function v = get_doppler(s_T, s_R)
    global T_r delta_t c f_c velocity 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;

    fprintf("doppler_freq = %E\n\n", f_d);
    v = (c * f_d) / (2 * f_c);

    fprintf("v: %E\norigin_velocity: %E\n", v, velocity);

end