Glossary of terms

This is to educate the applicant to understand the concept(s) of radiotelephony in general


A device for generating alternating current electricity by moving conductors across magnetic field lines.

An audio signal such as a voice modulates a carrier wave. This creates a signal which, when transmitted, carries information. Intelligence is determined by varying the intensity of the radio wave.

Also called an automatic alarm. A device which monitors the distress frequencies and alerts personnel when traffic is received. Its fundamental purpose is to stand watch when the radio operator is not on duty.

The maximum permitted band of frequencies as specified in the FCC authorization to be occupied by an emission. Includes a total of the frequency departure above and below the carrier frequency.

The radio frequency  assigned to a station by the FCC and specified in the station license or other instrument of authorization.

Unless transmitting distress calls, the minimum amount of output power necessary to carry on the telecommunications for which the station is licensed. (see part 80.63(a))

The electronics including the radio system aboard an aircraft which must be maintained by a General Radiotelephone Operator.

In marine radar , the bearing in degrees measured in a clockwise rotation.


The amount of frequency spectrum space taken up by 99% of a radiated signal.

A fixed land station used to communicate with mobile station radio stations installed in motor vehicles.

Phrase spoken just before a brief pause in radiotelephone conversation to allow the allow the other station to acknowledge your transmission.

A VHF radio station located on a ship's navigational bridge used for navigational purposes.


Agreed-upon frequency which stations used to initially call one another, both station switch over to another "working" frequency so others may use the calling frequency.

A unique identifier issued to a station by the FCC as an aid to enforcement of radio regulation. A call sign identifies the country of origin and individual station.

Any ship not licensed to carry more than 12 passengers.

An alternating -current wave radiated from a transmitter of constant frequency with no modulation present. A carrier is a radio wave intended to "carry" the modulation or information.

An indicator device which consist of an electron gun which focuses an electron beam on a fluorescent screen. The screen is painted when the beam is diverted by four deflection plates or a magnetic field deflection system.

Word used in radiotelephony to indicate that a transmission has ended and no response is expected.

Random echoes created by water waves, rain or snow that appear on a radar-scope and which tend to obscure weak echoes from small objects.

Land-based radio station for the maritime services. Class I stations provide long distance communications to ships at sea. Class II coast stations provide regional service. A public coast station is open to public correspondence; a limited coast station may not transmit telecommunication for the public.

Entity approved by the FCC to provide commercial radio license exams.

The basic document for controlling telecomunications in the United States.

The order of priority of communications in the mobile service is: (1) Distress messages, (2) Communications preceded by the urgency signal, (3) Communications preceded by the safety signal, (4) Radio direction finding communications, (5) Navigation and safe movement of aircraft, (6) Navigation and safe movement of ships and weather observation,  

(7) Government radio-telegram, (8) Government communications for which priority has been requested, Communications relating to previously exchanged, and; 

(9) All other communications.

The azimuth (compass direction) based on the north magnetic pole. True north is azimuth zero degrees, east is 90 degrees, south is 180 degrees and west is 270 degrees.

Ship required by international law to carry radio equipment, licensed radio operators and to keep logs.

An international satellite-based search and rescue system established by Canada, France the U.S.A. and Russia. Used to detect and locate land, sea and airborne radio beacons.

Piezoelectric material(such as quartz) that mechanically vibrates at specific frequencies when a-c current is applied. Crystals are very stable frequency oscillators.


Abbreviated dB. A means of measuring levels of voltage, current or power. Assuming no transmission line losses, an antenna with 3-dB gain radiates about twice the transmitter power. A 10-dB gain represents a power gain of 10, 20-dB: 100. dBd refers to antenna power gain over half wave dipole; dBi is the power gain over an isotropic antenna.

The process of extracting information from a transmitted radio frequency signal.

In FM, the maximum amount that a frequency-modulated signal changes from center frequency.

An automatic calling system which allows a specific station to be contacted. An "all ships" call can also be made for distress alerting and navigation safety communications.

Requiring immediate assistance. Distress traffic in radio communications receives the highest priority because distress calls indicate imminent disaster. MAYDAY in radiotelephone distress signal; SOS in radiotelegraphy.

An internationally recognized frequency set aside for distress traffic such as 2182 kHz (single sideband), 156.800 MHz (FM) and 500 kHz (telegraph), 121.5 MHz (AM) double sideband full carrier) is the aircraft distress channel. 

In radar, the fact that a return echo changes in frequency when it encounters a moving object. The radar echo is shifted higher or lower as the target moves closer or further away from the radar installation. The magnitude of the shift determines the speed of the remote object. Stationary objects do not show up on Doppler radar.

