Mission Considerations

When planning a mission with Hydrus, there are many factors need to be taken into consideration to ensure its success. Mission considerations encompass various aspects that directly impact the performance and outcomes of Hydrus operations.

Safety First

Keeping people and the environment safe is the highest priority and the most important Hydrus mission consideration. For safety related information, see General Safety Precautions

Environmental Considerations

Environmental considerations to be taken into account when operating Hydrus are described as follows.

Water Temperature

Hydrus is designed to operate within its designed Operating Temperatures. Extreme temperatures can affect the performance and efficiency of its components, including batteries and sensors. Hydrus may require additional cooling mechanisms for example the cooling fan on the desk charger for Hydrus when out of water an in high-temperature environments, while very low temperatures may impact its mobility and potentially lead to freezing of sensitive equipment.

Water Density

Hydrus is optimised for standard ocean density of 1025 kg/m³ and may require adjustments or considerations for operation in different water densities.

Note: If the user wishes to operate in water densities such as fresh water, other than standard ocean density, please contact Advanced Navigation

Currents and Tides

Currents and tides can affect the navigation and path-keeping ability of Hydrus. Understanding the impact of currents on energy consumption is important for mission planning, as navigating against strong currents may require additional power. Proper knowledge of local tidal patterns and current profiles is crucial for effective operation.

Visibility

Water clarity and visibility influence Hydrus's ability to use optical sensors effectively. Poor visibility can limit the effectiveness of the camera, impacting tasks such as mapping, object recognition, and obstacle avoidance. Considering the expected visibility conditions is important for selecting appropriate sensor systems and planning missions for Hydrus.

Biological Considerations

Hydrus users should consider potential impacts on marine life and ecosystems. The impellers or thrusters on Hydrus are designed to minimise harm to marine organisms. Care should be taken to avoid disturbing sensitive habitats like coral reefs or seagrass beds. Users should operate Hydrus with awareness of the presence of protected or endangered species.

Protected Areas

Some marine areas may be designated as protected zones due to their ecological significance or conservation status. When operating Hydrus in these areas, it is important to adhere to specific regulations and guidelines. Prior knowledge of protected areas helps avoid unauthorised entry and potential damage to sensitive habitats.

Marine Life Interaction

Hydrus may encounter marine life during its operations. Minimising negative impact on marine organisms is crucial. Hydrus should avoid close encounters, disturbances, or collisions with marine life. Operators should be aware of potential interactions with large marine animals, ensuring the safety of both Hydrus and the animals.

Hydrus Range and Endurance

There are numerous mission parameters and environmental variables that can impact the outcome of a mission. Hydrus has built-in functionality that alerts users in case a mission is not likely to be successfully completed due to mission parameters that exceed the estimated range of the unit based on the current battery charge level. Despite this, it is very difficult to foresee and compensate for unpredictable environmental conditions, particularly current speeds and directions.

By carefully aligning mission objectives with the capabilities of Hydrus, users can ensure optimal performance and successful mission executions. It is important to carefully factor in all mission parameters and environmental variables in order to plan missions within the capability limitations of the vehicle. This includes considering the estimated range of the Hydrus based on its current battery charge level.

Altitude

The altitude at which Hydrus operates affects the following:

  • Navigation: For Hydrus to navigate effectively throughout a mission, it relies on achieving and maintaining bottom lock, a condition where its Doppler Velocity Log (DVL) establishes a clear acoustic connection with the seafloor, enabling Hydrus to accurately calculate its velocity relative to the seabed. To maintain bottom lock, Hydrus needs to operate within a range of approximately 0.5 - 5 metres from the seabed. To achieve this, Hydrus travels to the Pause Vertical Position that positions it within the bottom lock distance range when necessary. By doing so, Hydrus avoids spending unnecessary time in mid-water without the bottom lock aiding the DVL, ensuring optimal navigation and data acquisition during underwater operations. By setting the pause vertical at an appropriate depth or altitude, Hydrus can descend directly at mission start and acquire DVL bottom lock. Thus minimising the adverse impact of surface currents in terms of positioning accuracy.

  • Risk of Collision: As Hydrus operates at lower altitudes, the image resolution improves because the camera can capture finer details. However, operating too low can lead to a potential collision with the seafloor or other obstacles. The altitude should be optimised to balance resolution and safety.

