The Back Page

Satellite Communications Glossary

Additive white Gaussian noise (white noise):

Statistically random radio noise characterized by a wide frequency spectrum that is continuous and uniform over a specified frequency band.

Adjacent channel interference:

Disruptions in a signal channel caused by power leaking from an adjacent channel, typically resulting from insufficient filtering, tuning, or frequency control in the reference or interfering channel (or both).

Antenna gain:

A measure of the change in an antenna's signal power amplification based on orientation or signal direction.

Attenuation:

A decrease in signal magnitude between two points.

Bandwidth:

The range of frequencies occupied by a signal; the information-carrying capacity of a communications channel.

Bandwidth–bit-time product:

The result obtained by multiplying the filter bandwidth by the bit time.

Binary phase-shift keying:

A modulation technique whereby a data stream is imprinted on a carrier wave using two wave phases (generally 0 and 180 degrees) to signal zeros and ones.

Bit:

A binary digit, the basic element of a signal, often rendered as zero or one, plus or minus, on or off.

Bit-error rate:

A measure of performance for a digital receiver; defined as the probability that a bit will be received incorrectly or as the ratio of bits received in error to total number of bits transmitted.

Bit time:

The period of time used to transmit a single bit.

Block coding:

A forward error-correction scheme characterized by a one-to-one correspondence between a finite set of input symbols and output codewords.

Channel:

The propagating medium (i.e., cable or electromagnetic path) connecting the transmitter and receiver.

Channel coding:

The application of processing algorithms, prior to transmission via a channel (and reverse algorithms at the receiver), used to improve data reliability.

Codec (coder/decoder):

The encoding device that converts a basic digital signal to a coded signal, and its reverse counterpart.

Codeword:

A sequence of symbols assembled in accordance with specific rules of a coding scheme that are assigned a unique meaning.

Coding gain:

The increase in efficiency that a coded signal provides over an uncoded signal. Expressed in decibels, the coding gain indicates a level of power reduction that can be achieved.

Concatenated code:

A forward error-correction method to achieve a relatively large coding gain by combining two or more relatively simple codes. A concatenated scheme such as a block code followed by a convolution code is particularly effective in combating bursty errors.

Continuous phase modulation:

A modulation technique achieving smooth phase transitions between signal states, thereby decreasing bandwidth requirements.

Convolutional code:

A forward error-correction scheme, whereby the coded sequence is algorithmically achieved through the use of current data bits plus some of the previous data bits from the incoming stream.

Crosslink:

A communication link between two satellites.

Decibel:

A unit for a logarithmic ratio, commonly used as a measure of the ratio of two powers.

Demodulation:

The process of separating a modulated signal from its carrier signal and converting it back to its original state.

Downlink:

The portion of a communications link used to transmit signals from a satellite to an Earth-based terminal (on land, ship, or aircraft).

Footprint:

The area of Earth with sufficient antenna gain to receive a signal from a satellite.

Forward error correction:

Error-correction method allowing detection and correction of bit or symbol errors without the need for requesting retransmission of the original data.

Frequency hopping:

The continual switching of transmitted frequencies based on a shared algorithm to minimize unauthorized interception or jamming of a radio transmission.

Gaussian minimum shift keying:

A phase-shift-keying modulation technique that passes the input stream through a Gaussian filter before sending it to the phase modulator. The smooth Gaussian pulses require less bandwidth to transmit and are less susceptible to adjacent channel interference.

Geosynchronous orbit:

An equatorial orbit roughly 35,800 kilometers above Earth in which a satellite can remain fixed relative to Earth's surface.

Hertz:

A measure of radio frequency equal to one cycle per second; often rendered in terms of kilohertz (one thousand cycles per second), megahertz (one million cycles per second), and gigahertz (one billion cycles per second).

Inclined orbit:

A satellite orbit for which the orbital plane is not in Earth's equatorial plane.

Interleaving:

A process of scrambling the order of symbols to be transmitted over a channel in such a way that, when they are descrambled (at the receiver), any burst of channel errors will be spread out in time and thus appear as random errors to the decoder.

Jammer-to-signal ratio:

The ratio in decibels of the power of a jamming signal to that of a desired signal at a receiving antenna.

Jamming:

The act of intentionally directing electromagnetic energy at a communication system to disrupt or prevent signal transmission.

Lobe:

A discrete segment of an antenna radiation pattern, characterized by a localized maximum bounded by identifiable nulls.

Main lobe:

The lobe of an antenna radiation pattern containing the highest gain.

Modulation:

The process of changing or regulating the characteristics of a carrier wave so that the variations represent meaningful information.

Multiplexing:

The process of coordinating multiple message channels for simultaneous transmission by multiple users.

Null:

A point in an antenna's radiation pattern at which the gain is zero.

Offset reflector:

A reflector antenna consisting of a feed placed at the focus of a paraboloid and directed at the center of a primary reflector cut from the paraboloid at an offset from the paraboloid's axis of symmetry. This arrangement avoids any blockage of the reflector's aperture by the feed.

Parity bit:

An extra bit inserted into a data sequence before transmission to enable error detection.

Phase-shift keying:

The broad class of modulation techniques that use discrete phases of a carrier wave to denote signal bits—for example, phases of 0 and 180 to represent zeros and ones.

Phased array:

A group of antenna elements whose radiation patterns are coordinated such that the radio waves are reinforced in certain directions and suppressed in others.

Power spectral density:

The distribution of signal power in the frequency domain.

Quarternary phase-shift keying:

A phase-shift keying modulation techniques that uses four distinct phases of the carrier wave (usually offset by 90 degrees) to signify a pair of bits.

Radio-frequency interference:

An electromagnetic disturbance that interrupts, obstructs, or degrades the performance of electronic equipment.

Radio-frequency spectrum:

The portion of the electromagnetic spectrum spanning roughly 3 kilohertz to 300 gigahertz. It includes the very high (30–300 megahertz), ultrahigh (300 megahertz–3 gigahertz), superhigh (3–30 gigahertz), and extremely high (30–300 gigahertz) frequency bands.

Rake receiver:

A radio receiver having multiple receptors using offsets of a common spreading code to receive and combine several multipath time-delayed signals.

Saturation:

The point at which an amplifier cannot deliver more power despite a further increase in input level.

Scintillation:

A random fluctuation of the received field strength caused by irregular changes in the transmission path over time.

Shannon limit:

The theoretical minimum relative power needed to provide reliable data transmission at a given bit rate. Thus, for a given noisy channel, this limit yields the maximum rate at which error-free communication is possible.

Sidelobes:

The distinct portions of an antenna's radiation pattern in any area other than the main lobe.

Signal-to-noise ratio:

A ratio of signal power to noise power in a channel.

Spot beam:

A narrow transmission beam from a satellite antenna focused at a limited area of Earth.

Spread-spectrum modulation:

A telecommunication technique in which a transmitted signal is spread out across a bandwidth range considerably greater than the frequency of the original information.

Transponder (transmitter/responder):

A device on a communication satellite that receives signals from Earth, alters their frequency, amplifies them, and retransmits them to Earth on a different frequency.

Turbo code:

A relatively new forward-error correction technique made up of a concatenated code structure plus an iterative feedback algorithm. Turbo codes have been shown to manifest error-performance very close to what is theoretically possible.

Uplink:

The portion of a communications link used to transmit signals from an Earth-based terminal (on land, ship, or aircraft) to a satellite.

Milsatcom Timeline

Back To Winter 2002 Table of Contents