What Do Orbits Have To Do With All of This? 

All About Orbits

Satellites' orbits take two shapes:
    1.  Circular
    2.  Elliptical (Egg-shaped)

Orbits range in size from 155 miles (250 km) to over 20,000 miles (32, 200 km).  The greater the altitude, the longer the orbital period (the time it takes to complete one orbit).  Satellites travel in set orbits around the Earth.

How Do They Get There?
Getting satellites into orbit is a two-step process:
1.  Launch phase-  Sets the satellite into an elliptical orbit.
    **Satellites are brought to launch phase by two types of vehicles**
       a.  Expendable Rockets-These rockets raise the satellite to an elevator of about 100 miles by lifting the rocket off the pad
            through the rise of a kerosene/liquid oxygen mixture and solid fuel rocket boosters.  Then, the satellite's apogee kick
            motor is fired on command when the satellite is in the correct position and puts it into circular orbit.
       b.  Space Shuttle-  performs the functions of the first two stages of an expendable launch vehicle.  The satellite, together
            with the third stage, are mounted in the cargo bay of the shuttle.  When the shuttle reaches its orbital elevation of 150
            to 200 miles, the satellite and the third stage assembly are ejected from the cargo compartment.  After all of its cargo
            has been thrown overboard, the shuttle returns to earth for refurbishing and reuse.
2.  Object Injection Phase-  The satellites apogee kick motor (AKM) supplies the energy to move the satellite from its elliptical
     orbit to its permanent circular rotation.

How Do They Stay Up There?
A satellite stays in orbit because of the balance between two factors:
1.  The satellite's velocity, which is the speed at which it would travel in a straight line
2.  The gravitational force between the satellite and the Earth.
Were it not for the pull of gravity, a satellite's velocity would send it flying away from the Earth in a straight line.  But, were it not for velocity, the gravity would pull the satellite back to Earth.

Kinds of Orbits
1.  Geosynchronous Earth Orbit (GEO) Satellites-  The majority of satellites in orbit around the earth are positioned at a point
     22,238 miles above the earth's equator in geosynchronous earth orbit (GEO) or what is sometimes called Clarke orbit.
     The satellite can maintain an orbit with a period of rotation around the earth exactly equal to 24 hours.  Because satellites
     revolve at the same rotational speed of the earth, they appear stationary from the earth's surface.
 
 


The Clarke Orbit


 


2.  Medium Earth Orbit (MEO) Satellites-  This type of orbit circles the earth passing directly over the North and South
     poles.  The satellites are coordinated with the earth's movement so that it always crosses the equator at the same local
     time on the earth.  In addition, because satellites in this orbit fly over all altitudes, their instruments can gather information
     on almost the entire surface of the earth.  Since MEO satellites fly closer to the earth, signals have to travel a shorter
     distance and thus have a stronger and improved signal and less transmission delay.  GEO satellites have a round trip
     signal time of 0.25 seconds, whereas a MEO satellite requires less than 0.1 seconds to complete the job.

3.  Low Earth Orbit (LEO) Satellites-  This orbit is contained within the earth's surface.  These satellites fly at the highest
     layer where there is almost no air to cause drag on the spacecraft and reduce its speed.  LEO satellites have a transmission
     delay of 0.05 seconds.  This type of orbit is often used for cellular phones satellites.

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                                     "http://udel.edu/~whick/project/orbits.html"            Created by Wendy Chick, Andrea Boyle,
                                         Last updated  May 18, 1999                              Mark Helfman, and Matt Poynton.

             Scen103