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∘ with just a single antenna, a 5× improvement compared to 14∘ from a compact mmWave radar baseline.
@article{arxiv.2510.13904,
title = {Millimeter Wave Inverse Pinhole Imaging},
author = {Akarsh Prabhakara and Yawen Liu and Aswin C. Sankaranarayanan and Anthony Rowe and Swarun Kumar},
journal= {arXiv preprint arXiv:2510.13904},
year = {2025}
}