The space segment

Satellite Communication

• A satellite communications system can be broadly divided into two segments– space segment (Satellite) – ground segment (tracking, telemetry, and

command , TTC)

The equipments

• 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 – 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• The three axes which define a satellite’s attitude are its

– roll, – pitch,– yaw

• 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 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 station-keeping 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 station-keeping

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 – 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.

Transponders

Units of Transponders

• The wide band receiver• The input demultiplexer• The power amplifier• The antenna subsystems

The wide band receiver

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

Inpu

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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 where omnidirectional characteristics are

required • Highly directional antennas 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