What was the first geostationary weather satellite?

ATS-1 was the first geostationary weather satellite. NASA launched Applications Technology Satellite 1 on 6 December 1966, and it operated from geostationary orbit above the equator.

A geostationary satellite circles Earth at the same rate that Earth rotates, making it appear to remain above one longitude. From that fixed viewpoint, ATS-1 could repeatedly observe a large portion of the same hemisphere. Its spin-scan cloud camera produced some of the first images showing weather systems moving across the full disk of Earth.

ATS-1 also tested communications and other space technologies, so it was not solely a meteorological spacecraft. Its weather observations nevertheless demonstrated the value of continuous satellite monitoring for hurricanes, cloud development, and large-scale atmospheric circulation.

TIROS-1, launched in 1960, was the first successful weather satellite overall, but it was in low Earth orbit rather than geostationary orbit. GOES-1 came later, in 1975, and began the long operational series of U.S. geostationary weather satellites.