NASA, with the co-operation of the U.S. Department of Interior, began a conceptual study of the feasibility of a series of Earth Resources Technology Satellites (ERTS). ERTS-1 was launched in July 23, 1972, and it operated until January 6, 1978. It represented the first unmanned satellite specifically designed to acquire data about earth resources on a systematic, repetitive, medium resolution, multispectral basis. It was primarily designed as an experimental system to test the feasibility of collecting earth resources data from unmanned satellites. About 300 individual ERTS-1 experiments were conducted in 43 US states and 36 nations. Just prior to the launch of ERTS-B on January 22nd 1975, NASA officially renamed the ERTS programme as the "LANDSAT" programme. All subsequent satellites in the series carried the Landsat designation. So far six Landsat satellites have been launched successfully, while Landsat-6 suffered launch failure. Table-4 highlights the characteristics of the Landsat series satellites. There have been four different types of sensors included in various combinations on these missions. These are Return Beam Vidicon camera (RBV) systems, Multispectral Scanner (MSS) systems, Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM).
After more than two decades of success, the Landsat program realised its first unsuccessful mission with the launch failure of Landsat-6 on October 5, 1993. The sensor included on-board was the Enhanced Thematic Mapper (ETM). To provide continuity with Landsat -4 and -5 the ETM incorporated the same seven spectral bands and the same spatial resolutions as the TM. The ETM's major improvement over the TM was addition of an eighth panchromatic band operating in 0.50 to 0.90-^m range and spatial resolution of 15m. Landsat-7 includes two sensors: the Enhanced Thematic Mapper plus (ETM+) and the High Resolution Multispectral Stereo Imager (HRMSI).
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