Ionospheric dispersion of electromagnetic pulses

When an electromagnetic pulse having a Gaussian envelope is passed through the ionosphere it remains a Gaussian pulse; however, its amplitude is reduced and its pulsewidth is increased with respect to what these parameters would be for free space propagation. It is pointed out that if this Gaussian pulse is chirped (i.e., if it is given a linear FM), then after passing through the ionosphere and following the pulse compression in the receiver, the signals envelope is still Gaussian. Furthermore, when the bandwidths for both signals are the same (or equivalently, when the pulsewidth of the compressed transmitted chirped pulse is the same as that of the transmitted unchirped Gaussian pulse) the amplitude degradation and pulsewidth spreading are identical for both signals. This not too surprising result applies as long as the time-bandwidth product is large enough for the chirped signal. The results obtained for chirped Gaussian pulse are compared, as to the pulse distortion introduced by the ionosphere and available bandwidth of the ionosphere, with the results obtained in the literature for other waveforms. It is pointed out that the amplitude degradation and pulsewidth spreading for a chirped signal having a 40 dB Taylor or Hamming weighting is about the same as obtained with the chirped pulse having a Gaussian envelope as long as the compressed pulsewidths of the transmitted signals are identical. Finally, the numerical results obtained previously in the literature are presented in a more convenient form.