Calibration tips for Ranger 2 USBL systems

Carrying out a USBL calibration is an essential step to get the most out of the system, over the years we have seen a number of simple steps which when missed can have a big impact on the calibration.

The best way to ensure you get the best out of your calibration is to read the manual and ensure that you understand what is trying to be achieved, the following tips will make sure you reach the next level:

• Always use the best Altitude Heading and Reference sensor (AHR) available – The our transceivers include onboard MEMS based Attitude Heading reference Sensors. Whilst these are great in locations where no other devices are available, a good quality GNSS derived heading sensor, FOG (Fiber Optic Gyro – such as a Sprint-Nav Mini) or LRG (Laser Ring Gyro – Such as a SPRINT-Nav) will vastly improve the system performance.
• Update offsets – In order to get the best out of the calibration a good knowledge must be known of the transceiver to GNSS and the GNSS. Errors in these offsets can provide the largest errors of the calibration.
• Ensure your subsea beacon is suitably deployed – When carrying out the calibration the required accuracy should determine the deployment method. For calibrations where ultimate accuracy is not required a tripod mounted beacon is a good option, for calibrations where a lower accuracy is required, beacons deployed on strops and a float may be utilised. Stropped beacons will be able to move in currents and so will reduce the accuracy of the calibration but may be much easier to deploy and retrieve.
• Enter up to date Sound Velocity profiles – Sonardyne’s USBL calibration software has the ability to estimate the average sound velocity anywhere in the world throughout the year. This provides a good basis for the calibration however a recently measured sound velocity profile will provide the best result.
• Maintain good acoustic address discipline – Ensure that there is only one beacon in the local area with the acoustic address. If multiple beacons with the same address are within range then the increase in noise will have a detrimental effect.
• Use the best correctional services available for your GNSS – The performance of the vessel GNSS will have a significant impact on the accuracy of the calibration. A simple GNSS solution will never result in a calibration of the same high accuracy of that as a GNSS with a real time correction service.
• Verify the new corrections – One of the most important steps is following the calibration. To ensure that no gross error has been made a verification run should be performed.

For more information on any of these notes please refer to the Ranger 2 manual.