Presenter: Dr. Stephan Howden, the University of Southern Mississippi, Division of Marine Science School of Ocean Science and Engineering

Cost: Free!

When: Friday, April 23, 11:30 a.m.

Where: Zoom Webinar (Zoom link forthcoming)

In an effort to extend the baselines over which cm-level 3-D kinematic positioning can be done in the marine environment using differential, dual-frequency GPS carrier phase observations, the Hydrographic Science Research Center at the University of Southern Mississippi deployed a GPS buoy in the Mississippi Bight in late 2004, along with GPS base-stations on Horn Island, MS, Gulfport, MS and Stennis Space Center, MS. The main obstacle to reliably using longer baselines for this level of accuracy was the short horizontal scales of variability in the tropospheric water vapor. An important objective of this work was to provide a database of measurements at a subtropical location of GPS data for Post Processed Kinematic (PPK) analysis, ancillary meteorological data at the receiver sites, and regional output from numerical weather prediction models. 

The positioning instrumentation on the buoy consisted of a survey grade GPS receiver and antenna, inertial measurement unit, and a 3-axis magnetometer. The oceanographic instrumentation consisted of a Doppler current sensor, a 600 kHz Acoustic Doppler Current Profiler, CTD and fluorometer. The meteorological instrument package consisted of ultrasonic and propeller anemometers, barometer, and a humidity and air temperature sensor. The GPS data were initially processed using the PPK technique with data from the closest GPS base station on Horn Island. However, this base-station and the next closest one at Gulfport, MS were destroyed during Hurricane Katrina, which precluded PPK processing throughout the duration of the storm.  

Since then, a stand-alone positioning technique termed Precise Point Positioning (PPP) matured and allowed for the reprocessing of the buoy GPS data throughout Katrina. A time series of water-level to the NAD 83(2011) ellipsoid was produced using PPP, which were then used in a tidal datum transfer procedure utilizing a Modified Range Ratio method and the National Oceanic and Atmospheric Administration (NOAA) tide gauge station in Waveland, MS (Station ID 8747766), to reduce the time series to water level to Mean Sea Level. The maximum water level during the storm was found to be about 3.6 m, relative to the transferred Mean Sea Level datum. The storm surge rose during more than a 24-hour period, but fell back to normal levels in less than 3 hours. The maximum speed of the current with respect to the seafloor was recorded to be about 4 knots towards the southeast as the storm surge moved back offshore.