An element set is a collection of parameters that are used, along with an orbit
propagator, to predict the motion of a satellite. The element set, or elset for short,
consists of identification data, the classical elements and drag parameters. The
figure below shows a typical two line elset.
The individual parameters are described in more detail below:
Satellite Identification
Satellite Number
Satellite number assigned by US Space Command. This number is assigned sequentially,
starting with 1 for the Soviet Sputnik 1 satellite, to each newly
identified satellite.
Intl Desig
The International Designator (or COSPAR number) is the internationally agreed upon
unique identifier for every orbiting object. The designator consists of three parts. The
first two digits are the launch year. The next three digits are the sequential launch
number for the year, starting with 001. The next three alphabetic characters give the
piece identifier starting with "A" for the payload and continuing for all know
objects associated with the launch.
Element Number
Element set number assigned by NORAD. This is nominally a sequential number that is
incremented each time a new elset is generated for the satellite. This number is
sometimes, arbitrarily, reset to zero by NORAD.
Rev. No.
Orbital rev (revolution) number assigned by NORAD. Again, a sequential number that is
incremented each time a satellite crosses the ascending node. This is generally a more
reliable number that Elset #.
Ephemeris Type
The theory type associated with this elset. Can be SGP, SGP4 or SDP4, depending your
preferences and the satellite's orbital period. This parameter is always 0.
Classical Elements
The classical elements describe the geometry of a satellite's orbit along with the
position of the satellite in its orbit at a specific time (the epoch). The mean motion and
eccentricity, together, describe the size and shape of the orbit. The inclination
Epoch
The epoch is the time at which the satellite is in the position specified by the
remaining elements. The epoch is usually set to the time of ascending nodal crossing
closest to the latest observation.
Mean Motion
This is given in the number of orbits (or revs) that the satellite makes in a day. This
number is directly related to the period and semi-major axis, both of which give the size
of the orbit. The smaller this number is, the greater the period.
Eccentricity
This is a unit-less number that defines the shape of an orbit. A circular orbit has zero
eccentricity. The greater the eccentricity, the more elliptical the orbit.
Inclination
The angle between the orbit plane and the ECI fundamental plane (i.e., the earth's
equatorial plane).
Right Ascension of the Ascending Node
This is the angle between the ECI x-axis and the point in the fundamental plane where
the satellite penetrates the plane going from south to north. Often abbreviated as RAAN.
Argument of Perigee
The argument of perigee is the angle, in the orbit plane, between the ascending node and
the perigee vector. The perigee vector points toward the perigee point in the orbit.
Mean Anomaly
An angle that determines the position of the satellite in its orbit relative to the
perigee vector.
Drag Terms
The drag terms are used by the orbit propagator to account for the gradual decrease in
orbital altitude due to atmospheric drag. This effect eventually leads to the decay or
re-entry of a satellite. The first two terms are used by the SGP-compatible orbit
propagator used in SatSpy. The third term should be used with an SGP4-compatible orbit
propagator.
NDot/2
First derivative of mean motion divided by 2. This gives the rate at which the mean
motion is changing.
