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Overview |
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Earth observation satellites are used to observe the earth surfaces, sea surfaces, ocean currents, clouds, etc., from space. Observation instruments are installed on satellites for remote sensing purposes. There is a relatively short development time for a long duration mission. Once launched, it has the advantage of being able to observe wide areas. Instrument operations can be easily performed from the console of a ground control station. Issues with satellite-based remote sensing are as follows: a lengthy development time from planning to actual operations, significant initial investments, risk of launch failure and inability to repair the satellite up in space.
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Subsystems of Earth Observation Satellites |
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A typical earth observation satellite consists of the following subsystems. |
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Observation Instrument |
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An observation instrument is a key element of the earth observation satellite. It acquires and transmits earth observation data to a data-recording unit and a telemetry unit. |
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Command Subsystem |
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A command subsystem sends commands to the rest of the units on the satellite.
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Telemetry Subsystem |
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A telemetry subsystem transmits observation data and status data of onboard units to the ground. |
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Data Storage Subsystem |
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A data storage subsystem temporarily stores image data and housekeeping data of onboard subsystems. |
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Navigation Subsystem |
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A navigation system generates information necessary for determination of the satellite position. |
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Attitude Control Subsystem |
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An attitude control subsystem generates information necessary for determination of the satellite orientation against the earth. |
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Propulsion Subsystem |
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A propulsion subsystem generates thrust necessary for correcting the orbit and the attitude of the satellite. |
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Power Subsystem |
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A power subsystem generates and distributes electric power required for operating the satellite. |
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Thermal Control Subsystem |
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A thermal control subsystem maintains temperatures of onboard units within specified ranges to assure proper operations and expected lifetime. |
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Processing Subsystem |
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This controls the entire satellite operations.
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Commands to Earth observation satellites (Command Processing): |
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Various commands are sent to the satellite subsystems for observation and housekeeping purposes, to determine the timing and locations for observations and to maintain the satellite in a healthy state. There are three different types of commands: commands uplinked from the ground station, commands stored in the satellite and executed at programmed times, and commands autonomously executed in predefined satellite conditions. |
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Image Data Transmission to the Ground (TLM (telemetry)): |
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Image and housekeeping data generated by earth observation instruments are transmitted from the satellite to the ground station in the form of digital signals. There are two different modes: the realtime mode transmits the data down to the ground as they are being generated, and the non-realtime mode stores the data in a data recorder and transmits it down to the ground when the satellite comes within the scope of the ground station. The range where the satellite and the ground station can communicate is limited. The data collected outside of that range will be transmitted to the ground when the satellite is in the range, where it can communicate with the ground.
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Satellites are delivered up to the space by a rocket or the space shuttle and placed in flight routes called "orbits". In general, a satellite orbit is an elliptical orbit, which has the earth as one of its focal points. A circular orbit, which is a special case of elliptical orbits, is usually used for earth observation satellites. In this case, the orbit is determined by six orbital parameters: altitude from the ground (altitude), angle of the plane of the orbit against the equator (inclination), time at which the orbit crosses the equator from south to north (ascending node) and so on. Although an infinite number of theoretical orbits exist, geostationary orbits, polar orbits, and sun-synchronous orbits are three major types of orbits most often used for earth observation missions. Selection of an orbit affects the observation area, the recurrence cycle, and the spatial resolution of the earth observation satellite. The maximum latitude of observation areas is determined by the orbit inclination. The recurrence cycle is determined by the orbit altitude. Spatial resolution decreases as the orbit altitude increases because the satellite is farther away from observation targets on the earth.
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Orbit Transition from Launch to Geostationary Orbit |
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A satellite launched from the ground first travels in an elliptical orbit called a transfer orbit.
In the next step, the apogee engine installed in the satellite is ignited at the apogee (the point which is the furthest from the earth), and the satellite moves into a semi-circular orbit called a drift orbit, the radius of which is the distance between the apogee and the center of the earth.
