In Situ Observation

An in-situ observation is a direct physical measurement made within the water column or at the sea surface, as opposed to a remote retrieval inferred from a satellite or radar. The sensor sits in the medium it samples: a CTD reading conductivity and temperature, an Argo float profiling to 2000 m, a moored ADCP measuring current. In-situ data carry the ground truth that calibrates and validates satellite altimetry, scatterometry, and radiometry, and they reach depths satellites cannot see. The trade is coverage: in-situ networks are sparse and costly, so observing systems blend them with remote sensing rather than choosing one.