Page 59 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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force

* ... ...
.....- ....
. ,*-a .- .. ...
. . . . . . .
'.*

Figure 2-9. Sun-synchronous orbit. The oblateness of the earth causes a
rotation of the orbital plane.

one degree per day. This would mean that an orbit established perpendic-
ular to the direction of the sun could be made to maintain this orientation
relative to the sun throughout the year. (Refer back to Figure 2-5 which
explained the fixed nature of orbital planes.) If a satellite were placed in
such an orbit at an altitude producing a period that was an integral multi-
ple of the sidereal day, then that satellite could be made to pass over the
same spot on the earth at the same solar time(s) each day, every day of the
year. Many remote sensing satellites are placed into such an orbit so as to
be able to control the conditions under which the information about an
area of interest is gathered throughout the year.

Constellations

As stated earlier, using combinations of the orbital elements and para-
meters, an orbit can be designed to give a desired coverage of the earth.
But there are purposes which require that the entire earth be covered at all
times, which is clearly impossible for a single satellite. In these cases, a
constellation of satellites is necessary. A constellation is simply a series of
two or more satellites purposely placed into mutually supporting orbits.
The satellites may be placed within the same orbital plane to effectively
decrease the functional orbital period, or satellites may be placed into dif-
ferent orbital planes to compensate for the movement of the observation
area due to the rotation of the earth. Figure 2- 10 illustrates a constellation
employing both these options (three planes of three satellites each).