Review of Hybrid Aerial Underwater Vehicle: Potential Applications in the Field of Underwater Marine Optics

Abstract

Hybrid Aerial Underwater Vehicle (HAUV) is a new type of unmanned system that can operate both in air and water, and complete underwater and air operations tasks by carrying corresponding sensors. Owing to this dual-medium operational capability, HAUVs hold significant promise for coordinated air–sea surveillance and monitoring efforts. Optical methods enable high-resolution sampling across both spatial and temporal scales, offering enhanced contextual information for the interpretation of discrete observational data. In order to evaluate the feasibility of ocean optical profiling systems based on HAUVs, this paper reviews the design features of current HAUV models and summarizes advanced techniques that support their cross-medium mobility. Subsequently, we summarized the types of commercial optical instruments commonly used for underwater observation and compared the field deployment methods. By analyzing the underwater motion performance of HAUVs and the requirements for optical observation platforms, we believe that multi-rotor HAUVs can provide new observation methods for future underwater optical acquisition due to their smooth entry and exit characteristics and the ability to maintain a controlled orientation during underwater operation. Finally, the paper explores prospective applications and outlines key obstacles to be overcome in the advancement of amphibious platforms for ocean optical profiling.

1. Introduction

Currently, unmanned vehicles, including Unmanned Aerial Vehicles (UAVs), Unmanned Surface Vehicles (USVs), and Unmanned Underwater Vehicles (UUVs), are essential tools for human exploration, understanding, and exploitation of the ocean [1]. These conventional vehicles can carry various exploration equipment or operational tools for ocean monitoring in different spatial dimensions. In addition, new and more complex application scenarios have emerged with the development of marine science and the increasing need for rapid ocean monitoring [2,3]. These missions include the monitoring of marine emergencies such as red tides and oil spills from the air, the detection of these events under water, and the sampling of the water in the area [4]. To meet the needs of air–sea cross-domain missions, researchers have made progress in the design and operation of a distributed heterogeneous autonomous sensor network that combines unmanned vehicles in coastal environments [5]. However, the harmonious integration of different devices into a single system is not an easy or cost-effective task, and the organization and control of such a system is therefore a major challenge.

An emerging concept, Hybrid Aerial Underwater Vehicles (HAUVs), has expanded significantly in recent years to provide a more convenient and efficient solution for air–sea cross-domain missions [4,6]. The capability of HAUVs to operate across both aerial and aquatic domains simplifies the execution of multi-environment monitoring tasks. A single platform can acquire data from various mediums, reducing the need for separate systems. Current HAUV design strategies typically involve adapting fixed-wing and rotary-wing UAVs to perform underwater functions. Nevertheless, HAUVs, as an emerging technology, encounter numerous technical and operational challenges in practical deployment and remain insufficiently developed to fully support the aforementioned applications [7,8]. Therefore, identifying and developing suitable design strategies for amphibious vehicles that can meet the demands of specific operational scenarios has become a key priority in current research.

The development of satellite remote sensing technology for ocean color in the 1980s led to widespread interest in the optical properties of the ocean [9,10,11]. The change and distribution of the color of seawater is determined by substances in the upper surface water, such as phytoplankton, non-pigmented particulate matter, and soluble organic matter. In order to determine the concentration of these substances from water color data, it is necessary to have an understanding of their optical properties and their effect on the light transmission process. This need has led to the development of a large number of new high-performance optical instruments. These have been designed to provide previously unmeasurable data on the optical properties of the ocean. These optical data can provide some basic information about the ocean, such as the concentration and type of particulate matter, the particle size distribution, the primary productivity, the turbidity of the water, etc. In addition, data on optical properties are correlated with data on physical, biological, chemical, and geological properties [12,13,14,15,16]. The current research focus of ocean optics is the use of optical property measurement technology as an important tool for the study of physical oceanography, chemical oceanography, biological oceanography, and geological oceanography, which inevitably requires different observation platforms to meet the needs. Compared to traditional marine optical measurement methods such as remote sensing, buoys, and ship surveys, more and more underwater mobile intelligent platforms can help to obtain fast and stereo optical data sets (e.g., Bio-Argo, AUV, ROV, and UG) [17,18,19,20,21,22,23,24,25]. It is necessary to further integrate new observing platforms and to extend the methods they use to observe.

