Machine Learning Methods for Earth Observation and Remote Sensing Using Spaceborne GNSS Reflectometry: Current status, challenges, and future prospects

Spaceborne GNSS reflectometry (GNSS-R) missions have been successfully launched in recent years, such as Technology Demonstration Satellite (TDS-1) in 2014, Cyclone Global Navigation Satellite System (CYGNSS) in 2016, Bufeng (BF)-1 A/B in 2019, and Fengyun (FY)-3E/3F/3G and Tianmu-1, launched successively in 2021. They provide a large amount of data to support spaceborne GNSS-R remote sensing applications, and spaceborne GNSS-R technology has also been widely used in various remote sensing fields by virtue of its advantages. With the rise of artificial intelligence (AI), many machine learning (ML) models have been developed for GNSS-R observations to estimate geophysical parameters. In particular, deep learning (DL) techniques have proved to have great potential to improve the accuracy of retrieval models in spaceborne GNSS-R applications, including ocean, land, cryosphere, atmosphere, and environment monitoring. This article provides the first comprehensive review of the application of ML in GNSS-R for Earth observation and remote sensing. The article first summarizes common ML algorithms as well as their basic concepts and theories. It then thoroughly reviews the progress of ML methods in the field of spaceborne GNSS-R and discusses the advantages, disadvantages, and challenges of ML models applied to GNSS-R. More importantly, it is imperative to adopt DL into the field of GNSS-R remote sensing and use it as a general model to tackle unprecedented, large-scale, and impactful challenges in areas such as ocean, land, cryosphere, atmosphere, hydrology, and environment remote sensing.