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Satellite Meteorology
Inclination Angle: the angle between the equatorial
plane and the satellite orbital plane.
The inclination angle is zero if the satellite orbital
plane coincides with the equatorial plane and if the
satellite rotates in the same direction as the earth.
If the two planes coincide but the satellite rotates in
an opposite direction to the Earth, then, the
inclination angle is 180 degree.
Satellite Meteorology
LEO: low Earth orbit
Polar orbit satellite
http://www.oso.noaa.gov/poes/index.htm
Altitudes are typically at 850 km.
Orbital periods are about 98-102 minutes.
Hence, each satellite will complete about 14 orbits in one day.
ADVANTAGES OF POLAR
ORBITS
•Since the orbit is lower than for the
Geostationary satellites, the data
resolution is higher.
•They provide global coverage,
necessary for NWP models and
climatic studies.
DISADVANTAGES OF POLAR
ORBITS
•Can not provide continuous
viewing of one location
The scan swath is about 3000 km wide.
Note that the orbit is directed to the northwest. The satellites do
not pass directly over the North pole or South Pole creating a
precession in the orbit so that is passes over locations further west
on subsequent orbits.
The scan swath is about 3000 km wide.
How many orbits are needed to make a global coverage?
Factors affecting data resolution
•Subpoint: the location on the earth that is directly below the satellite.
•Satellite footprint: the area being scanned by the satellite, similar to the
area being photographed with a camera.
•Nadir angle: the angle between the footprint and the subpoint.
Accordingly, the resolution gets worse with increasing nadir angle.
•Resolution is a function of: (a) Curvature of the earth. (b) to a lesser
extent, the increasing distance of the footprint from the satellite.
•Data at angles greater than 60° are not very useful
PARALLAX
Parallax refers to the displacement of cloud locations due to increasing
viewing angle.
Parallax-induced cloud
displacement gets real bad at
angles greater than 60° for
the GOES imagery.
Parallax also applies to
terrain features such as
mountains.
A sun-synchronous orbit is the orbit that is synchronized with the sun.
The satellite passes the equator and each latitude at the same time
each day. Because of this, the satellite receives consistent lighting.
How to keep an orbit sun-synchronized?
Geostationary Satellites
http://www.oso.noaa.gov/goes/index.htm
•Geostationary satellites orbit in the
earth's equatorial plane at a height of
35,800 km. Note that the typical
space shuttle orbit is only 225-250
km.
•At this height, the satellite's orbital
period matches the rotation of the
Earth, so the satellite seems to stay
stationary over the same point on the
equator. It always view the same
geographical area, day or night.
•This is ideal for making regular
sequential observations of cloud
patterns over a region with visible
and infrared radiometers
•High temporal resolution and
constant viewing angles.
WHY 35,800 km??
Gravitational force
Centrifugal force
Period: 24 h
ADVANTAGES OF GEOSTATIONARY ORBITS
•Make repeated observations over a given area
(constant view area)
•Get high temporal resolution data. GOES E and W
can give you a temporal resolution of 1 minute!! Hence,
GOES E and W can effectively monitor the severe
weather environment and track severe storms and
hurricanes in real time.
DISADVANTAGES OF GEOSTATIONARY
ORBITS
Due to the high orbit, the spatial resolution of the
data is not as great as for the polar orbiting
satellites
Poor spatial resolution in the polar regions
(parallax).