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 DISPLA Y.
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.
RCVR: Short for
RECEIVER.
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.
Stabilized PPI:
See STABILIZED DISPLAY.
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.
XMTR: Short for
TRANSMITTER.
