A SATELLITE IS an airborne autonomous platform that carries a set of sensors to capture information on the surface of the earth, including vegetation, water masses, ice, and the atmosphere. The scientific and technological field dealing with the conception, design, and control of satellites as well as processing and interpretation of space imaging is remote sensing.
The sensor of the satellite captures energy—electromagnetic radiation—of a certain wavelength and geographical area, producing a scene or image. The sensors capturing the energy reflected or emitted are called “passive,” while those that send the energy that will be returned by the surface and thereafter captured are termed “active,” like in satellites Radarsat or ERS-2. Active sensors operate in the region of microwave length, which allows them to capture information in almost any atmospheric condition, while passive sensors experience strong limitations with cloud coverage.
The resolution of the satellite determines the accuracy of the information provided and ultimately the field of application. There are four resolutions of interest: spatial, spectral, temporal, and radiometric. The spatial resolution resolves the smallest spatial feature represented (or the size of the image pixel). Spatial resolution is, in turn, conditioned by the IFOV (Instantaneous Field of View), the angle corresponding to a certain ground area of the earth surface captured at a certain moment.
The spectral resolution refers to the number of bands and the width of the bands captured by the sensor, expressing the capability of resolving features or phenomena at various wavelengths. Sensors are commonly multi-spectral, capturing various bands of the spectrum for the same area at the same time. Hyperspectral sensors register information in numerous (up to several hundred), very narrow, contiguous spectral bands along the spectrum. The temporal resolution is the frequency, or repeat time, with which a satellite passes over the same geographical area. The radiometric resolution indicates the capability of distinguishing between coverages with close response.
The regions of the electromagnetic energy spectrum operated are ultraviolet (UV), a wavelength in the range of three nanometers to 0.4 micrometers; visible (0.4–0.7 micrometers); infrared (IR), in the range of 0.7–1,000 micrometers; and microwave (one millimeter to one meter). Within the visible region blue, green, and red bands are differentiated; within the IR region near-, mid-, and thermal-infrared are distinguished.
There are various types of satellites for different mission objectives. Military satellites are deployed to accomplish restricted national defense goals of reconnaissance and surveillance, although images from some older decommissioned ones have been made public.
Meteorological observation has been one of the first civil applications, with a focus on weather prediction. Geostationary Operational Environmental Satellite (GOES) covers the hemisphere, including North and South America. European Meteosat, Japanese GMS, or Indian Insat serve similar purposes. Ocean monitoring and other atmospheric observation satellites are Seasat, Nimbus or SeaWiFS.
Earth observation, however, has been the more developed application. Landsat, a pioneering program initiated in 1967, continues today with Landsat-7. This mid-resolution satellite has long maintained Thematic Mapper as a sensor, allowing it to monitor long-term processes. The first SPOT was launched by France in 1986 with a revisit time of 26 days and five spectral bands—from visible to midinfrared—and a spatial resolution of 10 meters in panchromatic mode and 20 meters in multi-spectral. The satellite Terra integrates multiple sensors with multiple objectives, which makes it adequate for multiple applications. NOAA satellite series have addressed these same objectives with a single sensor (AVHRR).
Earliest initiatives responded to national schemes with durable scientific programs, such as NOAA, Landsat, SPOT, or IRS, but soon other countries sought to guarantee control over information about their resources and environmental monitoring by launching their own programs. Private initiatives started in 1999 with Ikonos-2, followed by Quickbird-2 and Orbview-3. Their commercial perspective led to reducing spectral resolution and increasing spatial and temporal resolution.
Geographic Information Science; Global Positioning Systems (GPS); Maps; Remote Sensing; Topographic Maps.
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