System Setup and Operation:
The Systems Engineering and Assessment (SEA) Ltd. SwathPlus bathymetric sonar was configured in a rigid pole-mount and deployed from the starboard side of the R/V Connecticut. A TSS DMS 2-05 motion sensor was mounted directly above and in-line with the transducers. An RTK-GPS antenna was mounted adjacent to the rigid-pole mount. Horizontal (x and y) and vertical (z) offsets between the transducers and motion sensor and antenna and motion sensor were precisely measured and recorded within the SwathPlus acquisition software (SEA Swath Processor (2005)). These offsets are used to establish the motion sensor as the common reference point for data acquisition. Additionally, the depth of the transducers below the water surface was measured and recorded within the SEA Swath Processor acquisition software. This depth was used to derive the speed of profile and acoustic ray path based on speed of sound profiles collected within the survey area. An Applied Microsystems SV Plus sound velocity profiler was used to collect speed of sound profiles at forty-two stations.
A patch test was conducted at the beginning of the survey in order to measure roll angle offsets. Roll angle offsets can have a strong effect on the accuracy of depth measurements. (The roll angle offset is the difference between the motion sensor vertical measurement and the nominal mount angle of the transducers (30 degrees)). The calculated roll angle offsets were stored within the SEA Swath Processor acquisition software.
Navigation:
The USGS established a Real-Time Kinematic Global Positioning System (RTK-GPS) that provided horizontal (x and y) and vertical (z) positioning during the survey. The RTK-corrected GPS signal was sent to the ship once every second from a base station positioned on the roof of the Air Force Antenna Station (USAF/AFRC/SNHA) in Ipswich, Massachusetts. Shipboard navigation data (x, y, and z) were stored in Hypack, Inc. Hydrographic Survey Software (
http://www.hypack.com) raw data files and used for positioning of geophysical instrumentation.
The offset between orthometric (NAVD88) and local chart datum (MLLW) as reported at the NOAA Tidal Station #8440452, Parker River National Wildlife Refuge, was applied to the RTK signal. This allows for elevations (z) received at the shipboard RTK antenna to be referenced to Mean Lower Low Water (MLLW) (that is, elevations are stored as distance of the RTK antenna above MLLW).
During post-processing, the distance of the RTK antenna above the water-line is removed from the stored RTK elevations. The resulting value is then applied to the bathymetric soundings, yielding data referenced to MLLW.
