Sky-wave backscatter: A means for observing our environment at great distances


  • Thomas A. Croft


For 45 years, men have measured and studied echoes from the distant surface of the earth, obtained with high-frequency signals (3 to 30 MHz) that travel to and from the scatterers via ionospheric refraction. During the last five years, much progress has been made in the understanding of this ‘sky-wave backscatter.’ An explanation of the various interacting phenomena is presented, as is a review of the current state of knowledge reflecting recent advances in observational methods and analytic techniques. New narrow-beam antennas, coupled with signal modulations that permit fine resolution in time delay, are beginning to yield information concerning the character of the scatterers, which now can be separately discerned. These narrow beams also permit study of polarization fading from small regions, and this shows promise as a means for learning the distant sea state. Doppler shifts of a fraction of a hertz on signals of tens of megahertz are separable, permitting isolation of sea returns from ground returns by virtue of the Doppler effect resulting from sea-wave speed; this also suggests a potential sea-monitoring principle. New modulation equipment permits short-wave broadcast stations to make sky-wave backscatter soundings with existing transmitters as an adjunct to normal operations, so that area coverage can be observed in real time. Despite these advances, there is little practical application of sky-wave backscatter as a means of environmental monitoring. This lack is attributed to the large remaining gaps in our understanding of the echoes and our inability to interpret the forms of data that can be acquired with equipment of reasonable cost. It is still not possible to calculate ionospheric structure from an observation of sky-wave backscatter although it is the opinion of many workers that this should be feasible. It is now at least possible to carry out the reverse process, calculating sky-wave backscatter from an assumed ionosphere of much complexity. Study of such synthetic data is yielding additional insight into the causes of diverse forms of observed sky-wave backscatter.