Researchers are now exploring the use of recent advances in open source Global Positioning System (GPS)-guided drone technology to design and test a low-cost and transportable small unmanned surface vehicle (sUSV).
In the effort to design a more repeatable and consistent platform to collect data for Structure from Motion (SfM) monitoring of coral reefs and other benthic habitats a research team comprising George T. Raber and Steven R. Schill used an sUSV to create underwater high-resolution orthophoto mosaics and digital surface models. The images can be used to create SfM models that provide very detailed images and measurements that can be used to monitor changes in biodiversity, reef erosion/accretion, and assessing health conditions. The University of Southern Mississippi, School of Biological, Environmental, and Earth Sciences, Hattiesburg, USA and The Nature Conservancy, Caribbean Division, Coral Gables USA were main contributors to this survey. The research titled, “Reef Rover: A Low-Cost Small Autonomous Unmanned Surface Vehicle (USV) for Mapping and Monitoring Coral Reefs”, is published in the open access journal Drones.
The imaging system uses two cameras to collect stereo photos that can be later processed using photogrammetry software to create underwater high-resolution orthophoto mosaics and digital surface models.
Materials and Methods
Researchers designed the Reef Rover system keeping in mind multiple goals and practical restraints. The system needed to have a very low draft in order to avoid potential obstacles such as shallow coral heads or other features close to the water surface. It also needed to be sufficiently stable on water to avoid getting toppled by small waves and water-resistant to avoid damage to the electronic components. A target budget of approximately $3000 USD was set to make the system affordable for varying budgetary constraints.
Such systems are becoming common tools used in marine mapping and conservation efforts worldwide. Similar to a UAV mission, programmed “float line” missions are uploaded to the Pixhawk that provide navigation waypoints for the vehicle to travel back and forth over a specific reef area while taking stereo photos. Ground control software Mission Planner (for PCs) and the Tower app (for android tablets and phones) which are both free and open source were used to plan, upload, and execute reef mapping missions.
The fully assembled Boat Vehicle version 2 measured 95 cm × 66 cm at the widest point (motor to motor) and weighed approximately 5 kg without batteries, making it easier to set up and transport. In the final configuration, without the external GPS antenna, it sat 16 cm above the water line at its tallest point.
Results from this project suggest the sUSV provides a repeatable, viable, and low-cost (<$3000 USD) solution for acquiring images of benthic environments on a frequent basis from directly below the water surface can be used successfully for mapping and monitoring coral reefs.
These advances will provide a greater ability to map larger areas in a more consistent manner and permit resource managers to be able to monitor small changes in coral reef environments at an increased temporal scale.
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