It has been suggested (Hines, 1972) that the atmospheric gravity waves that are generated by a tsunami may well produce an identifiable ionospheric signature that could be employed for purposes of tsunami warnings. The intrinsic difficulties to be faced by this suggestion derive from the geometrical spreading of the tsunami signature, from the possible occurrence of heights of null response, from the reflection and absorption of wave energy in the regions between the ocean surface and the ionospheric height of observation, from the time delay experienced by the tsunami signature in reaching that height, from a degradation of amplitude when the signature is detected via its effect on isopleths of electron concentration, and from the competition of 'noise' that would obscure the signature. These difficulties are assessed in the present paper and are found to be of only marginal consequence to the original suggestion, which is therefore reinfored as a proposal for an operational system.