The main purpose of this paper is to review the progress of existing cross-domain HAUVs, then analyze the requirements of ocean optical observation methods for platforms, evaluate the feasibility of amphibious profiling optical systems, and finally look at future application scenarios. The paper is structured as follows: a review of the current status of HAUVs is given in Section 2. Ocean optical instruments and deployment methods are presented in Section 3. The feasibility of a hybrid airborne underwater optical observation system is discussed in Section 4. Finally, the potential applications and future work for amphibious optical observation systems are described in Section 5.

2. Hybrid Underwater Vehicle Progress

The Hybrid Underwater Aerial Vehicle is an unmanned aerial vehicle that can move simultaneously in water and air, rely on its own energy to cross water and air media repeatedly, and perform underwater and air operations by carrying appropriate sensors [4,26,27]. This aircraft combines the advantages of stealth, the flexibility of underwater robots, and the speed of air vehicles. It has attracted great interest in various countries. The entire development process of HAUVs can be roughly divided into the conceptual design phase, the theoretical development phase, and the research phase of prototype testing. As early as the 1930s, before World War II, the former Soviet Union proposed the manned “flying submarine” plan [6]. It developed a design scheme that combined aircraft and submarines, and established a specialized technical research team. Their design scheme can search and determine the target from the air, then land and infiltrate the enemy ship’s route to set up ambushes and achieve the effect of surprise operations. However, due to technical limitations at the time, the plan did not enter the engineering research and development stage and was finally abandoned in 1938. Following the concept of the flying submarine, four notable prototypes have exemplified the manned concept: the LPL (Letayushchaya Podvodnaya Lodka), RFS-1(Reid Flying Submarine 1), and Convair models, along with DARPA’s (DARPA, the Defense Advanced Research Projects Agency) submersible aircraft. These four conceptual prototypes advanced related technological developments and demonstrated the theoretical viability of the design. However, none were ultimately implemented to function across both aerial and underwater environments. Since the beginning of the 21st century, with the rapid development of technologies such as autonomous control, structural compatibility design, computer simulation, new lightweight materials, and advanced power systems, especially the gradual maturity of unmanned aerial vehicles, underwater autonomous robots and biomimetic theory, research institutions have been continuously demonstrating and designing amphibious unmanned aerial vehicles in the sea and the air, and some institutions have conducted principle prototypes and preliminary tests. As an emerging field, HAUV technology remains in the early stages of exploration, with numerous innovative design concepts under active development and testing. These vehicles can be categorized in multiple ways depending on the classification criteria applied. In this paper, existing forms of HAUVs are grouped into two dominant designs on the basis of water entry strategies. One is the splash entry HAUV, which is characterized by the unique way of plunge-dive water entry. The other form is the Vertical Take-Off and Landing (VTOL) HAUV, which has a stable low-speed flight and a smooth water entry capability.

2.1. The Splash Entry/Exit Way HAUVs

HAUVs with a splash entry and exit path can usually be divided into two categories (as shown in Figure 1 and Figure 2): (1) Bio-inspired HAUVs designs are inspired by natural organisms capable of locomotion in both aerial and aquatic environments, such as cliff puffins, boobies, flying fish, frilled squids, dragonflies, and similar species. (2) Fixed-wing HAUVs are capable of high-speed transit across different media. For instance, they can rapidly transition between air and water or perform takeoff and landing maneuvers in a gliding fashion, similar to traditional seaplanes.

Bio-inspired HAUVs, drawing inspiration from natural amphibians like flying fish, seabirds, and remoras, offer several advantages: they enable efficient cross-domain transitions by mimicking natural behaviors, adapt to diverse environments with multi-functional capabilities, can be energy-efficient in specific modes (like flapping wings for slow underwater movement), and have miniaturization potential [28,29,30,31,32,33,34]. However, they also have notable disadvantages: limited underwater maneuverability, high mechanical demands for water-to-air transitions, design trade-offs where features optimized for one domain hinder performance in another, limited payload capacity restricting complex tasks, and technical complexity in control systems.

