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).
ADAPTIVE SYSTEM A system that automatically adjusts its parameters to improve its performance in response to changing conditions.
AGC (Automatic Gain Control) Circuit employed to vary gain or amplifier in proportion to input signal strength so that output remains at a constant level.
ALE (Automatic Link Establishment) A technique that permits radio stations to make contact with one another auto-matically.
AM (Amplitude
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.
AME (Amplitude
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 trans-mitter
and antenna
that modifies the characteristics of the load
presented
to the transmitter so that it transfers maximum power
to the antenna.
ANTENNA DIRECTIVE
GAIN The ratio of radiation intensity in
a certain
direction to the average radiation intensity.
ANTENNA POWER
GAIN The ratio of radiated power in a
given direction
to the antenna input power.
ARQ (Automatic
Repeat Request) Data transmission tech-nique
for error-free
data transfer.
ASCII (American
Standard Code for Information Interchange)
The standard
code for digital data interchange. ASCII uses a
coded character
set consisting of a 7-bit coded character (8 bits
including
parity check).
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.
BANDPASS
FILTER A filter that passes a limited band of frequen-cies.
It is used
to remove 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.
BASEBAND
The frequency band occupied by a signal prior to
radio frequency
carrier modulation or following demodulation.
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.
BINARY Number
system having base of 2, using only the
symbols 0
and 1.
BIT One binary
digit (0 or 1).
BLOS (Beyond
Line-of-Sight) Communications that occur over
a great distance.
BROADBAND
A term indicating the relative spectrum occu-pancy
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.
CCIR (International
Radio Consultative Committee) An
organization
of the International Telecommunications Union
(ITU) that
studies technical questions related to radio communi-cations.
CHANNEL A
unidirectional or bidirectional path for transmit-ting
and/or receiving
radio signals.
CIPHER TEXT
Encrypted data.
COLLOCATION
The act or result of placing or arranging side
by side.
COMSEC (Communications
Security) Scrambling or crypto-graphic
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.
D LAYER First
layer in the ionosphere. Reaches maximum ioniza-tion
when the
sun is at zenith and dissipates quickly toward sunset.
dB ( Decibel)
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 modu-lating
signal is
recovered from a modulated carrier.
DIPOLE ANTENNA
A versatile antenna that is usually a wire
fed at the
center of its length. Its orientation provides either
horizontal
or vertical polarization.
DIRECT WAVES
Travel in straight line, becoming weaker as
distance
increases.
DIRECTIONAL
ANTENNA An antenna that has greater gain in
one or more
directions.
DSP(Digital
Signal Processing) A recently developed technology
that allows
software to control digital electronic circuitry.
DTMF (Dual-Tone-Multi-Frequency)
Refers to DTMF signaling,
which is
typically used in telephone systems.
E LAYER The
mid-level of the ionosphere which reaches
maximum ionization
at noon. It begins dissipating toward sunset
and reaches
minimum activity at midnight. Irregular cloud-like
formations
of ionized gases occasionally occur in the E layer.
EMI (Electromagnetic
Interference) An electromagnetic
disturbance
that degrades communications performance .
Synonym:
Radio Frequency Interference (RFI).
ENCRYPTION
Process of translating information into an appar-ently
random message.
ERP (Effective
Radiated Power) Equivalent power transmitted
to the atmosphere,
which is the product of the transmitter power
output multiplied
by the gain of the antenna.
ERROR DETECTION
An error correction technique that uses
binary code
words to modify data messages by systematically
adding check
bits to detect errors in received words.
EXCITER The
part of the transmitter that generates the modu-lated
signal for
a radio transmitter.
F LAYER The
uppermost and most heavily ionized region of the
ionosphere.
Important for long-haul communications, since this
layer remains
ionized even after sunset.
FADING The
variation of the amplitude and/or phase of a
received
signal due to changes in the propagation path with time.
FEC (Forward
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.
FM (Frequency
Modulation) A form of modulation where the
frequency
of a carrier varies in proportion to an audio modulating
signal.
