An analysis of chlorophyll fluorescence algorithms for the moderate resolution imaging spectrometer (MODIS)

Next-generation ocean color sensors will include channels to measure passive chlorophyll fluorescence as well as traditional channels that use radiance ratios to estimate chlorophyll concentration. Because the chlorophyll fluorescence signal is small, these sensors have significantly higher signal to noise ratios in the channels used to measure fluorescence. Small changes in sensor performance, atmospheric transmissivity, and fluorescence efficiency could potentially result in significant changes in the performance of the fluorescence algorithms. We perform a sensitivity analysis on the present MODIS algorithms and derive the minimum chlorophyll concentrations that can be observed for various combinations of sensor performance, atmospheric conditions, and phytoplankton physiology. We show that the present sensor specifications will allow us to observe fluorescence at chlorophyll concentrations as low as 0.5 mg/m3 at the full resolution of the sensor (nominally 1 km2 at nadir) under optimum viewing conditions. Although band position changes over a ±4 nm range affect the absolute reading of individual bands by less than 2%, the impact on the total performance of the fluorescence algorithm can be greater than 70%.