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

Abstract

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 a!so suggests a potential sea-monitoring principle. New modulation equipment permi,•s short-wave broadcast stations to make sky-wave backscatter soundings with exis'Ang transmitters as an adjunct to normal operations, so that area coverage can be observed in real time. Despite these advances, there is little practical app!ication 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. CONTENTS

Observations of the ionosphere in the equatorial anomaly region using WISS during the total solar eclipse of 22 July 2009

“…The mechanism of oblique sounding is similar to an ionosonde but with a bistatic system configuration that can be used to acquire the ionospheric electron density profile. Oblique sounding can also estimate the quality of the ionospheric HF channel by taking advantage of oblique transmission and reflection of radio waves in the ionosphere (Croft, 1972).…”

Strengths and limitations of MST radar measurements of middle-atmosphere winds

“…In fact it is not; in reality the radar``sees'' the scatterers inside the envelope (the eddies with scales of the order of half a wavelength). Croft (1972) has extensively discussed this potential con¯ict between the roles of the motion of the envelope and the motions of the internal scatterers, and despite some exceptions concluded that in general Doppler radars really do measure the motions insider the envelope of the turbulence, rather than the motions of the envelope itself. Since these internal eddies are driven by the local wind, which will be due to both local winds and the gravity wave's own velocity ®eld, their motions are truly representative of the winds at this point in time.…”

A new inversion algorithm for backscatter ionogram and its experimental validation

“…The focusing effect determines that the trend of group distance in function of the elevation angle has a minimum value and enhances the power in the leading edge of the backscatter ionogram. This characteristic has been studied by many authors (Georges and Stephenson, 1969;Croft, 1967Croft, , 1972Ruelle and Landeau, 1994;Landeau et al, 1997). At the beginning, group path P decreases while β increases.…”

“…The oblique backscatter sounding is a powerful tool for detecting the ionosphere, and it can be used to monitor the ionosphere continuously at a remote distance (Croft, 1972). When a high-frequency (HF) signal is emitted with an angle with respect to the horizon, it can be reflected by the ionosphere, and then scattered by the terrestrial surface.…”