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A land remote sensing satellite system consists of five major components, each of which is critical to producing useful data:
- 1. Sensors: Optical systems gather light in various spectral (color) bands from Earth's surface and focus it on photosensitive surfaces that convert the light to digital electrical impulses that can be transmitted to Earth electronically. Landsats 4 and 5 collect light in seven spectral bands, ranging from the blue to the infrared. The thematic mapper sensor is capable of distinguishing objects as small as 30 meters across. Landsat 6 carries a higher resolution sensor, able to distinguish objects only 15 meters across.
- 2. Spacecraft and Transmitters: The spacecraft provides a stabilized platform and power for the sensors and their optics, the receiving and transmitting antennas, and the associated electronics necessary to control the spacecraft and to deliver data to Earth Some remote sensing spacecraft may also carry tape recorders to store data until the spacecraft is within sight of a receiving station.
- 3. Receiving Station and Other Communication Components: A ground station may receive data in digital form directly from the satellite as it passes overhead, or, if the satellite is not in a position to communicate with the ground station, through a system equivalent to NASA's Tracking and Data Relay Satellite System (TDRSS). In the latter case, data are passed from the remote sensing satellite to a communication satellite in geosynchronous orbit and then retransmitted to a ground facility. From the ground facility, the data are then passed directly to a processing laboratory.
- 4. Data Processing Facilities: Before the raw data can be converted into photographic images or computer tapes capable of being analyzed by the end user, they must be processed to remove geometric and other distortions inevitably introduced by the sensors. Data that have only had these distortions removed are generally referred to as unenhanced data. For remote sensing applications, large amounts of data manipulation are usually required.
- 5. Interpretation of the Data: After the unenhanced data are processed and converted to computer tapes or photographs, they must be interpreted to provide information for the end user. Part of the interpretation process may involve merging or layering sets of data, usually done with computer image processing programs. A variety of advanced techniques are available to turn remotely sensed data into new products for different users.