satellite communication-UNIT-II.pptx

Unit 2
Satellite Sub-Systems
SATELLITE SUB-SYSTEMS
CONTROL SEGMENT (MONITOR STATIONS)
The GPS-System is controlled by the US Army. The
“master control station” and four additional
monitoring stations were set up for monitoring the
satellites.
The Control Segment is composed of
A Master Control Station (MCS)
An Alternate Master Control Station,
Four dedicated Ground Antennas
Six dedicated Monitor Stations
ACE ECE IV SC- UNIT-II

Control Segment (Monitor Stations)
• The GPS-System is controlled by the US Army. The “master control
station” and four additional monitoring stations were set up for
monitoring the satellites.
• The Control Segment is composed of
A Master Control Station (MCS)
An Alternate Master Control Station,
Four dedicated Ground Antennas
Six dedicated Monitor Stations
• The flight paths of the satellites are tracked by dedicated U.S. Air Force
monitoring stations in
– Hawaii
– Kwajalein
– Ascension Island
– Diego Garcia
– Colorado Springs
• Monitor stations operated in England, Argentina, Ecuador, Bahrain,
Australia and Washington DC.
The flight paths of the satellites are tracked by
dedicated U.S. Air Force monitoring stations in
Hawaii
Kwajalein
Ascension Island
Diego Garcia
Colorado Springs
Monitor stations operated in England, Argentina,
Ecuador, Bahrain, Australia and Washington DC.

Control Segment
CONTROL SEGMENT

Control Segment
• During August and September 2005, six more monitor stations of the
National Geospatial-Intelligence Agency were added to the grid. Now, every
satellite can be seen from at least two monitor stations.
• This allows calculating more precise orbits and ephemeris data. For the end
user, a better position precision can be expected from this.
• In the near future, five more stations will be added so that every satellite can
be seen by at least three monitor stations. This improves integrity monitoring
of the satellites and thus the whole system.
CONTROL SEGMENT
During August and September 2005, six more monitor stations
of the National Geospatial-Intelligence Agency were added to
the grid. Now, every satellite can be seen from at least two
monitor stations.
This allows calculating more precise orbits and ephemeris
data. For the end user, a better position precision can be
expected from this.
In the near future, five more stations will be added so that
every satellite can be seen by at least three monitor stations.
This improves integrity monitoring of the satellites and thus
the whole system.

User Segment
• The User Segment is composed of hundreds of thousands of U.S. and allied
military users of the secure GPS Precise Positioning Service, and tens of millions
of civil, commercial and scientific users of the Standard Positioning Service.
• In general, GPS receivers are composed of an antenna, tuned to the frequencies
transmitted by the satellites, receiver-processors, and a highly-stable clock.
USER SEGMENT
The User Segment is composed of hundreds of
thousands of U.S. and allied military users of the
secure GPS Precise Positioning Service, and tens of
millions of civil, commercial and scientific users of
the Standard Positioning Service.
In general, GPS receivers are composed of an
antenna, tuned to the frequencies transmitted by
the satellites, receiver-processors, and a highly-
stable clock.

Spacecraft subsystem
overview
SPACECRAFT SUBSYSTEM OVERVIEW

AOCS (Attitude & orbit control
system)
AOCS (ATTITUDE & ORBIT CONTROL SYSTEM)

 At GEO orbit altitude the moon’s gravitational force is about twice as strong as the
sun’s
 Moon orbit is inclined to the equatorial plane by approximately 5 degrees
 The plane of the earth’s rotation around the sun is inclined to 23 degrees to the
equatorial plane
At GEO orbit altitude the moon’s gravitational force is about twice as
strong as the sun’s
Moon orbit is inclined to the equatorial plane by approximately 5 degrees
The plane of the earth’s rotation around the sun is inclined to 23 degrees
to the equatorial plane

Net gravitational force on the satellite tends to change the inclination of the satellite.
Approximately 0.86 degrees per year from the equatorial plane.
LEO satellites are less effected by this gravitational pull from the sun and moon.
At the equator there are bulges of about 65m at longitudes 162 degress East and 348
degrees East.
Satellite is accelerated towards one of two stable points on GEO orbit at the longitude
of 75 degree E and 252 degrees E

