Glossary

This glossary contains the definition of several technical terms used in the Celestial Mechanics section, in the orbital control context.

  • APOGEE

    The point in the orbit of the Earth or of an artificial satellite most distant from the center of the Earth. The opposite of perigee

  • ATTITUDE

    Space orientation of an artificial satellite.

  • BOX

    Maximum angular interval for control of the orbital positioning latitude and longitude, or of the space orientation of an artificial satellite.

  • MAXIMUM CIRCLE

    Intersection of a plane and a sphere where the plane passes through the center of the sphere.

  • ORBITAL CONTROL

    The process of placing the satellite in the required orbit and maintaining it there within the orbit and attitude boxes, by performing correction manoeuvers.

  • CELESTIAL SPHERE

    Fictitious sphere of arbitrary center and radius which contains all the heavenly bodies.

  • ORBIT ECCENTRICITY

    Orbital element that defines its shape. The bigger the eccentricity, the more elliptical the orbit. (see also geostationary orbit and inclined orbit).

  • EQUATOR

    Maximum circle of the terrestrial sphere perpendicular to the line joining the poles.

  • GMT (Greenwich Mean Time)

    Also known as Universal Time, corresponds to mean solar time referred to the meridian that by convention is the Greenwich meridian. To convert GMT time to Brasília time one must subtract 3 hours (or 2 hours during daylight saving time). For example, 04:00 pm GMT = 01:00 pm Brasília time.

  • ORBIT INCLINATION

    The angle between the satellite orbital plane and the equatorial plane (see also geostationary orbit and inclined orbit).

  • SOLAR INTERFERENCE ON EARTH STATIONS

    The satellite moves in orbit around the Earth which, in turn, moves around the Sun. The earth stations point their antennas at the satellite, which seems to be motionless vis-à-vis the Earth. Twice a year (around March and September) the Sun goes behind the satellite in its apparent motion in the sky, causing interference on the earth station antennas. The predictions are in GMT time.

  • LATITUDE and LONGITUDE

    Coordinates of the equatorial system whose references are the equator and the Greenwich meridian. Latitude is the angle, measured along the local meridian, from the equator and the satellite or station. Longitude is the angle, measured along the equator, from the Greenwich meridian and the local meridian.

  • CORRECTION MANOEUVER

    Consists of a sequence of firings of one or more thrusters of an artificial satellite so as to modify its orbit and/or altitude, to keep it within its orbital control boxes.

  • MERIDIAN

    The maximum circle running from one pole to the other.

  • GREENWICH MERIDIAN

    The meridian taken as the origin of Universal Time passing through the former Royal Greenwich Meridian Observatory in England.

  • LOCAL MERIDIAN

    The meridian passing through the poles, which contains the satellite or the station.

  • ORBIT

    The path of a heavenly body or satellite around another heavenly body.

  • GEOSTATIONARY ORBIT

    The orbit of a satellite around the Earth with inclination and eccentricity zero with a period equal to the Earth’s rotational period.

  • INCLINED ORBIT

    The orbit of a satellite around the Earth with eccentricity zero whose inclination is not maintained close to zero. The control of this orbit does not take into account the latitude box.

  • PERIGEE

    The point in the orbit of the Earth or of an artificial satellite least distant from the center of the Earth. The opposite of apogee.

  • MEAN SUN

    A virtual body that moves along the celestial equator at constant speed, matching the true Sun’s complete circuit (one year).

  • ARTIFICIAL SATELLITE

    Is a manufactured object that continually orbits Earth due to the Earth’s gravitational power, and can be used for communication, research, observation, etc.

  • GEOSTATIONARY SATELLITE

    An artificial satellite in orbit around the Earth with zero inclination (orbit on the equator) and eccentricity (circular orbit) and orbital period of one day, staying in that same spot above the Earth.

  • GEOSYNCHRONOUS SATELLITE

    An artificial satellite whose orbital track on the Earth is equal to one day, regardless of the inclination and eccentricity.

  • ATOMIC TIME

    Time whose unit is the atomic second with 9,192,631,770 radiation cycles of the radiation that corresponds to transitions between hyperfine states of the ground state of Cesium-133.

  • MEAN SOLAR TIME

    Time based on the diurnal apparent rotation of the mean Sun around the Earth.