Fixed-wing HAUVs, which use wing-generated lift for airborne flight, offer advantages such as efficient aerial performance (high-speed flight with low power consumption, suitable for covering large distances), versatile cross-domain functionality, innovative design adaptations, and rapid deployment [32,35,36,37,38,39]. However, they are also plagued by several drawbacks: take-off processes are problematic—surface glide take-offs demand lengthy distances and are easily disrupted by surface disturbances, while tilt-takeoffs require meticulous coordination of attitude and speed, with mismatches leading to insufficient lift and crashes; underwater efficiency is low—airborne propellers perform poorly underwater, necessitating retrofits such as water plungers, and wide wings generate significant drag during cross-domain movement, posing a risk of structural damage upon high-speed water entry.

Figure 1. Representative bio-inspired HAUVs. (a) Harvard University RoboBee [33] (b) Biomimetic flying squid [40] (c) MIT biomimetic bonito [41] (d) Beihang University hitchhiking robot [26] (e) AquaMAV [34] (f) HAUV with folding wings [42] (g) MIT biomimetic birds [28] (h) Chemical combustion HAUV [43].

Figure 1. Representative bio-inspired HAUVs. (a) Harvard University RoboBee [33] (b) Biomimetic flying squid [40] (c) MIT biomimetic bonito [41] (d) Beihang University hitchhiking robot [26] (e) AquaMAV [34] (f) HAUV with folding wings [42] (g) MIT biomimetic birds [28] (h) Chemical combustion HAUV [43].

Figure 2. Representative fixed-wing HAUVs. (a) Flying Fish [32] (b) Test Sub II [35] (c) Dipper (d) Triangular plate-like HAUV [36] (e) HAUV with foldable wings [38] (f) EagleRay [37].

Figure 2. Representative fixed-wing HAUVs. (a) Flying Fish [32] (b) Test Sub II [35] (c) Dipper (d) Triangular plate-like HAUV [36] (e) HAUV with foldable wings [38] (f) EagleRay [37].

2.2. The VTOL Way HAUVs

The VTOL way HAUVs usually fall into two categories: (1) Multi-rotor HAUVs, which retain the superior agility of their UAV counterparts, offering stable low-speed operation, vertical takeoff and landing (VTOL), and precise hovering at fixed positions. Such vehicles are particularly well-suited for operations in environments that are spatially limited or structurally complex. (2) Multi-modal HAUVs, integrate the hovering capability of multi-rotor systems with the extended air and underwater endurance typically seen in fixed-wing HAUVs or underwater gliders.

Multi-rotor Hybrid Aerial Underwater Vehicles (HAUVs) build on the structural framework of multi-rotor UAVs, with their propulsion systems directly providing the lift and torque needed for flight. Over the past five years, they have gained growing favor among researchers due to their significant advantages in flexibility and stability, particularly their unique hovering and Vertical Take-Off and Landing (VTOL) capabilities, which facilitate flight, diving, and cross-domain testing. Their structural layouts are categorized into single-layered and double-layered designs.

Double-layer multi-rotor HAUVs adopt a two-layer propulsion system to enable seamless movement in and out of water (Figure 3). For example, Brazil’s Federal University of Minas Gerais developed the HyDrone, a two-layer quadrotor with specialized lower-layer propellers to enhance underwater propulsion efficiency [44,45]. Rutgers University’s Naviator series, also a dual-layer quadrotor, uses its propulsion system for locomotion in air, underwater, and across the water surface, with pool tests validating its amphibious capabilities; the team has also conducted quantitative analyses to optimize its structures [46,47]. China’s Air Force Engineering University has developed a similar double-layer quadrotor prototype, primarily for control algorithm testing [48].

Single-layer multi-rotor HAUVs, meanwhile, feature varied designs (Figure 4). The University of California, Berkeley, built a miniature quadrotor that can accelerate from underwater, break the surface smoothly, and take off, though additional maneuvering tests are pending [49]. The National University of Singapore developed a tilt-quadrotor with propellers that adjust direction via a tilting mechanism—vertical for aerial flight and horizontal/downward for underwater vector propulsion, enhancing underwater maneuverability; it has undergone iterations and pool tests [50,51]. Tongji University and collaborators presented the TJ-FlyingFish, a quadrotor with tiltable dual-speed propulsion units, which has undergone thorough practicality, functionality, and maneuverability testing, functionality, and maneuverability testing [52].

