Glossary - G

Gain
a measure of amplification, generally expressed in dBs
(technology)

Galaxy
a large, gravitationally bound system of stars, gas, dust and dark matter generally containing between 100 million and 100 trillion stars
(astronomy)

Galilean satellites
the four largest natural satellites orbiting Jupiter
(planetary science)

Gamma rays
electromagnetic radiation with an energy greater than 100 keV; also known as gamma radiation
(physics, astrophysics)

Gateway
the point of interconnection between a satellite network and the PSTN/PSDN
(satellite communications)

Geostationary orbit
circular orbit at a height of 36,000 km above the equator with zero or near zero inclination
(orbits)

Geostationary Transfer Orbit (GTO)
an orbit with an apogee at the geostationary altitude of 36,000 kms used during orbit injection following the launch of a satellite
(orbits)

Geosynchronous Orbit (GSO)
a circular orbit at an altitude of 36,000 km but which does not necessarily have a zero inclination
(orbits)

Gimbal
a pivoted support which allows the object mounted on it to be rotated about an axis
(technology)

Global beam
an antenna beam which illuminates the whole Earth visible from a particular satellite location
(spacecraft technology, satellite communications)

Grade of Service
a measure of the probability of a user being unable to use a network because it is busy
(communications)

Gravitational waves
ripples in spacetime generated by the relative motion of massive bodies
(physics, astrophysics)

Gravity assist
the use of a planet’s gravitational field to accelerate of decelerate a spacecraft without entering orbit around the planet
(orbits)

Gravity waves
waves generated in a fluid or at the interface between two fluids when the force of gravity or buoyancy attempts to restore equilibrium; wind waves on a pond or the ocean are an example of gravity waves
(fluid dynamics)

Great circle
an imaginary circle on the surface of a sphere, the centre of which is co-located with that of the sphere
(mathematics)

Gregorian
a reflector antenna geometry employing a primary parabolic dish and a secondary concave hyperbolic reflector
(technology)

Ground segment
the earth stations and associated terrestrial infrastructure in a satellite system
(satellite communications)

Ground Segment as a Service (GSaaS)
the provision of mission planning, scheduling, telemetry downlinking, telecommand uplinking, data downlinking, processing and storage as a service to multiple satellite operators using shared ground segment infrastructure
(mission operations)

Guard band
the unused frequency band between two channels - the separation reduces mutual interference
(communications)

Guard time
the unused time in a TDMA system between burst transmissions - the time gap prevents the possibility that two bursts will overlap or that two consecutive bursts will be interpreted as a single burst by the receiver
(communications)