Satellite orbit information

Geostationary Orbit

 
The most common orbit used for satellite communications is the geostationary orbit (GEO). This is the orbit described above – the rotational period is equal to that of the Earth. The orbit has zero inclination so is an equatorial orbit (located directly above the equator). The satellite and the Earth move together so a GEO satellite 
appears as a fixed point in the sky from the Earth.

The advantages of such an orbit are that no tracking is required from the ground station since the satellite appears at a fixed position in the sky. The satellite can also provide continuous operation in the area of visibility of the satellite. Many communications satellites travel in geostationary orbits, including those that relay TV signals into our homes.

However, due to their distance from Earth GEO satellites have a signal delay of around 0.24 seconds for the complete send and receive path. This can be a problem with telephony or data transmission. Also, since they are in an equatorial orbit, the angle of elevation decreases as the latitude or longitude difference increases between the satellite and earth station. Low elevation angles can be a particular problem to mobile communications.

Low Earth Orbit/Medium Earth Orbit

        

        A low earth orbit (LEO), or medium earth orbit (MEO) describes a satellite which circles close to the Earth. Generally, LEOs have altitudes of around 300 – 1000 km with low inclination angles, and MEOs have altitudes of around 10,000 km. 

A special type of LEO is the Polar Orbit. This is a LEO with a high inclination angle (close to 90degrees). This means the satellite travels over the poles.

Satellites  that  observe our planet  such as  remote  sensing  and  weather  satellites often  travel  in a  highly  inclined LEO  so  they  can  capture  detailed  images  of  the  Earth’s surface  due  to  their  closeness  to  Earth.  A satellite  in  a  Polar orbit  will  pass  over  every  region  of  Earth  so  can  provide global  coverage.  Also  a  satellite  in  such  an  orbit  will sometimes  appear  overhead  (unlike  a  GEO  which  is  only overhead  to  ground  stations  on  the  equator ).  This  can  enable communication  in  urban  areas  where  obstacles  such  as  tall buildings  can  block  the  path  to  a  satellite.  Lastly,  the transmission  delay  is  very  small.        Any  LEO  or  MEO  system  however ,  for  continuous  operation,  requires  a   constellation  of  satellites.  The  satellites  also move  relative  to  the  Earth  so  wide  beam  or  tracking  narrow  beam  antennas  are  needed.

            

Elliptical Orbit

                A  satellite  in  elliptical  orbit  follows  an  oval - shaped  path.  One  part  of  the  orbit  is  closest  to  the  center  of  Earth  (perigee)  and   another  part  is  farthest  away  ( apogee ).  A  satellite  in  this  type  of  orbit  generally  has  an  inclination  angle  of  64  degrees  and  takes  about  12  hours  to circle  the  planet.  This  type  of  orbit  covers  regions  of  high  latitude  for  a  large  fraction  of  its  orbital  period.