The Earth resources satellite Landsat 3, here undergoing pre-flight tests, derived from the early Tiros and Nimbus weather satellites. Launched in Mar 1973, Landsat followed a polar orbit at a height of 917 km (570 mi) that took it around the Earth once in every 103 min. As the planet turned beneath it, Landsat passed over a different strip of ground on each orbit, until after 252 orbits (18 days) it had surveyed the whole Earth. Landsat 3 had two camera systems. One took ordinary color pictures, while the other analyzed five wavelengths: red and green visible light, and three infrared wavelengths, to distinguish rock, soil, water, and healthy and diseased vegetation.