# Towards mmWave Altimetry for UAS: Exploring the Potential of 77 GHz Automotive Radars
## 1 Introduction  
Accurate altitude estimation is paramount for ensuring the safety of unmanned aerial systems (UASs), particularly during the crucial phase of landing [1]. While various technologies contribute to this essential task, each method possesses unique limitations. Global positioning system (GPS)-aided attitude and the heading reference systems (AHRS) are prevalent in-flight navigation [2]. However, the slow update rate, typically at one pulse persecond (PPS), poses challenges in catering to the rapid descent during the landing phase. Furthermore, the altitude data are referenced from the mean sea level (MSL), whereas autonomous landing necessitates altitude information above ground level (AGL). Alternative technologies such as light detection and ranging (LiDAR) systems also contribute to altitude determination. However, LiDAR systems present limitations in adverse weather conditions with poor visibility such as heavy precipitation, fog, or dust [3]. Barotmetric altitude estimation requires recurrently applying corrections to the readings due to temperature-induced variations in atmospheric pressure. Radar altimeters emerge as the optimal solution to UAS altimetry, addressing key limitations in other technologies by offering a high update rate and precise AGL altitude. By virtue of electromagnetic (EM) waves traversing through atmospheric hindrances chiefly unfazed, the resilience of the radar altimeter in adverse weather conditions further solidifies its role as an essential tool for ensuring the safety of UAS operations. The two predominant waveforms employed in radar systems are Pulse Doppler and frequency-modulated continuous wave (FMCW) [4]. Continuous wave (CW) radars are limited to Doppler estimation only and, hence, do not find application in altimeters [5]. Pulse Doppler radar altimeters determine altitude by emitting short pulses of radio frequency (RF) energy and measure the round-trip delay for the echo to return from the surface of the ground. To avoid overlap between transmitted and received pulses, their duration must be shorter than the round-trip delay. Moreover, the time required for a solid-state switch to shift between the transmit and receive cycle is an essential consideration. Local oscillator (LO) inaccuracies further aggravate the timing situation, compelling a guard time interval. Collectively, these aspects require a long round-trip for the radar to function properly, hence compromising the minimum measurable altitude. Conversely, FMCW radars, owing to the simultaneous transmission and reception, are not strictly limited by a lower bound on altitude estimation. During the crucial landing phase, precise altitude estimation is vital until touchdown. In this context, FMCW radars emerge as the favored candidate. Moreover, continuous operation offers an additional benefit in terms of a low transmission peak power. This characteristic is significant for operations conducted in close proximity to the general public, as health authorities strictly regulate the transmission of peak power. Pulse Doppler radars typically operate with a duty cycle of 10%. Consequently, to attain the necessary average power, they are bound to transmit a higher peak power compared to FMCW radars. Over the past decade, mmWave automotive radars were largely built on a relatively higher volume silicon germanium (SiGe) architecture with limited abstraction for a customized waveform design [6]. However, with the advent of integrated 45-nanometer (nm) radio frequency complementary metal oxide semiconductor (RFCMOS) technology, the microcontroller unit (MCU), digital signal processor (DSP), hardware accelerator (HWA), and RF front-end (RFFE) are all housed on the same chip now [7]. This breakthrough has opened massive opportunities in terms of highly flexible waveform design leading to a high performance in a miniature and cost-effective package.