# Towards mmWave Altimetry for UAS: Exploring the Potential of 77 GHz Automotive Radars
## 1 Introduction
Initially designed for advanced driver assistance systems (ADASs) and limited only to high-end luxury cars, they now find widespread applications beyond the automotive domain [8]. Given the cost-effectiveness and wide availability, it is intrinsically appealing for application engineers and academicians to devise novel methods and techniques to stretch performance beyond hardware constraints. In the emerging proliferation of 5G networks and the associated interference in the universally allocated 4.2-4.4 GHz frequency band [9; 10] for radar altimeters, it is imperative that we explore novel avenues without compromising the performance requirements for a stable UAS operation. To cater to the burgeoning need of the hour, the versatility of automotive radars can be extended to the realm of UAS altimetry. However, diverging from the identification of pedestrians and obstacles towards the estimation of backscattering from the ground surface, a major transformation of dynamics is expected. To the best of the authors' knowledge, there is no dedicated minimum operational performance standard (MOPS) for radar altimeters of UASs. Accordingly, an adaptation from existing standardsfor commercial aviation is a reasonable approach given the largely similar application of radar altimeters in the landing stage of both platforms. While a few products on the market have incorporated mmWave FMCW radars, there has been limited discourse in the literature. Given the sizable potential of automotive radars, characterized by their flexibility in waveform development, cost-effectiveness, and widespread availability, this study aims to bridge the existing gap in the literature. The focus of this investigation is to explore the feasibility of mmWave FMCW automotive radars for UAS altimetry. Holistically, in radar altimeters, the performance metrics mainly include the maximum and minimum measurable altitude, antenna beamwidth, and range accuracy. These metrics are intricately related, and a careful consideration of the trade-offs is necessary in order to meet the operational requirements of a versatile altimeter. To achieve the requisite performance, there are two aspects to the problem statement of adapting automotive radars for altitude estimation. The first is the theoretical and mathematical conversation encompassing performance metrics in tandem with hardware constraints. Subsequently, there is a need to highlight the challenges and requirements for the adaptation of existing and novel signal-processing algorithms for high-accuracy altitude estimation. In the context of limited reference studies, this study endeavors to initiate fresh discourse on the specific subject of mmWave altimetry encompassing an in-depth deliberation on performance metrics. To the best of the authors' knowledge, a similar text is unreported. Furthermore, existing theoretical discussions on automotive radars often feature intricate mathematical expressions that may pose difficulty in understanding for emerging researchers. This article aims to address specific problem statements related to the application of mmWave radar technology in various scenarios. The primary focus revolves around developing a comprehensive understanding of intricate details crucial for specific applications. The methodology adopted involves an in-depth exploration of theoretical and mathematical aspects, aiming to provide a robust foundation for similar research endeavors. The outcome of applying this methodology has yielded valuable insights and knowledge, contributing to the refinement and enhancement of mmWave radar systems in a diverse context.