A one-way wide angle radio beam from a communications satellite in earth orbit to a station on the surface of the earth/ See uplink, transponder.

A artificial antenna device used to prevent an antenna from radiating an interfering signal into space. Its resisitance is similar to the impedance of an actual antenna.

Two-way communications with both stations transmitting and receiving on different frequencies.

In radar, a high-speed switching device that permits an antenna to be used for both transmitting pulses and receiving echoes.


A station in the earth-space service located on the surface of the earth or a ship or aircraft.

In radar, a return radio-frequency pulse reflected from a remote object which is detected  a few milliseconds or microseconds after transmission. The round trip time can accurately be converted to distance.

The amount of radiated power from a transmitter taking  into consideration transmission line losses and the gain of the antenna over a half wave dipole. The actual power that is radiated into space. 

An airborne distress alerting beacon operating in 121.5 MHz or 243 MHz that is detected by the COPAS-SARSAT polar-orbiting satellites. The ELT is the aviation counterpart of the maritime EPIRB. Its is battery operated and transmit automatically when activated.

A series of three alpha-numeric characters used to identify radio signal properties. For example: A1A, manual radiotelegraphy, (A=Double-sideband amplitude modulation, full carrier; 1=One channel of digital modulation; A=Morse code for manual reception)

A feature of the INMARSAT SafetyNET System that permits the addressing of messages to a group (or all vessels) in specific geographical areas.


The transmission and reception of fixed images by converting scanned lines to digital signals. The lines are redrawn on paper at the receiving site. An important maritime use is the transmission of weather maps for satellite.

The official telecommunications agency in the United States. Among its duties is the allocation and regulation of radio frequency assignment within  a framework of international agreements.

The conveying of information by transmitting light waves down a thin thread of glass or plastic. A signal modulates a light emitting diode (LED) which is fed into an optical fiber. Fiber optics cable are inexpensive, light in weight, immune to electromagnetic interference and can carry more signals than coaxial cables.

An amount of received signal power intensity at a given distance measured in volts per meter. 

The process of varying a radio signal to convey intelligence by changing the transmitting frequency.

A radio frequency or band of frequencies internationally or nationally  assigned by an authorized body.

VHF-FM transmitters in the maritime service are determined to be operating properly (100% modulation) when the maximum amount  by which the carrier frequency changes  either side of center frequency is plus-or-minus 5 kHz. 

A digital method of transmission commonly used in radioteletype. Unlike Morse on/off keying, an FSK carrier is always present but the frequency shifts when the key is down. The mark and space frequency is relatively close, between 60 and 850 Hz.

Simultaneous two-way communication in both directions on separate frequencies. Normal two-way conversation, similar to the telephone, is possible. 


The difference between the input and output current, voltage or power. In antennas, the power difference between a reference antenna and its effective radiated power (ERP). Gain is usually expressed in decibels (dB). 

License issued by the FCC to individuals qualified to service, maintain, repair and operate radio-telephony communications equipment. The GROL is granted by passing Elements 1 and 3.

An orbiting satellite positioned 22,285 miles above the equator has exactly a 24-hour orbital period. Since this is the same as the rotation of the earth, the satellite appears to hang motionless in space.

Term for one billion cycles per second. Also 1,000 megahertz (MHz).

An automated ship-to-shore distress alerting system that uses satellites and advanced terrestrial communications system. It is coordinated throughout by the IMO, the International Maritime Organization. It pick up radio distress messages and relay them to the proper authorities.

Also called Navstar, the GPS uses multiple satellites to provide worldwide positional fix capability. This is accomplished by measuring the propagation time of satellite signals at the GPS receiver.

A connection with earth  to establish ground potential. A common connection is an electrical or electronic circuit. The area directly below an antenna. With Marconi antennas, the ground becomes one-half of the antenna.

A rule applying to all ships in the Great Lakes region. Technical requirements are stated for radio equipment and operators. 


Two-way communications over two separate frequencies but not at the same time.

Any emission or radiation which interrupts or degrades a radio communications service operating accordance with the rules. Operators must never deliberately interfere with any radio signal. 

Spurious signals that show up at integer multiples of the main frequency.

A measure of frequency equal to one cycle per second identified as hertz with lower case h.

A one-half wave dipole antenna that is usually fed at its center and horizontally polarized.

The radio frequency band that occupies 3 MHz to 30 MHz. 


A four geostationary satellite satellite network operated by the International Mobile Satellite Organization. It provides telex, telephone, data, fax and SafetyNET (maritime safety information, weather and navigational warning) service for ships at sea. INMATSAT is responsible for the space segment of GMDSS.

