# Three-Dimensional Localization of Underwater Nodes Using Airborne Visible Light Beams
## 2 Related Work  
Accurate localization of underwater nodes is vital for applications such as marine research, environmental monitoring, and defense operations. Traditionally, acoustic-based methods have been the primary approach, as sound waves propagate efficiently in water [10; 11; 12]. In contrast, radio frequency (RF) and electromagnetic waves, commonly used for terrestrial and aerial localization, face significant challenges underwater [13; 14]. The high conductivity of seawater causes severe attenuation of RF signals, drastically limiting their range. While extremely low frequency (ELF) and very low frequency (VLF) waves have been explored for deep-sea communication, their feasibility is hindered by the need for large antennas and high power consumption, making them unsuitable for compact underwater nodes [15]. Another alternative, magnetic induction (MI) communication, has shown promise for cross-medium applications like air-to-water transmission [16]. However, despite its ability to penetrate the water surface, MI communication suffers from substantial path loss, restricting its practical use. Traditional underwater localization has predominantly relied on acoustic-based methods, which leverage sound waves due to their superior propagation characteristics in water. Various approaches, such as time of arrival (ToA) [17], time difference of arrival (TDoA) [18], and angle of arrival (AoA) [19], have been widely employed in underwater sensor networks. Acoustic localization systems are further classified into range-based methods [20; 21; 22], which require precise distance measurements between nodes, and range-free methods [23; 24], which use connectivity or hop-count data to estimate positions. These techniques have been extensively studied, leading to solutions such as long baseline (LBL) [25], short baseline (SBL) [26], and ultra-short baseline (USBL) [27] systems. Despite their widespread adoption, acoustic-based methods have several limitations. First, they suffer from high latency and limited bandwidth, restricting data transmission rates [28]. Second, the presence of multipath effects, Doppler shifts, and environmental noise significantly affects accuracy [29]. Moreover, acoustic localization often relies on surface or anchored reference nodes, which may not be viable in dynamic or covert operations. In recent years, VLC has emerged as a promising alternative to traditional acoustic and RF-based underwater communication [5; 30]. Recent studies have improved the understanding of VLC channel characteristics and performance, with a particular focus on light propagation across different water conditions [31]. The behavior of light beams as they transition through air-water interfaces has been examined [32]. VLC leverages high-bandwidth optical signals to achieve low-latency, high-data-rate transmission in underwater environments [33; 34]. Compared to acoustic methods, VLC offers significantly higher throughput and is not affected by Doppler shifts or multipath fading. Recent research has also investigated cross-medium VLC localization, where optical signals propagate from an airborne or surface source into the underwater environment. Some approaches rely on passive optical uplinks to improve positioning accuracy [35]. This method typically involves placing retroreflectors or light-sensitive surfaces on underwater nodes, which reflect or modulate incoming optical signals from surface or airborne sources back to receivers. This allows for range estimation or angle-of-arrival measurements using returned light, often improving localization accuracy without requiring active transmission from the underwater node. While such systems reduce onboard power requirements and hardware complexity, they still require careful alignment between transmitters and receivers. Moreover, the optoacoustic effect has been exploited for cross-medium localization, where high-energy laser beams are used to generate underwater acoustic signals [36, 37].