Evaluating multi-sensor agreement of satellite particulate backscatter retrievals by validation against in-water measurements

The particulate backscattering coefficient (bbp; m^-1) is a marine inherent optical property (IOP). It can be used for monitoring ecologically-relevant parameters such as phytoplankton carbon in the world’s oceans. In-water bbp is typically measured by fixed-angle volume scattering function (VSF) meters that can be deployed on a variety of platforms, some ship-based (e.g., flow-through, CTD casts, etc) and some autonomous (e.g., gliders, BGC-Argo floats, etc). Most commercial instruments measure bbp in a multi- or single-band configuration. Historically, in-water bbp measurements were often collected in relatively-accessible regions of the global oceans, leaving vast regions of the global oceans unmonitored. Estimates of bbp can also be obtained from ocean color semi-analytical algorithms (SAAs), which offer more complete spatio-temporal measurements of bbp globally.

It is important to assess satellite estimates of bbp to ensure they are useful for scientific applications and ocean monitoring. It is desirable to have satellite-derived bbp that are accurate and representative, with acceptable uncertainties, high precision, and low bias. The skill of SAAs, such as NASA’s Generalized Inherent Optical Properties algorithm framework (GIOP), can be assessed through validation analyses where remotely-sensed measurements are compared against in-water measurements. However, the diversity of in-water bbp measurements must be standardized (i.e., optically-weighted, spectrally-shifted, sub-pixel sampling accounted for, etc) before being compared to 5x5-pixel satellite extracts of bbp retrievals. Additionally, differing band-sets of ocean color sensors, such as MODIS and VIIRS, can lead to inconsistent estimates of bbp when using identical GIOP algorithm implementations.

This study: 1) discusses standardization of in-water bbp measurements that should be performed before comparing in situ bbp with satellite-retrieved estimates of bbp, 2) assesses MODIS and VIIRS GIOP-derived bbp through validation efforts, and 3) comments on methods to bring MODIS and VIIRS time series into agreement despite fundamental instrument design differences.