Since the Coastal Zone Color Scanner (CZCS)/Nimbus-7 was launched in 1978, a variety of studies have been conducted to retrieve ocean color variables from multi-satellites. Several algorithms and formulations have been suggested for estimating ocean color variables based on multi band data at different wavelengths. Chlorophyll-a (chl-a) concentration is one of the most important variables to understand low-level ecosystem in the ocean. To retrieve chl-a concentrations from the satellite observations, an appropriate algorithm depending on water properties is required for each satellite sensor. Most operational empirical algorithms in the global ocean have been developed based on the band-ratio approach, which has the disadvantage of being more adapted to the open ocean than to coastal areas. Alternative algorithms, including the semi-analytical approach, may complement the limits of band-ratio algorithms. As more sensors are planned by various space agencies to monitor the ocean surface, it is expected that continuous monitoring of oceanic ecosystems and environments should be conducted to contribute to the understanding of the oceanic biosphere and the impact of climate change. This study presents an overview of the past and present algorithms for the estimation of chl-a concentration based on multi-satellite data and also presents the prospects for ongoing and upcoming ocean color satellites.

Abstract
Introduction
Ocean Color Satellites
    Near-Polar Orbiting Ocean Color Satellites
    Geostationary Ocean Color Satellites
Chlorophyll-a ConcentrationRetrieval Algorithms
    Empirical Algorithms
    Band-ratio Algorithms: The band-ratio algorithms
    Band-difference Algorithms
    Hybrid Algorithms
    Semi-Analytical Algorithms
    Algorithms Based on Neural Network
    Algorithms for Geostationary Satellites
Accuracy Assessment andValidation
Prospect and Concluding Remarks
References

• Ji-Eun Park(Department of Science Education, Seoul National University)
• Kyung-Ae Park(Department of Earth Science Education/Research Institute of Oceanography, Seoul National University) Corresponding author
• Young-Je Park(Korea Ocean Satellite Center, Korea Institute of Ocean Science and Technology)
• Hee-Jeong Han(Korea Ocean Satellite Center, Korea Institute of Ocean Science and Technology)