ISS as a Platform for Optical Remote Sensing of Ecosystem Carbon Fluxes: A Case Study Using HICO

Data from the hyperspectral imager for coastal ocean (HICO), mounted on the International Space Station (ISS), were used to develop and test algorithms for remotely retrieving ecosystem productivity. Twenty-six HICO images were used from four study sites representing different vegetation types: grasslands, shrubland, and forest. Gross ecosystem production (GEP) data from eddy covariance were matched with HICO-derived spectra. Multiple algorithms were successful relating spectral reflectance with GEP, including: spectral vegetation indices (SVI), SVI in a light-use efficiency model framework, spectral shape characteristics through spectral derivatives and absorption feature analysis, and statistical models leading to multiband hyperspectral indices from stepwise regressions and partial least squares regression. Successful algorithms were able to achieve r² better than 0.7 for both GEP at the overpass time and daily GEP. These algorithms were successful using a diverse set of observations combining data from multiple years, multiple times during growing season, different times of day, with different view angles, and different vegetation types. The demonstrated robustness of the algorithms presented in this study over these conditions provides some confidence in mapping spatial patterns of GEP, describing variability within fields, as well as the regional patterns. The ISS orbit provides periods with multiple observations collected at different times of the day within a period of a few days. Diurnal GEP patterns were estimated comparing the half-hourly average GEP from the flux tower against HICO estimates of GEP (r² = 0.87) if morning, midday, and afternoon observations were available.

I. Introduction

Photosynthesis is possibly the most important biochemical process on Earth. It provides the energy that powers ecosystem processes, thus it is critically important for the understanding of ecosystem function [1]. Photosynthesis is one of the key processes removing CO₂, an important greenhouse gas, from the atmosphere, providing an important feedback to the climate system [2]–[5]. While it is well known that the global carbon cycle is changing [6]– [8] with terrestrial ecosystems contributing higher carbon uptake, identifying the mechanisms and locations responsible for high carbon uptake remains an important challenge for predicting future carbon–climate interactions [6], [9] . Satellite observations have the potential to provide timely data for regions where it is difficult or impossible to collect in-situ measurements, however the existing satellite-based observing systems cannot provide the long term, calibrated, and spectrally sensitive observations required to quantify global carbon-budget changes [10].