Communication links are subject to atmospheric effects that reduce the signal power received at the antenna (i.e., a fade). If the atmospheric losses are too high, the signal level could drop below the minimum measurable power of the receiver, causing data loss across the communication link. Previous attempts to forewarn fading using predictive algorithms have yielded sub-optimal results largely because they rely on real-time performance data from the communications link itself, meaning they have very little time to respond.
NASA Glenn Research Center developed the Vortex Radiometer (VR), an early-warning system for communication antennas that identifies atmospheric noise before it reaches the antenna and then optimizes mitigation strategies. The VR constantly senses incoming noise, which allows real-time measurement and characterization of fading and enables maximization of data throughput.
The VR creates concentric, annular antenna beam patterns that measure sky-noise temperature. Annular antenna patterns are created by imparting orbital angular momentum into the electric field received by the antenna using spiral phase plates placed in front of the antenna aperture, generating multiple radiometer channels. Data points are then collected by plotting the measured noise temperature of each radiometer channel as a function of time. Noise temperature increases as a noise source (e.g., weather-related noise, signal interference, etc.) traverses the antenna beam patterns. An algorithm is then used to correlate noise temperature peaks in adjacent beams and to determine when a fade will occur, how long the fade will last, and how intense the fade will be. With this information, effective and efficient strategies can be implemented using cognitive communication and antenna systems to autonomously select the optimum fade-mitigation technique and parameter.
The Vortex Radiometer system has been prototyped including the radiometer device and the algorithm for characterizing noise sources based on VR data. Simulations have shown that a VR system can instruct an existing cognitive antenna to switch between Ka- and X-band communications in order to avert interference from small-diameter noise sources.