  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.
