function doppler = get_doppler(s_T, s_R)
    global T_r delta_t len f_s c f_c velocity max_t slow_len;
    
    freqs = ((0:len-1) * f_s) / len;

    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);

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

    for i = 0: num_slow - 1
        while abs(s_R(init_idx + i * k)) == 0
            init_idx = init_idx + 1;
        end
        s_R_slow(i + 1) = s_R(init_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 = 1/T_r - freq_s_R;

    fprintf("s_R_freq = %d\n", max_index_s_R);
    fprintf("doppler_freq = %E\n\n", f_d);

    fprintf("%E\n", 2 * f_c * velocity / c);

    doppler_v = (c * f_d) / (2 * f_c);
    
    fprintf("doppler_v: %E\nvelocity: %E\n", doppler_v, velocity);

    doppler = doppler_v;
end