![UNIT II
SATELLITE SUBSYSTEMS[1] :
Attitude and orbit control system, telemetry,
tracking, Command and monitoring, power systems,
communication subsystems, Satellite antenna Equipment
reliability and Space qualification.](https://image.slidesharecdn.com/unit-2satellitesubsystems-240930154935-21660256/85/UNIT-2-Satellite-sub-systems-ppt-topics-pdf-2-320.jpg)
UNIT-2 Satellite sub systems ppt topics.pdf
- 1. Satellite Communication Satellite Subsystems Mr. P.Krishna Reddy M.Tech.(Ph.D.) Assistant Professor, Dept of ECE, DIET.
- 2. UNIT II SATELLITE SUBSYSTEMS[1] : Attitude and orbit control system, telemetry, tracking, Command and monitoring, power systems, communication subsystems, Satellite antenna Equipment reliability and Space qualification.
- 3. • A Satellite Is Made Up of Six Major Subsystems: ⚫ Attitude & orbit control system (AOCS) ⚫ T elemetryT racking Command and monitoring System (TT C &M) ⚫ Power supply system ⚫ Communication subsystems ⚫ SP ACECRAFTANTENNAS Satellite Subsystems:
- 4. AOCS (Attitude & orbit control system) Attitude and orbit control is used to control orbit of the satellite, besides helping maintain stabilization and its position. ➢The attitude and orbit of a satellite must be controlled so that the satellite’s antennas point toward the earth. ➢Satellite may deviate from its orbit due to the gravitational forces from the sun, moon and other planets.. ➢These forces changes cyclically over a 24-hour period, since the satellite moves around the earth.
- 5. 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
- 6. It may also deviate from its orbit because of other reasons like: ➢ Irregularities in the earths gravitational fields ➢ Solar pressure from the sun ➢ Variations in the earths magnetic field ▪ At the equator there are bulges of about 65m at longitudes 150 W1650E, with the result that the spacecraft experiences toward the stable point in GEO at longitudes 1050 west and 750 East. ▪ To maintain accurate station keeping, the spacecraft must be periodically accelerated in the opposite direction to the forces acting on it. This is done by firing rocket motors or thrusters, at periodic intervals.It can by controlled byTTC&M systems.
- 7. The control can be effected from the satellite it self and from ground. This AOC (Attitude and orbit control system) subsystem consists of four major parts. ➢ 1.Attitude control system ➢ 2.Orbit control system ➢ 3.Sensors ➢ 4.Propulsion system
- 8. Attitude control system T wo ways to make the satellite stable when is in orbit and weightless. ➢ Spinner satellites -- Satellite can be rotated at a rate between 30 and 100 rpm to create gyroscopic action, which maintain the spin axis in the same direction;such a spacecraft are called Spinners. ➢ Three-axis stabilized satellites – The rotation of the spacecraft about each axis can be commanded from earth by increase or decrease the appropriate momentum speed by using three momentum wheels.This is called three-axis stabilized satellites.
- 9. Spin Stabilization ⚫ Entire spacecraft rotates about vertical axis ⚫ Spinning sensors and payloads ⚫ Cylindrical geometry and solar arrays
- 10. ⚫ Satellite can be rotated at a rate between 30 and 100 rpm. This spin provides a powerful gyroscopic action to maintain the spin axis in the correct direction. ⚫ Such satellite consists of cylindrical drum covered by solar cells and the rocket motors ⚫ The transponders are mounted on the top of the drum. ⚫ It is driven by an electric motors in the appropriate direction to that of the drum, so that the antennas remains pointing towards the earth. ⚫ This opposite motion is called despun. ⚫ The despun section is kept stationary by counter rotation provided by small gas jets mounted on the periphery of the drum.
- 12. Dual Spin Stabilization ⚫ Upper section does not rotate (de-spun) ⚫ Lower section rotates to provide gyroscopic stability ⚫ Upper section may rotate slightly or intermittently to point payloads ⚫ Cylindrical geometry and solar arrays
- 13. Three-axis Body Stabilization A satellite can rotate about the three axis termed as yaw , roll and pitch. When a satellite is stabilization about these axes by using three momentum wheels, it is called as three axis body stabilization ⚫ Advantages ◦ No de-spin required for payloads ◦ Accurate pointing ⚫ Disadvantages ◦ Complex ◦ Added mass
- 15. Two types of motors used on satellites. ➢ T raditional 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.
- 16. Sensors ⚫For attitude control two types of sensors are used 1.Earth sensor 2.Sun sensor
- 17. Earth sensor ⚫ It is apassive infrared device, operating in 14-16 um wave length. ⚫ It senses the infrared rays coming from the horizon. ⚫ There is a sharp temperature difference between the space and earth’ s horizon,as space is cool and earth is warm. ⚫ Two earth sensors are used,positioned 50 north and 50 south of the spin axis.
- 18. Cont….. ⚫ When the spin axis of the satellite is correctly maintained the output of north and south sensors are in phase,otherwise they are in out of phase. ⚫ The phase difference pulses are sent to the earth station and they measure earth aspect angle.