FINE POSITIONING
Two ways to make the satellite stable in orbit
when it is weightless.
Satellite can be rotated at a rate between 30 and
100 rpm to create gyroscopic force that provides
stability (spinner satellites)
Satellites can be stabilized by one or more
momentum wheels, called three-axis stabilized
satellites

Two types of motors used on satellites.
Traditional bipropellant thruster
Bipropellants used are Mono-methyl Hydrazine
and Nitrogen tetraoxide.
They are hypogolic, i.e., they ignite
simultaneously on contact without any catalyst or
heater
Arc jets or ion thrusters
High voltage is used to accelerate ions
Fuel stored in GEO satellite is used for two purposes
Apogee kick motor (AKM) that injects the
satellite into its final orbit
Maintain the satellite in that orbit over its
lifetime.
ORBIT INSERTION & MAINTENANCE- GEO

TYPICAL SPIN STABILIZED SPACECRAFT

TYPICAL 3-AXIS SPACECRAFT

TT&C SUBSYSTEM

TYPICAL TTC&M SYSTEM
TELEMETRY MODES
TRACKING

COMMAND

POWER SYSTEMS-1

COMMUNICATION SUBSYSTEMS

REPEATERS & TRANSPONDERS

TYPES OF PAYLOADS/TRANSPONDERS

SIMPLIFIED DOUBLE CONVERSION TRANSPONDER
FOR 14/11 GHZ BAND

ONBOARD PROCESSING TRANSPONDER

SATELLITE ANTENNAS
Antennas form a very important element in communication system,
either terrestrial or extra terrestrial, depending on the mission type and
requirements.
"That part of a transmitting or receiving system which is designed to
radiate or to receive electromagnetic waves".
 we use antennas to overcome our inability to lay a physical
interconnection between two remote locations or an antenna can also be
viewed as a transitional structure (transducer) between free-space and a
transmission line (such as a coaxialline).
Antennas cannot add power, instead they can only focus and shape the
radiated power in space e.g. it enhances the power in some wanted
directions and suppresses the power in other directions

IMPORTANT DEFINITION
ANTENNA DIRECTIVITY
The directivity of an antenna is defined
as the ratio of the radiation intensity in a given
direction from the antenna, to the radiation
intensity averaged over all directions.
GAIN
Gain of an antenna is a measure of the
antenna’s capability to direct energy in one
direction, rather than all around.

ALL the major properties of a linear passive antenna are identical
whether it is used in transmit or receive mode. There is only one exception to
this rule called "reciprocity", and that is when the antenna contains
magnetically biased magnetic materials such as ferrites with resonantly
rotating electron spin systems.
The physical reason for reciprocity is that the only difference
between outgoing and incoming waves lies in the arrow of time. Since the
electromagnetic equations are invariant except for the signs of magnetic fields
and currents, under time reversal, there can be no difference between transmit
and receive mode in the physical current and field distributions. However, if we
have a magnet providing a steady bias field, under time reversed conditions we
would have to reverse the direction of this bias field. But for incoming and
outgoing waves, the bias field direction remains the same. Thus it is possible
for the system to be non-reciprocal.
RECIPROCITY

RELIABILITY

REDUNDANCY

Redundant TWTA configuration in HPA of a 6/4 GHz bent
pipe transponder.

BASIC TRANSMISSION THEORY
The calculation of the power received by an earth station from a
satellite transmitter is fundamental to the understanding of
satellite communications.
Consider a transmitting source, in free space, radiating a total
power 𝑃𝑡 watts uniformly in all directions as shown in below
Figure .

At a distance 𝑅 meters from the hypothetical isotropic source
transmitting RF power 𝑃𝑡 watts, flux density crossing the surface
of a sphere with radius 𝑅 is given by:
All real antennas are
directional and radiate
more power in some
directions than in other.
Any real antenna has a
gain 𝐺𝜃, as sketched
below.

