Fig.2. Test of bilateral filter (a) Raw echoes of different intensity targets; (b) Echo distribution of Fig.(a); (c) Echoes after Gaussian filter to Fig.(a); (d) Echo distribution of Fig.(c); (e) Echoes after bilateral filter to Fig.(a); (f) Echo distribution of Fig.(e); In the right panels,blue indicates the noise distribution and red indicates the signal distribution; and are the mean and standard deviation of the noise,respectively