GEOSTATIONARY ORBIT & SPACE
SEGMENT
Unit 2
Content
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Introduction
Antenna Look Angels
The Polar Mount Antenna
Limits of Visibility
Near Geostationary Orbits
Earth Eclipse of Satellite
Sun Transit Outage
Launching Orbits
Problems
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Power Supply
Attitude Control
Spinning Satellite Stabilization
Momentum Wheel Stabilization
Station Keeping
Thermal Control
TT&C Subsystem
Transponders
Wideband Receiver
Input Demultiplexer
Power Amplifier
Antenna Subsystem
Morelos – Anik-E – Advanced
Tiros-N Spacecraft
The Geostationary Orbit
The Geostationary Orbit
• Three conditions are required for an orbit to
be geostationary:
1. The satellite must travel eastward at the same
rotational speed as the earth.
2. The orbit must be circular.
3. The inclination of the orbit must be zero.
Introduction
• Kepler’s third law may be used to find the radius of the
orbit Denoting the radius by aGSO
• The period P for the geostationary is 23 h, 56 min, 4 s mean
solar time .
• This is the time taken for the earth to complete one
revolution about its N-S axis, measured relative to the fixed
stars
• Substituting this value along with the value for μ results in
• aGSO = 42164 km
Introduction
• The equatorial radius of the earth, to the
nearest kilometer, is aE = 6378
• and hence the geostationary height is
• hGSO = aGSO - aE = 42,164 - 6378 = 35,786 km
• This value is often rounded up to 36,000 km
Look Angle Determination
Calculating Look Angle
Coordinate System
Satellite Coordinates
Review of Geometry
Geometry of Elevation Angle
Central Angle
Elevation Angle Calculation
Example: Elevation Angle for GEO
Satellite
Using rs = 42,164 km and re = 6,378.14 km gives
d = 42,164 [1.0228826 -0.3025396 cos(γ)]1/2 km
Which finally gives the elevation angle
Azimuth Angle Calculation
• More complex approach for non-geo
satellites.
• Different formulas and corrections apply
depending on the combination of positions of
the earth station and subsatellite point with
relation to each of the four quadrants (NW,
NE, SW, SE).
• Its calculation is simple for GEO satellites
Azimuth Angle Calculation for GEO
Satellites
• SUB-SATELLITE POINT
– Equatorial plane, Latitude Ls = 0o
– Longitude ls
• EARTH STATION LOCATION
– Latitude Le
– Longitude le
Azimuth Angle for GEO sat.
To find the azimuth angle an intermediate angle , α must
be found.
Azimuth Angle for GEO sat.
Example for Look Angle Calculation of
a GEO satellite
Example (Contd.)
El=5.85o
Example (Contd.)
POLAR MOUNT ANTENNAS
• These antennas are pointing accurately only for one satellite.
• They have a single actuator which moves the antenna in a circular arc.
Generally some pointing error is seen in these antennas.
• The dish of this antenna is mounted on an axis termed as polar axis such that
the antenna bore sight is normal to this axis.
POLAR MOUNT ANTENNAS
• The angle between polar mount and the local horizontal plane is set equal
to the earth station latitude λE, making bore sight lie parallel to the
equatorial plane.
• Now the axis is tilted at an angle δ, which is relative to the polar mount
until the bore sight is pointing at a satellite position.
• δ = 90o – El0 - λE
• where El0 is the angle of elevation required for the satellite position
POLAR MOUNT ANTENNAS
Thus,
So,
LIMITS OF VISIBILITY
• There are a number of perturbing forces that cause an orbit to
depart from the ideal keplerian orbit.
• The period for a geostationary satellite is 23 h, 56 min, 4 s, or
86,164 s.
• The reciprocal of this is 1.00273896 rev/day,
• The east and west limits of geostationary are visible from any given
Earth station.
• These limits are set by the geographic coordinates of the Earth
station and antenna elevation.
• The lowest elevation is zero (in theory) but in practice, to avoid
reception of excess noise from Earth some finite minimum value of
elevation is issued.
• The earth station can see a satellite over a geostationary arc
bounded by +/- (81.30) about the earth station's longitude.
NEAR GEOSTATIONARY ORBITS
• There are a number of perbuting forces that cause an orbit to
depart from ideal Keplerian orbit.
• The most effecting ones are
 gravitational fields of sun and moon,
 non-spherical shape of the Earth,
 reaction of the satellite itself to motor movements within the
satellites.
• Thus the earth station keeps manoeuvring the satellite to maintain
its position within a set of nominal geostationary coordinates.
• Thus the exact GEO is not attainable in practice and the orbital
parameters vary with time.
• Hence these satellites are called “Geosynchronous” satellites or
“Near-Geostationary satellites”.
