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Improved Sensors for Better Satellite Measurement of Climate Impact

Researchers are working to renew sea-based sensors, dubbed MOBY-Refresh, to improve satellite accuracy in reading ocean colours. The ocean colours are essential as an indicator of Earth’s climate. The new sensors will allow for more precise and accurate measurements of the colours or wavelengths of sunlight. NOAA, Moss Landing Marine Laboratories (MLML), the University of Miami, and the National Institute of Standards and Technology (NIST) have collaborated on an upgrade to the sensor.

The ocean usually appears blue, but in some places, it seems blue-green. The difference was caused by the presence of single-celled plants called phytoplankton, which contains chlorophyll and reflect green in the sunlight.

Despite their small size, phytoplankton absorbs nearly as much carbon dioxide as all the trees and land plants combined. Therefore, they have a massive impact on the climate. Scientists are studying phytoplankton intensity by measuring the colour of the ocean with satellites and sea-based sensors. To ensure the accuracy of satellite measurements, researchers in the United States and many other countries rely on the Marine Optical Buoy (MOBY).

“MOBY measures the amount of light scattered out of the water at a single location in the Pacific Ocean across a range of wavelengths. Ocean-colour satellite sensors, including the MOBY site, monitor the oceans. The MOBY data are then delivered to satellite teams, who use the data to adjust the calibration of the satellite sensors, hence improving the accuracy of global data products such as chlorophyll concentration,” explained NIST researcher Carol Johnson.

MOBY comprises two main buoys: an optical buoy that measures and records light and a mooring buoy that keeps the optical buoy in place. A central mast extends underwater on the optical buoy. The mast has three pole-like arms, each with light-collecting optical fibres. Optical fibre collectors detect light at depths of 1 metre (3.3 feet), 5 metres (16.4 feet), and 9 metres (29.4 feet). Spectrographs at the bottom of the central mast measure light as a function of wavelength at each of the three depths.

MOBY is located 20 kilometres (12 miles) off the coast of Lanai, Hawaii because the atmosphere and water conditions are ideal for calibrating ocean-colour satellite sensors. The ocean water is clear and low in chlorophyll, representing the rest of the world’s oceans. The atmosphere is also clear, with few clouds, ensuring the global utility of the measurements.

MOBY is being upgraded as part of the Refresh project with a new optical system, support structures, and control system. According to Johnson, data analysis is ongoing, but preliminary comparisons to MOBY Buoy276, which is currently in the ocean, are very encouraging.

The upgrade to the optical system includes a better spectrograph that can measure sunlight at all three depths simultaneously, reducing environmental sources of uncertainty such as fluctuations in buoy tilt and arm depth, as well as changes in light as ocean waves focus it. In addition, the optical buoy is now made of carbon fibre rather than fibreglass and metal, which makes the structure more robust and increases its lifetime at sea.

In addition to MOBY-Refresh, the team is working on a similar instrument called MarONet, which will be used by a future NASA satellite mission called PACE (Plankton, Aerosol, Cloud, ocean Ecosystem). The optical system’s instrument design will be similar to Refresh but with a more portable buoy.

The optical system in the MarONet project can be disassembled and transported to a central location for calibration and characterisation. Calibration describes how the input affects the instrument output, whereas characterisation describes how this relationship changes in the presence of all possible influencing factors, such as ambient temperature.

The MarONet buoy will be deployed off the coast of Western Australia, with the central location for calibration and maintenance in Hawaii, where the central MOBY hub is. NIST’s role in this process will be to monitor the optical system for changes during transportation. The site planning for Australia has begun, led by Curtin University in Perth co-investigator David Antoine. The visual and mooring buoy equipment will be delivered to Australia in 2023.

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