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Orbital Principles 39
1 st pt.
(vernal
Figure 2-6. Orbital elements. Six independent quantities are required to
completely describe an orbit.
Finally, a parameter must be used to describe the actual position of the
orbiting body at any particular time or location within the orbit. Either an
angular measurement, such as the true anomaly v, or a time parameter,
such as time since perigee passage T,, may be specified.
ORBITAL PROPERTIES
This section discusses some of the properties of a satellite due to its
unique orbital position above the earth.
Field of View
Perhaps the most obvious benefit of being in space is the view one has
when looking back at the earth. The amount of the earth in view at any
one time is a function of the height above the surface as illustrated in Fig-
ure 2-7. In this figure, Re represents the mean radius of the earth (6,378
km or 3,444 nm), and h is the height of a satellite above this mean radius.
The point directly beneath the satellite on the surface of the earth is known
as the nadi,:
The angle Cp is known as the earth angle and represents the maximum
angular portion of the earth, measured to the tangential horizon, visible
from the satellite height. The equation for determining Cp for any given
altitude is:
