Page 36 - Satellite Communications, Fourth Edition

16 Chapter One

Integrated Program Office (IPO) and the Web page can be found at
http://www.ipo.noaa.gov/. As of 2005, a four-orbit system is in place,
consisting of two U.S. Military orbits, one U.S. Civilian orbit and one
EUMETSAT/METOP orbit. Here, METSAT stands for meteorological
satellite and EUMETSAT stands for the European organization for the
exploration of the METSAT program. METOP stands for meteorologi-
cal operations. These orbits are sun synchronous, meaning that they
cross the equator at the same local time each day. For example, the
satellites in the NPOESS (civilian) orbit will cross the equator, going
from south to north, at times 1:30 P.M., 5:30 P.M., and 9:30 P.M. Sun-
synchronous orbits are described in more detail in Chap. 2, but briefly,
the orbit is arranged to rotate eastward at a rate of 0.9856°/day, to
make it sun synchronous. In a sun-synchronous orbit the satellite
crosses the same spot on the earth at the same local time each day, so
that the same area of the earth can be viewed under approximately the
same lighting conditions each day. A sun-synchronous orbit is inclined
slightly to the west of the north pole. By definition, an orbital pass
from south to north is referred to as an ascending pass, and from north
to south as a descending pass.
The polar orbits are almost circular, and as previously mentioned
they are at a height of between 800 and 900 km above earth. The polar
orbiters are able to track weather conditions over the entire earth, and
provide a wide range of data, including visible and infrared radiometer
data for imaging purposes, radiation measurements, and temperature
profiles. They carry ultraviolet sensors that measure ozone levels, and
they can monitor the ozone hole over Antarctica. The polar orbiters
carry a NOAA letter designation before launch, which is changed to a
numeric designation once the satellite achieves orbit. For example,
NOAA M, launched on June 24, 2002, became NOAA 17 when success-
fully placed in orbit. The series referred to as the KLM satellites carry
much improved instrumentation. Some details are shown in Table 1.7.
Most of the polar orbiting satellites used in weather and environ-
mental studies, and as used for monitoring and in search and rescue,
have a “footprint” about 6000 km in diameter. This is the size of the
antenna spot beam on the surface of the earth. As the satellite orbits the
earth, the spot beam sweeps out a swath on the earth’s surface about
6000 km wide passing over north and south poles. The orbital period of
these satellites is about 102 min. Since a day has 1440 min, the number
of orbits per day is 1440/102 or approximately 14. In the 102 min the
earth rotates eastward 360°× 102/1440 or about 25°. Neglecting for the
moment the small eastward rotation of the orbit required for sun syn-
chronicity, the earth will rotate under the subsatellite path by this
amount, as illustrated in Fig. 1.7.