ALOS Data Users Handbook

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Section 3 Outline of ALOS Satellite System3.3.4 Attitude and Orbit Control Subsystem (AOCS)ALOS's Attitude and Orbit Control Subsystem (AOCS) has function to acquire information of satellite attitudeand location by using Earth Sensor (ESA), Inertia Reference Unit (IRU), Star Tracker (STT), and GPS Receiver(GPSR), and also has function to keep satellite correct attitude and orbit by driving Reaction Control Subsystem(RCS) gas jet, Reaction Wheel (RW) and Magnetic Torquer (MTQ).To make world map of 1/25,000 scale derived from PRISM's 2.5 meters resolution image, spacecraft's attitudemust be kept and determined precisely as we have never developed. The specifications of AOCS are as follows:• Pointing Stability (Maximum of satellite’s stagger)- Four ten-thousandths of degrees per 5 seconds or less (4.0e-4deg/5sec)• Pointing Determination Accuracy (error of satellite’s direction)- Four ten-thousandths of degrees (4.0e-4deg) at onboard processer- Two ten-thousandths of degrees (2.0e-4deg) at off-line processing on the ground• Location Determination Accuracy (error of orbit satellite): 1mALOS Star Tracker (STT) can determine satellite attitude with high accuracy of 0.0002deg comparing observedlocation and magnification of stars with its own star catalog. The large three hoods are installed to shield stardetector from direct or reflected Sun light. STT mormally operates by using STT-1 facing forward and STT-3facing backward. However in the case of interference of the Moon etc, STT will change its operation mode usingSTT-2 located at center to avoid malfunction for attitude detecting. Surface of the hoods are covered with MultiLayer Insulation (MLI) to keep STT's function normally controlling its thermal condition in the severeenvironment of direct Sunlight and darkness of night.There are two frequencies (L1 and L2) of GPS signals we can receive from GPS satellites. General GPSreceivers only receive L1 signal, but ALOS has dual frequency GPS receivers that can receive both L1 and L2signals. And those receivers can measure a phase of carrier wave of those signals. It is necessary to correct anerror of inflected radio wave in ionosphere (about 10m error occurs) to meet a high location accuracyrequirement of less than 1m, and the requirement can be achived by the dual-frequency phase measuring GPSreceivers. That’s why the GPS receiver is adopted by ALOS.3-26
ALOS Data Users Handbook3.3.5 Satellite Structure (STR)With high resolution observing sensors, ALOS's performance of observation will be changed for the worse bythermal distortion of the structure of spacecraft. To avoid this thermal distortion, the main truss structure ofALOS was made from carbon fiver reinforced plastics (CFRP) and aluminum joints connect those truss. Thermalexpansion of aluminum joints are designed to be cancelled by CFRP truss, and each CFRP truss in the busstructure are covered by silver thermal insulators to minimize those distortion. In addition, PRISM's wholesystems are covered by black polyimide insulators to keep and control its thermal range within 0.5 degree inorbit. In launch condition, maximum acceleration is three times as much as the land surface, so the satellite mustbe designed to endure those heavy loads.3.3.6 Thermal Control Subsystem (TCS)ALOS control its thermal condition by two ways to keep its component within proper thermal range. The one ispassive control installing proper insulators or radiators on the surface of the satellite to control its thermalcondition by their optical properties. The other is active control switching heaters, moving louvers and so onaccording to the satellite's thermal condition.Sheets covering the satellite, silver, gold and black, are thermal blankets and almost all satellites use thoseblankets to keep their thermal condition. Some thermal elements like heaters and heat pipes are integrated insidethis model. TTM thermal vacuum test was conducted to confirm their function and satellite's design for thermalcontrol.To trash the waste heat out, radiators are used to radiate the heat to deep space. Thermal insulators (Multi LayerInsulation: MLI) are used to protect direct solar radiation or inappropriate heat loss from inside the satellite.ALOS is designed to control some thermal-sensitive components and sensors actively using heaters, heat pipes,and some thermal elements.Some heaters are switched by thermostats, and others are controlled by the Heater Control Electronics (HCE) bytemperature data measured by thermometers. PRISM, the most thermal-sensitive sensor on ALOS, is designed tobe controlled within the range of 0.5 °C at any satellite thermal conditions.3-27
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ALOS Data Users HandbookSection 8 D
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ALOS Data Users Handbook8.5 Outline
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APPENDIXALOS Data Users Handbook
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Appendix 1 Acronyms and Abbreviatio
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ALOS Data Users HandbookAppendix 2
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ALOS Data Users Handbook

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