A series of satellites designed to study the earth's land, atmosphere, and oceans. The major instruments on ERS are an active microwave instrument, a radar altimeter, and an along-track scanning radiometer. The active microwave instrument operates in three modes: SAR image mode, SAR wave mode, and wind scatterometer mode. The scanning radiometer is made up of two instruments, an infrared radiometer and a microwave sounder. ERS-2 also has a global ozone monitoring experiment. ERS-1 was launched in 1991 and ERS-2 in 1995. Both satellites operate in sun-synchronous, near-polar orbits.