Ocean colour is the acorn from which Pixalytics eventually grew, and so we were delighted to see last weekâ€™s NASA announcement that one of their next generation ocean colour satellites is now more secure with a scheduled launched for 2022.
Unsurprisingly the term ocean colour refers to the study of the colour of the ocean, although in reality itâ€™s a name that includes a suite of different products, with the central one for the open oceans being the concentration of phytoplankton. Ocean colour is determined by the how much of the sunâ€™s energy the ocean scatters and absorbs, which in turn is dependent on the water itself alongside substances within the water that include phytoplankton and suspended sediments together with dissolves substances and chemicals. Phytoplankton can be used a barometer of the health of the oceans; in that phytoplankton are found where nutrient levels are high and oceans with low nutrients have little phytoplankton. Samâ€™s PhD involved the measurement of suspended sediment coming out of the Humber estuary back in 1995, and itâ€™s remained an active field of her research for the last 20 years.
Satellite ocean colour remote sensing began with the launch of NASAâ€™s Coastal Zone Colour Scanner (CZCS) on the 24th October 1978. It had six spectral bands, four of which were devoted to ocean colour, and a spatial resolution of around 800m. Despite only having an anticipated lifespan of one year, it operated until the 22nd June 1986 and has been used as a key dataset ever since. Sadly, CZCSâ€™s demise marked the start of a decade gap in NASAâ€™s ocean colour data archive.
Although there were some intermediate ocean colour missions, it was the launch of the Sea-viewing Wide Field-of-view (SeaWiFS) satellite that brought the next significant archive of ocean colour data. SeaWiFS had 8 spectral bands optimized for ocean colour and operated at a 1 km spatial resolution. One of Samâ€™s first jobs was developing a SeaWiFS data processor, and the satellite collected data until the end of its mission in December 2010.
Currently, global ocean colour data primarily comes from either NASAâ€™s Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the twin Aqua and Terra satellites, or the Visible Infrared Imaging Radiometer Suite (VIIRS) which is on a joint NOAA / NASA satellite called Suomi NPP. MODIS has 36 spectral bands and spatial resolution ranging from 250 to 1000 m; whilst VIIRS has twenty two spectral bands and a resolution of 375 to 750 m.
Until recently, there was also the ONR / NRL / NASA Hyperspectral Imager for the Coastal Ocean (HICO) mission on-board the International Space Station. It collected selected coastal region data with a spectral resolution range of 380 to 960nm and 90m spatial resolution. It was designed to collect only one scene per orbit and has acquired over 10,000 such scenes since its launch. However, unfortunately it suffered during a solar storm in September 2014. Its retirement was officially announced a few days ago with the confirmation that it wasnâ€™t possible to repair the damage.
In the same week we wave goodbye to HICO, NASA announced the 2022 launch of the Pre-Aerosol and ocean Ecosystem (PACE) mission in a form of ocean colour symmetry. PACE is part of the next generation of ocean colour satellites, and itâ€™s intended to have an ocean ecosystem spectrometer/radiometer called built by NASAâ€™s Goddard Space Flight Centre and will measure spectral wavebands from ultraviolet to near infrared. It will also have an aerosol/cloud polarimeter to help improve our understanding of the flow, and role, of aerosols in the environment.
PACE will be preceded by several other missions with an ocean colour focus including the European Sentinel-3 mission within the next year; it will have an Ocean and Land Colour Instrument with 21 spectral bands and 300 m spatial resolution, and will be building on Envisat’s Medium Resolution Imaging Spectrometer (MERIS) instrument. Sentinel-3 will also carry a Sea and Land Surface Temperature Radiometer and a polarimeter for mapping aerosols and clouds. It should help to significantly improve the quality of the ocean colour data by supporting the improvement of atmospheric correction.
Knowledge the global phytoplankton biomass is critical to understanding the health of the oceans, which in turn impacts on the planetâ€™s carbon cycle and in turn affects the evolution of our planetâ€™s climate. A continuous ocean colour time series data is critical to this, and so we are already looking forward to the data from Sentinel-3 and PACE.