Orbital Mechanics - Special Orbit Types

Special Orbit Types

Certain orbits are better suited for specific missions based on their Keplerian elements.

LEO (Low Earth Orbit)

  • No specified minimum altitude.
  • Relatively close to Earth (several hundred km).
  • Short orbital periods (approximately 90 minutes).
  • Many revolutions per day.
  • Limited swath areas (satellite view area on Earth's surface).
  • All manned space missions (except lunar missions) were LEO.
  • Many Earth-observing satellites utilize LEO for weather and imagery.
  • Example: International Space Station height of orbit ~350 km

GEO (Geostationary Orbit)

  • Remains over one location on Earth.
  • Period exactly equals one day.
  • Altitude: ~22,200 miles (36,000 km).
  • Inclination is exactly zero degrees.
  • A GEO satellite ONLY exists directly above the equator (sub-satellite longitude).
  • Can see approximately 70 degrees north and south of the equator.
  • Mainly used for communications or as a "permanent relay station" in space.
  • There is only one altitude with a period of 24 hours.
  • All geostationary orbits are in a "ring" around the Earth, called the geostationary belt.
  • The geostationary belt is a limited resource; when a satellite's life ends, it must be removed from its slot, usually by boosting it to a slightly higher orbit.
  • Difficult to maintain an exactly 24-hour period and zero inclination.
  • Orbits typically have a slight inclination, causing satellites to drift slightly north and south of the equator.
  • Slight east or west drift occurs due to imperfect period.
  • Small orbit-adjustment burns are performed (called station-keeps).
  • Satellites with a 24-hour period and non-zero inclination are called geosynchronous; geostationary and geosynchronous are often interchanged.
  • Urban Navigation Example: TV satellite dishes point south in the northern hemisphere because geostationary satellites "hover" above the equator.

Molniya Orbit

  • Used to overcome communication problems for Russia, given its large landmass near or north of 70 degrees latitude.
  • Highly inclined and highly elliptical orbit.
  • High inclination covers northern Russia.
  • High eccentricity -- Large apogee altitude -- Very slow velocity at apogee.
  • If apogee is over Russia, then satellite hangs over Russia (Kepler’s 2nd Law).

Polar Orbit

  • Inclination of a polar orbit is 90 degrees.
  • A satellite in polar orbit will eventually pass over every part of the world.
  • Well-suited for satellites gathering information about the Earth, such as weather satellites.
  • A special type of polar orbit called a Sun-synchronous orbit passes over the same part of the Earth at roughly the same local time every day.

Constellations

  • Groups of satellites in various orbits work together to accomplish a mission.
  • Example: GPS (Global Positioning System).