Airborne Sea-Surface Topography in an Absolute Reference Frame: Applications to Coastal Oceanography

J. Brozena,1 V. Childers,1 and G. Jacobs2
1Marine Geosciences Division
2Oceanography Division

Introduction: Water-level measurements are fundamental to Navy operations. Data from tide gages, satellite altimetry, and bottom pressure-gage/inverted echo sounders are assimilated into tide and circulation models to produce nowcasts and forecasts of parameters of interest (e.g., currents, acoustic propagation, and navigability of harbors and channels). Unfortunately, highly dynamic coastal ocean processes occur at temporal and spatial scales that cannot be captured by the available water level measurement systems. Additionally, there is no universal vertical datum that allows measurements from the different types of systems to be directly combined as heights. Instead, the delta or change in water level over time at a given location is the quantity generally assimilated into models. These time-derivatives of height are very useful, but they lack the information contained in the long-period average heights from mean currents, nonstandard water density, and wind set-up effects.

Airborne Measurement Techniques: NRL has been developing airborne measurement techniques, altimetry and gravity, to determine sea-surface height and residual height anomaly at the short time and space scales required for littoral regions. In our method, a precise radar altimeter measures the distance to the sea-surface beneath the aircraft. We combine these ranges with extremely accurate (cm-level) interferometric-mode Global Positioning System (GPS) estimates of the aircraft height above the reference ellipsoid. This produces sea-surface height profiles that are similar to conventional satellite altimetry. However, the airborne method is not restricted by the inherent orbital limitations in sampling in which there is a trade-off between sub-satellite track spacing and revisit times. Airborne altimetry may also be collected in close proximity to the coast where land within the large illuminated footprint of the satellite-borne radar corrupts the data.

We have also developed a methodology for providing a high-resolution geoid reference surface or vertical datum that can be used for any of the water-level measurement systems. The geoid is an equipotential surface of gravity that would approximate mean sea level (msl) if the ocean were motionless. The fit between the equipotential surface and sea level can be global or local. Our method involves calculating a precise local geoid from historical marine and terrestrial gravity data in the region augmented with airborne gravity data collected simultaneously with the airborne altimetry profiles. We then determine any offsets of the local geoid from GPS surveyed tide gages along the shoreline and with conventional altimetric mean sea-surface models away from shore. A gravitational geoid was calculated for the northern Gulf of Mexico to serve as a test msl reference surface. The local geoid and calibrated offsets provide a means to connect airborne, satellite, and tide-gage observations in an absolute (WGS-84) framework. It is also possible to transfer precise absolute vertical coordinates to bottom-moored water-level instrumentation by simultaneous collection of water levels from the bottom instruments and the airborne, GPS-referenced altimeter.

Flight Testing: In May 2003 we performed a series of 10 altimetry flights over the northern Gulf of Mexico near the mouth of the Mississippi River. The experiment was designed to test the accuracy and resolution of the airborne measurements while examining the temporal and spatial variability of the coastal sea-surface heights and related water-column temperature anomalies. Airborne expendable bathythermograph (AXBT) probes were dropped regularly to acquire the temperature data. Internal cross-over statistics and crossings of a GPS-surveyed tide gage indicate sea-surface topography accuracy of better than 5 cm rms.

We were fortunate to have a large warm-core eddy pinned against the slope in the test area throughout the experiment. The eddy showed a topographic relief of about 1 m (including a low, moat-like structure around the boundary of the eddy) and created a highly dynamic environment with coherent and rapidly varying sea-surface height anomalies across the region (Fig. 6). The AXBT data collected at the same time show wave-like temperature and topographic anomalies propagating up the continental slope (Fig. 7). Time and space scales of these features are hours to days and tens to a few hundred kilometers. The airborne altimetry revealed the structure and temporal evolution of the eddy and its interaction with the bottom topography of the slope in much greater detail than would be possible with space-borne altimeters. The airborne method also allowed the collection in situ water column measurements.

Figure 6 Image
Sea-surface height anomalies from a sequence of eight of the altimetry reflights. The contour interval is 5 cm. Black triangles show the locations of the MMS bottom-moored pressure gage/inverted echo sounders that will provide ground-truth when the data are retrieved. One side of the large warm-core eddy and the bounding, moat-like topographic low are visible in the figures over the entire 16-day period. Also prominent is a coastal high near the mouth of the Mississippi river that waxes and wanes over the period.
Figure 7 Image
Sea-surface height anomaly overlaid to the 20, 15, and 10°C isotherms (interpolated from the AXBT data) from every other flight of Fig. 6. Bathymetry of the continental slope is shown behind the surfaces, and the blue symbols and vertical blue lines show the position of the MMS bottom moorings. The eddy and bounding moat are again clearly seen. When animated, the entire data sequence clearly shows wavelike features in the topography and isotherms propagating shoreward and up the slope.

Uses: The new measurement capability can be used to improve our understanding of the littoral environment. Since the airborne altimeter and GPS equipment is relatively small and light, there is also good potential to utilize this method on unmanned airborne vehicles for access to denied areas of Navy interest. The high-resolution geoid msl reference used for this program is an initial step in providing a unified vertical datum for integrating water-level measurements of all types. This should help to improve the resolution and accuracy of the next generation of ocean circulation and tide models.

Acknowledgments: The Naval Oceanographic Office provided the 600 AXBT buoys deployed for this experiment. Our thanks to the Navy P-3 crew from the NRL Military Operations and Flight Support Detachment for an excellent job of flying and maintaining the aircraft.

[Sponsored by ONR]