1. A method for identifying ice crystal fabric and distribution characteristics of ice flow field in ice sheet based on polarimetric radar, the method comprising:providing an orthogonal arrangement receiving and transmitting antenna of the polarimetric radar;
acquiring radar data of four channels, EHHr, EHVr, EVHr and EVVr, by the orthogonal arrangement receiving and transmitting antenna of the polarimetric radar;
performing normalization processing on the radar data EHHr, EHVr, EVHr and EVVr obtained in step 1 to obtain the radar echo records of four channels RHH, RHV, RVH and RVV;
wherein, EHi represents an electric field intensity of incident wave in H direction, and EVi represents an electric field intensity of incident wave in V direction;performing deconvolution processing on the radar echo records of four channels RHH, RHV, RVH and RVV, to obtain the generalized reflection coefficient sequences of four channels RHH, RHV, RVH and RVV, recorded together as
calculating the eigenvalue and deflection angle of dielectric permittivity tensor in each layer from top to bottom by layer-by-layer recursive method, wherein the derivation from the first layer to the n-1 layer is shown in formula (2), and the interface reflection coefficient matrix formula, transmission coefficient matrix formula and reverse transmission coefficient matrix formula are shown in formulas (3) to (5), respectively:
wherein, Ri,i+1 represents the interface reflection coefficient matrix between the i th layer and the (i+1)th layer, Ti,i+1 represents the interface transmission coefficient matrix between the i th layer and the (i+1)th layer, Ti+1,i represents the reverse interface transmission coefficient matrix between the i th layer and the (i+1)th layer, Ri,i+1 represents the generalized reflection coefficient matrix of the interface between the i th layer and the (i+1)th layer in the radar echo record, e?jkidi represents the phase factor matrix of the i th layer, and di represents the thickness of the i th layer; the layer above the first ground surface is an isotropic air layer, the eigenvalue of the dielectric permittivity tensor is 1, and the rotation angle is 0, that is, ?11=1, ?12=1, ?13 =1 and ?1=0, are substituting into the formulas (1) to (3) to obtain the eigenvalues ?21, ?22 and the rotation angle ?2 of the dielectric permittivity tensor of the second layer, and then calculating the interface transmission coefficient matrix T12 and the reverse transmission coefficient matrix T21 by formulas (4) and (5), and then recursively in order to calculate the eigenvalue and rotation angle of the dielectric constant tensor of the n layer;calculating the eigenvalues a1(2), a2(2) and a3(2) of the second-order orientation tensor of each layer according to the eigenvalues ?i1, ?i2 and rotation angle ?i of the dielectric permittivity tensor of each layer obtained from S4, and according to the formulas (6) and (7) of the correlation between the orientation tensor and the dielectric permittivity tensor of ice crystal fabric;
wherein, ?? and ?|| represent the dielectric permittivity s that perpendicular to a C-axis and parallel to the C-axis respectively, which are measured in the laboratory; the second-order fabric orientation tensor calculated according to formula (6) and formula (7) is used to determine a fabric type of the ice crystal; anddetermining a stress state of the ice crystal according to the fabric type and the distribution characteristics of the C-axis, in which the direction of tensile force is the direction of ice flow; and extracting the flow direction of ice layer with different depth layer by layer, to obtain the spatial distribution characteristics of ice flow field.