After placed into the drift orbit, it will reach the final circular orbit by repeated fine tunings. |
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Geostationary Orbit |
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| If a satellite revolves around the earth at the same speed as the earth rotates (i.e. the orbital period is 24 hours), the satellite can always be seen at the same place from the ground. This is useful for constant observation for a single place on earth. The weather satellite "Himawari" is a satellite of this type. In this case, the altitude of the geostationary orbit is approximately 36,000 Km and the inclination is 0 degree (parallel to the equator). |
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Polar Orbits (Relation to Observation Areas) |
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A satellite in a polar orbit passes right over the North Pole and the South Pole, and its inclination is 90 degrees. If the orbit inclination is x degrees, the orbit is projected to the ground within latitudes +/- x degrees. It is important to understand the relationship between observation areas and an orbital inclination. Only the equatorial region can be observed when the orbital inclination is 0 degree. The areas of the southern and northern hemispheres between the equator and the 45-degree latitude line can be observed when the orbital inclination is 45 degrees. Finally, the entire surface of the earth can be observed when the orbital inclination is 90 degrees. Therefore, the orbital inclination needs to be 90 degrees to observe the entire earth surface. A sun- synchronous orbit is a type of orbit whose angle of the orbital plane against the sun is unchanging. This type of orbit is often used for earth observation satellites since the satellite appears at the same local time every day at any location on the ground. Typical earth observation satellites such as Landsat, Japan's Earth Resources Satellite (JERS) and SPOT are all revolving in sun-synchronous orbits.
This is not a question for a geosynchronous satellite since it always observes the same place. When using a polar orbit, it is critical for observation planning to consider how many days it takes to return to the same observation location.
This is the recurrence cycle.
A recurrence cycle is determined by an orbital altitude. |
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Earth observation satellites are classified by application, orbit, and attitude-control method as follows:
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Applications : |
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Meteorological observation (atmospheric observation)
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Land observation
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Sea observation
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Oceanic wind observation
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Orbits : |
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Geostationary orbits
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Polar orbits
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Attitude Control Methods : |
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Spin stabilized method |
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Three-axis stabilized method |
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The following controls are necessary for earth observation satellites.
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Descriptions of Controls |
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Orbit control |
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Orbit control is to ensure that a satellite flies in a designated orbit. Sometimes it is necessary to ignite thrusters to maintain the altitude of the orbit when the satellite flies in a low-altitude orbit, or it will gradually fall into a lower orbit because of air resistance. In the case of a geosynchronous satellite, it will not fall due to air resistance. However, the position of a geostationary satellite will fluctuate due to influence of non-uniformity of the earth's gravitational field and the gravitation forces of the sun and the moon. Firing thrusters is sometimes required to restore the satellite back to the designed position.
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Attitude control |
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Attitude control is for a satellite to maintain its attitude. The attitude of the satellite is calculated using data from an earth sensor, sun sensor or star sensor. Thrusters of the attitude control system are required to fire to restore the satellite back to its designed position when the satellite has drifted. This is a critical function in earth observation because it directly influences the image quality.
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Pointing control of observation instruments |
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Some earth observation instruments can change their observation direction, By commands sent from the earth. This is a critical function in earth observation because it directly influences the observational opportunities.
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Thermal control |
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Temperatures are controlled within acceptable temperature ranges for all subsystems, including observation instruments. There are two types of thermal control: passive control which depends on passive materials or devices such as surface treatments, heat pipes and radiators; and active control which depends on active devices such as coolers and heaters. It influences the performance and longevity of the observation instruments.
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Electric power control |
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This subsystem generates electric power required for onboard subsystems including observation instruments, converts to and delivers the right voltages. While it usually utilizes the electric power generated by solar cells, it also uses batteries when the satellite cannot acquire sufficient sun light during an eclipse in the shadow of the earth.In the event of an excess current anomaly, which may be caused by a short-circuit, a safe-guard function is activated and it separates the faulty circuit from the rest of the circuits to protect them.
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Command control |
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Commands are sent to onboard subsystems, including the observation instruments. There are two types of commands: stored commands which are temporarily stored in the control computer of the satellite and executed at a designated time, and realtime commands which are executed upon reception from the ground station.
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Telemetry control |
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This subsystem transmits housekeeping data of onboard subsystems including observation instruments, data of temperatures at various locations on the satellite, image data, and other data down to the ground.
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Communication control |
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Communication control is for command reception and telemetry transmission between the ground and the satellite.
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Copyright (c) 2000-2010 Earth Remote Sensing Data Analysis Center (ERSDAC)
All rights reserved.
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