Single-layer systems face greater challenges in continuous water exit, as rotors must handle sudden thrust changes at the surface. Thus, buoyancy control systems are often used when coherent surface movement is unnecessary. Oakland University’s Loon Copter uses a pump-and-drain mechanism to adjust weight: draining ballast tanks to float vertically for take-off, and filling them to submerge, with a 90° tilt positioning propellers horizontally for improved underwater maneuverability, validated in pool tests [54,62]. Georgia Tech’s GTQ-Cormorant uses buoyancy adjustment for vertical snorkeling, with pool tests confirming underwater and surface VTOL capabilities [55]. China’s Shanghai Maritime University developed a quadrotor prototype for cross-domain motion control testing (full amphibious capability unreported), while Shanghai Jiao Tong University’s Nezha-F, a small HAUV with a piston variable buoyancy system, balances aerial and underwater performance compactly without excessive actuators [53,57]. Recently, a series of new prototypes have continued to emerge, advancing the field [58,59,60,61,63,64].

Inspired by traditional Underwater Gliders (UG) and existing HAUV designs, Shanghai Jiao Tong University (SJTU) proposed a multi-modal HAUV (based on multi-rotor) concept capable of vertical flight, horizontal flight, underwater gliding, and stable water-air interface crossing. This multi-modal HAUV uses the UG motion mode underwater, adjusting net buoyancy and attitude via an internal mechanism to achieve low-energy, long-duration underwater gliding [65,66]. In the air, it can switch between multi-rotor UAV vertical flight and fixed-wing UAV horizontal flight, with the rotor propulsion system and fixed wing coordinating to provide appropriate power and lift for hovering, VTOL, or high-speed flight. SJTU made progressive developments: in 2017, it successfully developed the Nezha I, a principal prototype of a multi-rotor HAUV with vertical underwater locomotion; in 2018, it launched Nezha II, the first multi-modal HAUV prototype integrating horizontal flight, vertical flight, and underwater gliding. To date, the “Nezha” series has completed the third iteration, including the “Nezha III (tail-sitting)” and “Nezha III (VTOL)” prototypes (Figure 5). It is foreseeable that more multi-modal HAUVs will be developed to meet growing demands for rapid surveillance, expanding the application range and capabilities of amphibious vehicles [67,68].

2.3. Comparative Analysis of HAUV Types

While the previous subsections described splash-entry (bio-inspired and fixed-wing) and VTOL (multi-rotor and multi-modal) HAUVs separately, a direct parameter-based comparison provides clearer insights into their relative strengths and limitations.

Table 1. Comparison of representative HAUV types by parameters.

Type of HAUV	Transition Mechanism	Payload Capacity	Underwater Maneuverability	Aerial Endurance	Stability in Water Entry	Representative Applications
Bio-inspired	Plunge-dive biomimetic	Low (miniaturized)	Moderate (limited control)	Moderate	Sensitive to turbulence	Small-scale monitoring proof-of-concept research
Fixed-wing (splash)	Glide or tilt entry	Medium	Low (propellers are inefficient)	High (long range)	High risk of vibration	Wide-area patrols rapid deployment
VTOL Multi-rotor	Vertical descent/ascent	Medium–High	High (hover precise positioning)	Short–Moderate	Smooth controllable	Optical profiling localized surveys
Multi-modal HAUV	VTOL + gliding capability	Medium	High (gliding + hovering)	Moderate–High	Stable with buoyancy system	Long-duration surveillance adaptive missions

summarizes representative HAUV categories across critical parameters, including transition mechanisms, payload capacity, underwater maneuverability, aerial endurance, stability, and potential application scenarios.

This comparison highlights that bio-inspired systems are most promising for miniaturized applications but remain limited in payload; fixed-wing splash-entry designs excel in long-range aerial coverage but face challenges in underwater propulsion; VTOL multi-rotors offer superior stability for optical profiling at localized scales; and multi-modal HAUVs provide the most balanced solution for adaptive missions, albeit with added system complexity.

