function [range, range_idx] = get_range(s_T, s_R)
    global T_r delta_t len c r_0 range_t freqs;

    range_N = T_r / delta_t;
    s_T_first = [s_T(1:range_N), zeros(1, len - range_N)];
    s_R_first = [s_R(1:range_N), zeros(1, len - range_N)];
    
    figure(1);
    plot(range_t, s_T_first, color='red');
    hold on;
    plot(range_t, s_R_first, color='blue');

    s_T_fft = fft(s_T_first, len);
    s_R_fft = fft(s_R_first, len);

    figure(2);
    plot(freqs, s_T_fft, color='red');
    hold on;
    plot(freqs, s_R_fft, color='blue');
    
    t = conj(s_T_fft);
    p = ifft(s_R_fft .* t);
    norm_p = real(p).^2 + imag(p).^2;
    % plot(range_t, norm_p);
    
    [~, range_idx] = max(norm_p);
    range = range_t(range_idx) * c / 2;
    fprintf("range = %f, r_0 = %f\n", range, r_0);
end