Satellite Orbits

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Satellite Orbits

• Satellite Meteorology/Climatology
• Professor Menglin Jin

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Satellite Orbits

• At what location is the satellite looking?
• When is the satellite looking at a given
  location?
• How often is the satellite looking at a given
  location?
• At what angle is the satellite viewing a given
  location?

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Atmospheric Remote Sensing Sensors, Satellite
Platforms, and Orbits

• Satellite orbits and platforms
• Low Earth orbit
• Sunsynchronous and repeat coverage
• Precessing
• Geosynchronous orbit
• Sensor scanning modes
• Whiskbroom and pushbroom scanners
• Active and passive microwave radiometers

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Types of orbits

• Sunsynchronous orbits An orbit in which the
  satellite passes every location at the same time
  each day
• Noon satellites pass over near noon and midnight
• Morning satellites pass over near dawn and dusk
• Often referred to as polar orbiters because of
  the high latitudes they cross
• Usually orbit within several hundred to a few
  thousand km from Earth

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Types of orbits

• Geostationary (geosynchronous) orbits An orbit
  which places the satellite above the same
  location at all times
• Must be orbiting approximately 36,000 km above
  the Earth
• Satellite can only see one hemisphere

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Geosynchronous Meteorological SatellitesWMO
Member States
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Low Earth Orbit Concepts
Descending node
Ascending node
Perigee
Ground track
Orbit
Inclination angle
Equator
South Pole
Orbit
Apogee
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Sun-Synchronous Polar Orbit
Earth Revolution
Equatorial illumination angle
Satellite Orbit

• Satellite orbit precesses (retrograde)
• 360 in one year
• Maintains equatorial illumination angle constant
  throughout the year
• 1030 AM in this example

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Sun-Synchronous Orbit of Terra
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Spacing Between Adjacent Landsat 5 or 7 Orbit
Tracks at the Equator
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Timing of Adjacent Landsat 5 or 7 Coverage Tracks
Adjacent swaths are imaged 7 days apart
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Polar-Orbiting Satellite in Low Earth Orbit (LEO)
Example from Aqua
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Tropical Rainfall Measuring Mission Orbit
(Precessing)

• A precessing low-inclination (35), low-altitude
  (350 km) orbit to achieve high spatial resolution
  and capture the diurnal variation of tropical
  rainfall
• Raised to 402 km in August 2001
  

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TRMM Coverage
1 day coverage
2 day coverage
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Definition of Orbital Period of a Satellite

• The orbital period of a satellite around a planet
  is given by
• where
• T0 orbital period (sec)
• Rp planet radius (6380 km for Earth)
• H orbit altitude above planets surface (km)
• gs acceleration due to gravity (0.00981 km s-2
  for Earth)

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Orbital Characteristics of Selected MissionsLow
Earth Orbit Precessing Missions
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Ellipse

• An ellipse is defined as follows For two given
  points, the foci, an ellipse is the locus of
  points such that the sum of the distance to each
  focus is constant.
• BTW, Locus-A word for a set of points that forms
  a geometric figure or graph

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Keplers laws

• 1. Satellites follow an elliptical orbit with the
  Earth as one focus

Foci
Perigee
Apogee
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Period of orbit
Period of orbit
4?2
T2 r3
Gme
Radius of the orbit
Gravitational constant
Mass of the Earth

• Valid only for circular orbits (but a good
  approximation for most satellites)
• Radius is measured from the center of the Earth
  (satellite altitudeEarths radius)
• Accurate periods of elliptical orbits can be
  determined with Keplers Equation

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Sunsynchronous image (SMMR)
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Geostationary Image (GOES-8)
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Space-time sampling

• Geostationary
• Fixed (relatively) field of view
• View area of about 42 of Earths surface
• Sunsynchronous
• Overlapping views
• See each point at several viewing angles
• Other orbits (walking orbits)
• Passes each location at a different time of day
• Earth Radiation Budget Satellite
• Useful when dirunal information is needed

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Scanning techniques

• Vidicom
• Like television camera sees everything at once
• Swinging
• Results in a zig-zag pattern of scanning
• Spinning
• Satellite spins in order to create image
• Pushbroom
• Multiple scanning elements, relies on forward
  motion of satellite