Tide corrections from KGPS and a precise

geoid

John Brozena – Naval Research Labjohn.brozena@nrl.navy.mil

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

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 currentswater column density anomaliesnon-standard average air pressurewind set-uptidal 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.

Tampa Bay MSL-Datum Offset (Parker et al., 2001)

39 cm MSL-MLL offset produces a max

5 cm datum error

meters

Local NRL Geoid

meters

Local Residual Geoid (EGM96- NRL Local Geoid)

Shows high frequency information not contained in EGM96 field

Tide Gage MSS-geoid MSL-geoid MSL-MLWpcb 0.11 0.04 0.20pensb 0.04 0.04 0.19wavms 0.10 0.16 0.24

Goddard MSS Model – NRL Local Geoid

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

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

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.

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

Bertram Gulfport SurveyEach 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.

Sea Surface Height for Day 129

GPS Reference

Red=local geoid

Blue=EGM96

Sea Surface Height with 200 s Gaussian Filter

Squat Correction- 1st Iteration

Final (speed-based) Squat Correction

Squat and Tide Adjusted SSH

RMS residual = 4.0 cm

RMS residual = 4.1 cm

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

Chesapeake Bay Airborne Survey

SummaryCurrent project completedMethod works extremely well for this case

Good gravity coverage/geoidConstant 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