The presence of unwanted atmospheric noise or man-made signals that obstructs or inhibits the reception of radio communications.

A point-to-point radio communications service open to public correspondence.

A United States agency headquartered in London specializing in safety shipping and preventing ships from polluting the seas.

Worldwide method of substituting words for individual letters to increase understanding.

A signal consisting of two sinusoidal audio tones transmitted alternately. This signal activates automatic devices to inform the operator that traffic is being received on a distress frequency.

A signal consisting of a series of twelve dashes sent in one minute. This signal activates automatic devices to inform the operator that traffic is being received on a distress frequency.

The worldwide governing body controlling wire and telecommunications.

An electrically charged frequency sensitive portion of the upper atmosphere that has the ability to reflect radio waves back to earth.

A theoretical antenna in free space that radiates an RF signal equally well in all directions. Usually used as a reference point in the measurement of antennas. The gain of a half-wave antenna over an isotropic radiator is 2.15 dB.




One thousand cycles per second.

A vacuum tube used to generate and simplify microwave alternating current signals. The most common are multicavity (amplifiers) and reflex (oscillators).


A mobile communications  service between land-based movable and permanently located base stations - or between land-based movable stations.

This band occupies 30 kHz to 300 kHz.

 The entity to which a radio station is licensed by the Federal Communications Commission.

 Ship and aircraft stations are licensed for life. GROL, GMDSS, MP and RR has no expiration date.

 The unobstructed distance to the horizon. Communication above the VHF level is usually by direct "line of sight" radio-wave propagation. Range depend upon antenna height and terrain.

 Diary of radio communications kept by  the station operator. It must contain frequencies used, any technical problems encountered , what action has been taken to correct technical problems, and any distress traffic has been intercepted. The log is the written report of the station's performance and activities.

Acronym for LOng RAnge Navigation. Loran-C is a system of radio transmitters broadcasting low-frequency pulses to allow ships to determine their position. The difference in time it takes for pulses from different transmitters to reach a ship allows Loran-C to determine the ship's location very accurately.


An oscillator tube that produces high-power microwave pulses for radar.

A one-quarter wavelength antenna fed at one end and operated against a good RF grounding system.

A permit earned by passing a 24 questions examination on regulations, operating techniques and practices in the maritime services. The MROP is granted by passing Element 1 Radio Law. MROP holders may not make internal adjustments to radio transmitting equipment.

Relating to navigation or commerce on the seas.

A two-way mobile communications service between ships, or ships and coast stations.

A land station at a fixed location established for the automatic retransmission of signals to extend the range of communication of ship and coast stations.

Important navigational and meteorological information.

A person licensed to command a merchant ship.

A word spoken three times during radiotelephone distress messages.

This radio frequency band occupies 300kHz ro 3,000 kHz (or0.3 MHz to 3 MHz.)

One million cycles per second.  Also 1,000 kilohertz per second.

Radio waves generally beginning at 1,000 MHz or 1 GHz. Most microwave activity is in the 1 to  50 GHz range.

Land-based rescue authorities who collect, store and exchange distress and alerting information.

A modulator/demodulator that converts digital signals to audio tones for transmission over wirelines or via radio wave. The process is revered at the receiver.

The process of modifying a radio wave so that information may be transmitted. The desired signal is superimposed onto a higher "carrier" radio frequency. A radio wave has three basic properties that can be varied: amplitude, frequency and phase.

Radio waves can bounce back and forth between the earth and the ionosphere or follow more than one route to a receiving point.

The combining of two or more signals and transmitting them through a signal a single cable radio-frequency channel. The signals are separated at the receiving end.           


The fundamental unit of distance used in navigation. One nautical mile = 1.15 statute miles (or 6,080 feet). One knot is one nautical mile per hour.

Safety communications exchanged between ship and/or coast stations concerning concerning the maneuvering of vessels.

Navigational Telex, an international, automated system for instantly distributing maritime navigational warnings, weather forecast, search and rescue notices and similar information to ships on 515 kHz worldwide.


A radio navigation system relying on eight land transmitters throughout the world. Ships carry special receivers to listen to the transmitters and determine from the information carried exactly where the ship is at all times. Omega relies on phase differences between received signals.

Performing equally well in all directions.

Word spoken in radiotelephone conversation to indicate that it is the other station's turn to speak.

A method of tracking remote objects by refracting high frequency (HF) radio signals off of the ionosphere down towards the earth and analyzing the return echoes.

Driving a transmitter over its designed parameters causes adjacent frequency interference. Can be caused by shouting into a microphone. Peak modulation should not exceed 100%.


The internationally recognized radiotelephone urgency signal. The words "PAN PAN" are spoken three times in succession to indicate that an urgent message will follow.

