AAL	ATM Adaptation Layer
AOS	Acquisition of Signal. The time at which a radio signal is first heard from a satellite, usually just after it rises above the horizon.
ACSS	Access Control Sub-System
ADSL	Asymmetric Digital Subscriber Line
Amplifier	A device used to boost the strength of an electronic signal.
AM	Amplitude Modulation. The base-band signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.
Analogue	Transmission method which uses a variable and continuous voltage to represent variations in the same quantity. Analogue is a form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analogue signal is responsive to changes in light, sound, heat and pressure.
ADC	Analogue-to-Digital Conversion. Process of converting analogue signals to a digital representation. DAC = Digital-to-Analogue Conversion represents the reverse translation.
Anomalistic Period	The elapsed time between two successive perigees of a satellite.
Antenna	A device that concentrates a beam of electromagnetic waves to a focal point so as to send/receive radio signals. Depending on their use and operating frequency, antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flat or convoluted surface.
Aperture	The effective cross sectional area of the antenna which is exposed to the satellite signal. The larger the aperture the stronger the signal the antenna receives or transmits.
Apogee	The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geo-synchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit at this distance.
Argument of Perigee	The polar angle that locates the perigee point of a satellite in the orbital plane; drawn between the ascending node, geo-centre, and perigee; and measured from the ascending node in the direction of satellite motion.
Ascending Node	The point on the ground track of a satellite orbit where the sub-satellite point(SSP) crosses the equator from the Southern Hemisphere into the Northern Hemisphere.
ASTRA	The provider of Satellites and capacity for TV distribution and data communications.
ASTRA 1H	19.2° East, other ASTRA satellites on 28.2, 24.2/23.5, 5.2° East
ATM	Asynchronous Transfer Mode
Attenuation	The loss in power of electromagnetic signals between transmission and reception points.
Attitude Control	The orientation of the satellite in relationship to the earth and the sun.
Audio Subcarrier	The carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier.
AZ/EL Mount	Antenna mount that requires two separate adjustments to move from one satellite to another;
Azimuth	The angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can be determined for any point on the surface of the earth giver the latitude and longitude of that point. It is defined with respect to due north as a matter of easy convenience.