Acquisition - (1) An image captured by a satellite sensor. (2) The process of searching for and locking onto a received signal.

Acquisition Date - The date the image was acquired. Format: YYYY/MM/DD

Acquisition of Signal - The time a site receives a signal from a spacecraft.

Algorithm - In the context of remote sensing, algorithms generally specify how to determine higher-level data products from lower-level source data. For example, algorithms prescribe how atmospheric temperature and moisture profiles are determined from a set of radiation observations originally sensed by satellite sounding instruments.

Altitude - The angle, in degrees, above the level horizon where an object in the sky appears (the height in space).

Analog - 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 analog signal is used to transmit audio (such as voice, radio, stereo, and control tones) and is responsive to changes in light, sound, heat and pressure.

Analog-to-Digital Conversion - The process of converting analog signals to a digital representation.

Anomoly - (1) A deviation from the norm. (2) The angular distance between the position of a planet and its last perihelion (point nearest to the sun), or between that of a satellite and its last perigee (point nearest to the center of the earth).

Antenna - A device for transmitting and receiving radio waves.

Aperture - The diameter of an opening; the diameter of the primary lens or mirror of a telescope. A cross-sectional area of the antenna that is exposed to the satellite signal.

Ascending Node (AN) - Direction a satellite is traveling relative to the Equator. An ascending node implies a northbound Equatorial crossing.

Asychronous - Not synchronized.

Attitude - The angular orientation of a remote sensing system with respect to a geographical reference system. The orientation of the sensor along with information about the accuracy and precision with which this orientation is known. This data is required to perform proper calibration of instrument data. The attitude is usually stored in Euler angle or quaternion form and may be 1) calculated by the on-board computer and telemetered to the ground or 2) calculated by ground processing facilities using a variety of attitude sensor data.