A receiving or transmitting reflector (dish) antenna that focuses the RF energy into a narrow beam to insure maximum gain.

Unwanted spurious signals at frequencies removed from the operating frequency. Parasitics can cause distortion, loss of power and possible interference with to others. They are usually eliminated by shielding, RF chokes and neutralization.

The rules issued by the FCC governing commercial radio operators.

The group of rules issued by the FCC governing stations in the international fixed public radio communication services.

The group of rules issued by the FCC governing radio broadcast services.

The group of rules issued by the FCC governing stations in the maritime services.

The group of rules issued by the FCC governing stations in the aviation services.

Any ship carrying more than 12 paying passengers. (Six passengers when used in reference to the Great Lakes Radio Agreement.)

Method of measuring the output power of a single sideband signal since no carrier is transmitted.

An land-based satellite beacon that is detected by the COPAS-SARSAT polar-orbiting satellites.

A system of substituting easily understood words for corresponding letters.

A cathode ray tube display map used to highlight targets objects illuminated by a scanning radar antenna.

Any ship propelled by machinery.

Maritime service rules require that priority be given to: (1) distress calls concerning grave and imminent danger, (2) urgent messages concerning safety of life and property, and (3) traffic concerning navigational safety and weather warnings. (See part 80.91)

The method of radio wave travel which may be along the earth's surface, directly through space or reflected from the upper atmosphere.

Any third-party telecommunication (message, image or voice traffic) that must be accepted for transmission.

In radar, a repetitious burst of radio-frequency energy of very short duration which is directed at a remote object.

Also, the same as PRF, the pulse repetition frequency. The number of radar pulses transmitted in a second. PPRs usually vary between 800 and 2,000 per seconds.

The time between the leading edge  edge of one transmitted pulse and the leading edge of the next pulse. The period of the pulse repetition rate.



Acronym  for RAdio Detection And Ranging. A method of tracking objects by analyzing reflected microwave radio signals or echoes. Doppler radar is used to measure speed.

The time required for a radio frequency signal to travel from the transmitter to a target one nautical mile away and back. A radar mile is considered to be 12.3 microseconds.

The art  of determining the direction of a radio signal using a radio receiver with a signal strength indicator and a rotatable directional antenna. The location of a radio signal requires bearings to be taken by two radio stations.

An electromagnetic wave that travels at the speed of light; 186,282 statute miles or 162,000 nautical mile per second. its frequency ranges from 10 kHz to 3,000 GHz.

The FCC licensed operator in charge of the station who is responsible for its proper use and operation.

A means of exchanging  alphanumeric code by direct printing.

Radio operations are classified into services according to the nature and purpose of the transmission.

Method of transmitting voice over radio waves.

The direction when the reference point is the ship's heading. The number of degrees port or starboard of the bow.

A receiver/transmitter installation that receives signals on one channel and instantly retransmits them on another frequency usually at higher power from tall antennas. A duplexer keeps the transmitter from overloading the receiver. The advantage of a repeater is increased radio range. A repeater on a satellite is called transponder.

In GMDSS, the unit responsible for organizing and conducting search and rescue operations within a specific geographical area.

In radar the capability to separate two objects that are close to each other in range or bearing and to show them as two distinct echoes on the radarscope.

Permit allowing certain radio privileges in the aviation, broadcast and maritime services. No examination is required.

A word in radiotelephone conversation to indicative that you have received and understood all of the other station's transmission.


In radio transmission indicating that a station is about to transmit an important navigation or weather warning.

International agreement which spells out certain safety requirements for on-board radio equipment and operators.

Radio equipment on ships is considered in terms of its range the areas in which the vessel will travel. There are four watchkeeping areas. Sea area A1 is within VHF communications range (20-30 miles); A2 within the coverage of a shore based MF coast station (about 100 miles); A3 within coverage  of an INMARSAT geostationary satellite and A4 is the remaining polar areas of the world.

These are portable devices which are taken into a survival craft when abandoning ship. When switched on, they transmit a series of dots on a rescuing ship's 9-GHz radar display. SARTS's can also notify persons in distress that a rescue ship or aircraft is within range.

The word "SECURITY" is spoken three times prior to the transmission of a safety message.

Other than broadcast to the general public, persons may not divulge the content of telecommunications nor use the information obtained to benefit anyone other than the intended recipient.

A coded transmission to a particular radio station. Other stations do not hear it.

The ability of a radio receiver to separate the desired signal from other signals.

A method of minimizing mutual interference by spacing stations using the same frequency at required distance intervals.

The ability of a radio receiver  to separate the desired signal from other signals.

