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UNIT - IIIThe Space Segment

Power supply

Attitude control

Station keeping –Thermal Control

Telemetry, Tracking and command (TT&C)

Antenna subsystem

Transponders  

Wide Band Receiver 

Earth Segment

Receive only home TV System

Community Antenna TV1

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file:///C:/Users/hp/Downloads/satellite%20communications%20by%20Dennis%20Roddy4thedition.pdf

Link to Download the Text Book

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Satellite Communication SegmentsSatellite Communication Segments

A satellite communications system can be broadly divided into two segments

a ground segment a space segment.

The space segment will obviously include the satellites, but it also includes the ground facilities needed to keep the satellites operational, these being referred to as the tracking, telemetry, and command (TT&C) facilities.

 

Space systems are complex pieces of equipment designed to perform specific functions for a specified design life. Space systems are not merely robots launched into space and then left to perform their mission. To keep a space system functioning over many years requires almost constant attention through a complex network of equipment and the involvement of many people.

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Space SystemsSpace Systems

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•Space systems have three distinct segments:

Space Segment: The satellites placed into orbit or components used to launch the satellites.

Control Segment: The personnel, equipment and facilities responsible for the operation and control of the satellite and, in many communications systems, control of users' transmissions through the satellites.

User Segment:The personnel, equipment and facilities that use the capabilities provided by the satellite payload.

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The payload refers to the equipment used to provide the service for which the satellite has been launched.

The bus refers not only to 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.

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.

DefinitionsDefinitions

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Satellites require power to operate the electrical equipment that is on board.

Satellites which have high power sensors or transmit strong or continuous signals require more power than those which have low power sensors and radios.

For example, a satellite with a radar emitter and receiver requires a significant amount of power. 

Communications satellites which receive, process, amplify and retransmit signals sent from users on the Earth or from other satellites require more power than a scientific satellite with only few sensors and a small radio to transmit the data to researchers on the ground.

Power SupplyPower Supply

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Power SupplyPower Supply

Primary power from Solar cells.

Individual cells - small amounts of power

Arrays of cells in series parallel connection.

Higher powers can be achieved with solar panels arranged in the form of rectangular solar coils.

Solar sails must be folded during the launch phase and extended when in geostationary orbit.

Satellite Communication SegmentsSatellite Communication Segments

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The Earth will eclipse a geostationary satellite twice a year (Spring and Autumn).

During eclipse, storage batteries must be provided. Eg. Ni-Cd , Ni-H2 etc…

Stabilization and Attitude controlStabilization and Attitude control

Slide 216

Stabilization and attitude control are necessary to ensure that the satellite maintains the proper attitude. 

Satellites are subjected to a number of forces in space such as particles streaming from the Sun, meteorites, atmospheric drag, gravity from the Moon, gravity gradients and other perturbations.

These forces cause satellites to wobble, spin, drift, or move in other ways not desired. Most satellites which provide visual or electronic images of the Earth or its environment maintain three axis stabilization (roll, pitch and yaw).

Many communications satellites are designed to rotate about their longitudinal axis (roll) and thus have only two axis stabilization.

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Two and three axis stabilization allow sensors and antennas to be pointed in specific directions. 

Devices such as momentum wheels on the satellite help to stabilize the satellite while in orbit.

Position, velocity and attitude data from on board sun sensors, star trackers, horizon scanners and other devices is transmitted to ground control stations.

When momentum wheels and other such passive devices cannot compensate or adjust the orbit, the satellite controllers send signals to the satellite to fire thrusters in short spurts to control roll, pitch, yaw and to make corrects in orbital altitude.

To reduce size, mass, complexity and cost some small satellites are designed to tumble freely through space without any stabilization or attitude control.

Stabilization and Attitude controlStabilization and Attitude control

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Attitude ControlAttitude Control The attitude of a satellite refers to its orientation in space.

Attitude control is necessary, for example, to ensure that directional antennas point in the proper directions.

Controlling torques may be generated in a number of ways. Dampers – Energy Absorbers

Passive attitude control refers to the use of mechanisms which stabilize the satellite without putting a drain on the satellite’s energy supplies.

Examples of passive attitude control are spin stabilization and gravity gradient stabilization.

