GLOSSARY OF TERMS


 

This glossary assists you in understanding the basic terms that are frequently used when referring to satellite orbits.
ALTITUDE

The shortest distance of the satellite from the Earth's surface at a given time. This can also be called the Height.

This can also refer to an observer's distance above sea level (as in aircraft altitude).

It is symbolized by the letter "A".
 
APOGEE

CLICK HERE
ARGUMENT OF LATITUDE

The angle formed from the satellite orbit's ascending node to the satellite orbit's perigee point in the direction of the satellite's travel. It is basically the addition of the satellite orbit's Argument of Perigee (w) and the satellite's True Anomaly (n).

It is symbolized by the Greek letter "m".

m = w + n
 
ARGUMENT OF PERIGEE CLICK HERE
ASCENDING NODE CLICK HERE
CARTESIAN COORDINATES

A coordinate system based solely on linear components (no angles). A three-dimensional Cartesian coordinate system has three components; normally symbolized by the letters x, y and z.
DESCENDING NODE CLICK HERE
ECCENTRIC ANOMALY CLICK HERE
ECCENTRICITY CLICK HERE
ELLIPSE The oval shape that is assumed by all satellite orbits.
EPOCH CLICK HERE
EQUATORIAL COORDINATES

A coordinate system with one axis parallel to the equatorial plane of the Earth. Normally, its components are Right Ascension (R.A. or a) and Declination (Dec. or d).
 
EQUATORIAL PLANE

The plane formed by the Earth's equator projected outward into infinity.
FOCI

The two points on the inside of an orbit's ellipse such that all rays emanating from one of the foci will reflect off the inner walls and pass through the other focus.
FOCUS DISTANCE CLICK HERE
GEOCENTRIC

With respect to the true (gravitational) center of the Earth.
GEODETIC

With respect to the point created by the intersection of the Nadir (anti-zenith) line and the Earth's equatorial plane. This is mainly used to correct for the spheroid (non-spherical) Earth.
HEIGHT

The shortest distance of the satellite from the Earth's surface at a given time. This can also be called the Altitude.

It is symbolized by the letter "h".
 
KEPLER'S EQUATION CLICK HERE
INCLINATION CLICK HERE
MEAN ANOMALY CLICK HERE
MEAN MOTION CLICK HERE
ORBIT

The path that a satellite assumes about the Earth.
ORBIT PLANE The plane that is formed by the satellite's orbit motion as it sweeps out one complete orbit.
ORBIT PROPAGATION Using a satellite's orbit elements to predict where the same satellite will be within its orbit at a future (or past) time.
PERIGEE CLICK HERE
PERIOD CLICK HERE
POLAR COORDINATES A coordinate system based on one linear term and two angular terms. For instance, a satellite's location might be indicated by using its equatorial angular coordinates (a, d) and its linear range (r).
 
PRECESSION The apparent periodic motion of a satellite orbit's ascending node (and other elements) due to gravitational and other external forces.

A satellite's orbit will precess according to its orbit inclination and orbit period.
 
PROGRADE The perceived counter-clockwise motion of a satellite when its motion in its orbit is viewed from the north of the Earth's equatorial plane. A prograde orbit is quantitatively expressed by an orbit inclination of less than 90 degrees.
RADIANS A method of measuring angles based on the units of pi (p). p/2 radians is equivalent to 90o, p radians is equivalent to 180o, 2p radians is equivalent to 360o, etc.

To convert from degrees to radians, multiply by p/180o.
 
RANGE The direct distance of the satellite from a point on the Earth.

It is symbolized by the letter "r".
RETROGRADE

The perceived clockwise motion of a satellite when its motion in its orbit is viewed from the north of the Earth's equatorial plane. A retrograde orbit is quantitatively expressed by an orbit inclination of greater than 90 degrees.
RIGHT ASCENSION OF THE ASCENDING NODE CLICK HERE
SEMI-MAJOR AXIS CLICK HERE
SEMI-MINOR AXIS

CLICK HERE
TRUE ANOMALY CLICK HERE


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Glossary of Terms Was Last Modified On May 23, 2010