Page 57 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
44
A = 71,089,195.3 km2
!2 = 73.85' = 1.29 rad
tview = 8.59 min (not a lot of time if you have much to com-
municate between the satellite and a ground station!)
# Revlday = 16
T = 5,384.7 sec = 89.75 min
T,/T = 16

USEFUL ORBITS

There are many orbits which have come into common use. This section
presents four of the most common.

Low-Earth Orbii

There is no specified cut-off altitude, but as the name implies, low-
earth orbits (sometimes designated LEO) represent orbits which come rel-
atively close to the surface of the planet. All the manned spacecraft, with
the exception of the Apollo missions to the moon, have been in low-earth
orbits, as are many of the earth-observing satellite systems. As the previ-
ous sections have implied, low-earth orbits are characterized by short
orbital periods, many revolutions per day, high orbital velocities, and lim-
ited swath areas on the earth's surface.

Geostationary Orbit

In the last section, an orbit was defined which had an orbital period
exactly equal to one sidereal day. Any orbit with this period can be called
geosynchronous. A geostationary orbit is defined as a circulal; geosyn-
chronous orbit with an inclination equal to zero (equatorial). A satellite
placed in such an orbit would appear, to an observer on the surface of the
earth, to remain stationary in the sky. The higher altitude in Figure 2-7
shows the approximate position of a geostationary satellite and its associ-
ated field of view. Geometrically, a satellite at geostationary altitude can
see a footprint about 70 degrees north and south of the equator.
The benefits of a satellite so positioned are obvious. A single sensor
placed at a longitude equal to the center of the United States could moni-
tor the weather pattern for the entire country continuously. A communi-
cations relay station placed at a longitude in the middle of the Atlantic or