Page 55 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
earth beneath the orbital plane. The distance between the first ascending
(south to north) passage through the equator shown in Figure 2-8 (1) and
the next ascending passage (2) is determined by the period of the orbit
which, as we saw in equation 2-6, is a function of the orbital semi-major
axis only.
Another point to notice in Figure 2-8 is that the ground track is con-
strained between about 45" north and 45" south latitudes. This corresponds
directly to the orbital inclination, and constrains the ground track only; the
area on the surface of the earth visible to the satellite is a function of the
height of the satellite within the orbital plane as mentioned earlier.

Maximum Time in View

The footprint of the satellite's view on the earth moves continuously
with the satellite as it travels around the earth. This means that the satel-
lite can only view a particular spot on the surface (or conversely, a ground
observer could only see an orbiting satellite) for a limited amount of time.
This amount of time depends on the satellite's altitude (which corresponds
to the satellite velocity) and orbital inclination, as well as on the latitude
and distance from nadir of the ground observer during the satellite pass.
For planning purposes, the maximum time that a satellite can be in view,
associated with tangential horizon to tangential horizon passage directly
over the observer's location, can be found from:

(2-18)

Note that use of this equation requires computation of the arc cosine in
degrees. Of course, any oblique pass not directly over the observer's loca-
tion will result in less time that the satellite will be in view.

Number of Revolutions per Day

The number of times a day that a satellite will completely circle the
globe depends on its orbital period, which is a function of the semi-major