Glossary of Radar Terms and Abbreviations

RADAR (radio detection and ranging)

AFC:  Automatic frequency control.

Aerial: Antenna.

Afterglow: The slowly decaying luminescence of the screen of the cathode-ray tube after excitation by an electron beam has ceased. See PERSISTENCE.

Amplify: To increase the strength of a radar signal or echo.

Antenna:  A conductor or system of conductors consist­ing of horn and reflector used for radiating or receiving radar waves. Also called AERIAL.

Anti-clutter Control:  A means for reducing or eliminating interferences from sea return and weather.

Attenuation: The decrease in the strength of a radar wave resulting from absorption, scattering, and reflection by the medium through which it passes (waveguide, atmosphere) and by obstructions in its path. Also attenuation of the wave may be the result of artificial means, such as the inclusion of an attenuator in the circuitry or by placing an absorbing device in the path of the wave.

Automatic Frequency Control (AFC): An electronic means for preventing drift in radio frequency or maintaining the frequency within specified limits. The AFC maintains the local oscillator of the radar on the frequency necessary to obtain a constant or near constant difference in the frequency of the radar echo (magnetron frequency) and the local oscillator frequency.

Azimuth: While this term is frequently used for bearing in radar applications, the term azimuth is usually restricted to the direction of celestial bodies among marine navigators.

Azimuth-stabilized PPI: See STABILIZED PPI.

Beam Width: The angular width of a radar beam between half-power points. See LOBE.

Bearing: The direction of the line of sight from the radar antenna to the contact. Sometimes called AZIMUTH although in marine usage the latter term is usually restricted to the directions of celestial bodies.

Bearing Cursor: The radial line inscribed on a transparent disk which can be rotated manually about an axis coincident with the center of the PPI. It is used for bearing determination. Other lines inscribed parallel to the radial line have many useful purposes in radar plotting.

Blind Sector: A sector on the radarscope in which radar echoes cannot be received because of an obstruction near the antenna. See SHADOW SECTOR.

Cathode-.ray Tube (CRT): The radarscope (picture tube) within which a stream of electrons is directed against a fluorescent screen (PPI). On the face of the tube or screen (PPI), light is emitted at points where the electrons strike.

Challenger: See INTERROGATOR.

Clutter: Unwanted radar echoes ret1ected from heavy rain, snow, waves, etc., which may obscure relatively large areas on the radarscope.

Contact: Any echo detected on the radarscope not evaluated as clutter or as a false echo.

Contrast: The difference in intensity of illumination of the radarscope between radar images and the background of the screen.

Corner Reflector: See RADAR REFLECTOR.

Cross-band Racon: A racon which transmits at a frequency not within the marine radar frequency band. To be able to use this type of racon, the ship’s radar receiver must be capable of being tuned to the frequency of the cross-band racon or special accessory equipment is required. In either case, the radarscope will be blank except for the racon signal. See IN-BAND RACON.

Crystal: A crystalline substance which allows electric current t to pass in only one direction.

CRT: Cathode-ray tube.

Definition: The clarity and fidelity of the detail of radar images on the radarscope. A combination of good resolution and focus adjustment is required for good definition.

Double Stabilization: The stabilization of a Heading Upward PPI display to North. The cathode-ray tube with the PPI display stabilized to North is rotated to keep ship’s heading upward.

Duct:  A layer within the atmosphere where refraction and reflection results in the trapping of radar waves, and consequently their propagation over abnormally long distances. Ducts are associated with temperature inversions in the atmosphere.

Echo: The radar signal reflected back to the antenna by an object; the image of the reflected signal on the radarscope. Also called RETURN.

Echo Box. A cavity, resonant at the transmitted frequency, which produces an artificial radar target signal for tuning or testing the overall performance of a radar set. The oscillations developed in the resonant cavity will be greater at higher power outputs of the transmitter.

Echo Box Performance Monitor:  An accessory which is used for tuning the radar receiver and checking overall performance by visual inspection. An artificial echo as received from the echo box will appear as a narrow plume from the center of the PPl. The length of this plume as compared with its length when the radar is known to be operating at a high performance level is indicative of the current performance level.