- 19. Sun sensor ⚫ It operates in the visible spectrum and use a photocell for detecting solar radiations. ⚫ There are two solar sensors,one parallel to the spin axis and the other canted 350. ⚫ Pulses from the sensor send to the earth station to determine Solar aspect angle.
- 22. Example
- 23. Example
- 26. Telemetry,Tracking,Command and monitoring System (TT C & M System) Function of TTC&M: Provides the means of monitoring and controlling the satellite operations. Telemetry: a system that reliably and transparently conveys measurement information from a remotely located data generating source to users located in space or on Earth. Tracking: a system that observes and collects data to plot the moving path of an object. Command: a system by which control is established and maintained.
- 27. T TC&M
- 29. Telemetry Modes
- 32. Tracking
- 33. Command
- 35. ⚫ Decoder reproduce command message and produce lock/enable and clock signal. ⚫ Command logic validates the command. 1.default is to reject if any uncertainty of validity 2.drives appropriate interface circuitry . ⚫ Command decoder decodes detects PCM encoding and outputs binary streams in NRZ format ⚫ The function of interface circuitry are latching relays with telesales, pulse commands, level commands and serial and parallel data commands.
- 38. Power supply system ⚫ In communication satellite the electric power supply system provides electrical energy required to run the space craft,when the satellite is in stationary orbit. ⚫ This sub system is further divided into three parts 1.Solar array . 2.Battery. 3.Power supply control circuit. Solar cells are photovoltaic cells that converts solar radiation into electricity and then it is converted to the required voltage level in a power supply. The conversion efficiency of solar cell is around 12% to 15% and typical satellite solar cells generate 1 to 2 KW of power.
- 40. Power systems-3
- 41. Power ⚫ Provides,stores, distributes,and controls electrical power. ⚫ Need power for (basically everything) communications, computers, scientific instruments, environ. control and life support,thermal control,and even for propulsion (to start the rocket engine). ⚫ Solar array:sunlight ➔ electrical power ◦ max.efficiency = 17% (231W/m2 of array) ◦ degrade due to radiation damage 0.5%/year ◦ best for missions less than Mars’ dist.from Sun ⚫ RadioisotopeThermoelectric Generator (RTG): nuclear decay ➔ heat ➔ electrical power ◦ max.efficiency = 8% (lots of waste heat!) ◦ best for missions to outer planets ◦ political problems (protests about launching 238PuO2) ⚫ Batteries – good for a few hours,then recharge
- 42. Power ⚫Dynamic Power Sources ◦ Like power plants on Earth. ⚫Fuel Cells ◦ Think of these as refillable batteries. ◦ The Space Shuttle uses hydrogen-oxygen fuel cells. ⚫The design is highly dependent on: ◦ Space Environment (thermal,radiation) ◦ Shadowing ◦ Mission Life
- 43. Communication subsystems ⚫ Communication:a system that enabling the transfer of information from one point to another . ⚫ T ransmits data to ground or to relay satellite (e.g. TDRS) ⚫ Receives commands from ground or relay satellite
- 44. Repeaters andTransponders A transponder consists of BPF to select the particular channel’s band of frequency, a down converter & an output amplifier.
- 45. Types of payloads/Transponders there are two types of transponders. Those are • Bent pipe transponders and • Regenerative transponders. Bent Pipe Transponders: Bent pipe transponder receives microwave frequency signal. It converts the frequency of input signal to RF frequency and then amplifies it. Bent pipe transponder is also called as repeater and conventional transponder. It is suitable for both analog and digital signals.
- 46. Regenerative Transponders Regenerative transponder performs the functions of Bent pipe transponder. i.e., frequency translation and amplification. In addition to these two functions, Regenerative transponder also performs the demodulation of RF carrier to baseband, regeneration of signals and modulation. Regenerative transponder is also called as Processing transponder. It is suitable only for digital signals. The main advantages of Regenerative transponders are improvement in Signal to Noise Ratio (SNR) and have more flexibility in implementation.
- 47. Fig. shows a typical single conversion bent pipe transponder used on many satellites for 6/4 GHz band. The local oscillator is at 2225 MHz to provide the appropriate shift in frequency from 6 GHz to 4 GHz. Band pass filter after the mixer removes unwanted frequencies. The output power amplifier is usually a solid state power amplifier(SSPA)unless a very high output power is required.
- 48. ⚫ Redundancy is provided for high-power amplifiers in each transponder by including a spare TWTA(travelling Wave tube amplifier) or solid-state amplifier(SSPA) that can be switched if primary power amplifiers fail. ⚫ The lifetime of high power amplifier’s is limited and they represent least reliable component in most transponders. ⚫ Providing a spare HPA in each transponder increases the probability that the satellite will reach the end of its working life with all its transponders working.
- 51. ⚫ Transponders for use in the 14/11-GHz bands normally employ a double frequency conversion scheme ⚫ Incoming 14-GHz carrier is translated to an IF around 1GHz. ⚫ The amplification & filtering are performed at 1 GHz & a relatively high-level carrier is translated back to 11 GHz for amplification by HPA.
