Applies to both the satellites themselves and to the overall system of geostationary observations used by the United States. The current operational series of GOES satellites were preceded by the ATS and SMS satellites, with GOES-1 being launched on 16 October 1975. The early GOES (1 through 7) were spin- stabilized spacecraft, while the latest GOES are three-axis stabilized. Two GOES satellites are normally in operation, one at 75° W longitude and the other at 135° W longitude. Before launch, GOES satellites are given a letter designation (e.g., GOES-J) that is changed to a number designation (e.g., GOES-9) when the satellite becomes operational. The current generation of GOES satellites supports separate imager and sounder systems, SEM and DCS. The imager is a five-channel scanning radiometer with a 1-km resolution visible channel, along with slightly lower resolution images in the midinfrared, water vapor, and thermal IR bands. The sounder has 18 thermal infrared bands and a low-resolution visible band.