Face: The viewing surface (PPI) of a cathode-ray tube. The inner surface of the face is coated with a fluorescent layer which emits light under the impact of a stream of electrons. Also called SCREEN.

Fast Time Constant (FTC) circuit:  An electronic circuit designed to reduce the undesirable effects of clutter. With the FTC circuit in operation, only the nearer edge of an echo having a long time duration is displayed on the radarscope. The use of this circuit tends to reduce saturation of the scope which could be caused by clutter.

Fluorescence: Emission of light or other radiant energy as a result of and only during absorption of radiation from some other source. An example is the glowing of the screen of a cathode-ray tube during bombardment by a stream of electrons. The continued emission of light after absorption of radiation is called PHOSPHORESCENCE.

FTC: Fast time constant.

Gain ‘(RCVR) Control:  A control used to increase or decrease the sensitivity of the receiver (RCVR). This control, analogous to the volume control of a broadcast receiver, regulates the intensity of the echoes displayed on the radarscope.

Heading Flash:  An illuminated radial line on the PPI for indicating own ship’s heading on the bearing dial. Also called HEADING MARKER.

Heading- Upward Display: See UNSTABILIZED DISPLAY.

In-band Racon:  A racon which transmits in the marine radar frequency band, e.g., the 3-centimeter band. The transmitter sweeps through a range of frequencies within the band to insure that a radar receiver tuned to a particular frequency within the band will be able to detect the signal. See CROSS­BAND RACON.

Intensity Control:  A control for regulating the intensity of background illumination on the radarscope. Also called BRILLIANCE CONTROL.

Interference: Unwanted and confusing signals or patterns produced on the radarscope by another radar or transmitter on the same frequency, and more rarely, by the effects of nearby electrical equipment or machinery, or by atmospheric phenomena.

Interrogator: A radar transmitter which sends out a pulse that triggers a transponder. An interrogator is usually combined in a single unit with a responsor, which receives the reply from a transponder and produces an output suitable for feeding a display system; the combined unit is called an INTERROGATOR­RESPONSOR.

Kilohertz (kHz): A frequency of one thousand cycles per second. See MEGAHERTZ.

Lobe: Of the three-dimensional radiation pattern transmitted by a directional antenna, one of the portions within which the field strength or power is everywhere greater than a selected value. The half power level is used frequently as this reference value. The direction of the axis of the major lobe of the radiation pattern is the direction of maximum radiation. See SIDE LOBES.

Megacycle per second (Mc): A frequency of one million cycles per second. The equivalent term MEGA­HERTZ (MHz) is now coming into more frequent use.  

Megahertz: A frequency of one million cycles per second. See KILOHERTZ.

Microsecond: One millionth of one second.

Microwaves: Commonly, very short radio waves having wave lengths of 1 millimeter to 30 centimeters. While the limits of the microwave region are not clearly defined, they are generally considered to be the region in which radar operates.

Minor Lobes: Side Lobes.

North-upward Display: See STABILIZED DISPLAY.

Paint: The bright area on the PPI resulting from the brightening of the sweep by the echoes. Also, the act of forming the bright area on the PPI by the sweep.

Persistence:  A measure of the time of decay of the luminescence of the face of the cathode-ray tube after excitation by the stream of electrons has ceased. Relatively slow decay is indicative of high persistence. Persistence is the length of time during which phosphorescence takes place.

Phosphorescence: Emission of light without sensible heat, particularly as a result of but continuing after absorption of radiation from some other source. An example is the glowing of the screen of a cathode-ray tube after the beam of electrons has moved to another part of the screen. It is this property that results in the chart like picture which gives the PPI its principal value. PERSISTENCE is the length of time during which phosphorescence takes place. The emission of light or other radiant energy as a result of and only during absorption of radiation from some other source is called FLUORESCENCE.

Plan Position Indicator (PPI): The face or screen of a cathode-ray tube on which radar images appear in correct relation to each other, so that the scope face presents a chart like representation of the area about the antenna, the direction of a contact or target being represented by the direction of its echo from the center and its range by its distance from the center.

