Glossary - D

Data broadcast
one-way transmission of data from a central location to a number of remote receivers
(communications)

Data collection
one-way transmission of data from a number of remote transmitters to a central receiving location, usually for the monitoring of remote (and often unmanned) sites
(communications)

DBS
broadcasting of satellite television by specially designed television satellites operating in accordance with the regulations defined at WARC 77
(regulation, broadcasting)

Declination
the angle between a celestial body’s position in the sky and the celestial equator
(astronomy)

Decoder
a device used with a receiver to descramble encrypted broadcasts
(broadcasting)

Delay
the time taken for a signal to travel between two points; delays result from the transmission process itself and from the electronic processing of signals (ie during digitisation or because of buffering in packet based systems)
(communications)

Demand Assigned Multiple Access (DAMA)
a multiple access technique in which a transmission channel is only assigned to an earth station for the duration of a call
(communications)

Demodulator
a device which recovers a baseband signal from a modulated carrier
(communications)

Depolariser
device used to convert circular polarisation to linear polarisation
(technology)

Descending node
the point at which an orbit crosses a reference plane (generally a planet’s equatorial plane or the plane of the ecliptic) going from north to south
(orbital geometry)

Descent burn
the firing of a rocket motor to slow a spacecraft's approach to a planet, moon, asteroid or comet in preparation for landing
(space exploration)

Digital Circuit Multiplication Equipment
equipment designed to allow several digital speech circuits to share a single speech channel by exploiting the statistics of the periods of silence in speech.
(communications)

Digital Speech Interpolation (DSI)
a technique used in digital systems utilising the quiet periods in speech traffic to statistically combine several speech channels over a single bearer channel
(communications)

Dish
slang for a parabolic antenna
(technology)

Doppler shift
change in frequency of a signal caused when a signal source approaches or recedes from the receiver; the frequency increases for an approaching signal source and decreases for a receding one
(communications)

Downlink
1: signal path from a spacecraft to the receiving earth station
(communications)

Downlink
2: the signal received from a spacecraft
(communications)

Drag
the force (usually due to atmospheric friction) which acts to reduce the velocity of an object
(spaceflight)

DTH
broadcasting of satellite television by general purpose communications satellites operating in accordance with the regulations defined for the Fixed Satellite Service
(regulation, broadcasting)

Dynamic allocation
the allocation of transmission capacity depending on demand
(communications)

Dynamic routing
routing that adjusts automatically to changes in network topology and traffic
(communications)