Sentinel-6: maintain the sea-level reference

Sentinel-6 continues the reference ocean-altimetry record established by TOPEX/Poseidon and the Jason series. The first spacecraft, Sentinel-6 Michael Freilich, launched in 2020. The mission is a partnership involving the European Union, ESA, EUMETSAT, NASA and NOAA, with CNES support. Its value includes keeping successive observations comparable across spacecraft generations.

The preparation photograph shows Sentinel-6 Michael Freilich mounted on a payload adapter at Vandenberg in November 2020. It documents the spacecraft before launch. A long sea-level record comes from later observations, orbit determination, corrections and processing; it cannot be read from the appearance of the vehicle or the size of its launch enclosure.

The radar altimeter sends pulses toward the ocean and measures their return. Travel time provides information about the distance between spacecraft and reflecting surface. To turn that range into sea-surface height, the spacecraft’s position must be known relative to a defined reference. A shorter radar range alone could reflect a change in spacecraft position rather than a rise in the ocean.

The atmosphere also affects signal travel. A microwave radiometer helps estimate the water-vapor contribution to the delay, and additional corrections account for other effects. Precise orbit information and environmental corrections belong to the measurement itself. They are not optional decorations added after an otherwise complete height estimate.

Successive reference-altimetry missions are compared so that an instrumental offset does not look like a sudden ocean change. Overlap and cross-calibration — open definition connect new observations with the earlier record. The task is particularly important when investigating gradual change across many years: a small mismatch between instruments can matter to a long-term interpretation.

For a fictional coastal project, distinguish a global mean sea-level series from the water level at one harbor at a particular moment. Tides, waves and regional conditions affect local observations. Use the appropriate spatial and temporal product, preserve its reference and uncertainty, and compare it with suitable local information. A consistent global record and a local harbor measurement answer related but different questions.