Plotting Head: Reflection plotter.

Polarization: The orientation in space of the electrical axis of a radar wave. This electrical axis which is at right angles to the magnetic axis may be horizontal, vertical, or circular. With circular polarization, the axes rotate, resulting in a spiral transmission of  the radar wave. Circular polarization is used for reducing rain clutter.

PPI repeater: Radar repeater.

Pulse: An extremely short burse of radar wave transmission followed by a relatively long period of no transmission.

Pulse Duration: Pulse Length.

Pulse Length: The time duration, measured in micro seconds, of a single radar pulse. Also called PULSE DURATION.

Pulse Recurrence Rate (PRR): Pulse repetition rate.

Pulse Repetition Rate (PRR): The number of pulses transmitted per second.

Racon: Radar beacon which, when triggered by a ship’s radar signal, transmits a reply which provides the range and bearing to the beacon on the PPI display of the ship. The reply may be coded for identification purposes; in which case, it will consist of a series of concentric arcs on the’ PPI. The range is the measurement on the PPI to the arc nearest its center; the bearing is the middle of the racon arcs. If the reply is not coded, the racon signal will appear as a radial line extending from just beyond the point where the echo would be painted if detected. See IN-BAND RACON, CROSS-BAND RACON, RAMARK.             .

Radar Indicator: A unit of a radar set which provides a visual indication of radar echoes received, using a cathode-ray tube for such indication. Besides the cathode-ray tube, the radar indicator is comprised of sweep and calibration circuits, and associated power supplies.

Radar Receiver: A unit  of a radar set which demodulates received radar echoes, amplifies the echoes, and delivers them to the radar indicator. The radar receiver differs from the usual superheterodyne communications receiver in that its sensitivity is much greater; it has a better signal to noise ratio, and it is designed to pass a pulse type signal.

Radar Reflector: A metal device designed for reflecting strong echoes of impinging radar signals towards their source. The corner reflector consists of three mutually perpendicular metal plates. Corner reflectors are sometimes assembled in clusters to insure good echo returns from all directions.

Radar Repeater: A unit which duplicates the PPI display at a location remote from the main radar indicator installation. Also called PPI  REPEATER, REMOTE PPI.

Radar Transmitter: A unit of a radar set in which the radio-frequency power is generated and the pulse is modulated. The modulator of the transmitter provides the timing trigger for the radar indicator.

Ramark:  A radar beacon which continuously transmits a signal appearing as a radial line on the PPI, indicating the direction of the beacon from the ship. For identification purposes, the radial line may be formed by a series of dots or dashes. The radial line appears even if the beacon is outside the range for which the radar is set, as long as the radar receiver is within the power range of the beacon. Unlike the RACON, the ram ark does not provide the range to the beacon.

Range Markers: Equally spaced concentric rings of light on the PPI which permit the radar observer to determine the range to a contact in accordance with the range setting or the range of the outer rings. See VARIABLE RANGE MARKER.

Range Selector:  A control for selecting the range setting for the radar indicator.


Reflection Plotter: An attachment fitted to a PPI which provides a plotting surface permitting radar plotting without parallax errors. Any mark made on the plotting surface will be reflected on the radarscope directly below. Also called PLOTIING HEAD.

Refraction: The bending of the radar beam in passing obliquely through regions of the atmosphere of different densities.

Relative Motion Display:  A type of radarscope display in which the position of own ship is normally fixed at the center of the PPI (some sets have off-center capability in relative mode of operation) and all detected objects or contacts move relative to own ship.

Remote PPI: Radar repeater.

Resolution: The degree of ability of a radar set to indicate separately the echoes of two contacts in range, bearing, and elevation. With respect to:

  • Range – The minimum range difference between separate contacts at the same bearing which will allow both to appear as separate, distinct echoes on the PPI.
  • Bearing – The minimum angular separation between two contacts at the same range which will allow both to appear as separate, distinct echoes on the PPI.
  • Elevation – The minimum angular separation in a vertical plan between two contacts at the same range and bearing which will allow both to appear as separate, distinct echoes on the PPI.

