Monopulse autotrack methods using software-defined radios

Space missions with large-aperture Ka-band antennas face challenging pointing requirements. We propose a monopulse autotrack technique to mitigate this challenge. The proposed system receives an X-band uplink and estimates angle error to assist with Ka-band downlink pointing. Rather than rely on high-order modes or passive RF hardware, we implement the monopulse using traditional digital carrier-recovery methods. Modern space-born software-defined radios provide accurate and flexible phase tracking capability. This capability can be leveraged for both phase and amplitude monopulse sensors. With appropriate integration and filtering the proposed system provides pointing estimates with RMS error less than 0.01 degrees even at receive power levels as low as −150 dBm. By avoiding implementations that use passive hardware, the system designer has greater flexibility. For example, both 3-element and 4-element systems can be realized, and antenna spacing is flexible and need not be equal or mutually orthogonal. Furthermore, this method can operate both as a closed-loop autotrack system or as an open-loop calibration system where absolute angle estimates are required, and can reuse hardware that supports radio functionality. Simulation results are presented and practical degradations are discussed. NASA's Europa Clipper mission is used as an example to evaluate the efficacy of this method for use in deep space.