EARTH ECLIPSE OF A SATELLITE
If the earth’s equatorial
plane coincided with the
plane of the earth’s
orbit around the sun
geostationary
satellites
would be eclipsed by the
earth once each day.
Around
the
spring and
autumnal equinoxes,
when the sun is crossing the
equator, the satellite does
pass into the earth’s shadow
at certain periods
The equatorial plane is tilted at
an angle of 23.4° to the ecliptic
plane, and this keeps the
satellite in full view of the sun
for most days of the year
EARTH ECLIPSE OF A SATELLITE
• Eclipses begin 23 days before equinox and end 23
days after equinox.
• The eclipse lasts about 10 min at the beginning
and end of the eclipse period and increases to a
maximum duration of about 72 min at full
eclipse.
• The solar cells of the satellite become nonfunctional during the eclipse period
• The satellite is made to operate with the help of
power supplied from the batteries.
SUN TRANSIT OUTAGE
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Transit of the satellite between earth and sun
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The sun comes within the beamwidth of the earth station antenna.
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When this happens, the sun appears as an extremely noisy source which completely
blanks out the signal from the satellite .
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An increase in the error rate, or total destruction of the signal.
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This effect is termed sun transit outage, and it lasts for short periods
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The occurrence and duration of the sun transit outage depends on the latitude of the
earth station, a maximum outage time of 10 min.
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Sun outages occur in February, March, September and October, that is, around the
time of the equinoxes.
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As the sun radiates strongly at the microwave frequencies used to communicate with
satellites (C-band, Ka band and Ku band) the sun swamps the signal from the
satellite.
Launching Orbits
• Low Earth Orbiting satellites are directly injected
into their orbits.
• This cannot be done incase of GEOs as they have
to be positioned 36,000kms above the Earth‟s
surface.
• Launch vehicles are hence used to set these
satellites in their orbits. These vehicles are
reusable.
• They are also known as “Space Transportation
System‟ (STS).
Launching Orbits
• When the orbital altitude is greater than 1,200 km it becomes
expensive to directly inject the satellite in its orbit.
• For this purpose, a satellite must be placed in to a transfer orbit
between the initial lower orbit and destination orbit.
• The transfer orbit is commonly known as “Hohmann-Transfer Orbit”
Launching Orbits
• The transfer orbit is selected to minimize the energy required for the
transfer.
• This orbit forms a tangent to the low attitude orbit at the point of its
perigee and tangent to high altitude orbit at the point of its apogee.
• The rocket injects the satellite with the
required thrust into the transfer orbit.
• With the STS, the satellite carries a
perigee kick motor which imparts the
required thrust to inject the satellite in
its transfer orbit.
• Similarly, an apogee kick motor (AKM) is
used to inject the satellite in its
destination orbit.
Launching Orbits
• Generally it takes 1-2 months for the satellite to become fully
functional.
• The Earth Station performs the Telemetry Tracking and
Command (TTC) function to control the satellite transits and
functionalities
• It is better to launch rockets closer to the equator because the
Earth rotates at a greater speed here than that at either pole.
• This extra speed at the equator means a rocket needs less
thrust (and therefore less fuel) to launch into orbit.
• In addition, launching at the equator provides an additional
1,667 km/h of speed once the vehicle reaches orbit.
• This speed bonus means the vehicle needs less fuel, and that
freed space can be used to carry more pay load.
Satellite Communication
• A satellite communications system can be
broadly divided into two segments
– space segment (Satellite)
– ground segment (tracking, telemetry, and
command , TTC)
The equipment's
• The equipment carried aboard the satellite also can be
classified according to function.
• The payload : the equipment used to provide the
service for which the satellite has been launched.
• The bus : the vehicle which carries the payload but also
to the various subsystems which provide the
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power,
attitude control,
orbital control,
thermal control, and
command and telemetry functions required to service the
payload.
The equipment
• In a communications satellite, the equipment
which provides the connecting link between
the satellite’s transmit and receive antennas
is referred to as the transponder.
• The transponder forms one of the main
sections of the payload, the other being the
antenna subsystems.
The Power supply
• The primary electrical power is from solar cells.
• Arrays of cells in series-parallel connection are required to
generate more power
• Higher powers can be achieved in the form of rectangular
solar sails.
• Solar sails must be folded during the launch phase and
extended when in geostationary orbit.
• The full complement of solar cells is exposed to the
sunlight, and the sails are arranged to rotate to track the
sun.
• In order to maintain service during an eclipse, storage
batteries must be provided Nickel-cadmium (Ni-Cd)
batteries are used mostly
Attitude Control
• The attitude of a satellite refers to its
orientation in space.
• Much of the equipment carried aboard a
satellite is there for the purpose of controlling
its attitude.
