmospheric conditions, target size, shape and reflectivity. Doubling the output power will only increase the maximum detectable range by about 20 percent. But, doubling the height of the scanner will increase the detectable range by over 40 percent (that's assuming the output power is adequate for the range).

Minimum Detectable Range: The minimum range at which an object can be detected due to antenna height, vertical beam width and transmitted pulse length.

Off Center PPI: Offsets the sweep from the center of the screen to a location on the edge of the PPI to permit better viewing of a specific area without changing range selections. Also called Sector Display or Sector Scan.

Offset EBL: An electronic bearing line that can be moved to any position on the screen.

Pulse Duration: The length of the radar pulse. Also called Pulse Length.

Pulse Length Error: A range distortion of a radar return caused by the duration of the pulse. The error is always on the far side of the return and is equal to the range equivalent of one half the pulse length. For accurate range determination always use the leading edge of a radar return.

PRF: Pulse Repetition Frequency. The number of radar pulses transmitted each second. The pulse transmission rate is automatically lengthened for longer ranges.

Radar Beacons: Transmitters operating in the marine radar frequency band, which produce distinctive indications on the radar scope.

Range Control: Selects the scale used by the display and sets the distance between the range rings to correspond with the range selected. The PRF and Pulse Length are adjusted automatically as the range setting is changed.

Range Rings Control: Range rings are displayed on the screen when selected by this control and provide a rough estimate of the distance to a target displayed on the screen. The distance between the range rings is usually displayed in nautical miles in the corner of the screen along with the scale selected by the range control.

Radar Range: A measurement of the distance an object can be seen by radar based on the height of the antenna, the height of the object and atmospheric bending of the radar beam.

Refraction: The bending of the radar beam as it travel to and from a target due to atmospheric density gradients.

Resolution: The radar's ability to display multiple objects individually when they are close together.

Resolution in Bearing: The ability to display multiple objects individually that are closely spaced at the same range. Shorter pulse lengths increase resolution

Resolution in Range: The ability to display multiple objects individually that are closely spaced at the same bearing. Smaller horizontal beam width increases resolution.

STC: Sensitivity Time Control. An electronic circuit designed to automatically increase the gain as the electron beam is deflected from the center to the edge of the scope. Also called anti-clutter gain control.

Vertical Beam Width: The beam width of the radar pulse measured in the vertical plane. Most radar units have a vertical beam width of 20 to 25 degrees.

VRM: Variable Range Marker. An adjustable range ring used to measure the distance to a target. When the VRM is adjusted over the leading edge of a return with the cursor control, the distance to the object is usually displayed on the bottom of the screen.

Zoom Control: Zooms the display in on the area around the cursor or between the cursor and the display center to enhance the view of a target area.


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