A mobile satellite station located on board a vessel.

The three-minute duration of time during a continuous watch on a distress frequency when a maritime radio operator must not transmit.

Two-way communications with both stations transmitting and receiving  on the same frequency. Only one station may transmit at a time.

Method of transmitting radiotelephony  where one sideband is filtered out and the carrier suppressed or reduced. SSB is more efficient that double sideband signals since it takes less radio spectrum. Emission: J3E

Acronym for SIplex Teleprinter On Radio. Error-free teleprinter of news and weather over the high-frequency bands.

The radiotelegraphy distress signal sent three times followed by "DE" ("this is") and the callsign of the station in danger.

The ratio of the current values at the maximum  and minimum points on a transmission line. There is no standing waves when the load impedance matches the line impedance. SWR of 3:1 and less is generally considered satisfactory. Standing waves are measured with an SWR meter of reflectometer.

Any construction permit, license or special temporary authorization issued by the FCC for activating a radio station.

5,280 feet. Unit of distance commonly used on land in the United States. One statute mile equals 0.8684 nautical miles.

The height, thickness and intensity of the ionosphere from which radio waves are reflected vary according to a cycle of approximately 11 years.

A mobile station on a lifeboat, life raft or other survival equipment aboard a ship or aircraft intended for emergency purposes.

In radar, the line across the cathode-ray tube that is synchronized with the rotation of the scanning antenna.


In radar, a term frequently used to denote a boat, buoy, island or other object that is radar conspicuous and produces an echo in the radarscope.

The transfer of sound, images or other intelligence  by electromagnetic means.

The process of sending and receiving information through the use of Morse code.

The process of exchanging information through the use of speech transmission.

A mechanical typewriter-like device that prints text sent over wire or radio circuits. In a radioteleprinter, a modem converts the audio output of a receiver into electrical impulses to drive the individual keys. See Modem.

Radio messages exchanged between stations.

A device that converts one form of energy into another. In depth sounder applications, electronic impulses are converted into sound impulses and vice versa.

A relay station used to improves the reception of weak television and FM broadcast signals in remote locations. Translator equipment rebroadcast the input signal on another frequency or channel.

The conduit by which radio frequency energy is transferred from the transmitter to the antenna.

A device in a orbiting  satellite that received uplink (transmitted) signals from earth and downlinks (retransmits) them back to earth. A transponder is a wide coverage space repeater. See Uplink, downlink.

A vacuum tube used to amplify UHF and microwave frequencies.

Radio equipment that has met FCC specifications. all transmitters in the Fixed, Aviation and Maritime services must be "type accepted." Type acceptance is based on data provided submitted by the manufacturer. 'Type Approval" is granted after tests are made by FCC technical personnel.


This radio frequency band occupies 300 MHz to 3,000 MHz (0.3 GHz to 3 GHz)

Sometimes called Greenwich Mean Time (GMT), the time appearing at the zero meridian near Greenwich, England. UTC is the standard for time throughout the world.

New FCC licensing system using one integrated database to electronically issue and maintain radio operator licenses.

The ground-based frequency on which an orbiting satellite receives its radio signals from earth. See downlink, transponder.

The information carrying portion of the signal just above the amplitude modulated (AM) carrier frequency which is reduced or eliminated before transmission.

Th Urgent message concerning the safety of a ship, aircraft, other vehicle or person. Urgency traffic has slightly lower priority than distress traffic.


Standard method of orienting maritime antennas operating at frequencies above 30 MHz: perpendicular to ground or water. Polarization is determined by the direction of the electric component of the electromagnetic field. Vertically polarized antennas produce vertically polarized waves.

Traffic management service operated by the U.S. Coast Guard in certain water areas to prevent ship collisions , grounding and environmental harm.

This radio frequency band occupies 30 MHz to 300 MHz. (or 3,000 kHz to 30,000 kHz)

Notification from the FCC of a rule infraction. A written response must be made within 10 days containing a full explanation and action taken to prevent reoccurrence.

This band occupies 3 kHz to 30 kHz.

A ship not required to carry radio equipment, licensed radio operators or keep logs. When radio equipment is carried, however, appropriate listening listening watches must be maintained on 2182 kHz.


The precise standard frequency and time transmissions of the National Bureau of Standards.

The act of keeping close observation on distress frequencies for any distress messages.

A new low-loss circular or rectangular hollow pipe-like feedline used at UHF and microwave frequencies. The wave are carried inside the pipe.

Phrase spoken in radiotelephony to acknowledge  that a message has been received and that the receiving station will comply. "Wilco" is short for "will comply".

A frequency establishing for conducting communications after first being established on a calling frequency.