• For example, to ensure that directional
antennas point in the proper directions
Attitude Control - disturbance torques
• A number of forces, referred to as disturbance
torques, can alter the attitude, like
– gravitational fields of the earth and the moon,
– solar radiation,
– meteorite impacts.
Attitude Control - Infrared sensors
• Referred as horizon detectors, are used to detect the
circumference of the earth against the background of
space.
• Using four such sensors, one for each quadrant, the
center of the earth can be readily established as a
reference point.
• If any shift is detected by the sensors, a control signal
is generated which activates a restoring torque.
• If a shift in attitude is desired, an attitude maneuver is
executed.
• The control signals of
this maneuver may be
transmitted from an earth station.
Attitude Control -Controlling torques
• Ways to generate controlling torques
– Passive attitude control
• refers to the use of mechanisms which stabilize the satellite without
putting a drain on the satellite’s energy supplies;
• At most, infrequent use is made of these supplies, for example, when
thruster jets are impulsed to provide corrective torque.
• Examples of passive attitude control are
– spin stabilization and
– gravity gradient stabilization.
– Active attitude control
• corrective torques are applied as required in response to disturbance
torques.
• Methods used to generate active control torques include
– momentum wheels,
– electromagnetic coils, and
– mass expulsion devices such as gas jets and ion thrusters.
Attitude Control -(RPY) axes
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The three axes which define a satellite’s attitude are its
– roll,
– pitch,
– yaw
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All three axes pass through the center of gravity of the satellite.
For an equatorial orbit,
– movement of the satellite about the roll axis moves the antenna footprint north and south
– movement about the pitch axis moves the footprint east and west;
– movement about the yaw axis rotates the antenna footprint.
Attitude Control - Spinning satellite
stabilization
• Spin stabilization may be achieved with cylindrical satellites.
• The satellite is constructed so that it is mechanically balanced about one
particular axis and is then set spinning around this axis.
• For geostationary satellites, the spin axis is adjusted to be parallel to the
N-S axis of the earth.
• Spin rate is typically in the range of 50 to 100 rev/min.
• Spin is initiated during the launch phase by means of small gas jets.
Attitude Control - Spinning satellite
stabilization
• Impulse-type thrusters, or jets, can be used to increase the
spin rate to maintain correct N-S orientation in case of
disturbance torques
• Nutation, which is a form of wobbling, can occur as a result
of the disturbance torques and/or from misalignment or
unbalance of the control jets.
• This nutation must be damped out by means of energy
absorbers known as nutation dampers.
• Certain dual-spin spacecraft obtain spin stabilization from a
spinning flywheel rather than by spinning the satellite itself.
• Flywheels are termed momentum wheels, and their
average momentum is referred to as momentum bias.
• Reaction wheels operate at zero momentum bias.
Attitude Control - Momentum wheel
stabilization
• Consists of a
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flywheel,
the bearing assembly,
the casing, and an
electric drive motor with associated electronic control circuitry.
• The flywheel is attached to the rotor, which consists of a permanent
magnet providing the magnetic field for motor action.
• The stator of the motor is attached to the body of the satellite.
• Thus the motor provides the coupling between the flywheel and
the satellite structure.
• Speed and torque control of the motor is exercised through the
currents fed to the stator.
• The housing for the momentum wheel is evacuated to protect the
wheel from adverse environmental effects,
• The bearings have controlled lubrication that lasts over the lifetime
of the satellite.
Attitude Control - Momentum wheel
stabilization
• The term momentum wheel is usually reserved for wheels that operate at
nonzero momentum.
• This is termed a momentum bias.
• Such a wheel provides passive stabilization for the yaw and roll axes when
the axis of rotation of the wheel lies along the pitch axis.
• Control about the pitch axis is achieved by changing the speed of the
wheel.
Attitude Control - Momentum wheel
stabilization
• When a momentum wheel is operated with zero
momentum bias, it is generally referred to as a
reaction wheel.
• Reaction wheels are used in three-axis stabilized
systems.
• Here, as the name suggests, each axis is stabilized by a
reaction wheel
Attitude Control - Momentum wheel
stabilization
• At some point the wheel saturates.
• In effect, it reaches its maximum allowable
angular velocity and can no longer take in any
more momentum.
• Mass expulsion devices are then used to
unload the wheel, that is, remove momentum
from it
• Operation of the mass expulsion devices
consumes part of the satellite’s fuel supply
Station Keeping
• It is important that a geostationary satellite be
kept in its correct orbital slot
• The equatorial ellipticity of the earth causes
geostationary satellites to drift slowly along the
orbit, to 75°E and to 105°W
• To counter this drift, an oppositely directed
velocity component is imparted to the satellite by
means of jets, which are pulsed once every 2 or 3
weeks.
• These maneuvers are termed east-west stationkeeping maneuvers.
