Redondo Outfall Inspection Seattle, Washington

For this project, an Ultra Short Baseline (USBL) transceiver was mounted on our survey vessel, the M/V Carta and a transponder was mounted on the ROV. The Carta was equipped with an inertial navigation system that output position, heading, pitch and roll. These data in addition to the USBL measurements were interfaced into survey navigation software to provide real-time ROV positions during the inspection.

The survey was controlled horizontally using a local GPS base station setup on client supplied control at the project site. The control point was a PK nail in Redondo Beach Drive to the west of a traffic island at the intersection of Redondo Shore Drive South. RTCM differential corrections were broadcast to the survey vessel using radio telemetry.

The ROV "flew" above the outfall and the time was logged when the ROV was at each anchor block. A file was logged simultaneously in the navigation software throughout the inspection. A calibration routine was performed with the system following the inspection. The calibration was conducted to solve for the alignment of the USBL transceiver in the pitch, roll and heading axis relative to the vessel coordinate frame. These values were applied to the raw data in order to refine the final positions.

Similar to GPS positioning, the accuracies of a USBL system increase with multiple observations at a location. The single point position taken at each block was not reliable enough to generate a valid as-laid location of the outfall, so an alternative approach was used. Longer term measurements along the outfall were recorded to the beacons on blocks 51, 97 and 141 during the inspection before they were released. These positions were used to control the overall location of the outfall and the ROV positions were used to infill the sections between.

Back to top