ADAPTIVE EXCISION FILTER A signal- processing technique that improves data transmissions. It seeks and suppresses narrowband interference in the demodulator input and reduces the effects of co- channel interference (interference on the same channel that is being used).
ALE (Automatic Link Establishment) A technique that permits radio stations to make contact with one another automatically.
Modulation) A technique used to transmit information in which the amplitude
of the radio frequency carrier is modulated by the
audio input and the full carrier and both sidebands are transmitted.
Modulation Equivalent) A method of single sideband
transmission where the carrier is reinserted to permit reception by
conventional AM receivers.
AMPLITUDE The peak- to- peak magnitude of a radio wave.
ANTENNA COUPLER/ TUNER A device between the transmitter and antenna that modifies the characteristics of the load presented to the transmitter so that it transfers maximum power to the antenna.
ARQ (Automatic Repeat Request) Data transmission technique for error- free data transfer.
ASK (Amplitude Shift Keying) A form of modulation in which a digital signal shifts the amplitude of the carrier.
ASYMMETRICAL KEY SYSTEM A key management system that allows two- way secure communications among all users that have a public key and a private key.
ASYNCHRONOUS A data communication system that adds start-and-stop signal elements to the data for the purpose of synchronizing individual data characters or blocks.
ATMOSPHERIC NOISE Radio noise caused by natural atmospheric processes (primarily by lightning discharges in thunderstorms).
AUTOMATIC CHANNEL EQUALIZER A signal processing technique that improves data transmissions by compensating for variations in the channel characteristics as data is received.
A filter that passes a limited band of frequencies. It removes noise
and spurious signals generated in the exciter or output
frequency harmonics from the power amplifier.
BANDWIDTH The range of frequencies occupied by a given signal.
BAUD A unit of signaling speed equal to the number of symbols, i. e., discrete signal conditions per second.
BER (Bit Error
Ratio) The number of erroneous bits divided by the total
number of bits communicated.
BICONICAL ANTENNA An antenna used for fixed station use; designed to cover the 100 to 400 MHz range.
BINARY Number system having base of 2, using only the symbols 0 and 1.
BIT One binary digit (0 or 1).
Line- of- Sight) Communications that occur over a great
BROADBAND A term indicating the relative spectrum occupancy of a signal as distinguished from a narrowband signal. A broadband signal typically has a bandwidth in excess of twice the highest modulating frequency. Synonym: Wideband.
CARRIER A radio frequency signal that may be modulated with information signals.
CHANNEL A unidirectional or bi- directional path for transmitting and/ or receiving radio signals.
CIPHER TEXT Encrypted data.
CNR Combat Net Radios.
COLLOCATION The act or result of placing or arranging side by side.
COMSEC (Communications Security) Scrambling or cryptographic techniques that make information unintelligible to unauthorized persons.
COSMIC NOISE Random noise originating outside the earths atmosphere.
CRYPTOGRAPHY A COMSEC technique that translates (encrypts) information into an apparently random message and then interprets (deciphers) the random message by decryption.
CW (Continuous Wave) A radio wave of constant amplitude and constant frequency. Also, Morse code.
DAMA (Demand Assigned Multiple Access) A technique that matches user demands to available satellite time.
The standard unit for expressing transmission gain or
loss and relative power ratios.
DE- INTERLEAVING Process used by a demodulator to reverse interleaving and thus correct data transmission errors used in FEC coding.
DEMODULATION The process in which the original modulating signal is recovered from a modulated carrier.
DIFFRACTION When a VHF or UHF wave comes to a sharp edge, a portion of the wave bends around the edge and continues propagation as if a very low power radio was placed at the top of the ridge.
A versatile antenna that is usually a wire fed at the center of its length.
Its orientation provides either horizontal or
Travel in straight line, becoming weaker as distance
DIRECTIONAL ANTENNA An antenna that has greater gain in one or more directions.
DOWNLINK Radio paths back from a satellite.
Signal Processing) A recently developed technology that
allows software to control digital electronic circuitry.
An effect where radio waves can bend between air layers of
Interference) An electromagnetic disturbance that
degrades communications performance. Synonym: Radio Frequency Interference (RFI).
ENCRYPTION Process of translating information into an apparently random message.
Radiated Power) Equivalent power transmitted to the
atmosphere, which is the product of the transmitter power output multiplied by the gain of the antenna.
EXCITER The part of the transmitter that generates the modulated signal for a radio transmitter.
