Ranger 2 computes a range and bearing solution to a subsea target relative to the vessel mounted transceiver. To convert this relative acoustic solution into a real-world position suitable for survey and dynamic positioning (DP) systems, Ranger 2 relies on aiding from vessel sensors (GNSS position along with heading, attitude and accurate lever arms).
GNSS acts as the top-level reference for the entire USBL solution. Regardless of acoustic or attitude performance:
- GNSS defines where the vessel is in global coordinates
- All subsea positions are referenced back to this GNSS position
If the GNSS solution is unstable or noisy, that behaviour is inherited by the USBL output.
Most subsea workflows assume GNSS is “truth” because it’s the top of the reference chain. However, GNSS can be degraded by:
- Poor correction service / loss of corrections
- GPS Spoofing
- Multipath (cranes, structures, turbine towers, masts)
- Poor antenna location or installation
- RF interference
- Latency, low update rate or timing issue
GNSS installation and configuration best practices
- Mount the GNSS antenna in a location with a clear, unobstructed view of the sky, minimising the risk of signal masking or multipath from vessel structures.
- For short-term operations where the USBL transceiver is mounted on an over-the-side pole, consider mounting the GNSS antenna directly above the pole. This helps minimise errors arising from antenna offsets and simplifies lever-arm definition.
- Accurately measure and record the X, Y, and Z offsets between the GNSS antenna and the USBL transceiver. These lever-arm measurements are critical and will be required when configuring the USBL software.
- Where possible, provide position data directly from the GNSS receiver to the USBL software to minimise position latency and timing uncertainty that can be introduced by computer-based GNSS solutions.
- The GNSS accuracy recommendation is RTK Float, DGPS is also acceptable.
GNSS and USBL calibration, CASIUS
USBL calibration relies on clean and stable aiding inputs.
Sonardyne calibration guidance identifies aiding sources and lever arms as major contributors to system error.
A poor GNSS quality while attempting to undertake a CASIUS can result in:
- Degraded calibration quality
- Inconsistent calibration results between runs
- Increased residuals
- Reduced repeatability
This is particularly noticeable in shallow water operations, where GNSS and MRU errors can be proportionally larger. Calibration should therefore be viewed as a system-level validation, not just an acoustic alignment exercise.
Please contact our service support team if you require any other Ranger 2 or Gyro USBL support. Alternatively, you can browse other knowledge base articles.