Sea Technology

AUG 2013

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rors. After the brief initialization, survey lines were collected on the Columbia River. Two areas of interest were surveyed, including one of open sky and one close to a high embankment. The IMUs were installed near the center of rotation aboard the 9-meter SEAHORSE Geomatics training and development vessel. Both sensors were carefully aligned to the keel, and offsets were measured to each GNSS antenna and a preproduction Norbit Group AS (Trondheim, Norway) Wide-Band Multibeam Sonar (WBMS). Conditions on the river were generally very calm on the day of testing. Results from this test represent a typical inland bathymetric multibeam survey. Image generated using STIM300 data. Difference between STIM300 and FSAS. conditions. As reference, a high-grade, tactical IMU was used alongside the STIM300. The iMAR Navigation GmbH (Sankt Ingbert, Germany) FSAS is a 0.75-degree-per-hour gyro bias IMU with a history of excellent performance. The IMUs were coupled with NovAtel's OEM628 GNSS receivers. Both systems utilized NovAtel's ALIGN GNSS dual-antenna heading functionality. SPAN provided real-time position, attitude and heave solutions, and the raw data collected were post-processed using NovAtel's Inertial Explorer. Post-processing allows for a better solution as data can be processed forward and backward. The majority of errors incurred in real time can also be fltered and smoothed. All results presented in this article are from the post-processed solutions. In this test, both systems were initialized while the vessel was docked. When the vessel departed, a few simple S-turn maneuvers were executed to observe and estimate IMU er- 16 st / August 2013 Processing the Data Real-time data were acquired using HYPACK Inc. (Middletown, Connecticut) HYPACK/HYSWEEP multibeam collection and processing software. During the collection process, the GNSS solution was corrected with the local Oregon Real-time GPS Network (ORGN) NTRIP. FSAS data were captured with HYPACK in real time, while both the FSAS and STIM300 were also set up for raw data collection in NovAtel format. These raw data were later processed in Inertial Explorer, which has the ability to use publicly available base-station data, such as CORS (Continually Operating Reference Stations), or precise point positioning (PPP) corrections. Base-station data were combined with real-time results to generate a post-processed kinematic (PPK) solution. A post-processed position, attitude and heave solution was provided at 40 hertz for both the FSAS and STIM300 IMUs into the HYPACK/HYSWEEP software. HYPACK/ HYSWEEP was then utilized for subsequent bathymetric data processing and analysis in both cases. This allowed for a fair comparison of the two IMUs over the same period using the same sonar data. The post-processed solution for the STIM300 and FSAS were differenced, with results computed for heading, roll, pitch and heave. Mean value differences in heading and roll indicate installation misalignment between the two IMUs of approximately 0.4 degrees. These misalignments were corrected during HYPACK data processing. The remaining

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