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Landsat 7 is the latest satellite of the Landsat program. It was launched on April 15th, 1999. The primary goal of Landsat 7 is to refresh the global archive of satellite photos, providing up-to-date and cloud free images. Although the Landsat Program is managed by NASA, data from Landsat 7 is collected and distributed by the USGS.
Satellite Specifications
Landsat 7 was designed to last for five years, and has the capacity to collect and transmit up to 532 images per day. It is in a polar, sun-synchronous orbit, meaning it scans across the entire earth's surface. It takes 232 orbits, or 15 days, to do so. The satellite weighs 1973 kg, is 404 cm long, and 274 cm in diameter. Unlike its predecessors, Landsat 7 has a solid state memory of 378 gigabits (roughly 100 images). The main instrument on board Landsat 7 is the Enhanced Thematic Mapper Plus (ETM+). Its main features are:
- A panchromatic band with 15m spatial resolution
- Full aperture, 5% absolute radiometric calibration
- A thermal infrared channel with 60m spatial resolution
On May 31, 2003 the Scan Line Corrector (SLC) in this instrument failed. The effect of this is that the raster scanning of the image is angled rather than parallel, causing narrow bands of each image not to be acquired. As of 2004 the images are being released with the missing points filled with previous years' data. This may change later to use data acquired at a few weeks' interval. There is not yet any plan for a replacement Landsat.
 A false color IR image of Washington DC, taken by Landsat 7. 
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