SATELLITE COMMUNICATION
SATELLITE COMMUNICATION
a satellite is a celestial body that orbits around a planet
In essence, a communications satellite is a microwave repeater in
the sky that consists
of a diverse combination of one or more of the following: receiver,
transmitter, am
plifier, regenerator, filter, onboard computer, multiplexer,
demultiplexer, antenna, waveguide, and about any other electronic
communications circuit ever developed
A satellite radio repeater is called a transponder
It consists of one or more satellite space vehicles, a ground-based
station to control the operation of the system, and a user network of
earth stations that provides the interface facilities for the transmission
and reception of terrestrial communications traffic through the satellite
system
Transmissions to and from satellites are categorized as either bus or
payload
Bus It includes control mechanism that support the payload
operation
Payload The actual user information conveyed through the system.
Passive Reflector A type of satellite wherein it simply bounces
signals from one place to another.
Two satellites which are commonly used in satellite communication are
Active and passive satellites.
Passive satellites: It is just a plastic balloon having a metal coated
over it. This sphere reflects the coming microwave signals coming from
one part of the earth to other part. This is also known as passive
sphere. Our earth also has a passive satellite i.e. moon.
Active satellites: It basically does the work of amplifying the
microwave signals coming. In active satellites an antenna system,
transmitter, power supply and a receiver is used. These satellites are
also called as transponders.
Some history of satellite communication
The first artificial Earth satellite was Sputnik 1. Put into orbit by
the Soviet Union on October 4, 1957, it was equipped with an onboard radio-transmitter that worked on two frequencies: 20.005 and
40.002 MHz.
Echo 1 was the world's first artificial communications satellite capable
of relaying signals to other points on Earth. It soared 1,600 kilometers
(1,000 mi) above the planet after its Aug. 12, 1960 launch
The first American satellite to relay communications was Project
SCORE in 1958, which used a tape recorder to store and
forward voice messages
Johannes Kepler A German astronomer who discovered the laws
that governs satellite motion.
Keplers Law
1). the planets move in ellipses with the sun at one focus
2.) the line joining the sun and a planet sweeps out equal areas in
equal intervals of time
3.) the square of the time of revolution of a planet divided by the cube
of its mean distance from the sun gives a number that is the same for
all planets
Satellite Elevation Categories
Satellites are generally classified as having either a low earth orbit
(LEO), medium earth orbit (MEO), or geosynchronous earth
orbit (GEO). Most LEO satellites operate in the 1.0-GHz
to 2.5-GHz frequency range
Satellite Orbital Patterns
Apogee. The point in an orbit that is located farthest from Earth
Perigee. The point in an orbit that is located closest to Earth
Major axis. The line joining the perigee and apogee through the
center of Earth;
sometimes called line of apsides
Geosynchronous Satellite
High-altitude earth-orbit satellites operating primarily in the 2-GHz to
18 GHz frequency spectrum with orbits Satellite 22,300 miles above
earths surface
Advantages of Geosynchronous Satellite: It remain almost
stationary in respect to a given earth station. Available to earth within
their shadows 100% of the time. No need to switch from one
geosynchronous satellite to another as they orbit overhead The effects
of Doppler shift are negligible
Disadvantages of geosynchronous Satellite: It requires
sophisticated and heavy propulsion device onboard to keep them in a
fixed orbit Much longer propagation delays Requires higher transmit
power and more sensitive receivers because of the longer distances
and greater path loss. High precision spacemanship is required
Azimuth Angle
Azimuth angle is defined as the horizontal pointing angle of an earth
station antenna
Frequency Bands Available for Satellite Communications
Angle of Inclination
The angle between the earths equatorial plane and the orbital
plane of a satellite measured counter clockwise at the point in the orbit
where it crosses the equatorial plane traveling from south to north
called the ascending node.
SATELLITE SYSTEM LINK MODELS
Uplink Model The primary component within the uplink section of a
satellite system is the earth station transmitter
Transponder A typical satellite transponder consists of an input
band limiting device (BPF), an input low-noise amplifier (LNA), a
frequency translator, a low-level power amplifier, and an output
bandpass filter
Downlink Model An earth station receiver includes an input BPF, an
LNA, and an RF-to-IF down-converter
Equatorial Orbit
It is when the satellite rotates in an orbit directly above the
equator, usually in a circular path.
Polar Orbit
It is when the satellite rotates in path that takes over the North
and the South poles in an orbit that is close to earth and passes
over and very close to both the North and South Poles.
Noise Density
The noise power normalized to a 1 Hz bandwidth, or the noise
power present in a 1Hz bandwidth
Link Budget
It identifies the system parameters and is used to determine the
projected carrier-to-noise ratio and energy Bit-to-noise density
ratio at both the satellite and earth station receivers for a given
modulation scheme
