Steven W. Ellingson

In past work, we described the use of a reconfigurable intelligent surface (RIS) mounted on the rim of an axisymmetric prime focus-fed reflector to create nulls in the close-in sidelobes. In this paper, we show that similar performance is possible in an offset Gregorian reflector system using a RIS on the rim of the subreflector. Applications include radio astronomy, where offset Gregorian reflectors are common and observations are subject to deleterious levels of interference from satellites entering through sidelobes. We show that an efficient RIS replacing the outer one-third of the subreflector surface, employing passive elements with 1-bit phase-only control, can create a null in the peak of the second sidelobe in the quiescent pattern. This is achieved using a simple unconstrained optimization algorithm to set the states of the RIS elements. The algorithm yields a deep null with just 0.2~dB reduction in main lobe directivity, despite lacking any constraints on main lobe pattern. Compared to our previous approach of mounting the RIS on the rim of the main reflector, the subreflector-based approach demonstrated in this paper requires a much smaller RIS and can implemented in existing systems by replacing the subreflector.

A common problem in justice applications is localization of a user of a cellular network using a call detail record (CDR), which typically reveals only the base station and sector to which the user was connected. This precludes precise estimation of location. Instead, one is limited to estimating a region of plausible locations (RPL) using static information such as sector antenna orientation, beamwidth, and locations of nearby base stations. In this paper, we propose a method for RPL estimation in which the shape bounding the RPL is derived from a model of the antenna pattern via the Friis Transmission Equation, and the size of the RPL is determined by mean distance to nearby base stations. The performance of the proposed method is evaluated by “best server” analysis of measurements acquired from drive testing in the vicinity of Winter Garden, Florida, observing three 700 MHz-band LTE cellular networks serving this area. Of the 16 sectors evaluated, the aggregate error rate (i.e., fraction of users located outside the RPL estimated for the associated sector) is found to be 1.3%, with worst per-sector error rate of about 13.3% and error rates below 1.8% for 13 of the 16 sectors. The principal difficulty is shown to be estimation of RPL size, which entails a tradeoff between minimizing RPL area (yielding the “tightest” localization) and minimizing error rate.

Anthropogenic interference has been a long-standing problem for radio astronomy. In a previous paper, we presented a study of interference mitigation methods based on the concept of coherent time-domain canceling, which ideally allows one to “look through” interference, as opposed to avoiding the interference or deleting the afflicted data. The focus of that paper was on “reference antenna” methods, in which a separately acquired signal containing the interference waveform is used to identify the interference waveform in the primary signal. In this paper, we shift focus to methods in which the reference signal is instead a parametric model of the waveform, so that no additional antenna is needed. As in our previous paper, we present a rigorous theoretical analysis of performance. Findings are demonstrated using real-world interference from the Iridium system. We find that interference suppression is possible if the product of the interference-to-noise ratio and the number of statistically independent samples is greater than 1, and that suppression increases linearly with this product. However good performance is achieved only for interferers whose bandwidth is much less than the sample rate, and algorithm parameters must be carefully selected to avoid undesirable distortion of the noise spectral baseline.

Radio astronomy is vulnerable to interference from a variety of anthropogenic sources. Among the many strategies for mitigation of this interference is coherent time-domain canceling (CTC), which ideally allows one to “look through” interference, as opposed to avoiding the interference or deleting the afflicted data. However, CTC is difficult to implement, not well understood, and at present this strategy is not in regular use at any major radio telescope. This paper presents a review of CTC including a new comprehensive study of the capabilities and limitations of CTC using metrics relevant to radio astronomy, including fraction of interference power removed and increase in noise. This work is motivated by the emergence of a new generation of communications systems which pose a significantly increased threat to radio astronomy and which may overwhelm mitigation methods now in place.

Radio astronomy is vulnerable to interference from a variety of anthropogenic sources. Among the many strategies for mitigation of this interference is coherent time-domain canceling (CTC), which ideally allows one to "look through" interference, as opposed to avoiding the interference or deleting the afflicted data. However, CTC is difficult to implement, not well understood, and at present this strategy is not in regular use at any major radio telescope. This paper presents a review of CTC including a new comprehensive study of the capabilities and limitations of CTC using metrics relevant to radio astronomy, including fraction of interference power removed and increase in noise. This work is motivated by the emergence of a new generation of communications systems which pose a significantly increased threat to radio astronomy and which may overwhelm mitigation methods now in place.

Dynamic modification of the pattern of a reflector antenna system traditionally requires an array of feeds. This letter presents an alternative approach in which the scattering from a fraction of the reflector around the rim is passively modified using, for example, an electronically reconfigurable reflectarray. This facilitates flexible sidelobe modification, including sidelobe canceling, for systems employing a single feed. Applications for such a system include radio astronomy, where deleterious levels of interference from satellites enter through sidelobes. We show that an efficient reconfigurable surface occupying about 11% of the area of an axisymmetric circular paraboloidal reflector antenna fed from the prime focus is sufficient to null interference arriving from any direction outside the main lobe with little change in the main lobe characteristics. We further show that the required surface area is independent of frequency and that the same performance can be obtained using 1 b phase control of the constituent unit cells for a reconfigurable surface occupying an additional 6% of the reflector surface.

The geometrical theory of diffraction (GTD) is a popular method for the analysis of diffraction from edges of electrically large conducting surfaces. It is also easy to use, as it involves only algebraic expressions and geometrical considerations. However, GTD is awkward to teach because a rigorous derivation of the diffraction coefficients requires mathematical skills that are beyond the reach of most students and many practitioners. This article presents an alternative derivation of the GTD edge diffraction equations. This approach begins with a relatively simple stationary-phase analysis of a radiation integral formed from equivalent currents postulated to flow along the edge. This analysis yields expressions in GTD form but with unspecified diffraction coefficients. Diffraction coefficients in the original GTD (Keller) form or the uniform GTD (UTD) (Kouyoumjian and Pathak) form can then be introduced as an alternative to the equivalent currents in the final expressions. This approach is suitable for a concise introduction to GTD edge diffraction for students with no previous experience in asymptotic high-frequency methods.