Page 86 - Satellite Communications, Fourth Edition

66 Chapter Two

Example 2.21 Determine the subsatellite height, latitude, and LST for the satel-
lite in Example 2.16.
Solution From Example 2.16, the known components of the radius vector r in
the IJK frame can be substituted in the left-hand side of Eqs.(2.52) through
(2.54). The known values of a E and e E can be substituted in the right-hand side
to give

4685.3 a 6378.1414 hb cos l SS cos LST
2 2
21 0.08182 sin l SS
5047.7 a 6378.1414 hb cos l SS sin LST
2 2
21 0.08182 sin l SS
2
6378.1414 (1 0.08182 )
3289.1 a hb cos l SS cos LST
2 2
21 0.08182 sin l SS
Each equation contains the unknowns LST, l , and h. Unfortunately,
ss
these unknowns cannot be separated out in the form of explicit equa-
tions. The following values were obtained by a computer solution.

l ss > 25.654°

h > 1258.012 km
LST 132.868°
>

2.9.10 Predicting satellite position
The basic factors affecting satellite position are outlined in the previ-
ous sections. The NASA two-line elements are generated by prediction
models contained in Spacetrack report no. 3 (ADC USAF, 1980), which
also contains Fortran IV programs for the models. Readers desiring
highly accurate prediction methods are referred to this report.
Spacetrack report No. 4 (ADC USAF, 1983) gives details of the models
used for atmospheric density analysis.

2.10 Local Mean Solar Time and
Sun-Synchronous Orbits
The celestial sphere is an imaginary sphere of infinite radius, where the
points on the surface of the sphere represent stars or other celestial
objects. The points represent directions, and distance has no signifi-
cance for the sphere. The orientation and center of the sphere can be
selected to suit the conditions being studied, and in Fig. 2.14 the