Page 18 - SATELLITE AND RADAR COMMUNICATION SYSTEM

Low Earth Orbit (LEO)

 A low Earth orbit (LEO) is, as the name suggests, an orbit that is relatively close
to Earth’s surface.
 It is normally at an altitude of less than 1000 km but could be as low as 160 km
above Earth – which is low compared to other orbits, but still very far above
Earth’s surface.

 By comparison, most commercial aeroplanes do not fly at altitudes much greater
than approximately 14 km, so even the lowest LEO is more than ten times higher
than that.
 LEO satellites must travel very fast so gravity does not pull them back into the
atmosphere.
 However, individual LEO satellites are less useful for tasks such as

telecommunication, because they move so fast across the sky and therefore re-
quire a lot of effort to track from ground stations.
 Satellites in this orbit travel at a speed of around 7.8 km per second; at this
speed, a satellite takes approximately 90 minutes to circle Earth, meaning the
International Space Station (ISS) travels around Earth about 16 times a day.
 LEO satellites don’t stay in fixed position relative to the surface, and are only
visible for 15 to 20 minutes each pass.

Advantages of LEO:

A LEO satellite’s proximity to earth
compared to a GEO satellite gives it a
better signal strength and less of a time
delay, which makes it better for point
to point communication. A LEO
satellite’s smaller area of coverage is
less of a waste of bandwidth.

Disadvantages of LEO:
A network of LEO satellites is needed,
which can be costly. LEO satellites have
to compensate for Doppler shifts cause
by their relative movement.
Atmospheric drag effects LEO satellites,
causing gradual orbital deterioration. Figure 1.7: Low Earth Orbit

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Applications: Satellite telephone, International Space Station (ISS)

Introduction to Satellite Communication System