22-08-2012, 11:23 AM
EARTH OBSERVATION SATELLITE
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INTRODUCTION
EARTH OBSERVATION SATELLITE:-
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc.
Most Earth observation satellites carry instruments that should be operated at a relatively low altitude. But altitudes below say 500-600 kilometers are in general avoided, though, because of the significant air-drag at such low altitudes making frequent orbit raising manoeuvres necessary
To get a (close to) global coverage with such a relatively low orbit it has to be polar or at least close to polar. As such a rather low orbit will have an orbital period of roughly 100 minutes the Earth will rotate around its polar axis with about 25 deg between successive orbits with the result that the ground track is shifted towards west with these 25 deg in longitude.
For spacecraft carrying instruments for which an altitude of 36000 km is suitable the Geostationary orbit is sometimes the preferred choice. From such an orbit one gets uninterrupted coverage of more than 1/3 of the Earth. With 3 geostationary spacecraft positioned over the equator at longitudes separated with 120 deg the whole Earth is covered except the extreme polar regions. This type of orbit is mainly used for meteorological satellites
LITERATURE VIEW OF EARTH OBSERVATION
SATELLITE
During World War I reconnaissance aircraft flew over enemy forces in order to observe troop movements. By using ordinary cameras mounted onto the aeroplanes, the precursors of modern remote sensing systems were developed. Based upon the photographs it was possible to observe the position and strength of enemy forces.
During World War II the technique was further developed. As part of preparations for the Normandy Invasion (D-day), aerial photographs were used to map coastal conditions to identify the most suitable sites on which to land. By measuring waves close to the coast it was possible to determine wavelength and thereby calculate water depth. Furthermore, infrared film was used to identify green vegetation and distinguish it from camouflage nets.
Development of other remote sensing technologies continued apace. Mapping experiments were performed with airborne radar systems. TIROS 1, the first weather satellite, was sent into orbit in 1960. It provided the US Weather Bureau with daily images of cloud formation and represented a milestone in weather forecasting.
The development of non-photographic remote sensing technology progressed rapidly after the first mapping satellite, Landsat 1, was put in orbit in 1972. It was equipped with a new type of sensor known as a multispectral scanner (MSS). With this new technology, data were produced in the form of digital chorological matrices enabling substantial advances in image processing.
Today the scanner is a very important instrument in remote sensing. It is used on land, in aircraft and onboard satellites. The detectors on each scanner are designed to receive radiation in specific channels. The number of channels, their width, and their location in the electromagnetic spectrum vary for each sensor, resulting in different spectral resolution characteristics. This combination of factors determines the uses for which the sensor's imagery are most suited.
WORKING OF EARTH OBSERVATION SATELLITE
Radar sensors:-
A radar sensor system emits the radiation that it ultimately records and is therefore classified as an active sensor. Passive sensors, on the other hand, are dependent upon receiving reflected sunlight or thermal infrared emissions. Examples of these passive systems are the multispectral sensors discussed in the above section.
In simple terms, the radar sensor sends pulses of energy down towards the surface of the Earth. A portion of the energy is reflected and returns as an 'echo' signal. The strength of the returned 'echo' will depend on the roughness and moisture content of the surface and the degree and orientation of sloping in relation to the radar beam. The delay of the 'echo' reveals the distance to the reflecting surface.
Radar sensors use energy emitted at longer wavelengths which can penetrate clouds and haze effectively, and can therefore acquire imagery at night. This provides a significant advantage over passive satellites that are hampered by clouds and require sunlight to acquire detailed imagery.
Radar sensor systems are used both in aircraft and satellites. Their images can reveal topographical details, and if the same area is sensed from two different angles, the object's distance from the satellite can be calculated, and consequently, its height above sea level inferred (interferometry). These data can then be used in three-dimensional mapping. Such terrain models are used, for example, in the control system of missiles which can find their own way to targets. The missile control system can compare the landscape over which it is passing with the installed terrain model and can navigate automatically to its target. The data can also be used for a range of other applications, such as assessing the impact of flooding.
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.
Commands to Earth observation satellites (Command Processing):-
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.
Image Data Transmission to the Ground (TLM (telemetry)):-
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.
CONCLUSION
At the macro level, ISRO has three satellite programmes: geo-stationary, remote sensing and small/experimental satellites. The geo-stationary satellites are largely communication satellites used for telecommunications, television broadcasting, internet and other purposes, while remote sensing or earth observation satellites send back pictures and other data for use.
Just as the invention of the mirror allowed humans to see their own image with clarity for the first time, Earth observations from space have allowed humans to see themselves for the first time living on and altering a dynamic planet.