FADING The variation of the amplitude and/ or phase of a received signal due to changes in the propagation path with time.
Error Correction) A system of error control for data
transmission whereby the receiver can correct any code block that contains
fewer than a fixed number of bits in error.
FLTSATCOM (Fleet SATCOM) A group of Navy satellites.
FM (Frequency Modulation) A form of modulation where the frequency of a carrier varies in proportion to an audio modulating signal.
FOOTPRINT The line of sight (LOS) areas covered by a satellite.
FREQUENCY The number of completed cycles per second of a signal, measured in hertz (Hz).
FREQUENCY HOPPING The rapid switching (hopping) of radio system frequency for both the receiver and transceiver from frequency to frequency in apparently random patterns, using a common timing reference.
Shift Keying) A form of modulation in which a digital
signal shifts the output frequency between discrete values.
GAIN The ratio of the value of an output parameter, such as power, to its input level. Usually expressed in decibels.
GEOSTATIONARY ORBIT The speed of a stable satellite orbit depends upon its distance above the earth. If a satellite is placed in a stable orbit 22,300 miles above the equator, it must travel just fast enough to make a rotation around the earth in 24 hours. Since that is exactly the same speed that the earth rotates, a satellite placed in that orbit will hover over the same spot on earth as they both rotate around together. This is called a geostationary orbit.
GROUND REFLECTED WAVE The portion of the propagated wave that is reflected from the surface of the earth between the transmitter and receiver.
GROUND WAVE A radio wave that is propagated over the earth and ordinarily is affected by the presence of the ground.
HF (High Frequency) Normally, the band from 3 to 30 MHz. In practice, the lower end of the HF band extends to 1.6 MHz.
Hz (Hertz) Basic unit for frequency.
Frequency) A frequency used within equipment as an
intermediate step in transmitting or receiving.
Opposition to current flow of a complex combination of resistance and
reactance. Reactance is the opposition to AC current flow
by a capacitor or an inductor. An ideal antenna coupler will act so as to
cancel the reactive component of antenna impedance, i. e., by providing an
equal inductive reactance if the antenna has a capacitive reactance or an
equal capacitive reactance if the antenna presents an inductive reactance.
INTERLEAVING A technique that increases the effectiveness of FEC codes by randomizing the distribution of errors in communication channels characterized by error bursts.
A region of electrically charged particles or gases in the
earths atmosphere extending from 50 to 600 kilometers (approximately
30 to 375 miles) above the earths surface.
ISB (Independent Sideband) Double sideband transmission in which the information carried by each sideband is different.
Deliberate interference that results from transmission on
operating frequencies with the intent to disrupt communications.
KEK (Key Encryption Key) Used in digital encryption.
KEY A variable that changes the mathematical algorithm in cryptography.
KEY GENERATOR A device or process that generates the variable for a cryptographic encoding system.
LF (Low Frequency) The frequency range from 30 to 300 kHz.
LNA Low noise receive amplifier.
LOBE Area of strong radiation
of Sight) A term that refers to radio signal propagation in a
straight line from the transmitter to a receiver without refraction; generally extends to the visible horizon.
LPD (Low Probability
of Detection) Techniques for minimizing the
probability that the transmitted signal is detected by an unauthorized party.
LSB (Lower Sideband) The difference in frequency between the AM carrier signal and the modulation signal.
Usable Frequency) The lowest frequency in the HF band at which the received
field intensity is sufficient to provide the required
signal- to- noise ratio.
In an antenna radiation pattern, the lobe containing the
direction of maximum radiation intensity.
M- ARY PSK (M- ary Phase Shift Keying) A method of increasing the data rate of radio transmissions. M refers to the number of phases used in the modulation scheme.
MODEM (MOdulator- DEModulator) A device that modulates and demodulates signals. The modem converts digital signals into analog form for transmitting and converts the received analog signals into digital form.
MODULATION The process, or result of the process, of varying a characteristic of a carrier in accordance with a signal from an information source.
MUF (Maximum Usable Frequency) The upper limit for the frequencies used at a specified time for radio transmission between two points via ionospheric propagation.
MULTIBAND Military radios that combine VHF and UHF, HF and VHF, or HF- VHF- UHF capabilities.
The propagation phenomenon that results in radio signals
reaching the receiving antenna by two or more paths.
