Structured Radio Wave Generation for Target State Estimation via Satellite Formations

This paper discusses next-generation radar systems utilizing structured radio waves, which allow for the integrated control of wavefront structure, polarization state, and phase. By leveraging the azimuthal information inherent in Orbital Angular Momentum (OAM) modes, structured radio wave radar mitigates image distortion issues common in conventional plane-wave Synthetic Aperture Radar (SAR). Furthermore, it enables advanced three-dimensional imaging and the simultaneous acquisition of moving target velocity (Doppler components) through a single observation.

To maximize these benefits within the context of rapidly evolving small satellite constellations, we propose a method for spatial radio wave synthesis utilizing a formation of multiple plane-wave radar satellites. A distinctive feature of this method is its ability to spatially synthesize structured radio waves by synchronizing and controlling the phases of satellites arranged in a circular formation, thereby eliminating the need for specialized OAM hardware on individual satellites. Numerical analyses confirmed that in wide-area formation configurations where the inter-satellite spacing is significantly larger than the wavelength, grating lobes exhibiting a mosaic-like vortex phase structure are formed around the main lobe.

This paper details a novel observation strategy that actively utilizes these grating lobes as multiple effective observation beams, even though they are typically suppressed as sources of ghosting artifacts. Through theoretical analysis based on the array factor, we revealed the influence of the number of satellites and OAM mode orders on the structure of the grating lobes, providing design guidelines for constellations aimed at estimating target states, including shape and velocity, through the analysis of their echoes.