4 Rec. ITU-R S.673-2
Orbit
The path, relative to a specified frame of reference, described by the centre of mass of a satellite or
other body in space, subjected solely to forces of natural origin, mainly the force of gravity; by
extension, the path described by the centre of mass of a body in space subjected to forces of natural
origin and occasional low-energy corrective forces exerted by a propulsive device in order to
achieve and maintain a desired path.
Orbital elements (of a satellite or other body in space)
The parameters by which the shape, dimensions and position of the orbit in space and the period of
the body can be defined in relation to a specified frame of reference.
NOTE 1 – In order to determine the position of a body in space, at any instant it is necessary to know, in
addition to its orbital elements, the position of its centre of gravity in its orbit at one given instant.
NOTE 2 – The frame of reference used is a direct rectangular coordinate system OXYZ, in which the origin
is at the centre of mass of the primary body and the third axis OZ is perpendicular to the principal reference
plane, also called the basic reference plane, or simply the reference plane.
NOTE 3 – For an artificial earth satellite, the reference plane is the Earth’s equatorial plane and the third
axis OZ has a South to North orientation.
Orbital plane (of a satellite)
The plane containing the centre of mass of the primary body and the velocity vector of a satellite,
the frame of reference being that specified for defining the orbital elements.
Periapsis
The point in the orbit of a satellite or planet which is situated at the minimum distance from the
centre of mass of the primary body.
Perigee
The point in the orbit of an earth satellite which is situated at the minimum distance from the centre
of the Earth.
NOTE 1 – The perigee is the periapsis of an earth satellite.
Period of revolution (of a satellite)
Orbital period (of a satellite)
The time elapsing between two consecutive passages of a satellite through a characteristic point on
its orbit.
NOTE 1 – If the characteristic point on the orbit is not specified, the period of the revolution considered is,
by convention, the anomalistic period.
Point and shoot beam
A satellite transmit and/or receive beam that is formed and pointed as required by traffic levels. The
beam will hop around in time and/or space division multiplex schemes.
Polar orbit (of a satellite)
A satellite orbit, the plane of which contains the polar axis of the primary body.
Primary body (in relation to a satellite)
The attracting body which primarily determines the motion of a satellite.