# Towards mmWave Altimetry for UAS: Exploring the Potential of 77 GHz Automotive Radars
## 2 Literature Review and Related Work
### Legacy and Contemporary Literature  
The early use of radars was limited to military applications with a rich history of research and development. There have been many academic contributions in terms of textbooks dedicated to radar concepts and theory [21; 22]. These resources offer a comprehensive coverage of the terminologies, fundamental principles, and algorithms, as well as frameworks for diverse types of radar waveforms and systems. Nevertheless, with the recent advancements in the field of automotive radars such as mmWave time division multiplexing multiple-input multiple-output (TDM-MIMO) [23], it is more appropriate and convenient to consult surveys and state-of-the-art review articles for an informed literature review [24; 25; 26]. Such works offer stimulating discussions on trends in the industry and the direction of research moving forward. Nonetheless, such reviews are devoid of depth and focus on the breadth of the research. On the other hand, concise instructional materials, emphasizing implementation details and practical facets, are presently being  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Year** & **Application** \\ \hline
[11] & 2010 & Autonomous robot navigation \\
[12] & 2017 & Fluid level sensing \\
[13] & 2018 & Material classification \\
[14] & 2018 & Blood glucose level detection \\
[15] & 2018 & Traffic monitoring \\
[16] & 2020 & RCS analysis of 9 mm bullet \\
[17] & 2021 & UAS detection and localization \\
[18] & 2022 & Vital sign measuring \\
[19] & 2022 & Blood pressure monitoring \\
[20] & 2023 & Indoor positioning system \\ \hline \hline \end{tabular}
\end{table}
Table 1: Emerging applications of mmWave radars.  
disseminated by manufacturers of mmWave radars. These resources are presented in the form of whitepapers and application notes intended for application engineers [27; 28; 29]. They offer a basic level of understanding from the perspective of product development with restricted introductory content and limited experimental work.  
More recently, researchers have presented dissertations that delve into the intricate details of mmWave radars tailored for specific applications. The theoretical and mathematical nuances expounded in these works establish a substantial repository, offering valuable insights for similar endeavors to draw upon. In line with the aim of this work, the notable dissertation in [30] stands as a testament to stretching the boundaries of automotive radars for applications beyond conventional constraints. The cited work put forth the basis for the resolution of velocity ambiguity well beyond the maximum measuring capacity of automotive radars. Likewise, a consolidated theoretical and practical framework for the applicability of mmWave radars for vital signs application was offered in [31]. Such works lay the groundwork for exploring the untapped potential of automotive radars. This article aims to coalesce the strengths of the existing forms of literature and fill the looming gap. There is an evolving need to present a tutorial-themed piece of literature to equip the impending and enthusiastic researchers to better understand the theoretical aspects of cutting-edge technologies. To combine the strengths of the available spectrum of the literature, the theoretical basics have been embraced from legacy textbooks. Application notes were consulted for a better understanding of the hardware capabilities. Although not written for a particular application, a tutorial-themed in-depth mathematical understanding of three-dimensional signal processing in mmWave automotive radars was offered [32]. This article follows a similar tutorial tone but instead takes an application-specific tangent towards UAS altimetry.