  • Image quality: Operating at altitudes that puts Hydrus closer to objects, structures or the sea floor generally results in better image quality. Turbidity (the level of water turbidity or cloudiness) reduces image quality, the closer Hydrus is to the target, the less distortion and blurring occur due to light scattering and other factors. Reduced ambient light, and effect of LED light illumination is reduced under various conditions. Choose an appropriate altitude to achieve high-quality images.

Pause Vertical Position

Note: Pause vertical is only available in Subsonus Assisted Mission Mode.

Considerations for pause vertical are important when planning missions. This parameter can be defined as either depth from the surface or bottom altitude.

If specified as depth from the surface, Hydrus utilises its depth sensor to measure the distance from the water's surface and maintains its position at the designated depth. When the bottom altitude option is chosen, Hydrus relies on its Doppler Velocity Log (DVL) to measure its altitude from the seabed.

At the very start of a Subsonus assisted mission, Hydrus descends down to the predetermined pause vertical position. This initial dive serves several purposes:

  • Avoiding strong surface currents and swells,

  • Establishing communications with Subsonus,

  • Stabilising the Inertial Navigation System (INS) before mission commencement,

  • Acquiring DVL bottom lock.

Prior to configuring the pause vertical parameter, it is advisable to consult nautical charts and sonar data for depth information at the mission site. Whether depth from the surface or bottom altitude is chosen, it is imperative to adhere to two conditions:

  • Ensuring the depth remains within Hydrus' maximum limit, and

  • Verifying that the water is not too shallow to accommodate the specified parameter.

Hydrus Transit Between Tasks

Hydrus is capable of performing multiple mission tasks sequentially. If a mission involves multiple tasks, the vehicle will automatically transition between the end and start points of each task at the specified vertical position, as defined in the global mission parameters. This vertical position can be specified as either depth from the surface or bottom altitude. Upon completing a mission task, Hydrus ascends to the designated vertical depth or altitude, then horizontally manoeuvres to the starting point of the next task. This default transit behaviour applies to travel between all mission tasks in the mission.

To override the automatic transit behaviour, you can manually insert a transect task, which acts as a "bridge" between the preceding and succeeding mission tasks. When adding a transit task manually, set the Type parameter under Coverage to None in order to conserve storage space and battery power. Utilising transect mission tasks for transit between missions is advisable in situations where known obstructions, such as underwater infrastructure (e.g., jetties, pipes, or netting), can be avoided by guiding Hydrus to take shortcuts between the obstacles. This can save both time and battery power as avoiding obstructions autonomously can take longer and use up more energy.

It is recommended to leverage available data and prior knowledge of the site when planning missions. Utilising previously acquired data, such as depth profiles or the locations of obstacles and reference points, can significantly enhance the efficiency of mission planning.

Bluewater Missions

Blue water missions, where Hydrus operates in depths deeper than 50 metres, require careful attention to the power and propulsion capabilities of Hydrus. Its thrusters are designed to counteract the effects of water currents, allowing the vehicle to maintain its desired course and altitude even in challenging conditions. However this may not be possible for long periods of time without recharging the unit.

Using Hydrus in Standalone Mission Mode

During standalone missions, in Standalone (GNSS / Dead Reckoning) mission mode, Hydrus operates without a Subsonus. Missions in this mode are executed without an established communication link until the Hydrus has been reconnected to WiFi upon retrieval. This means there is no feedback to the user about the mission status for the entire duration of the mission. To safeguard the unit and ensure mission success, a designated lookout / spotter, ideally equipped with binoculars, should be assigned to monitor the water surface throughout the mission. This individual should actively scan the water surface within the mission area and stand by to visually locate Hydrus in case of mission abort due to a bailout or other unforeseen circumstances. See Mission Modes for more information.

Retrieval Location

Strong wind, swell and surface current are all factors that can cause Hydrus to drift when floating on top of the surface after reaching the retrieval location. When retrieving Hydrus from an anchored vessel, it is advisable to set the retrieval point upstream and upwind. This location should typically be directly ahead of the boat. By establishing a home point in this position, especially during windy conditions or when facing strong surface currents, you ensure ample time for locating and retrieving the unit without the need to weigh anchor.

Note: In Subsonus assisted mission mode, Hydrus can be commanded to stand-by for a resurface command at a specified vertical position upon reaching the retrieval point. This is a good option for avoiding prolonged surface time in challenging surface conditions.

Battery Recharge Downtime

During mission planning, considerations should be made to allocate sufficient time for recharging the battery using the Desk Charger in between mission runs. The charging infrastructure and its availability at the deployment location should be assessed to ensure safe, uninterrupted and efficient charging during Hydrus downtime.