Satellite Glossary

Beam:  A flow of electromagnetic radiation (radio waves) concentrated in a particular direction from the source (the satellite’s antenna). The shape and intensity of the beam is formed by the radiation pattern from the satellite’s antenna(s).  The intersection of the satellite beam with the earth’s surface is often referred to as the (beam’s) footprint.

Bus: A term that is applied to two concepts: (1) the “Bus Equipment” (enabling equipment) on a satellite and (2) the “General Model” from a specific manufacturer on which multiple-production satellite spacecraft are often based.

Communications Payload: The equipment to provide a repeating of the desired radio signals for the satellite mission.

Communications Satellites:  Transmission “repeaters” in space where signals are up-linked from a ground terminal, received, processed, amplified by the satellite, and re-transmitted to one or more user terminals of various types.

Design Life: The period over which the satellite is planned to be able to provide full performance required of the satellite and accounts for effects of Radiation, Cycle Life, Operating Life, and Calendar Life.

Effective Isotropic Radiated Power (EIRP):  The hypothetical power that would have to be radiated by an isotropic antenna (a theoretical point source of electromagnetic waves which radiates power in all directions) to give the same signal strength as the actual source antenna in the direction of the antenna’s strongest beam. EIRP tells us how much signal reaches the receiving antenna and if the desired signal is strong enough to overcome background noise.

Fuel Life: The primary determination of the life of a GEO satellite based on the fuel needed to maintain the orbit at the designated longitude.  The average GEO satellite is retired due to Fuel End of Life has been 22.5 years or 150% of the average Design Life.

Gain-to-noise-Temperature (G/T): A figure of merit of antenna performance, where G is the antenna gain in decibels (at the receive frequencies) and T is the equivalent noise temperature of the receiving system (in degrees kelvin). G/T tells us how sensitive the transponder is to being able to receive and amplify the desired uplink signal to the maximum power available from the satellite. 

Geostationary Earth Orbit (GEO): A special case of GSO which is a circular orbit in Earth’s equatorial plane, and usually with zero (or close to zero) inclination and eccentricity. A satellite in GEO remains “fixed” in the same position in the sky to observers on the surface.  A GEO orbit means a ground antenna can be pointed to a “spot” and it does not need to “track” or move to follow the satellite’s orbit.

Geo-Synchronous Orbit (GSO):  An Earth-centered orbit 35,786km (22,236 mi) above Earth’s equator (and following the direction of Earth’s rotation) which provides an orbital period that matches Earth’s rotation.

Geostationary Transfer Orbit (GTO) A type of geocentric orbit used for satellites that are destined for geosynchronous (GSO) or geostationary orbit (GEO) immediately following the satellite launch. GTO is an intermediate step for satellites reaching their final orbit.

High-throughput satellite (HTS): A communications satellite that provides more data throughput (in bits) than a classic satellite (typically by a factor of ~20 or more) for the same amount of allocated orbital spectrum. This is achieved by significant frequency re-use and spot beam technology which enables frequency re-use across multiple narrowly focused spot beams.  HTS satellites designs significantly reduces cost-per-bit thus are the most efficient for point-to-point and internet data transmission, but are not useful for broadcast uses like Direct-To-Home (DTH) TV.

Isolation: The difference in signal level between the amplitude of the desired signal in a transponder and the amplitude of leaked power to or from an undesired signal location (possibly in another transponder or from a nearby satellite) – and is consider for both the transmit and receiving signal strengths.

Launch Vehicle: A rocket-propelled vehicle used to carry a payload, usually a satellite, from Earth’s surface to space, usually to Earth orbit. 

Link Budget: An accounting of all of the power gains and losses that a signal experiences from a ground transmitter, to the satellite and back to a ground as a design aid, to determine the overall system characteristics that ensure that the information is received intelligibly with an adequate signal-to-noise ratio. Randomly varying gains such as rain fading and antenna mis-pointing are taken into account by adding some margin depending on the anticipated severity of its effects. 

Orbit: The gravitationally curved trajectory of an object such as the trajectory of a planet around a star or a satellite around the Earth. 

Phased Array Antenna: A computer-controlled array (spaced geometrical) group of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. This enables the creation of beam(s) in any shape and position that can be dynamically changed, including the power levels to any portion of that shape, also known as “digital beam-forming”.

Processed Payload: A communications payload using a Digital Payload Processor (DPP) to provide a changeable mission definition that enables variable spectrum allocation, variable channel connectivity, flexible signal distribution to different beams, and reconfigurable missions through variable uplink and downlink coverage areas (beams).

Redundancy A methodology of including spare equipment and/or functionality to overcome lower levels failures to meet the requirements of the satellite performance specification.

Reliability: A measure of the expectation of the satellite to meet all requirements of the satellite performance specification as a mathematical estimate of the probability of full mission success. It is NOT a prediction of total failure of the satellite.

Satellite:  The term is usually reserved for man-made objects that have an active purpose in orbit around the Earth.

Satellite Retirement: The process of decommissioning a satellite at the end of its useful life – not just its possible fuel life – including (for GEO satellites) 1) De-Orbit (changing the orbit to a safer level to avoid any collisions or interference), 2) Passivation (eliminating stored energy to preclude later uncontrolled release or explosions) and 3) Shut-Down (final electrical and control turn-off of the satellite).  This may follow operations in an Inclined Orbit and/or maybe using a Life Extension Service.

Space Environment: The regions a satellite moves through and/or operates at, characterized by the properties of the local vacuum pressure, neutral particles, magnetic fields, plasma, total radiation, micrometeoroids, and orbital debris. Satellite designs need to account for each of these properties and multiple techniques will be implemented to analyze, prevent, lessen, and mitigate harmful effects of these environments.

Space Situational Awareness (SSA): The knowledge and characterization of space objects and their operational environment and is a critical component of Space Domain Awareness (SDA) – the ability to understand current and predicted operational environments. SSA covers the areas of Orbits Awareness, Conjunction Prediction, Collision Avoidance, Safe Retirement, Debris Limitation and Removal, RF interference, and operations in the Natural Space Environment. The GEO and LEO orbits face different challenges in SSA.

Transponder: The series of interconnected equipment that forms a communications “channel” of a fixed frequency bandwidth between the satellite receiving and transmitting antenna(s). A satellite will have multiple transponders, often with different uplink and/or down areas (beams) and frequencies and different levels of amplification, filtering, and frequency shifts.