What types of sensors are used in underwater navigation and exploration? (Part 2 of 2)

Underwater navigation and exploration require special sensors for object detection and imaging. Part 1 of this blog addressed cameras and sonar. LiDAR is also used for underwater applications.

Studying the depths and shapes of underwater terrain or bathymetry can be performed by bathymetric LiDAR. The airborne approach to scanning the seafloor uses a green wavelength of 532 nm to penetrate the water instead of the infrared wavelength of 1,064 nm used for airborne topographic LiDAR. A bathymetric LiDAR sensor consists of:

• A Global Positioning System (GPS) receiver to provide the aircraft position,
• An inertial measurement unit (IMU) to determine the roll, pitch and yaw of the aircraft,
• A laser scanner to generate the signal in a particular pattern and
• A sensor to read the returning signal.

These types of sensors are characterized as shallow-water (<10m) or deep-water systems (>10m). Improvements to new systems include:

• The use of multiple sensors in the aircraft,
• More system integration with additional sensors,
• Faster transmission to data products,
• Reflectance calibration between flight paths,
• Greater point density,
• Enhancements for fresh water capture and
• Enhanced classification of point clouds.

An example of an airborne bathymetric LiDAR system is the Leica Chiroptera 4X from Leica Geosystems. It simultaneously captures 140,000 points per second in the bathymetric channel (full waveform) and up to 500,000 points per second in the topographic channel. The 4X indicates a point density increase by a factor of 4 compared to previous versions. With the addition of the HawkEye 4X deep water module, water penetration down to 50m depth can be achieved.

Since available bathymetric LiDAR systems can weigh several hundred pounds, researchers at Georgia Tech Research Institute (GTRI) developed a new approach. Called Active Electro-Optical Intelligence, Surveillance, and Reconnaissance or AEO-ISR, the technology would reduce the size and weight allowing modest-sized unmanned aerial vehicles (UAVs).