A Concise Survey of G4U
The general four-component model-based decomposition with unitary transformation of coherency matrix (G4U) is a state-of-the-art four-component decomposition, which has received extensive attentions recently. A literature survey is carried out to indicate the overall influence, improvement, development, evaluation, and application of G4U. Totally, 137 literatures are found mentioning G4U in Google Scholar\c{opyright} until October 7, 2019, which can be attributed into 4 categories in terms of the degree of concentration and 17 subcategories according to the focus of attention. Among these literatures, 61 of them simply mention G4U mainly because it is a new four-component model-based decomposition, a typical model-based decomposition, or even a target decomposition. There are also 9 literatures which improve G4U and develop G4U-like decompositions with unitary transformation of coherency matrix. 20 literatures generally use G4U as a typical target decomposition for comparison or a pseudo-color visualization technique to show the performance of some developed approaches. There are also 47 literatures dedicated to critically evaluate and deeply apply G4U in the remote sensing of forestry, agriculture, wetland, snow, glaciated terrain, earth surface, manmade target, environment, and damages caused by earthquake, tsunami, and landside, which indicates the value and significance of G4U in the true sense.