Satellite S2A MSI RGB composite image of Lucinda Jetty taken on 25 March 2017 (Image: Craig Donlon, European Space Agency)


IMOS Task Team goes troppo to solve radiometry challenges

Images of the ocean from space show a range of colours that indicate levels of algae and other factors influencing ocean colour. Observations much closer to the ocean surface are required to validate images produced by satellite techniques and to improve the accuracy of inferences based on those images. The IMOS Radiometry Task Team met in tropical north Queensland to tackle shared challenges.

Representatives of the Australian ocean colour remote sensing research community from CSIRO, Curtin University and University of Technology Sydney recently joined forces to investigate the experimental uncertainties associated with a variety of commonly used multi- and hyperspectral radiometers for the calculation of above-water remote sensing reflectance; a key parameter in ground-truthing ocean colour satellites and other bio-optical studies.

Accurate sea-truth radiometric measurements of ocean colour are required to monitor the temporal stability of airborne and space-based ocean colour sensors, as well as the accuracy of any atmospheric correction procedures applied to the remote sensing data. Thus, the accuracy of any sea truth radiometric measurements needs to be established. 

In addition to ground-sensor performance, deployment and post-processing methodologies can contribute significantly to the overall accuracy of radiometric measurements. In Australia, there are a relatively small number of geographically isolated researchers and technicians that study ocean optics. They are from different backgrounds with different perspectives and use different instrumentation. IMOS one-year funding provided the opportunity for a group of these researchers to form a Radiometry Task Team in which to compare their approaches.

First, the group pooled their sensors and sent them to a commercial radiometric calibration facility where lamp-based calibrations were performed for each sensor at three different internal temperatures encompassing the range of temperatures encountered in above-water Australian field conditions. 

Next, the task team inter-compared the calibrated instruments in the field at the IMOS/CSIRO funded Lucinda Jetty Coastal Observatory (LJCO) in tropical North Queensland.  Situated at the end of a 5.76 km long bulk sugar loading facility, this unique location adjacent to the Great Barrier Reef World Heritage area exhibits a wide range of Great Barrier Reef water inherent optical property (IOP) throughout the year. 

Suspended some 20 metres above the ocean surface, the dedicated work platform has long-term IOP, SeaPRISM radiance and DALEC above water remote-sensing reflectance installations. 

The task team assembled for a two-day campaign (9-10 Nov 2016) in the beautiful fishing village of Dungeness; woke every morning to a view of the warm tropical mist-clad mountains of Hinchinbrook Island, looking past the invitingly glassy but crocodile-inhabited mangrove channel. After a 15-minute drive out over the jetty, the team performed consistent measurements for each of the radiance and irradiance device types, and two integrated DALEC above water reflectance systems and the autonomous SeaPRISM (Figs 1, 2 and 3). 

The tropical solar zenith angles approaching one or two degrees at noon allowed observations of the cosine behaviour of the irradiance sensors over a large angular range.

Sky conditions were nearly ideal for the 2-day sampling period, with only a few wispy clouds in the morning, rapidly burning off in the hot tropical sun. The mornings were typically still and very humid until a sea breeze system developed, bringing wind speeds of 10 - 15 knots as measured by a permanently installed Vaisala Weather Transmitter (WXT520). These hot conditions ensured that all sensors that recorded internal temperatures were reading 35+ degrees Celsius; far hotter than typical temperatures factory calibrations are performed at. Fortunately, pre-deployment calibrations were performed for all instruments at these elevated temperatures, so suitable instrument responsivities were applied to field data in order to achieve improved radiometric accuracy. 

The field deployment was an excellent chance for the often regionally separated researchers and technicians to meet, compare stories, and discuss methodologies and other technical considerations pertinent to ocean colour radiometry. 

With the ongoing processing and analysis of the datasets acquired in this study, the community is learning more about the experimental uncertainties and resultant confidence which can be placed in using field radiometers to validate aerial and satellite-based ocean colour remote measurements.

Article written by Matt Slivkoff and David Antoine, IMOS Satellite Ocean Colour Sub-Facility, Remote Sensing & Satellite Research Group, Department of Physics and Astronomy, Curtin University.

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The Lucinda Jetty Coastal Observatory with field participants, L to R: M. Slivkoff, J. Lovell, D. Boadle, M. Doblin, B. Baker, D. Antoine, and T. Schroeder (Photo: D. Boadle)

Irradiance radiometers lined up, with a view of Pelorus and Orpheus Islands in the background. 4 x Ramses, 2 x HyperOCR, 1 x USSIMO, and 1 x MS8 sensors are visible (Photo: M. Slivkoff)