For a transmitter with output 𝑃𝑡 watts driving a lossless antenna
with gain 𝐺𝑡, the flux density in the direction of the antenna
boresight at distance 𝑅 meter is:
The product 𝑃𝑡𝐺𝑡 is often called the effective isotopically radiated power or
EIRP

If we had an ideal receiving antenna with an aperture area of 𝐴 𝑚2, as shown in
Figure 4.3, we will collect power 𝑃𝑟 watts given by:
A practical antenna with a physical aperture area of 𝐴𝑟 𝑚2 will not deliver the
power transmitted, and some is absorbed by lossy components. This reduction
in efficiency is described by using an effective aperture 𝐴𝑒 where:
And 𝜂𝐴 is the aperture efficiency of the antenna.
The power received by a real antenna is
This expression is known as the link equation, and it is essential in the
calculation of power received in any radio link.

The term 4𝜋𝑅𝜆 2 is known as the path loss, 𝐿𝑝. Therefore, we can write the 𝑃𝑟
as:
In Communication systems, decibel quantities are commonly used to simplify
calculations:
In practice, we will need to take account of more complex situation in which
we have losses in the atmosphere due to attenuation by oxygen, water vapor,
and rain, losses in the antennas at each end of the link.
where 𝐿𝑎 is the attenuation in the atmosphere, and 𝐿𝑡𝑎,𝐿𝑟𝑎 are the losses
associated with the transmitting and receiving, respectively.

Is a useful concept in communications receivers, since it
provides a way to determining how much thermal noise is
generated by active and passive devices in the receiving system.
The noise power is given by:
where 𝑘 denotes Boltzman’s constant = 1.39×10−23 J/K = -228.6
dB W/K/Hz, 𝑇𝑠 is the system noise temperature of source in
kelvin degrees, and 𝐵𝑛 represents the noise bandwidth in which
the noise power is measured, in Hz
Calculate the total noise power at the output of the IF amplifier
of the receiver in the below figure.
NOISE TEMPERATURE
SYSTEM NOISE TEMPERATURE AND G/T

The link equation can be rewritten in terms of 𝐶𝑁 at the
earth station:
Thus, , and can be used to specify the quality of
a receiving earth station or a satellite receiving system.
G/T RATIO FOR EARTH STATIONS

Design of Downlinks
The design of any satellite communication is based on
meeting a minimum C/N ratio for a specified percentage of
time.
Any satellite link can be designed with very large antenna to
achieve high C/N ratios under all conditions, but cost will be
high.
The art of good system design is to reach the best
compromise of system parameters that meet the specification
at the lowest cost.
For example if a satellite link is designed with sufficient
margin to overcome a 20 dB rain fade rather than a 3 dB fade,
earth station antennas with seven times the diameter are
required.

All satellite communication links are affected by rain attenuation.
In the 6/4 GHz band the effect of rain on the link is small
In the 14/11GHz (Ku) band and even more in the 30/20 GHz (Ka) band, rain
attenuation becomes important.
Rain attenuation is very variable phenomenon, both with time and place.
C/N is simplified by the used of link budget
A link budget is a tabular method for evaluating the received power and noise
power in a radio link.
Link budget must be calculated for an individual transponder, and must be
repeated for each of the individual links.
Link budgets are usually calculated for the worst case, the one in which the
link will have the lowest C/N ratio.
Design of Downlinks
Link Budget

Factors which contribute to a worst case scenario include: an earth
station located at the edge of the satellite coverage zone where the
received signal is typically 3 dB lower than the center of the zone.
This is because the satellite antenna pattern, maximum path length
from the satellite to the earth station, low elevation angle at the earth
station giving the highest atmospheric path attenuation in clear air and
maximum rain attenuation on the link causing loss of received signal
power and increase in receiving system noise temperature.
The calculation of carrier to noise ratio in a satellite link is based on the
two equations for received signal power and received noise power
A receiving terminal with a system noise temperature TsK and a noise
bandwidth Bn Hz has a noise power Pn

satellite communication-UNIT-II.pptx

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