Page 114 - Satellite Communications, Fourth Edition

94 Chapter Three

3.7 Sun Transit Outage
Another event which must be allowed for during the equinoxes is the
transit of the satellite between earth and sun (see Fig. 3.8), such that
the sun comes within the beamwidth of the earth-station antenna. When
this happens, the sun appears as an extremely noisy source which com-
pletely blanks out the signal from the satellite. This effect is termed sun
transit outage, and it lasts for short periods—each day for about 6 days
around the equinoxes. The occurrence and duration of the sun transit
outage depends on the latitude of the earth station, a maximum outage
time of 10 min being typical.

3.8 Launching Orbits
Satellites may be directly injected into low-altitude orbits, up to about
200 km altitude, from a launch vehicle. Launch vehicles may be classi-
fied as expendable or reusable. Typical of the expendable launchers are
the U.S. Atlas-Centaur and Delta rockets and the European Space
Agency Ariane rocket. Japan, China, and Russia all have their own
expendable launch vehicles, and one may expect to see competition for
commercial launches among the countries which have these facilities.
Until the tragic mishap with the Space Shuttle in 1986, this was to
be the primary transportation system for the United States. As a
reusable launch vehicle, the shuttle, also referred to as the Space
Transportation System (STS), was planned to eventually replace expend-
able launch vehicles for the United States (Mahon and Wild, 1984).
Where an orbital altitude greater than about 200 km is required, it
is not economical in terms of launch vehicle power to perform direct
injection, and the satellite must be placed into transfer orbit between
the initial LEO and the final high-altitude orbit. In most cases, the
transfer orbit is selected to minimize the energy required for transfer,
and such an orbit is known as a Hohmann transfer orbit. The time
required for transfer is longer for this orbit than all other possible trans-
fer obits.
Assume for the moment that all orbits are in the same plane and
that transfer is required between two circular orbits, as illustrated in
Fig. 3.10. The Hohmann elliptical orbit is seen to be tangent to the low-
altitude orbit at perigee and to the high-altitude orbit at apogee. At the
perigee, in the case of rocket launch, the rocket injects the satellite with
the required thrust into the transfer orbit. With the STS, the satellite
must carry a perigee kick motor which imparts the required thrust at
perigee. Details of the expendable vehicle launch are shown in Fig. 3.11,
and of the STS launch in Fig. 3.12. At apogee, the apogee kick motor
(AKM) changes the velocity of the satellite to place it into a circular orbit