With Active attitude control, there is no overall stabilizing torque present to resist the disturbance torques.

Methods used to generate active control torques include momentum wheels, electromagnetic coils.

Satellite Communication SegmentsSatellite Communication Segments

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Roll, pitch, and yaw axes

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Spin stabilization in the geostationary orbit. The spin axis lies along the pitch axis, parallel to the earth’s N-S axis.

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.

Spinning satellite stabilizationSpinning satellite stabilization

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Momentum wheel stabilizationMomentum wheel stabilization

The gyroscopic effect of a spinning satellite was shown to provide stability for the satellite attitude.

Stability also can be achieved by utilizing the gyroscopic effect of a spinning flywheel, and this approach is used in satellites with cube-like bodies.

These are known as body-stabilized satellites.

The complete unit, termed a momentum wheel, consists of a flywheel, the bearing assembly, the casing, and an electric drive motor with associated electronic control circuitry.

The term momentum wheel is usually reserved for wheels that operate at nonzero momentum. This is termed a momentum bias.

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.

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A gyroscope or "circle" is a spinning wheel or disc in which the axis of rotation is free to assume any orientation.

When rotating, the orientation of this axis is unaffected by tilting or rotation. 

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Momentum wheel stabilizationMomentum wheel stabilizationAlternative momentum wheel stabilization systems: (a) one-wheel, (b) two wheel, c) three-wheel.

Station KeepingStation Keeping

Slide 2

In addition to having its attitude controlled, it is important that a geostationary satellite be kept in its correct orbital slot.

This results in the satellite drifting back through its nominal station position, coming to a stop, and recommencing the drift along the orbit until the jets are pulsed once again. These maneuvers are termed east-west station-keeping maneuvers.

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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.

Station KeepingStation Keeping

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The rectangular box shows the positional limits for a satellite in geostationary orbit in relation to beams from a 30-m and a 5-m antenna.

Assuming 38,000 km (typical) for the slant range, the diameter of the 30-m beam at the satellite will be about 80 km.

This beam does not encompass the whole of the box and therefore could miss the satellite. Such narrow-beam antennas therefore must track the satellite.

The diameter of the 5-m antenna beam at the satellite will be about 464 km, and this does encompass the box, so tracking is not required.

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Thermal ControlThermal Control

Satellites are subject to large thermal gradients, receiving the sun’s radiation on one side while the other side faces into space.

In addition, thermal radiation from the earth

Equipment in the satellite also generates heat

Thermal blankets and shields may be used to provide insulation. Radiation mirrors are often used to remove heat from the communications payload.

These mirrored drums surround the communications equipment shelves in each case and provide good radiation paths for the generated heat to escape into the surrounding space.

In order to maintain constant temperature conditions, heaters may be switched on (usually on command from ground) to make up for the heat reduction.

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The TT&C subsystem performs several routine functions aboard the spacecraft.

The telemetry, or telemetering, function could be interpreted as measurement at a distance.

Specifically, it refers to the overall operation of generating an electrical signal proportional to the quantity being measured and encoding and transmitting this to a distant station, which for the satellite is one of the earth stations.

Telemetry and command may be thought of as complementary functions.

The telemetry subsystem transmits information about the satellite to the earth station, while the command subsystem receives command signals from the earth station, often in response to telemetered information.

The command subsystem demodulates and, if necessary, decodes the command signals and routes these to the appropriate equipment needed to execute the necessary action.

TT&C SubsystemTT&C Subsystem

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Block diagram form the TT&C facilities used by Canadian Telesat for its satellites.

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TranspondersTransponders

They are Communications channel between the receive and transmit antennas in a communications satellite.

The bandwidth allocated for C-band service is 500 MHz, and this is divided into subbands, one for each transponder.

Atypical transponder bandwidth is 36 MHz, and allowing for a 4-MHz guard band between transponders, 12 such transponders can be accommodated in the 500-MHz bandwidth.

By making use of polarization isolation, this number can be doubled. Polarization isolation refers to the fact that carriers, which may be on the same frequency but with opposite senses of polarization, can be isolated from one another by receiving antennas matched to the incoming polarization.

Carriers with opposite senses of polarization may overlap in frequency, this technique is referred to as frequency reuse.

