The ATSR-1 instrument on-board the ERS-1 satellite is a low-resolution conical imaging spectrometer operating in the mid-infrared and thermal spectrum. Data are acquired through four bands with a 1 kilometre spatial resolution. The (A)ATSR instruments are unique in their use of along track scanning to provide two views of the surface and thus improve atmospheric correction. The surface is first viewed along the direction of the orbit track, at an angle of 55°, as the spacecraft flies towards the scene. Then, 150 seconds later, or when the satellite has moved approximately 1000 km forward along the ground track, a second observation is made of the same scene at the sub-satellite point.