There are two types of missions or mission modes available:

• Subsonus Assisted - This mission mode requires a Subsonus USBL

• Standalone (GNSS / Dead Reckoning) - This mision mode allows for missions to be carried out with a standalone Hydrus unit.

Subsonus Assissted Mission Mode

In Subsonus Assisted mission mode Hydrus is used in conjunction with Subsonus. The integration of the Hydrus and Subsonus systems enables highly accurate positioning and mid-water vehicle tracking, even in the presence of varied bottom currents. By utilising the Subsonus acoustic positioning capabilities, a precise location of Hydrus can be determined and monitored in real-time. This ensures that Hydrus stays on track during missions, even in challenging conditions. In case of prolonged acoustic communication outage, Hydrus will autonomously enter dead reckoning navigation mode and navigate independently.

In addition to tracking, the Subsonus also facilitates acoustic communication during the mission. This allows for two-way communication between Hydrus and a surface vessel or a control station via the Subsonus web UI. Acoustic communication is required for transmitting mission instructions, receiving status updates, and making any necessary adjustments to the course or operations of Hydrus.

Standalone Mission Mode

Standalone (GNSS / Dead Reckoning) Hydrus missions are very quick and easy to set up and deploy. They are ideal for rapid, on the fly mission planning and execution, or when a Subsonus is not available or when it is not feasible to use Subsonus.

In Standalone (GNSS / Dead Reckoning) mission mode, without the aid of a Subsonus, Hydrus will acquire a GNSS fix at the surface, and then carry out the mission based on dead reckoning navigation, with the autopilot relying on the sensor data from the INS and DVL. At regular intervals throughout the mission, or when accuracy is deemed poor, the unit will resurface in order to acquire a GNSS fix. This will give Hydrus an exact position reference and will enable the system to make any navigation adjustments if required.

Upon completion of the mission, Hydrus will return to the planned recovery location and automatically surface at that location. Once at the surface, it will utilise its thrusters to stay directly above the recovery location based on its current GNSS position. In case Hydrus is tipped upside down while at the surface, it will dive down, correct its orientation and then resurface again.

Standalone (GNSS / Dead Reckoning) missions are:

• Suitable for shallow waters and locations that are difficult or impossible to reach with a boat equipped with Subsonus.

• Suitable for mission depths up to 30 metres. Deeper missions are possible, however, the deeper the mission in standalone mode, the more drift prone the accuracy is. If available, consider Subsonus assisted mission mode for deeper missions.

• Suitable when launching missions from shore, a jetty, small boat.

• Not recommended in open water scenarios with large swell or choppy conditions.

Apart from the absence of Subsonus functionality, the process of planning missions in Standalone (GNSS / Dead Reckoning) mission mode remains the same. Note that at the end of a mission, Hydrus will go straight to the surface in the specified recovery location. Consequently, it is important that the area around the retrieval point is clear of overhead obstacles, for example a jetty or a boat.