Responder Beacon: Transponder beacon.

Scan: To investigate an area or space by varying the direction of the radar antenna and thus the radar beam. Normally, scanning is done by continuous rotation of the antenna.

Scanner:  A unit of a radar set consisting of the antenna and drive assembly for rotating the antenna.

Scope: Short for RADARSCOPE.

Screen: The face of a cathode-ray tube on which radar images are displayed.

Sea Return: Clutter on the radarscope which is the result of the radar signal being reflected from the sea, especially near the ship.

Sensitivity Time Control (STC): An electronic circuit designed to reduce automatically the sensitivity of the receiver to nearby targets. Also called SWEPT GAIN CONTROL.

Shadow Sector:  A sector on the radarscope in which the appearance of radar echoes is improbable because of an obstruction near the antenna. While both blind and shadow sectors have the same basic cause, blind sectors generally occur at the larger angles subtended by the obstruction. See BLIND SECTOR.

Side Lobes: Unwanted lobes of a radiation pattern, ie., lobes other than major lobes. Also called MINOR LOBES.

Stabilized Display (North-Upward):  A PPI display in which the orientation of the relative motion presentation is fixed to an unchanging reference (North). This display is North-Upward, normally. In an UNSTABILIZED DISPLAY, the orientation of the relative motion presentation changes with changes in ship’s heading. See DOUBLE STABILIZATION.


STC: Sensitivity time control.

Strobe: Variable range marker.

Sweep: determined by the time base or range calibration, the radial movement of the stream of electrons impinging on the face of the cathode-ray tube. The origin of the sweep is the center of the face of the cathode-ray tube or PPI. Because of the very high speed of movement of the point of impingement, the successive points of impingement appear as a continuously luminous line. The line rotates in synchronism with the radar antenna. If an echo is received during the time of radial travel of the electron stream from the center to the outer edge of the face of the tube, the sweep will be increased in brightness at the point of travel of the electron stream corresponding to the range of the contact from which the echo is received. Since the sweep rotates in synchronism with the radar antenna, this increased brightness will occur on the bearing from which the echo is received. With this increased brightness and the persistence of the tube face, paint corresponding to the object being “illuminated” by the radar beam appears on the PPI.

Swept Gain Control: Sensitivity time control.

Trace: The luminous line resulting from the movement of the points of impingement of the electron stream on the face of the cathode-ray tube. See SWEEP.

Transponder:  A transmitter-receiver capable of accepting the challenge (radar signal) of an interrogator and automatically transmitting an appropriate reply. See RACON.

Transponder beacon:  A beacon having a transponder. Also called RESPONDER BEACON.

Trigger: A sharp voltage pulse usually of from 0.1 to 0.4 micro-seconds duration, which is applied to the modulator tubes to fire the transmitter, and which is applied simultaneously to the sweep generator to start the electron beam moving radially from the sweep origin to the edge of the face of the cathode-ray tube.

True Motion Display: A type of radarscope display in which own ship and other moving contacts move on the PPI in accordance with their true courses and speed. This display is similar to a navigational (geographical) plot. See RELATIVE MOTION DISPLAY.

Unstabilized Display (Heading-Upward):  A PPI display in which the orientation of the relative motion presentation is set to ship’s heading and, thus, changes with changes in ship’s heading. In this Heading Upward display, radar echoes are shown at their relative bearings. A true bearing dial which is continuously set to ship’s course at the 000˚ relative bearing is normally used with this display for determining true bearings. This true bearing dial may be either manually or automatically set to ship’s course. When set automatically by a course input from the gyrocompass, the true bearing dial is sometimes called a STABILIZED AZIMUTH SCALE. The latter term which appears in manufacturer’s instruction books and operating manuals is more in conformity with air navigation rather than marine navigation usage. See DOUBLE STABILIZATION.

Variable Range Marker: A luminous range circle or ring on the PPI, the radius of which is continuously adjust able. The range setting of this marker is read on the range counter of the radar indicator.

VRM: Variable range marker.


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