To provide a comprehensive overview of the global research landscape, the distribution of HAUV-related studies by country/region is summarized in Table 2. This table highlights the geographical diversity of the field and emphasizes the dominant contributions from China and the United States.

In summary, HAUV research has developed along multiple directions, ranging from biomimetic design and propulsion mechanisms to cross-domain control strategies and prototype demonstrations. While these studies vary in technical approach and application focus, they collectively reflect the growing attention to versatile aerial–aquatic platforms. To provide a clearer picture of the global research landscape, we summarize in Table 2 the distribution of HAUV-related studies by country and region. The table highlights the leading contributions from China and the United States, along with representative studies from the United Kingdom, and other countries. This international distribution underlines the increasing worldwide interest in HAUVs and indicates significant potential for future cross-disciplinary and cross-border collaborations.

Table 2. Distribution of HAUV-related studies by country/region.

Country/Region	Representative Studies	Number of Studies
China	Nezha series (Nezha-F [57], Nezha-IV [58], Nezha-SeaDart [59], Nezha-H [64], Nezha-X [61]); WuKong [60]; TJ-FlyingFish [52]; Wu et al. flying boat [32]; Plunge-diving gannet [29,30,31,42]; Buoyancy system [68]; Control and stability works [26,27,48,53,66,67]; Early Soviet “flying submarine” concept [6]; Morphable quadrotors [50];	~20
USA	Loon Copter [54]; MIT hybrid fish [28,41]; aquatic microrobots [33]; Naviator [46,47]; DARPA submersible aircraft [6]; GTQ-Cormorant [55]; Miniature quadrotor [49]; Dynamic modeling and fixed-wing concepts [35,37,39]; Squid-like soft-morphing vehicle [40]; Morphable quadrotors [51]; Fixed-wing aquatic UAV [36]	~14
UK	AquaMAV and morphing aquatic MAV (Imperial College London, Kovac Lab) [33,43]; Wing model [2]; IROS aquatic-aerial works [44]	4
Others (Europe collaboration)	Hitchhiking robots (Science Robotics, ETH/Kovac collaboration) [26]; HyDrone project and propeller configuration [44,45]	2–3
Total		~40

Table 2. Distribution of HAUV-related studies by country/region.

Country/Region	Representative Studies	Number of Studies
China	Nezha series (Nezha-F [57], Nezha-IV [58], Nezha-SeaDart [59], Nezha-H [64], Nezha-X [61]); WuKong [60]; TJ-FlyingFish [52]; Wu et al. flying boat [32]; Plunge-diving gannet [29,30,31,42]; Buoyancy system [68]; Control and stability works [26,27,48,53,66,67]; Early Soviet “flying submarine” concept [6]; Morphable quadrotors [50];	~20
USA	Loon Copter [54]; MIT hybrid fish [28,41]; aquatic microrobots [33]; Naviator [46,47]; DARPA submersible aircraft [6]; GTQ-Cormorant [55]; Miniature quadrotor [49]; Dynamic modeling and fixed-wing concepts [35,37,39]; Squid-like soft-morphing vehicle [40]; Morphable quadrotors [51]; Fixed-wing aquatic UAV [36]	~14
UK	AquaMAV and morphing aquatic MAV (Imperial College London, Kovac Lab) [33,43]; Wing model [2]; IROS aquatic-aerial works [44]	4
Others (Europe collaboration)	Hitchhiking robots (Science Robotics, ETH/Kovac collaboration) [26]; HyDrone project and propeller configuration [44,45]	2–3
Total		~40

3. Ocean Optical Instruments and Deployment Methods

Optical techniques allow sampling at much higher spatial and temporal resolution, providing a more informed context for interpreting the results from discrete samples [69]. In marine optics, there are two main types of physical quantities used to describe the characteristics of light transmission in water media, namely Inherent Optical Properties (IOPs) and Apparent Optical Properties (AOPs) [70]. IOPs are those that depend only on the water medium and are independent of the ambient light field in the water medium, such as beam absorption coefficient, volume scattering function, beam attenuation coefficient, single scatter probability, and backscatter coefficient. AOPs are those that depend not only on the water medium but also on the geometric structure of the surrounding light field. Commonly used apparent optical properties include irradiance reflectance, remote sensing reflectance, diffuse attenuation coefficient, and mean cosine. The measurement of apparent optical properties is obtained indirectly by measuring spectral radiance and spectral irradiance and is a passive measurement method. Measurement of inherent optical properties is an active method, and the types of instruments used to measure IOPs include: spectral absorption meters, spectral attenuation meters, volume scattering function meters, and spectral backscatter meters.

