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Frequency conversion in the ionosphere for over-the-horizon radar
Phys. Rev. Applied 24, 054040 – Published 13 November, 2025
DOI: https://doi.org/10.1103/c8ch-xhc1
Abstract
This paper proposes a method for generating tunable, low-frequency signals in the ionosphere and propagating them back to the ground. The concept is based on a ground-based, mobile, modulated rf source, which generates signals in the ionosphere at frequencies in the megahertz range. These low-frequency signals propagate to the observation point on the ground. The low-frequency signal is parametrically generated by a ground-based, modulated high-frequency rf source. By tuning the modulation frequency to the local plasma frequency in the ionosphere, a resonant condition generates a low-frequency signal that propagates to the observation point on the ground. The ionospheric electrons are subjected to the modulated rf source and experience a nonlinear force—a ponderomotive force—which resonantly drives a current. The frequency of the driving current is at the modulation frequency, that is, the local plasma frequency. The driving current, driven by a resonance, increases in time, and generates a low-frequency signal. In an example, we find that a 1-MW source at 94 GHz and modulated at 9 MHz can generate a 9-MHz signal 500 km from the source at an intensity greater than the minimum detectable level. The low-frequency signal from the ionosphere can propagate back to the ground-based detector by means of surface waves. This concept may have applications in compact, mobile, and low-cost over-the-horizon radar and as a method to monitor ionospheric parameters.
