Our industry has experienced a recent explosion of innovative technologies providing teams with increasingly effective survey and inspection methods. One such innovation being dynamic underwater laser scanning. But with new approaches, come new challenges and risks, which can be daunting for those looking to take advantage of these innovative systems.
In order to overcome these challenges and reduce risk in your dynamic laser scanning operations, the 2G Team has put together a series of articles and a white paper that highlight the most common issues and their solutions. You can pre-register for the Reducing Risk for Dynamic Laser Scanning Operations white paper here and we will send it to you as soon as it’s released!
Part 1 of the Reducing Risk series focuses on factors you should consider when choosing an Inertial Navigation System (INS) for your dynamic laser scanning operations as well as 2G’s Top 3 Proven INSs.
INS Requirements for Dynamic Laser Scanning
1) Ring Laser Gyroscope or Fibre Optic Gyroscope
When choosing an INS for dynamic laser scanning, it is important the INS include either a Ring Laser Gyroscope (RLG) or a Fibre Optic Gyroscope (FOG). Although, Microelectromechanical Systems (MEMS) based gyroscopes are available in the industry, unfortunately, they are unable to provide the accuracy required for dynamic laser scanning.
An RLG is composed of a closed loop created by mirrors through which two beams of light travel in opposing directions. When the system is rotated, the total travel time of one of the beams of light will be longer in relation to the travel time of the other beam of light. It is this difference in speeds that allows the system to calculate the rotation of the vehicle.
An FOG is based on the same principle as the RLG, however, the two beams of light instead travel through a coil of optical fibre. Most FOGs and RLGS on the market have an average heading accuracy of 0.02⁰ and a roll/pitch accuracy of 0.01⁰. The smaller the accuracies for the gyroscope, the lower the errors in position estimates are for the INS as a whole.
Alternatively, a MEMS Inertial Measurement Unit (IMU) is comprised of an oscillating or vibrating component. Any changes in the component’s vibrations are used to calculate a change in direction. Unfortunately, high-grade MEMS IMUs exhibit an average heading accuracy of 0.75⁰ and a roll/pitch accuracy of 0.2⁰, which create much higher positioning errors than what is acceptable for dynamic laser scanning.
2) Software Able to Run Kalman Filter
It is equally essential that the INS can run an advanced Kalman filter. A Kalman filter is an algorithm that uses the sensor measurements and the error models for the sensors to estimate and correct errors in the navigation data. For example, in dynamic laser scanning, sensor inputs from the IMU, DVL, depth sensor, and USBL or LBL are taken to estimate the position and orientation of the vehicle.
After a Kalman filter is applied to the navigation data, the software should provide the system’s pose, the position and orientation of the system.
The pose information will comprise of the following;
- X co-ordinate
- Y co-ordinate
- Z co-ordinate
The majority of INSs run a forward Kalman filter that corrects data in real-time. Various INSs are also about to run a Forward-Backward Kalman filter during the post-processing of data, which will smooth the data and provide increased navigational accuracy.
3) Integration with Doppler Velocity Log
Last, but certainly not least, the INS you choose needs to include software that enables the system to co-locate and calibrate to your Doppler Velocity Log (DVL). Since the INS and DVL both capture navigational data relative to their own sensor origin, it is necessary to be able to calibrate the systems relative to one another. This ensures the navigational data will all be within the same frame of reference, which significantly improves the final positional accuracy. This calibration process is very similar to a standard patch test.
2G Proven Inertial Navigation Systems
Now that we’ve covered how the proper INS will reduce the risk in your offshore survey operations, we’ve provided our top three proven Inertial Navigation Systems. These three systems have been used with 2G’s Underwater Laser Scanners in a multitude of trials and offshore survey work. They are proven to provide the required navigational accuracy for dynamic laser scanner and have enabled our customers to make the most of their laser data.
Looking to Further Reduce Risk in your Dynamic Laser Scanning Operations?
Register to receive our latest white paper “Reducing Risk for Laser Scanning Operations: Challenges and Solutions for System Integration and Mobilization”
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