Space Segment

space segment satellites (TT&C)

equipment carried aboard the satellite can be classified acc equipment carried aboard the satellite can be classified acc. to function. payload -equipment used to providethe service transponder forms one of the main sections of the payload Bus- refers not only the vehicle which carries payload, but also to the various subsystems which provide power, attitude control, orbital control,thermal control, and command and telemetry functions required to service the payload.

Anatomy

POWER SUPPLY primary electrical power -solar cells required arrays of cells in series-parallel connection are required -Higher powers can be achieved with solar panels arranged in the form of rectangular solar sails. capable of greater power output than cylindrical arrays

ATTITUDE CONTROL Attitude -orientation in space to ensure that directional antennas point in the proper directions. Stabilizing a satellite is attitude control Passive Active

Passive systems stabilize the satellite without putting a drain on the satellite’s energy supplies Eg: spin stabilization & gravity gradient stabilization(interaction of the satellite with the gravitational field of the central body) Active control maintains the satellite attitude by sensing its orientation along the three axes and forming corrections based on these measurements. momentum wheels(3-axis/body satabilization) electromagnetic coils(for satellites relatively close to the earth), and mass expulsion devices

active control subsystem has three components one that senses the current attitude of the platform one that computes the deviations in the current attitude from the desired attitude one that controls and corrects the computed errors.

Yaw- towards earth’s center Pitch-normal to orbital plane Roll-perpen.to other.

Sensors are used to determine the position of the satellite axis with respect to specified reference directions (commonly used reference directions are Earth, sun or a star). Earth sensors sense infrared emissions from Earth and are used for maintaining the roll and the pitch axis. Sun and star sensors are generally used to measure the error in the yaw axis. The error between the current attitude and the desired attitude is computed and a correction torque is generated in proportion to the sensed error.

Momentum wheel stabilization uses reaction wheels or momentum wheels to correct orbit perturbations

The basic control technique used in momentum wheel is to speed up or slow down the momentum wheel depending upon the direction in which the satellite is perturbed. The satellite rotates in a direction opposite to that of speed change of the wheel.

momentum wheel consists flywheel the bearing assembly, the casing, and an electric drive motor with associated electronic control circuitry.

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

In reaction wheels tech,3reaction wheels are used, one for each axis In reaction wheels tech,3reaction wheels are used, one for each axis. They can be rotated in either direction depending upon the active correction force. momentum wheel is operated with zero momentum bias

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.

Spin is initiated during the launch phase by means of small gas jets. overall effect is -spin rate will decrease, and - -the direction of the angular spin axis will change. Impulse-type thrusters, or jets, can be used to increase the spin rate and to shift the axis back to its correct N-S orientation. Spin rate is typically in the range of 50 to 100 rev/min.

two types simple spinner and dual spinner In simple spinner configuration, the satellite payload and other subsystems are placed in the spinning section, while the antenna and the feed are placed in the de-spun platform. In dual spinner configuration, the entire payload along with the antenna and the feed is placed on the de-spun platform and the other subsystems are located on the spinning body. In both configurations, solar cells are mounted on the cylindrical body of the satellite.

The requirements on the attitude and orbit control differ during the launch phase and the operational phase of the satellite. Orbit control is required to correct for the effects of perturbation forces. During the launch phase, control system maintains the correct attitude of the satellite so that it is able to maintain a link with the ground Earth station and controls its orientation such that the satellite is in the correct direction for an orbital manoeuvre.

When the satellite is in orbit, the attitude control system maintains the antenna of the satellite pointed accurately in the desired direction. The precision with which the attitude needs to be controlled depends on the satellite antenna beam width. Spot beams and shaped beams require more precise attitude control as compared to Earth coverage or regional coverage antennas.

Thermal Control sun’s radiation thermal radiation from the earth and the earth’s albedo Equipment in the satellite

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

TT & C support the function of spacecraft management which are vital for successful operation of all satellite. treated separately from communication management

functions of a TT&C system Monitor the performance of all satellite sub systems & transmit the monitored data to the satellite control centre Support the determination of orbital parameters. provide a source to earth stations for tracking Receive commands from the control centre for performing various function of the satellite.

Telemetry subsystem 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(ES) Data include attitude information environmental information Such as the magnetic field intensity and direction, the frequency of meteorite spacecraft information temperatures,power supply voltages, and stored-fuel pressure

telemetry data rates are in the range 150-100bps. telemetry signal is commonly used as a beacon by ground stations for tracking. Distributed telemetry systems are increasingly being favored.

digital encoders are locked in each sub system of the satellite and data from each encoder are located in each sub system of satellite data from each encoder are sent to a central encoder via a common, time shared bus. reduces number of wire connections also permits easy expansion of the initial design and facilities testing during assembly of the satellite.

Command Subsystem Receives commands transmitted from the ES, verifies reception and executes these commands. Examples Transponder switching Switch matrix reconfiguration Antenna pointing control Controlling direction and speed of array drive Battery reconditioning Thruster firing Switching heaters of the various sub systems.

demands verification of each command by the satellite control centre before execution. To reduce the impact of high bit error,coding and repetition of data are employed. by combining the outputs of two receive chains bit rates around 100bps.

Tracking Subsystem angular and range tracking. angular tracking –monopulse technique range of a satellite - round-trip time delay of the signal. time delay is obtained by measuring the phase difference between the transmitted and the received tunes. -transmission of pseudo-random digital data.

Transponder

input band limiting device (a band pass filter) Input low-noise amplifier (LNA), designed to amplify the signals received from the earth station A frequency translator (composed of an oscillator and a frequency mixer) used to convert the frequency of the received signal to the frequency required for the transmitted signal An output band pass filter A power amplifier (this can be a traveling-wave tube or a solid state amplifier)

The uplinked signal to satellite is 6GHz The uplinked signal to satellite is 6GHz.it is received at the satellite and then amplified using a Low Noise Amplifier(L.N.A). This amplified signal is then down converted at 4GHz. It is sent through a filter and then power amplifier(TWT). The local oscillator frequency of the down converter is 2225MHz for C band and Ex-C band. This signal is then retransmitted at earth ground station.

wideband receiver

first stage - low-noise amplifier (LNA). amplifier adds little noise to the carrier being amplified, and at the same time it provides sufficient amplification for the carrier to override the higher noise level present in the following mixer stage mixer stage-which requires a local oscillator signal for frequency-conversion process. The power drive from the LO to the mixer input is about 10 dBm. The oscillator frequency must be highly stable and have low-phase noise.

second amplifier- provide an overall receiver gain of about 60 dB.

Input demultiplexer input demultiplexer separates the broadband input [covering the frequency range 3.7 to 4.2 GHz], into the transponder frequency channels.

channels are usually arranged in even-numbered and odd-numbered groups to reduce adjacent channel interference output from 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, and the channelizing is achieved by means of channel filters connected to each circulator

Power amplifier A separate power amplifier provides the output power for each transponder channel each power amplifier is preceded by an input attenuator-to permit the input drive to each power amplifier to be adjusted to the desired level. fixed section and a variable section

Traveling-wave tube amplifiers (TWTAs)

To provide amplification over a very wide bandwidth.

Transparent Transponder Amplification and Frequency Translation and No Processing Figure Transparent Transponders

Processing/Regenerative Transponder Amplification and Frequency Translation along with Signal Processing