3.1. Inherent Optical Instruments and Observation Methods

The IOPs of seawater and its components are unaffected by sunlight and can be directly measured in situ. Absorption and angular scattering describe the mechanisms through which monochromatic (i.e., narrowband) and collimated (i.e., highly directional) light is removed from a beam through energy conversion and directional deflection, respectively. The attenuation coefficient (c(λ)) is the sum of the absorption coefficient (a(λ)) and the scattering coefficient (b(λ)) [71]. In situ optical instruments available on the market can provide estimates of beam attenuation, backscattering, side scattering, and absorption coefficients, which serve as indirect indicators of particle concentration. The parameters of some commonly used IOP instruments are given in

Table 3. Commercially available in situ IOP optical instruments.

Instrument
	Spectral absorption and attenuation sensor (ac-s)	ECO BB9	ECO fluorometers and scattering sensors	RBR tridente
Company	WET Labs, Inc., Philomath, OR, USA	WET Labs, Inc., Philomath, OR, USA	WET Labs, Inc., Philomath, OR, USA	RBR Ltd., Ottawa, ON, Canada
weight in air /water	5.9 kg/0.8 kg	3.1 kg/1.8 kg	0.4 kg/0.21 kg	0.4 kg/0.02 kg
observation parameters	c(λ), b(λ), a(λ), bp(λ)	Fluorescence, bp(λ)	Fluorescence, bp(λ)	Fluorescence, bp(λ)
deployment platforms	CTD rosettes, Float profiler, AUV, ROV, Buoy
Compatibility with HAUVs	Limited—heavy and high power, suitable only for large HAUVs	Moderate—feasible for mid-size HAUVs with endurance capacity	High—light weight, low power, suitable for small HAUVs	High—excellent compatibility with miniaturized HAUVs

, together with an evaluation of their compatibility with HAUV platforms.

As this type of instrument is usually part of an active observing system with its own light source, which has a high degree of modularity and automation in the observation, it can be flexibly deployed on different platforms. These instruments have been routinely deployed on slow-drop packages (packages that fall under their own weight [18]), towed systems, AUVs and profiling floats, submarines, seafloor tripods, and moorings [19,20,21,22,23,24,25]. Consideration of the deployment platform should include the potential for the platform to interfere with particles (e.g., by inducing turbulence that breaks up aggregates) and the ability of the platform to cope with the weight, size, power, and data streaming requirements of a particular sensor.

3.2. Apparent Optical Sensors and Observation Methods

AOPs are strongly associated with the biological, chemical, and lithogenic components within the water column, making them valuable bio-optical indicators for profiling water column characteristics [72]. In recent decades, considerable scientific attention has focused on ocean surface reflectance due to its critical role in ocean color remote sensing applications. For these applications, many radiometric measurements have been collected for (i) the spectral diffuse attenuation coefficients K_d, K_u, and K_Lu for E_d, E_u, and L_u, respectively; (ii) the spectral irradiance reflectance, R = E_u/E_d; and (iii) the spectral remote-sensing reflectance just above the sea surface, R_rs(z = 0₊) = L_w(z = 0₊)/E_d(z = 0₊) where L_w is the spectral water-leaving radiance and z = 0₊ is just above the sea surface.

Radiance and irradiance sensors are typically categorized based on their ability to distinguish light across different spectral wavelengths [70]. Radiance refers to the light flux measured per unit solid angle and per unit projected area. It is typically obtained by narrowing the radiometer’s field of view and assuming the radiance remains spatially uniform or varies minimally within that solid angle. Plane irradiance, often simplified as irradiance, is quantified using a cosine collector and represents the light flux per unit surface area. Field measurements commonly include both downward and upward irradiance, defined as the radiant energy passing through a horizontal plane per unit time in the respective directions. At present, internationally representative marine radiometers include the RAMSES series (TRIOS), the OCR series (Seabird), and the C-OPS radiation measurement system (BioSpheric). Figure 6 shows typical commercial hyperspectral radiometer products.

