06-satellite communications.ppt - Satellite Communications...

Satellite Communications

outline

Categorization of satellites

Frequency bands & their applications

Satellite Link Performance Factors

Satellite communication configuration

Capacity Allocation Strategies

Satellite-Related Terms

Earth Stations – antenna systems on or near earth

Uplink – transmission from an earth station to a
satellite

Downlink – transmission from a satellite to an
earth station

Transponder – electronics in the satellite that
convert uplink signals to downlink signals

Satellite-Related Terms

Satellite radio is a microwave transmission system utilizing
a non-terrestrial (non-land-based) relay station positioned in
space; i.e. a communications satellite can be thought of as
a big microwave repeater in the sky. The figure below
illustrates how a communication satellite in orbit around the
earth can provide a network connection across an ocean.
The satellite contains a transponder that consists of a radio-
wave receiver and transmitter. The transponder accepts an
incoming signal, amplifies it, and transmits the amplified
signal back toward the ground at a slightly different angle
than it received it. A ground station on one side of the
ocean transmits a signal to the satellite, which then sends
the signal to a ground station on the other side
.

How the Satellite Operates

How Satellites Operate

A single satellite usually contains multiple transponders
that operate independently (typically six to twelve);
each transponder uses a different radio frequency (i.e.,
channel), making it possible for multiple
communications to proceed simultaneously.
Communication satellites can be grouped into
categories. A Geo-stationary Earth Orbit (GEO) is an
orbit such that, when viewed from the ground, a
satellite in the orbit appears to be at the same point in
the sky at all times. It has been determined that such an
orbit is about 35,785 kilometers from the earth. A
satellite orbiting in such an orbit is sometimes known as
a Geo-synchronous Satellite.

VSAT

A new development in the communication satellite world
is the development of low-cost micro-stations – known
as Very Small Aperture Terminals (VSAT). These tiny
terminals have 1-meter or small antennas (versus 10
meters for a standard geostationary earth orbit (GEO)
satellite antenna). These terminals can put out about 1
watt of power; the uplink speed is 19.2 kbps while the
downlink speed is 512 kbps or more

In many VSAT systems, the micro-stations do not have
enough power to communicate directly with one another
(via the satellite). Instead, a special ground station
(called the Hub) with a large, high-gain antenna is
needed to relay traffic between VSATs as shown below
.

VSAT