FOT (Frequency
of Optimum Transmission) The highest
frequency
predicted to be available for sky wave transmission for
a given path
and time for 85 percent of the maximum usable
frequency
(MUF).
FREQUENCT
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.
FSK (Frequency
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.
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) Nominally, 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.
IF (Intermediate
Frequency) A frequency used within equip-ment
as an intermediate
step in transmitting or receiving.
IMPEDANCE
Opposition to current flow of a complex combina-tion
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 reac-tance
if the antenna
presents an inductive reactance.
INCIDENT
ANGLE The angle at which sky waves enter the
ionosphere.
INTERLEAVING
A technique that increases the effectiveness of
FEC codes
by randomizing the distribution of errors in communi-cation
channels
characterized by error bursts.
IONCAP (Ionospheric
Communications Analysis and Prediction)
A popular
and effective propagation prediction program that
predicts
system performance at given times of day as a function
of frequency
for a given HF path and a specified complement of
equipment.
IONOSPHERE
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.
IONOSPHERIC
SOUNDING An automated propagation predic-tion
technique.
ISB (Independent
Sideband) Double sideband transmission in
which the
information carried by each sideband is different.
JAMMING Deliberate
interference that results from transmis-sion
on operating
frequencies with the intent to disrupt commu-nications.
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 vari-able
for a cryptographic
encoding system.
LOS (Line
of Sight) A term that refers to radio signal propaga-tion
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.
LPI (Low
Probability of Intercept) Techniques for minimizing
the likelihood
of the intelligence on a transmitted signal being
recovered
by an unauthorized party.
LQA (Link
Quality Analysis) A technique for real-time channel
evaluation
in which radios measure and store values indicating
the relative
quality of a radio link at different assigned frequencies.
LUF (Lowest
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.
MAIN LOBE
In an antenna radiation pattern, the lobe containing
the direction
of maximum radiation intensity.
MFSK (Multi-tone
Frequency Shift Keying).
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.
MULTIPATH
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 ground-wave path. The effect of multipath spread is
minimized
by selecting a frequency as close as possible to the MUF.
NVIS (Near-Vertical
Incidence Sky wave) A technique for
transmitting
over relatively short ranges by ionospheric refrac-tion
using very
high incident angles.
OHM Unit
of measurement of resistance. Its symbol is ?.
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.
OTAR (Over-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 simulta-neous
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.
POLARIZATION
The orientation of a wave relative to a refer-ence
plane. Usually
expressed as horizontal or vertical in radio
wave terminology.
POWER AMPLIFIER
The part of the transmitter that boosts the
output power
of the radio signal to the desired wattage before
sending it
to the transmitting antenna.
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.
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.
RMS (Root
Mean Square).
RTCE (Real-Time
Channel Evaluation) Techniques used to
select frequencies,
adjust data rates, or change modulation
schemes in
adaptive radio systems.
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.
SID (Sudden
Ionospheric Disturbance) Abnormally high
ionization
densities caused by solar flares, resulting in a sudden
increase
in radio wave absorption.
SIDEBAND
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.
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 char-acter
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.
SPORADIC
E Layer found in the E Layer of the ionosphere.
Supports
propagation of sky waves at the upper end of the HF
band and
beyond.
SPREAD SPECTRUM
A technique used to overcome deliberate
radio communications
interference, in which the modulated
information
is transmitted in a bandwidth considerably greater
than the
frequency content of the original information.
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.
STORE AND
FORWARD A technique where information is
stored until
a communication link is established and then sent.
SUNSPOT CYCLE
Eleven-year cycle of sunspots which
generate
bursts of radiation that increase levels of ionization.
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.
TAKE-OFF
ANGLE The angle between the axis of the main
lobe of an
antenna pattern and the horizontal plane at the
transmitting
antenna.
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.
VERTICAL
WHIP ANTENNA An omnidirectional antenna that
has low take-off
angles and vertical polarity.
VOCODER A
device that converts sounds into a data stream
that can
be sent over an HF channel. Short for voice coder-decoder.
WAVELENGTH
Distance between point on loop of wave to
corresponding
point on adjacent wave.