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satellite subsystems ppt. reference: satellite communication by timothy pratt
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Satellite Communication
Spacecraft subsystems
Spacecraft
Spacecraft subsystem overview
Satellite Subsystems(Cont..)
Power system Solar cells Used by transmitter and other electrical systems Communication sub system Major part Small weight and volume Composed of one or more antennas which receive and
transmit over wide bandwidths at microwave frequencies Receiver – transmitter unit is known as transponder 2 types1. linear or bend pipe transponder
Amplifies the received signal and retransmits it at a usually different lower frequency
2. Base band processing transponderUsed only with digital signal. Converts the received signal to baseband, processes it retransmits
Satellite antennas Most are designed to operate in a
single band Such as Cu band or K band
4 or more antennas can be used for multiple frequencies
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
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
Attitude and Orbit Control System(AOCS)
Attitude and orbit change reasons Gravitational field of Sun and Moon
Irregularities in Earths gravitational field
Solar pressure from Sun
Variations in Earths magnetic field
Solar pressure and Earths magnetic field can generate eddy currents in satellites metallic structure
The Earth is not a perfect sphere Satellite is accelerated towards the two stable
points To maintain accuracy of satellite position, it is
accelerated in between by using thrusters, which can be controlled from Earth via TTC&M
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.
Attitude Control System
Two ways to make a satellite stable in its orbit
1. Create a gyroscopic force by rotating the body of the satellite between 30 to 100 rpm Gyroscope a wheel mounted such that the
force gravity acts on centre of mass, and no torque is acting on the wheel. Without torque acting on the wheel, without torque to change its direction of motion, a spinning gyroscopic wheel will remain pointing in same direction
Gyroscopic force provides stability of spin axis and keeps it pointing in the same direction. Such satellites are known as spinners
Eg: Hughes (Boeing 376)
2. The satellite can be stabilized by one or more momentum wheels. This is called 3 axis stabilized satellite.
Eg: Hughes (Boeing) 701 The momentum wheel is usually a solid metal
disk driven by an electric motor Either there must be one momentum wheel for
each of the three axes , or a single momentum wheel can be mounted on gimbals and rotated to provide a rotational force about any of the 3 axes
As per principle of angular momentum, increasing the speed of the momentum wheel causes the satellite to precess in the opposite direction
Orbit insertion & Maintenance- GEO 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.
Definition of axis
TTC&M
Telemetry Modes
Tracking
• Determines the current orbit of the satellite
Command
Typical TTC&M system
Power systems-1
Power systems-2
Power systems-3
Communication subsystems
Repeaters and Transponders
Types of payloads/Transponders
Transponders
In Transponders, two amplifiers are used is series so that though one fails other will make up
Output back off the degree to which output backup power is reduced below its peak output to reduce intermodulation products
In FDMA systems, back off is 2 to 7 db
Output amplifier is usually a solid state power amplifier
Travelling wave tube amplifier is used for higher output powers
M by N redundancy providing M amplifiers instead of N in a ring
Eg: 16 by 10 redundancy Transponder lasts till the last time of
satellite..
Basic Transponder elements
On Board Processors Methods to conserve uplink bandwidth
Use different modulation techniques on uplink and downlink
Provide a baseband processor on the satellite On board processing is advantageous to
switch between uplink(eg. MF-TDMA) and downlink(eg. TDM) so that small earth stations may access each other via directly via the satellite
The processor can provide data storage and also perform error correction independently
Spacecraft Antennas
Wire antennas: monopoles and dipoles
Horn antennas Reflector antenna Array antenna
Antenna Theory
Wire antennas Used primarily at VHF and UHF to
provide communications for TTC&M systems
Omnidirectional coverage
Horn antennas: Used at microwave frequencies when
relatively wide beams are required, as for global coverage
A flared section of waveguide provides an aperture several
wavelengths wide Also provides a good match between
the waveguide impedance and free space
Horns are used as feeds for reflectors either singly or in clusters
Reflector antennas: Usually illuminated by one or more
horns Provide a larger aperture than
provided with a horn alone Horns and reflectors are examples of
aperture antennas that launch a wave into free space from a waveguide
Important terms Antenna pattern:
Plot of field strength in the far field of the antenna when the antenna is driven by the transmitter
Usually measured in decibels (db) Gain:
Measure of antennas capability to direct energy in one direction rather than all around
Reciprocity: Antenna has same gain and pattern
at a given frequency whether receiving or transmitting
To provide a separate beam for each earth station would also require one antenna feed per earth station if multiple-feed antenna with a single reflector were used. A compromise between one beam per station and one beam for all stations has been used in many satellites by using zone – coverage beams
Footprint
Spot Beams Small geographic area
Zonal Coverage Less than one-third of
the earth’s surface Earth Coverage
One-third of the earths surface with approximate antenna beam width of 17 degrees.(hemi beams)
Outer space at geostationary distances is a harsh environment
Total vacuum; Sun irradiates the satellite with 1.4 K W heat and light on each square meter
When surfaces are in shadow, heat is lost to the infinite sink of space and surface temp falls to zero
Electronic devices cant withstand such temp Temperature must stay b/n 0 to 75 degree
celcius Quality control or quality assurance
Space Qualification
Quality control or quality assurance
Each satellite component is tested individually to ensure that it meets the specification
Testing in 3 parts Mechanical model contains structural and
mechanical parts Ensures all moving parts operate correctly in
vaccum Thermal modelEntire satellite will be
modelled in a chamber- shake and brake test Electrical modelchecks all electronic parts
Space qualification makes the GEO satellites expensive
Many electronic and mechanical components have less life time Hence back up or redundant unit will
be provided If one unit fails, back up device
automatically takes charge
ReliabilityFamiliar to automobile users.Chances for failure in starting and ending years is more compared to others
Redundancy
Next lecture earth station technology
Reference : Satellite communications engineering , Wilburt L Pritchard Robert A Nelson and Henri G Suyderhoud