- 54. Factors affecting payload design
- 56. SatelliteAntennaTypes Four main type of antennas are used on satellite. These are 1. WireAntennas:Monopole & Dipole 2. HORN:Efficient,Low Gain,Wide Beam 3. REFLECTOR: High Gain, Narrow Beam, May have to be deployed in space 4. PHASED ARRAY:Complex,Electronically steered 56
- 57. ⚫ These are used primarily at VHF & UHF to provide communications for the TTC&M systems. ⚫ They are positioned with great care on the body of the satellite in an attempt to provide Omni directional coverage. ⚫ An antenna pattern is a plot of the field strength in far field on antenna. ⚫ It is usually measured in decibels(dB). Wire Antennas:
- 58. Horn Antennas ⚫ These are frequencies used at microwave when relatively wide global beams are required, as for coverage. ⚫ A horn is a flared section of waveguide that provides an aperture several wavelengths wide and a good match between the waveguide impedance used as feeds for and free space. ⚫ Horns are also reflectors
- 59. Reflector Antennas ⚫ The most often used antenna for satellite systems, particularly for those operating above 10 GHz, is the parabolic reflector antenna. ⚫ These are usually illuminated by one or more horns and provide a lager aperture compared to horn antenna. ⚫ Parabolic reflectors offer a much higher gain than that achievable by the horn antenna alone.
- 60. ArrayAntennas: ⚫ Array antennas are also used on satellites to create multiple beams from a single aperture. ⚫ Multiple beams are formed by combining the radiation from several small elements made up of dipoles, horns etc. ⚫ Iridium and Globalstar used these kind of antennas to generate up to 16 beams from a single aperture for their LEO mobile telephone systems. ⚫ These antennas generate high gain output signals which are preferred for long distance communication.
- 65. Equipment Reliability & Space qualification: ⚫ Once satellite is in geostationary orbit,there is little possibility of repairing components that fail or adding more fuel for station keeping. ⚫ The component that make up the satellite must therefore have very high reliability. ⚫ T wo approaches are used:Space qualification of every part of the satellite to ensure that it has a long life expectancy in orbit & redundancy of most critical components to provide continued operation when one component fails.
- 66. Space Qualification: distances, is a harsh ⚫ Outer space, at geostationary orbit environment. ⚫ The sun irradiates the satellite with 1.4 kW of heat and light on each square meter of exposed surface. ⚫ When surfaces are in shadow, surface temperature will fall toward absolute zero. ⚫ Electronic equipment can’t operate at such extremes of temperature & heated or cooled so that it’s temp. stays within the range 0˚to 75 ˚C.This requires a thermal control system. ⚫ The first stage in ensuring high reliability in a satellite is by selection and screening of every component used. ⚫ Past operational & test experience of components indicates which components can be expected to have good reliability. ⚫ Each component is tested individually to ensure that it meets its specification. This process is known as quality control or quality assurance & is vital in building any equipment that is to be reliable.
- 67. ⚫ Once individual components and subsystems have been space qualified, the complete satellite must be tested as a system to ensure that its many systems are reliable. ⚫ When a satellite is designed, three prototype models are often build and tested. Mechanical model- It contains all the structural & mechanical parts that will be included in the satellite & is tested to ensure that all moving parts operate correctly in vacuum, over a wide temperature range. It also subjected to vibration & shock testing. Thermal model- It contains all electronics packages and other components that must be maintained at the correct temperature. Electrical model- contains all the electronic parts of the satellite and is tested for correct electrical performance under total vacuum and a wide range of temperatures. ⚫ Testing carried out on models is designed to overstress the system and induce failure in any weak components. ⚫ Temperature cycling will be carried out to 10% beyond expected extremes;structural loads 50% above those expected may be applied. ⚫ Electrical equipment will be subjected to excess voltage and current drain to test for food electronic and thermal reliability .
- 68. Reliability: ⚫ We need to be able to calculate reliability for two reasons: We want to know what the probability is that subsystem will still be working after a given time period, & we need to provide redundant components or subsystems where probability of failure is too great to be accepted. ⚫ Reliability is a mathematical technique which gives an idea about the probability of failure. a b
- 69. Reliability Cont.. ⚫ The reliability of a component can be expressed in terms of probability of failure after time t. ⚫ For most electronic equipment,probability of failure is higher at beginning of life- the burn – in period. ⚫ As the component ages,failure becomes more likely . ⚫ Mathematically reliability of device or sub-system is defined as,
- 70. Where ti = time after which ith device fails. ⚫ MTBF is related to average failure by The failure rate ⅄ is often given as average failure rate per 109 hours. In terms of reliability of subsystem or device it is given by The other improvement parameter in reliability study is the meantime before failure (MTBF) and is given by
- 71. Redundancy: ⚫ By using redundant components, reliability of subsystem can be increased. ⚫ The redundant device can incorporate with the existing operating devices/ components either in series or in parallel or may be mixed.
- 72. END