AGI's Introduction to Orbits Notes

What Is an Orbit?

  • Johannes Kepler discovered in the 1600s that planets orbit in ellipses, not circles.
  • Satellites (natural or human-made) also orbit Earth in an elliptical pattern.
  • Elliptical orbits remain fixed in space, and Earth spins under a fixed satellite orbit.
  • An orbit is a closed path around which a planet or satellite travels.

All Sorts of Orbits!

  • Low Earth Orbit (LEO)
  • Polar Orbit
  • Medium Earth Orbit (MEO)
  • Highly Elliptical Orbit (HEO)
  • Geostationary (GEO)
  • Molniya Orbit (Moly)

Low Earth Orbit (LEO)

  • LEOs orbit relatively close to the Earth (e.g., several hundred kilometers, km) with no minimum altitude.
  • LEO orbits are characterized by short orbital periods; roughly 90 minutes.
  • Many revolutions per day and limited swath areas (area that a satellite can see).
  • All staffed space missions except lunar missions have been LEO.
  • Many Earth-observing satellites are in LEO orbits.

Geostationary (GEO)

  • Achieved by placing a satellite at an altitude where the orbital period exactly equals one day.
  • The orbit is about 22,300 miles above Earth, and the inclination is exactly zero degrees.
  • A geostationary satellite stays in one spot with respect to the Earth.

GEO

  • There is only one altitude above Earth with an orbital period of 24 hours.
  • All geostationary orbits are in a ring around Earth, called the geostationary belt.
  • The geostationary belt is a limited resource.

GEO - Urban Navigation

  • To determine which direction is south in the urban United States without a compass or GPS: look for a building or house with a TV satellite dish.
  • Geostationary satellites can only hover above the equator; therefore, all northern hemisphere dishes are communicating with geostationary satellites toward the south.

Molniya (“Moly”)

  • Geostationary satellites for Russian communications pose severe challenges since a majority of its land mass is too far north for geostationary belt satellites to see.
  • The solution was to create a type of orbit called a Molniya orbit; it allows long-term communications over northern Russian land mass.

Molniya

  • Molniya ground trace differs from most conventional ground traces.
  • The image below clearly illustrates the satellite hang time over Russia.

Polar

  • Polar orbit has a 90-degree inclination.
  • A satellite will eventually pass over all of Earth.
  • Polar orbit satellites can gather information about the entire Earth, e.g., weather satellites.

Constellations

  • Single satellites are often insufficient to perform a mission.
  • Groups of satellites in various orbits work together to accomplish the mission.
  • This grouping of satellites is called a constellation, e.g., the GPS system.

Orbital Pattern Animation Satellites

  • TDRS 4
  • TDRS 10
  • DIRECTV
  • DIRECTV BHOSTAR 12
  • SIRIUS-1
  • DIRECTV PROSTAR
  • DIRECTV 75
  • ECHOSTAR 10
  • SIRIUS
  • ECHOSTAR 9
  • Molniya
  • ECHOSTAR 5
  • ECHOSTAR 1
  • TDRS 7
  • TDRS 5
  • BELSTAR/INSTELSAT 709

Now That You Know the Basics

  • If Norway wanted to obtain satellite imagery for all of its major urban areas, what type of orbit would be appropriate?
  • Could researchers at McMurdo Station in Antarctica use geostationary satellites for communications? Use your new understanding of orbits to answer these questions.

Answer These Questions

  • Planets orbit in circles. T / F
  • Name three types of orbits.
  • A geostationary satellite can orbit around the moon. T / F
  • How long is the orbit period of a geostationary satellite?
  • Molniya orbits are commonly used to view Russia. T / F
  • A satellite in an equatorial orbit (inclination = 0) will eventually pass over the entire Earth. T / F

Answer Key

  • Planets orbit in circles. F
  • Name three types of orbits: Low Earth Orbit (LEO); Polar Orbit; Medium Earth Orbit (MEO); Highly Elliptical Orbit (HEO); Geostationary (GEO); and Molniya Orbit (Moly)
  • A geostationary satellite can orbit around the moon. F
  • How long is the orbit period of a geostationary satellite: 24 hours
  • Molniya orbits are commonly used to view Russia. T
  • A satellite in an equatorial orbit (inclination = 0) will eventually pass over the entire Earth. F