Tide corrections from KGPS and a precise geoid

1. Tide corrections from KGPS and a precise geoid John Brozena, Randy Herr, Vicki Childers

2. Goals of Program • Determine the extent to which we can approximate a tidal datum from a precise gravimetric geoid and an offset determined from one or more tide gauges or GPS buoys • i.e. Can we extrapolate a tidal datum from a tidal gauge out into the open water using a geoid and KGPS measurements?

3. NRL/NAVO Bertram Test • Three day survey in May 2002 • Conducted aboard the Bertram from Gulfport tide gauge • Sea surface height plus ship attitude information measured while repeatedly traveling between four tide gauges: Gulfport, Ship Island, Waveland, and Point Cadet. • The first three of these have been referenced to the ellipsoid by static GPS surveys.

4. Bertram Instrumentation • Ashtech Z12 dual-frequency GPS receiver • GPS antenna mounted on mast above bridge • Inertial navigation system and tilt meter for attitude information • The Gulfport tide gauge was set up as the GPS base station

5. Initial Reference Frame Concerns • GPS is referenced to an ellipsoid whereas tide gauges are referenced to a tidal datum • At each GPS-surveyed tide gauge, the antenna was referenced to the gauge’s tidal staff and to its electric gauge • On the boat, the antenna must be referenced to the sea surface

6. Roll Ship-Antenna Geometry: Changes in Attitude • GPS solution locates the antenna atop the boat • Need to relate antenna position to sea surface (or need instantaneous keel depth) • Vertical distance changes with static draft, dynamic draft, and boat-antenna lever-arm geometry Squat

7. Ship as Tide Gauge • “Reducing” the GPS position to the water level yields an instantaneous measurement of water surface with respect to the ellipsoid • If the local offset between the ellipsoid and the tidal datum can be determined, the ship can become a continuous free-floating tide gauge

8. Connecting the Tidal Datum to the Ellipsoid • Tidal datum is defined locally as a long-term average of low tides as measured by a tide gauge. • Global geoid is the gravitational equipotential surface that approximates MSL on a global basis. • MSL deviates from the geoid due to oceanographic and atmospheric effects • Effects can be approximated by a geoid with a constant or slowly varying offset.

10. Chesapeake Bay Airborne Survey • P-3 airborne sea-surface height measurement over 8 tide gauges in the bay • SSH calculated using GPS heights and a radar altimeter • Geoid 99 is a relatively good geoid for the region but high frequency information is lacking • 3 cm rms accuracy for survey

12. Bertram Gulfport Survey • Each day began at the Gulfport tide gauge with half hour GPS collection at port • Bertram traveled to each of the other three tide gauges repeatedly, returning to the Gulfport gauge in between • Half hour occupations at the other tide gauges were also performed several times daily to help constrain biases associated with antenna-boat geometry, the geoid, and the links into each gauge.

14. Sea Surface Height for Day 129

15. Sea Surface Height with 200 s Gaussian Filter

16. Summary • Work is in progress • Encouraged by our first view of the data • Expect to demonstrate how the sea surface as measured with GPS can be related to the tidal datum using a combination of geoid, hydrodynamic models, and tide gauges. • Thanks for the great job from the NAVO crew