MULTIPATH SPREAD The range of timed differences that it takes for radio signals to reach the receiving antenna when they arrive from several routes, which may include one or more sky wave paths and/ or a groundwave path. The effect of multipath spread is minimized by selecting a frequency as close as possible to the MUF.
NULL Area of weak radiation
OHM Unit of measurement of resistance.
OMNIDIRECTIONAL ANTENNA An antenna whose pattern is non- directional in azimuth.
ON- OFF KEYING Turning the carrier on or off with telegraph key (Morse code). Same as CW.
The- Air- Rekeying) This technique developed by Harris
eliminates the need for manual loading of encryption keys and provides a more secure method of key management.
PARALLEL TONE MODEM Carries information on simultaneous audio tones, where each tone is modulated at a low- keying rate.
PHASE In a periodic process such as a radio wave, any possible distinguishable state of the wave.
PICKET FENCING Form of multipathing common to vehicular mounted radios
POLARIZATION The orientation of a wave relative to a reference plane. Usually expressed as horizontal or vertical in radio wave terminology.
PREAMBLE A known sequence of bits sent at the start of a message, which the receiver uses to synchronize to its internal clock.
PROPAGATION The movement of radio frequency energy through the atmosphere.
PSK (Phase Shift Keying) PSK is similar to FSK except that it is the phase of the carrier rather than the frequency that is modulated.
PUBLIC KEY CRYPTOGRAPHY A type of key management system used in the commercial sector. Under this standard, each user generates two keys, a public key and a private key. The strength of such a system lies in the difficulty of deriving the private key from the public key.
RADIATION PATTERN Pattern determined by an antennas design and strongly influenced by its location with respect to the ground. Radiation patterns are frequency dependent.
RAU Remote Access Unit.
REFRACTION The bending of a radio wave as it passes obliquely from one medium to another with different indices of refraction.
SATCOM Satellite Communications.
SCRAMBLING A COMSEC technique that involves separating the voice signal into a number of bands, shifting each band to a different audio frequency range, and combining the resulting bands into a composite audio output that modulates the transmitter.
SERIAL TONE MODEM Carries digital information on a single audio tone.
SHORT WAVE Radio frequencies above 3 MHz.
The spectral energy, distributed above or below a carrier,
resulting from a modulation process.
SKY WAVE A radio wave that is reflected by the ionosphere.
Management Center) Regulates and assigns the satellite
resources to users.
SNR (Signal- to- Noise Ratio) The ratio of the power in the desired signal to that of noise in a specified bandwidth.
SOFT- DECISION DECODING An error- correction technique where a group of detected symbols that retain their analog character are compared against the set of possible transmitted code words. A weighing factor is applied to each symbol in the code word before a decision is made about which code word was transmitted.
SSB (Single Sideband) A modulation technique in which the carrier and one sideband (upper or lower) are suppressed so that all power is concentrated in the other sideband.
SURFACE WAVES Travel along the surface of the earth and may reach beyond the horizon.
SYMMETRIC KEY SYSTEM A key management system in which the same key encrypts and decrypts data.
SYNCHRONOUS A form of data communications that uses a preamble to alert the data receiver that a message is coming and to allow it to synchronize to an internal bit clock.
TACSAT Tactical Satellite.
TAKE- OFF ANGLE The angle between the axis of the main lobe of an antenna pattern and the horizontal plane at the transmitting antenna.
TCM (Trellis Coded Modulation) A coding technique that provides maximum data rate capability to PSK data streams by improving the noise margin.
TEK (Traffic Encryption Key) Used in digital encryption.
TIU Telephone Interface Unit.
TRAFFIC The information moved over a communications channel.
TRANSCEIVER Equipment using common circuits in order to provide transmitting and receiving capability.
TRANSEC (Transmission Security) Techniques that prevent signal detection or jamming of the transmission path.
UHF (Ultra High Frequency) The portion of the radio spectrum from 300 MHz to 3 GHz.
UPLINK Radio paths up to a satellite.
USB (Upper Sideband) The information- carrying band and is the frequency produced by adding the carrier frequency and the modulating frequency.
VERTICAL WHIP ANTENNA An omnidirectional antenna that has low take- off angles and vertical polarity.
VHF (Very High Frequency) The portion of the radio spectrum from approximately 30 MHz to 300 MHz.
VOCODER A device that converts sounds into a data stream that can be sent over a radio channel. Short for voice coder- decoder.
Distance between point on wave to corresponding point on adjacent wave.