In-water radiometric profiling relies primarily on deployments such as freefalls, which operate down to a few tens of meters from deployment platforms [73,74]. These systems enable vertical sampling from the surface to several tens of meters while minimizing interference from the deployment platform [75]. They have replaced the previous winch systems, which required extensive analysis to quantify the superstructure disturbances affecting the radiometric data [76]. Beyond eliminating disturbances from the platform’s superstructure, several further deployment criteria must be fulfilled to ensure measurement accuracy. These include (i) employing radiometric instruments and deployment mechanisms (e.g., free-fall systems) designed to reduce self-shading—an effect where the measurement apparatus itself disturbs the surrounding light field [77]; (ii) collecting a sufficient number of measurements—from just below the sea surface to several meters depth—to ensure statistical reliability, preferably under conditions where the water column exhibits vertically uniform optical properties; (iii) monitoring the pitch and roll angles of all measurement units (e.g., free-fall sensors and surface reference devices); and (iv) routinely logging system offset values to maintain data consistency. Some representative platforms are shown in Figure 7.

4. Feasibility of Hybrid Aerial Underwater Optical Observation System

Drawing from the preceding analysis, the performance characteristics of the two dominant HAUV types can be contrasted: (1) Most current HAUV platforms are adapted from established fixed-wing or multi-rotor UAV architectures, each offering distinct strengths and limitations. However, these systems still require further refinement, as their diving capabilities remain limited—making them primarily applicable to shallow-water environments (depths less than 100 m). (2) Most splash entry/exit HAUVs have the ability to fly at high speed in the air [4]. In contrast to the VTOL capabilities of multi-rotor HAUVs, fixed-wing systems face greater challenges in smoothly transitioning between air and water. Their cross-domain operations are harder to control and more susceptible to failure due to environmental disturbances such as wind and wave interference. Similar conclusions have been drawn in recent stability analyses of trans-media HAUVs, which emphasized that posture disturbances and hydrodynamic impacts are critical challenges for reliable operation [27]. In addition, the optical sensor can be damaged by impact forces during water entry. Therefore, for underwater optical observation of the ocean, the VTOL type of HAUV is more suitable.

To further clarify the impact of these technical challenges on optical measurements, we provide a qualitative ranking by degree of influence. The most critical factor is platform stability and hydrodynamic disturbances, because even minor posture fluctuations can significantly bias AOPs measurements, such as diffuse attenuation and remote-sensing reflectance [27]. The second most influential issues are self-shading and platform-induced turbulence, which modify the natural light field and particle distribution and thereby degrade both inherent and apparent optical observations [77]. Impact forces during water entry are assessed as having a medium influence: while they primarily threaten sensor survivability and alignment, they also interrupt data continuity if damage or misalignment occurs [7]. Finally, limitations in energy supply and endurance represent a moderate constraint, as they primarily restrict the achievable profiling duration and spatial coverage rather than directly degrading the quality of optical measurements [54].

Mitigation approaches include active control and attitude-stabilization algorithms demonstrated on amphibious multirotor prototypes (e.g., Naviator; TJ-FlyingFish) to minimize posture errors [47,52]; structural optimization (streamlined hulls, foldable arms) to reduce turbulence and self-shadowing [38,57]; protective housings and damping mechanisms to limit water-entry shocks on sensitive radiometers and IOP sensors [43]; and hybrid propulsion with buoyancy regulation to extend endurance while maintaining stable underwater orientation [68]. Collectively, these strategies target the primary error sources that currently limit HAUV-based optical measurements.

