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Ground-truthing for satellites

12 December 2017
Dr Callum Whyte, from the Scottish Association for Marine Science (SAMS), summarises how they have been validating satellite data for the identification of target algal species.
During spring and summer this year a SAMS team, consisting of Dr Phil Anderson, Sharon McNeill and Dr Callum Whyte, made several trips into the Lyn of Lorn, Argyll and Bute, aboard the SAMS research vessel Seol Mara. They were attempting to match the ocean colour images from the OLCI sensor, flying aboard the satellite Sentinel 3, with actual water samples.

Using sophisticated algorithms, scientists at Plymouth Marine Laboratory (PML) analyse these images to try to detect specific species of potentially harmful, bloom forming micro-algae in the sea. To test the accuracy of these algorithms the collected water samples are analysed by experienced taxonomists at SAMS, who identify and enumerate the species of micro-algae that were present in the water when the satellite passed overhead.

In addition to collecting water samples, they also deployed a boom-mounted spectral radiometer. Similar to the sensors aboard Sentinel 3, this also obtains colour images of the sea, albeit with a much smaller coverage. However, unlike the sensor aboard the satellite, the light for these images does not have to travel up through the atmosphere and, therefore, contains less distortion. If the algorithms used to analyse these images are to be accurate it is important that they take account of this atmospheric distortion. It may seem to be an impossible task to take a boat out to sea at the right time and location to coincide with a satellite passing far above and, in Scotland at least, with a cloudless sky. Yet the crew of the Seol Mara achieved just this, not once but on several occasions.

​The spectral radiometer has been operated on a bow-sprite from the Seol Mara for three trips from SAMS/Dunstaffnage to Loch Creran. These data indicate that there is a species signature in the spectra but we await further analysis of the in situ samples to assess their correspondence with algal content of the water column. SAMS are procuring a suite of lifting platforms for the spectral radiometer and similar instruments and aim to integrate the units during the coming winter. These platforms will be capable of 40 minutes flight and will operate from the Seol Mara or similar boats, giving the ability to make swathe measurements, increase coverage and also to operate away from the boat shadow. 

This “ground-truthing”, while time consuming, plays an important part in improving the accuracy of these satellite-based observations and this work will continue next year.