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TranspondersTransponders

Section of an uplink frequency and polarization plan. Numbers refer to frequency in megahertz.Section of an uplink frequency and polarization plan. Numbers refer to frequency in megahertz.

MHz

Guard band

Transponder BW

12 transponders

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Satellite Transponder ChannelsSatellite Transponder Channels

Uplink Freq. Range Downlink

Freq. Range

Channelling Scheme

Freq. Convertor

Signals are channelized in to freq. bands representing individual transponder BW

TV signal

Telephony

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Wide Band ReceiverWide Band Receiver

Duplicate Receiver

Redundant Receiver

Low Noise Amplifier

Total noise power is expressed as equivalent noise temperature.

Frequency Conversion

Overall Receiver gain

Tunnel Diode, FET – AmplifiersDiodes- Mixers

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Input De-multiplexerInput De-multiplexer

Redundant Receiver

Receiver CouplerPower Splitter

It separates the broadband input in to transponder frequency channels

Frequency channels are arranged in even and odd numbered groups – Greater frequency separation

Each filter has a bandwidth of 36 MHz.

Filters

Ch

ain o

f Circu

lators

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Power AmplifierPower Amplifier

Traveling-wave tube amplifiers (TWTAs) are widely used in transpondersto provide the final output power required to the transmit antenna.

In the TWT, an electron-beam gun assembly consisting of a heater, a cathode, and focusing electrodes is used to form an electron beam.

A magnetic field is required to confine the beam to travel along the inside of a wire helix.

The rf signal to be amplified is coupled into the helix at the end nearestthe cathode and sets up a traveling wave along the helix.

The electric field of the wave will have a component along the axis of the helix.

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Schematic of TWT and Power SuppliesSchematic of TWT and Power Supplies

Electron Beam Gun

wire helix

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Power transfer characteristics of a TWTPower transfer characteristics of a TWT

At low-input powers, the output-input power relationship is linear; that is, a given decibel change in input power will produce the same decibel change in output power.

At higher power inputs, the output power saturates, the point of maximum power output being known as the saturation point.

The saturation point is a very convenient reference point, and input and output quantities are usually referred to it.

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Transfer curve for a single carrierTransfer curve for a single carrier

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The Antenna SubsystemThe Antenna Subsystem

The antennas carried aboard a satellite provide the dual functions of receiving the uplink and transmitting the downlink signals.

The gain of the paraboloidal reflector, relative to an isotropic radiator, is given by

Wavelength , reflector diameter, aperture efficiency= 0.55.

These are fed from horn arrays, and various groups of horns can be excited to produce the beam shape required. Separate arrays are used for transmit and receive. Each array has 146 dual-polarization horns.

The transmit and receive signals are separated in a device known as a diplexer, and the separation is further aided by means of frequency filtering. Polarization discrimination also may be used to separate the transmit and receive signals using the same feed horn.

The polarization separation takes place in a device known as an orthocoupler, or orthogonal mode transducer (OMT).

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Satellite Transponder ChannelsSatellite Transponder Channels

Antenna Subsystem for INTELSAT VI Satellite

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Earth SegmentEarth Segment

The earth segment of a satellite communications system consists of thetransmit and receive earth stations.

The simplest of these are the home TV receive-only (TVRO) systems, and the most complex are the terminal stations used for international communications networks.

Receive-Only Home TV Systems

Planned broadcasting directly to home TV receivers takes place in the Ku (12-GHz) band. This service is known as direct broadcast satellite (DBS) service.

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Block diagram showing a home terminal for DBS TV/FM reception.

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The outdoor unit

This consists of a receiving antenna feeding directly into a low-noise amplifier/converter combination. A parabolic reflector is generally used, with the receiving horn mounted at the focus.

The receiving horn feeds into a low-noise converter (LNC) or possibly a combination unit consisting of a low-noise amplifier (LNA) followed by a converter. The combination is referred to as an LNB, for low-noise block.

The indoor unit for analog (FM) TV

The signal fed to the indoor unit is normally a wideband signal covering the range 950 to 1450 MHz. This is amplified and passed to a tracking filter which selects the desired channel.

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Community Antenna TV SystemCommunity Antenna TV System

One possible arrangement for the indoor unit of a community antenna TV (CATV) system