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Null-steering using adaptive antenna systems is a robust method to global positioning system (GPS) receivers against hostile jamming. Conventionally, multi-antenna arrays have been utilized for such receivers, but the size and weight are often prohibitive especially for space-limited airborne applications. In this paper, we present another approach based on antenna pattern and polarization diversity to enable nullsteering, and propose a design with co-located elements to achieve a small footprint. As a proof-of-concept, we present an antenna prototype with a size of only 5 × 5 × 1.9 cm 3 (0.26λ × 0.26λ × 0.1λ) at the L1 band, capable of steering two nulls. We present the simulated and measured parameters of the antenna, study its null-steering performance in various interference scenarios with a simple power minimization method, and demonstrate its capability in suppressing up to two jamming signals incident from the angular region close to horizon. INDEX TERMS Adaptive antennas, global positioning system, interference suppression.

A novel ultrawideband pattern diversity antenna, with sum and difference radiation patterns used in monopulse tracking, is proposed. To achieve pattern diversity, a crescent‐shaped radiating element is fed from two sides at an angle close to 90°, by two coaxial feeds. The ground is a corner‐shaped cavity. The sum and difference patterns can be obtained by in‐phase and out‐of‐phase excitations of the two coaxial ports. A single‐element antenna is first studied to show the operating principle of the design. A modified design with two elements is then proposed and studied through simulations and measurements of a fabricated prototype, showing excellent radiation characteristics over a large frequency bandwidth. The application of this antenna as a reflector feed is then studied. It is shown that the antenna can theoretically feed a prime focus reflector with 60% (for the sum pattern) efficiency over a 48% fractional bandwidth (5.5–9 GHz), with a maximum efficiency of 75%. The antenna is compact (5 × 4.1 × 2 cm3) and has low cost and simple fabrication. To the best of authors’ knowledge, this is the first attempt to feed a reflector with this class of antennas.

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We study the problem of persistent monitoring of finite number of inter-connected geographical nodes for event detection via a group of heterogeneous mobile agents. We use Poisson process model to capture the probability of the events occurring at the geographical nodes. We then tie a utility function to the probability of detecting an event in each point of interest and use it in our policy design to incentivize the agents to visit the geographical nodes with higher probability of event occurrence. We show that the design of an optimal monitoring policy to maximize the utility of event detection over a mission horizon is an NP-hard problem. By showing that the reward function is a monotone increasing and submodular function, we then proceed to propose a suboptimal dispatch policy design with a known optimality gap. To reduce the time complexity of constructing the feasible search set and also to induce robustness to changes in event occurrence and other operational factors, we preform our suboptimal policy design in a receding horizon setting. Our next contribution is to add a new term to our optimization problem to compensate for the shortsightedness of the receding horizon approach. This added term provides a measure of importance for nodes beyond the receding horizon's sight, and is meant to give the policy design an intuition to steer the agents towards areas with higher importance on the global map. Finally, we discuss how our proposed algorithm can be implemented in a decentralized manner. We demonstrate our results through a simulation study.