1. Tide corrections from KGPS and a precise geoid
John Brozena – Naval Research Lab

2. Goals
Approximate tide datum from a precise gravimetric geoid and an offset determined from one or more tide gauges or GPS buoys
Determine hydrographic tide correction from KGPS vertical shipboard positions referenced to geoid + offset

3. Connecting the Tide Datum to the Ellipsoid
Global geoid is the gravitational equipotential surface that approximates MSL on a global basis.
Local MSL deviates from the global geoid due to oceanographic and atmospheric effects
mean currents
water column density anomalies
non-standard average air pressure
wind set-up
tidal nodes
Tidal datum is defined locally as a long-term average of low tides below local MSL (tide gauge).
Approximate tide datum by a geoid with a constant or slowly varying offset.

4. Tampa Bay MSL-Datum Offset (Parker et al., 2001)
39 cm MSL-MLL offset produces a max 5 cm datum error

5. Local NRL Geoid
meters

6. Local Residual Geoid (EGM96- NRL Local Geoid)
meters
Shows high frequency information not contained in EGM96 field

7. Goddard MSS Model – NRL Local Geoid
Tide Gage MSS-geoid MSL-geoid MSL-MLW
pcb
pensb
wavms

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

9. 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
Roll
Squat

10. NRL/NAVO Bertram Experiment
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.

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

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
GPS Reference
Red=local geoid
Blue=EGM96

15. Sea Surface Height with 200 s Gaussian Filter

16. Squat Correction- 1st Iteration

17. Final (speed-based) Squat Correction

18. Squat and Tide Adjusted SSH
RMS residual = 4.0 cm

19. RMS residual = 4.1 cm

20. 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

21. Chesapeake Bay Airborne Survey

22. Summary Current project completed
Method works extremely well for this case
Good gravity coverage/geoid
Constant datum offset assumption valid for region
Next step: 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 gauge/GPS buoy.
Thanks for the great job from Randy Herr & NAVO crew