function velocity = get_velocity(s_T, s_R, origin_velocity)
    global T_r delta_t c f_c max_t slow_len;
    freq_s_T = 1 / T_r;

    slow_freqs = ((0:slow_len - 1) * (1 / T_r)) / slow_len;
    range_t_slow = 1:slow_len;

    num_slow = max_t / T_r;
    s_R_slow = zeros(1, slow_len);
    k = floor(T_r / delta_t);

    idx = 1;

    while abs(s_R(idx)) == 0
        idx = idx + 1;
    end

    for i = 0:num_slow - 1

        while abs(s_R(idx + i * k)) == 0
            idx = idx + 1;
        end
        s_R_slow(i + 1) = s_R(idx + i * k);
    end

    s_R_fft = fft(s_R_slow);
    [~, max_index_s_R] = max(s_R_fft);
    freq_s_R = slow_freqs(max_index_s_R);

    % figure(2);
    % subplot(2, 1, 1);
    % plot(range_t_slow, abs(s_R_slow));
    % title(sprintf('s_R'));
    % subplot(2, 1, 2);
    % plot(slow_freqs, abs(s_R_fft));
    % title(sprintf('s_R_fft, freq = %E', freq_s_R));
    % xlabel('频率 (Hz)');

    f_d = freq_s_T - freq_s_R;
    velocity = (c * f_d) / (2 * f_c);

    fprintf("predict velocity: %f\n", velocity);
    fprintf("origin velocity: %f\n", origin_velocity);
    fprintf("err = %f\n", velocity - origin_velocity);
    fprintf("\n");

end