An ideal amphibious optical profiling system must take into account factors such as payload, air and underwater endurance, attitude, and shape. For inherent optical observation systems, the measurement of inherent optical parameters is an active observation method that does not rely on the attitude of the underwater platforms. Amphibious inherent optical platforms are relatively simple, and the only problem to be solved is air endurance. This requires large motor and battery reserves, which further affects the air endurance, and a balance must be struck between the two. For an apparent optical observation system, the apparent optical parameters require the platform to maintain a certain geometric attitude underwater, and both require synchronous measurement of water depth. Therefore, it is necessary to design a shape that can ensure posture underwater and adjust the center of gravity and buoyancy (Figure 8). Attitude adjustment can also assist some attitude adjustment algorithms to keep the observation attitude horizontal. In addition, the color of the amphibious drone platform should be black to avoid the influence of light; the arms and hull of amphibious drones should avoid self-shadowing as much as possible, or use Monte Carlo methods to eliminate shadow effects [77]. In short, as optical sensors become more modular and functional, more amphibious optical systems will emerge in the future, making optical measurements more convenient, automated, and intelligent.

5. Amphibious Optical Observation Systems: Potential Applications and Future Work

At present, it is important to note that the application of Hybrid Aerial Underwater Vehicles (HAUVs) in underwater optical observation is still at the conceptual and experimental stage. Most of the reported platforms have been tested only in laboratory pools or near-shore demonstrations, rather than in routine ocean observation campaigns. For instance, Rutgers University’s Naviator platform has successfully carried out seamless transitions between air and water and demonstrated the ability to support optical sensors in pool trials [46,47]. Similarly, Tongji University’s TJ-FlyingFish prototype has shown stable underwater hovering and maneuverability with tiltable propulsion units, which makes it a promising candidate for integrating radiometers and scattering sensors [52]. In addition, the Nezha series developed by Shanghai Jiao Tong University represents progressive advancements toward multi-modal amphibious operation, including vertical flight, underwater gliding, and real-sea trials. These cases represent successful proof-of-concept demonstrations, but they remain limited compared to the requirements of operational deployments.

Compared to existing intelligent mobile optical observation platforms, amphibious optical systems have advantages both in the air and underwater, making them particularly suitable for emergency and precision observation. For example, in the event of a red tide outbreak, an amphibious optical system can initially conduct small aerial patrols using a hyperspectral camera integrated into a drone. The location of the red tide can be pinpointed using real-time image processing by a shore-based computer. The amphibious optical drone then conducts optical observations of the seawater profile, obtaining underwater optical information and inverting parameters of phytoplankton, CDOM, and other particulate matter. The specific schematic diagram is shown in Figure 9. By conducting integrated observation of suspicious targets, thereby reducing observation costs and shortening response time, new observation methods can be provided for pollution spills, red tides, and conventional fixed point observation. In particular, with its excellent networking and observation capabilities, it can make observations more intelligent and efficient.

Overall, it is essential to enhance the theoretical framework by incorporating concrete application scenarios, proposing novel design strategies, conducting rigorous scheme evaluations, and actively performing experimental verification—rather than remaining constrained by the boundaries of current research. The main purpose of this article is to expand the scope of application based on existing amphibious drones. However, much remains to be done. To achieve rapid observation in the field of ocean optics, we should propose new shapes, structures, and dynamic layouts based on specific application scenarios to support the development of ocean optics and ocean remote sensing. Further research is needed in the following areas: 1. Conduct in-depth research on the hydrodynamic process of amphibious unmanned aerial vehicles entering and exiting the water, and optimize existing control algorithms; 2. Optimize the existing structural layout to make it easier for the body to maintain a stable posture underwater while avoiding self-shadowing; 3. Experiment with different types of optical probe combinations to link ocean optics and the ocean.

This review highlights the potential of HAUVs as emerging platforms for ocean optical observations. By integrating aerial and underwater capabilities, HAUVs simplify cross-domain monitoring and enable rapid, cost-effective responses to dynamic marine environments. Among current designs, VTOL-type HAUVs show greater suitability for optical profiling due to their stability and smooth transitions across the air–sea interface, which are essential for accurate measurements of apparent optical properties. While significant challenges remain in hydrodynamic stability, platform–sensor interactions, and endurance optimization, the integration of HAUVs with modern optical instruments offers a transformative pathway to complement satellite and buoy observations, enhance emergency monitoring, and advance marine remote sensing. Continued research on control strategies, structural design, and sensor integration will be key to realizing HAUVs as reliable operational tools in ocean science.