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Satellite Orbits

Satellite Orbits. Satellite Meteorology/Climatology Professor Menglin Jin. 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?

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Satellite Orbits

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  1. Satellite Orbits Satellite Meteorology/Climatology Professor Menglin Jin

  2. 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?

  3. 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

  4. 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

  5. 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

  6. Geosynchronous Meteorological SatellitesWMO Member States

  7. Low Earth Orbit Concepts Descending node Ascending node Perigee Ground track Orbit Inclination angle Equator Orbit South Pole Apogee

  8. 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 • ~10:30 AM in this example

  9. Sun-Synchronous Orbit of Terra

  10. Spacing Between Adjacent Landsat 5 or 7 Orbit Tracks at the Equator

  11. Timing of Adjacent Landsat 5 or 7 Coverage Tracks Adjacent swaths are imaged 7 days apart

  12. Polar-Orbiting Satellite in Low Earth Orbit (LEO) Example from Aqua

  13. 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

  14. TRMM Coverage 1 day coverage 2 day coverage

  15. 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 planet’s surface (km) gs = acceleration due to gravity (0.00981 km s-2 for Earth)

  16. Orbital Characteristics of Selected MissionsLow Earth Orbit & Precessing Missions

  17. 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

  18. Kepler’s laws 1. Satellites follow an elliptical orbit with the Earth as one focus Foci Perigee Apogee

  19. Period of orbit Period of orbit 42 T2= r3 • Valid only for circular orbits (but a good approximation for most satellites) • Radius is measured from the center of the Earth (satellite altitude+Earth’s radius) • Accurate periods of elliptical orbits can be determined with Kepler’s Equation Gme Radius of the orbit Gravitational constant Mass of the Earth

  20. Sunsynchronous image (SMMR)

  21. Geostationary Image (GOES-8)

  22. Space-time sampling • Geostationary • Fixed (relatively) field of view • View area of about 42% of Earth’s 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

  23. 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

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