tide corrections from kgps and a precise geoid john brozena, randy herr, vicki childers
TRANSCRIPT
Tide corrections from KGPS and a precise
geoid
John Brozena, Randy Herr, Vicki Childers
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?
NRL/NAVO Bertram Test
Three day survey in May 2002Conducted aboard the Bertram from Gulfport tide gaugeSea 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
The Gulfport tide gauge was set up as the GPS base station
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
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
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
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.
Chesapeake Bay Airborne Survey
P-3 airborne sea-surface height measurement over 8 tide gauges in the baySSH calculated using GPS heights and a radar altimeterGeoid 99 is a relatively good geoid for the region but high frequency information is lacking3 cm rms accuracy for survey
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.
Sea Surface Height for Day 129
Sea Surface Height with 200 s Gaussian Filter
Summary
Work is in progressEncouraged by our first view of the dataExpect 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