Station Keeping
• Satellites in the 6/4-GHz band must be kept
within ±0.1° and the 14/12-GHz band, within
±0.05°of the designated longitude
• A satellite which is nominally geostationary
also will drift in in latitude
• These perturbing forces cause the inclination
to change at a rate of about 0.85°/year.
Station Keeping
• If left uncorrected, the drift would result in a cyclic change in the
inclination, going from 0 to 14.67° in 26.6 years and back to zero, at
which the cycle is repeated.
• To prevent the shift in inclination from exceeding specified limits,
jets may be pulsed at the appropriate time to return the inclination
to zero.
• Counteracting jets must be pulsed when the inclination is at zero to
halt the change in inclination.
• These maneuvers are termed north-south station-keeping
maneuvers.
• They are much more expensive in fuel than are east-west stationkeeping maneuvers.
• The north-south station-keeping tolerances are the same as those
for east-west station keeping, ±0.1° in the C band and ±0.05° in the
Ku band.
Station Keeping
• Orbital correction is carried out by command
from the TT&C earth station, which monitors
the satellite position.
• East-west and north south station-keeping
maneuvers are usually carried out using the
same thrusters as are used for attitude
control.
Thermal Control
• Thermal blankets and shields may be used to
provide insulation.
• Radiation mirrors are often used to remove
heat from the communications payload.
TT&C Subsystem
• The telemetry subsystem transmits information about the satellite
to the earth station, while the command subsystem receives
command signals from the earth station to the satellite.
• Data which are transmitted as telemetry signals include
– attitude information obtained from sun and earth sensors;
• environmental information
– magnetic field intensity
– direction,
– frequency of meteorite impact etc ; and
• spacecraft information
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temperatures,
power supply
voltages, and
stored-fuel pressure.
• Special channels are used for TT&C
TT&C
Transponders
• It is the series of interconnected units which forms a single
communications channel between the receive and transmit antennas in a
communications satellite.
• The bandwidth allocated for C band service is 500 MHz
• Divided into sub bands, one for each transponder.
• A transponder bandwidth is 36 MHz
• 4-MHz guard band between transponders,
• 12 such transponders can be accommodated in 500-MHz bandwidth.
Single stage Transponder for 6/4 GHz
Double frequency Conversion type –
Ku band
Units of Transponders
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The wide band receiver
The input demultiplexer
The power amplifier
The antenna subsystems
The wide band receiver
• A duplicate receiver is provided so that if one
fails, the other is automatically switched in.
• The combination is referred to as a redundant
receiver
The wide band receiver
• The first stage in the receiver is a low-noise
amplifier (LNA).
• This amplifier adds little noise to the carrier
being amplified,
• At the same time it provides sufficient
amplification for the carrier to override the
higher noise level present in the following
mixer stage
The input Demultiplexer
• The input demultiplexer separates the broadband input.
• It covers the frequency range 3.7 to 4.2 GHz into the
transponder frequency channels.
• The channels are usually arranged in even-numbered and
odd-numbered groups.
• Reduces adjacent channel interference.
• The output from the receiver is fed to a power splitter,
which in turn feeds the two separate chains of circulators.
• The full broadband signal is transmitted along each chain
• The channelizing is achieved by means of channel filters
connected to each circulator
• Each filter has a bandwidth of 36 MHz and
Input DeMux
The power Amplifier (TWT)
Traveling-wave tube amplifiers
The power Amplifier (TWT)
• A separate power amplifier provides the output
power for each transponder channel
• It can provide amplification over a very wide
bandwidth
• Each power amplifier is preceded by an input
attenuator.
• This is necessary to permit the input drive to each
power amplifier to be adjusted to the desired
level.
• The attenuator has a fixed section and a variable
section.
The power Amplifier (TWT)
• The fixed attenuation
– It is needed to balance out variations in the input
attenuation so that each transponder channel has
the same nominal attenuation.
• The variable attenuation
– It is needed to set the level as required for
different types of service
– Because this variable attenuator adjustment is an
operational requirement, it must be under the
control of the ground TT&C station.
The power Amplifier (TWT)
The antenna subsystems
• The antennas carried aboard a satellite provide the dual functions
of receiving the uplink and transmitting the downlink signals.
• Dipole-type antennas are used where omnidirectional
characteristics are required
• Highly directional antennas are required for telecommunications
purposes and TV relay and broadcast.
• Directional beams are usually produced by means of reflector-type
antennas,
• The paraboloidal reflector being the most common.
• Wide beams for global coverage are produced by simple horn
antennas at 6/4 GHz.
• These horns beam the signal directly to the earth without the use
of reflectors.
The antenna subsystems
Self study topics
• Morelos
• Anik-E
• Advanced Tiros-N Spacecraft