给出eci坐标系转j2000坐标系的matlab代码

以下是eci坐标系转j2000坐标系的matlab代码：

function [r_j2000] = eci2j2000(r_eci, t_utc)
% ECI to J2000 coordinate conversion
% Input:
%   - r_eci: position vector in ECI coordinates [km]
%   - t_utc: UTC time in format 'yyyy-mm-ddTHH:MM:SS.SSSSSS'
%
% Output:
%   - r_j2000: position vector in J2000 coordinates [km]

% Earth rotation angle (ERA) calculation
jd = juliandate(t_utc); % Julian date
t_ut1 = ut1utc(jd); % UT1-UTC time difference [s]
[~, ERA] = earth_rotation_angle(jd + t_ut1/86400); % Earth rotation angle [rad]

% Transformation matrix from ECI to J2000 coordinates
R_3 = [cos(ERA) sin(ERA) 0; -sin(ERA) cos(ERA) 0; 0 0 1]; % Rotation about Z-axis
eps = obliquity(jd); % Mean obliquity of the ecliptic [rad]
R_1 = [1 0 0; 0 cos(eps) sin(eps); 0 -sin(eps) cos(eps)]; % Rotation about X-axis
M_eci2j2000 = R_1 * R_3; % Transformation matrix

% Convert position vector from ECI to J2000 coordinates
r_j2000 = M_eci2j2000 * r_eci;

end

其中，`juliandate`、`ut1utc`、`earth_rotation_angle`和`obliquity`是自定义的函数，分别用于计算儒略日、UT1-UTC时间差、地球旋转角和黄赤交角。