Millimeter wave (mmWave) radars are popular for perception in vision-denied contexts due to their compact size. This paper explores emerging use-cases that involve static mount or momentarily-static compact radars, for example, a hovering drone. The key challenge with static compact radars is that their limited form-factor also limits their angular resolution. This paper presents Umbra, a mmWave high resolution imaging system, that introduces the concept of rotating mmWave "inverse pinholes" for angular resolution enhancement. We present the imaging system model, design, and evaluation of mmWave inverse pinholes. The inverse pinhole is attractive for its lightweight nature, which enables low-power rotation, upgrading static-mount radars. We also show how propellers in aerial vehicles act as natural inverse pinholes and can enjoy the benefits of high-resolution imaging even while they are momentarily static, e.g., hovering. Our evaluation shows Umbra resolving up to 2.5$^{\circ}$ with just a single antenna, a 5$\times$ improvement compared to 14$^{\circ}$ from a compact mmWave radar baseline.

翻译：毫米波（mmWave）雷达因其紧凑尺寸而在视觉受限场景的感知应用中广受欢迎。本文探讨了涉及静态安装或瞬时静态紧凑雷达的新兴应用场景，例如悬停无人机。静态紧凑雷达面临的核心挑战在于其有限的外形尺寸同时限制了角分辨率。本文提出Umbra，一种毫米波高分辨率成像系统，引入了旋转毫米波"逆针孔"的概念以实现角分辨率增强。我们阐述了毫米波逆针孔的成像系统模型、设计与评估。逆针孔因其轻量化特性而具有吸引力，可实现低功耗旋转，从而升级静态安装雷达。我们还展示了飞行器中的螺旋桨如何作为天然逆针孔，即使在瞬时静态（如悬停）状态下也能获得高分辨率成像的优势。评估结果表明，Umbra仅用单天线即可实现高达2.5$^{\circ}$的分辨率，相较于紧凑毫米波雷达基线14$^{\circ}$的分辨率实现了5$\times$的提升。