NPL: does vicarious calibration of satellite ocean colour measurements give the same results from different platforms — comparing MarONet and the MOBY buoy
On 17 September 2026 an engineer from the Optical Radiometric Metrology team at the UK’s NPL described research on system vicarious calibration (SVC) of satellite ocean colour measurements. The work checks whether adjustment factors derived from different in situ platforms are consistent and how differences affect products such as chlorophyll concentration.
- Ocean colour, the spectrum of water-leaving radiance, carries information on phytoplankton, suspended sediments and dissolved organic material; satellite products such as chlorophyll concentration, phytoplankton biomass and water clarity are based on it. The water-leaving signal is only a small fraction of the radiance measured by a satellite — most comes from atmospheric scattering — so the products are highly sensitive to sensor calibration and atmospheric correction.
- Vicarious calibration compares satellite measurements with well-characterised, SI-traceable in situ measurements at dedicated sites and derives adjustment factors for the sensor’s on-orbit response. According to NPL only two platforms in the world operate at the level needed for primary vicarious calibration: MarONet and the MOBY buoy off the island of Lānaʻi, Hawaii, funded by NOAA and operated jointly by San José State University, the University of Miami and NIST.
- The study, carried out with Curtin University, is to establish whether factors from different platforms give consistent results, what drives any differences and what this means for products such as chlorophyll concentration. This is a prerequisite for moving to a network of vicarious calibration sites; NPL announces a description of more than a month of work with the MOBY team in Honolulu in the next post.
- According to the MOBY website (Moss Landing Marine Laboratories), the buoy has measured light near the sea surface at 1, 5 and 9 m since July 1997 and since late 1996 has been the primary basis for vicarious calibration of all three US ocean colour sensors and many international sensors. Water-leaving radiance is at most 10% of the signal measured by a satellite above the atmosphere, so a 5% uncertainty in it implies 0.5% in the satellite-measured radiance; without MOBY, SeaWiFS values would have been in error by 25% at 490 nm and 75% at 412 nm.
Source: NPL — Ocean colour, vicarious calibration and trip to Hawai’i (17.09.2026)