How Does Airborne Bathymetric LiDAR Work?

Bathymetric LiDAR, Fonctionnement
Lidar Bathymétrique par drone

How Does Airborne Bathymetric LiDAR Work?

LiDAR technology (Light Detection and Ranging) uses light pulses in the green spectrum to penetrate water and measure the depth of the seabed.

Unlike traditional methods, this technology relies on the use of laser (light beam ) to measure the depth of water.

In this article, we will detail the operation of an aerial bathymetric LiDAR system, covering its laser, wavelength, interaction with water, as well as GPS and INS (inertial navigation system) positioning technologies, which are essential for obtaining centimetre-accurate georeferenced surveys.

1. The Laser: The Primary Tool of LiDAR

The core of a LiDAR system is the laser. In the case of aerial bathymetric LiDAR, a specific laser is used: the green LiDAR, which operates at a wavelength of 532 nm (nanometres). This particular wavelength belongs to the visible light spectrum and is especially suited for underwater surveys because it can penetrate the water's surface.

Why green light ?

Unlike other wavelengths (such as red or blue), green light has the ability to penetrate the upper layers of water before being absorbed, while remaining strong enough to return to the sensor after reaching the seabed. This property makes it the ideal wavelength for bathymetric surveys in shallow waters (up to approximately 15 to 20 metres), where other technologies, such as sonar, are not suitable (e.g., rivers or coastlines).

 

 

 

bathymétrie lidar
Absorption Spectrum of Visible Light in Water

The Principle of Reflection and Refraction

When a laser beam passes through the water's surface, it undergoes a phenomenon of refraction. This means that the direction of the beam changes depending on the angle of incidence and the density of the water. Part of the beam's energy is reflected back towards the airborne sensor, while the rest continues to penetrate deeper before being reflected by the seabed.

Diagram 

shema lidar bathymetrique laser refraction eau temps de retour
Shema réfléxion / réfraction d’un lidar bathymétrique

2. The Inertial Navigation System (INS) and GPS: Centimetric Accuracy

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  • Centrale inertielle INS pour LiDAR bathymétrique
  • Récepteur GNSS Emlid Reach RS2+ pour positionnement RTK
  • Satellite GNSS en orbite pour positionnement géodésique

To achieve centimetric accuracy in bathymetric LiDAR surveys, simply sending laser pulses is not enough. It is essential to combine this technology with precise positioning systems.

The Role of GPS: Accurate Positioning

GPS plays a crucial role in the LiDAR system by enabling the precise localisation of the sensor (or rather its INS) at all times during the survey.
Centimetric GPS systems provide precise coordinates of the onboard GPS antenna, typically with an accuracy of a few centimetres using RTK or PPK.

The Inertial Navigation System (INS): Measuring Micro-Movements

An Inertial Navigation System (INS) is a set of sensors that measure the orientation, angular velocity, and acceleration of the device. These sensors track the movements of the sensor in real-time, compensating for vibrations or deviations that might affect the accuracy of the surveys.

Absolute Position Calculation: GPS + INS + Laser

The system operates by combining data from the GPS, INS, and Laser.

  1. GPS Positioning: Centimetric georeferencing of the INS
  2. INS and Motion Compensation: The INS adjusts these measurements based on the dynamic movements (roll, pitch, yaw) of the sensor to ensure that the collected data is corrected.
  3. Laser Return: The system measures the time it takes for the laser pulse to return to the sensor after hitting the seabed. By combining this return time with the GPS and INS data, the system can calculate the absolute position of the seabed with a centimetric accuracy.

3. Data Processing: Millions of Points

Each laser pulse sent towards the water surface produces a return that is analyzed in real-time. The bathymetric LiDAR system captures millions of data points during a flight, each representing a precise point of the underwater topography.

Classified cloud point and DTM

The raw data are initially processed as point clouds. Each point corresponds to a position x, y, z (latitude, longitude, depth / elevation). These point clouds are then used to create a Digital Terrain Model (DTM), a "3D" representation of the seabed.

 

4. Practical Applications of Airborne Bathymetric LiDAR

Airborne bathymetric LiDAR is a versatile solution used across various industries :

  • Coastal Management: Accurate mapping of coastal areas to combat erosion or monitor changes in the coastline.
  • Environmental Surveys: Monitoring aquatic ecosystems and tracking natural habitats in shallow waters.
  • Projets d’ingénierie : Planification de travaux de construction d’infrastructures côtières ou portuaires.
  • Hydrology: Watershed studies, flood management, and flood forecasting.

 

For more information or to arrange a demonstration, contact us.