### Evolution and advances in remote sensing satellites and sensors for the study of environments  
There are eight distinct eras of remote sensing; some running parallel in time periods, but are distinctly unique in terms of technology, concept of utilization of data, applications in science, and data characteristics (e.g., Table 1). These are discussed below:  
**Airborne remote sensing era:** The airborne remote sensing era evolved during the first and the Second World War ([PERSON] and [PERSON], 1992, [PERSON], 1983). During this time remote sensing was mainly used for the purposes of surveying, reconnaissance, mapping, and military surveillance. **Rudimentary spaceborne satellite remote sensing era:** The spaceborne remote sensing era began with launch of \"test of concept\" rudimentary satellites such as Sputnik 1 from Russia and Explorer 1 by the United States at the end of 1950s ([PERSON], 1993, [PERSON] et al., 1986). This was soon followed by the first meteorological satellite called Television and Infrared Observational Satellite-1 (TIROS-1) by the United States also in the late 1950s ([PERSON] et al., 1986). **Spy satellite remote sensing era:** During the peak of the cold war, spy satellites such as Corona ([PERSON] et al., 1988) were widely used. Data was collected, almost exclusively, for military purposes. The data was not digital, but was produced as hard copies. However, the spin-off of the remote sensing developed for military purposes during the above 3 eras spilled over to mapping and slowly into environmental and natural resources applications. **Meteorological satellite sensor remote sensing era:** The early meteorological satellite sensors consisted of geo-synchronous Geostationary Operational Environmental Satellite (GOES) and polar-orbiting National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) ([PERSON], 2002). This was an era when data started being available in digital format and were analyzed using exclusive computer hardware and software. This was also an era when global coverage became realistic and environmental applications practical. **Landsat era:** The Landsat era begins with the launch of Landsat-1 (then called Earth Resources Technology Satellite) in 1972 carrying multi spectral scanner (MSS) sensor. This was followed by other path-finding Landsat satellites 2 through 3 which carried MSS and 4 and 5 which carried Thematic Mapper (TM). The Landsat 7 carries Enhanced Thematic mapper (ETM+) sensor. The Landsat-6 failed during launch. The Landsat-8 carrying Operational Land Imager (OLI) is planned for launch in 2011. The Landsat era also has equally good sun-synchronous Land satellites such as Systeme Pour l'Observation de Ia Terre (SPOT) of France and, Indian Remote Sensing Satellite (IRS) of India ([PERSON], 2000). These satellites have high resolution (nominal 2.5-80 meter) and have global coverage potential. At this resolution, only Landsat is currently gathering data with global wall to wall coverage. This is, by far, the most significant era that kick started truly wide environmental application of remote sensing data locally and globally. **Earth Observing System era:** The Earth Observing System (EOS) era ([PERSON], 2005, [PERSON] et al., 2001, [PERSON], 2000, [PERSON], 1983) began with launch of Terra satellite in 1999 and has brought in the global coverage, frequent repeat coverage, high level of processing (e.g., georectified, at-satellite reflectance), easy and mostly free access to data. The Terra\(\backslash\)Aqua satellites carrying sensors such as Moderate Resolution Imaging Spectroradiometer (MODIS) and Measurements of Pollution in the Troposphere (MOPITT) have daily re-visit and various processed data. Applications of sensor data have become wide spread and applications have multiplied. Institutions and individuals who never used remote sensing have begun to take an interest in remote sensing.