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Zhang, SJ, Sandwell DT.  2017.  Retracking of SARAL/AltiKa Radar Altimetry Waveforms for Optimal Gravity Field Recovery. Marine Geodesy. 40:40-56.   10.1080/01490419.2016.1265032   AbstractWebsite

The accuracy of the marine gravity field derived from satellite altimetry depends on dense track spacing as well as high range precision. Here, we investigate the range precision that can be achieved using a new shorter wavelength Ka-band altimeter AltiKa aboard the SARAL spacecraft. We agree with a previous study that found that the range precision given in the SARAL/AltiKa Geophysical Data Records is more precise than that of Ku-band altimeter by a factor of two. Moreover, we show that two-pass retracking can further improve the range precision by a factor of 1.7 with respect to the 40 Hz-retracked data (item of range_40 hz) provided in the Geophysical Data Records. The important conclusion is that a dedicated Ka-band altimeter-mapping mission could substantially improve the global accuracy of the marine gravity field with complete coverage and a track spacing of <6 km achievable in similar to 1.3 years. This would reveal thousands of uncharted seamounts on the ocean floor as well as important tectonic features such as microplates and abyssal hill fabric.

Garcia, ES, Sandwell DT, Smith WHF.  2014.  Retracking CryoSat-2, Envisat and Jason-1 radar altimetry waveforms for improved gravity field recovery. Geophysical Journal International. 196:1402-1422.   10.1093/gji/ggt469   AbstractWebsite

Improving the accuracy of the marine gravity field requires both improved altimeter range precision and dense track coverage. After a hiatus of more than 15 yr, a wealth of suitable data is now available from the CryoSat-2, Envisat and Jason-1 satellites. The range precision of these data is significantly improved with respect to the conventional techniques used in operational oceanography by retracking the altimeter waveforms using an algorithm that is optimized for the recovery of the short-wavelength geodetic signal. We caution that this new approach, which provides optimal range precision, may introduce large-scale errors that would be unacceptable for other applications. In addition, CryoSat-2 has a new synthetic aperture radar (SAR) mode that should result in higher range precision. For this new mode we derived a simple, but approximate, analytic model for the shape of the SAR waveform that could be used in an iterative least-squares algorithm for estimating range. For the conventional waveforms, we demonstrate that a two-step retracking algorithm that was originally designed for data from prior missions (ERS-1 and Geosat) also improves precision on all three of the new satellites by about a factor of 1.5. The improved range precision and dense coverage from CryoSat-2, Envisat and Jason-1 should lead to a significant increase in the accuracy of the marine gravity field.

Wessel, P, Sandwell DT, Kim SS.  2010.  The Global Seamount Census. Oceanography. 23:24-33. AbstractWebsite

Seamounts are active or extinct undersea volcanoes with heights exceeding similar to 100 m. They represent a small but significant fraction of the volcanic extrusive budget for oceanic seafloor and their distribution gives information about spatial and temporal variations in intraplate volcanic activity. In addition, they sustain important ecological communities, determine habitats for fish, and act as obstacles to Currents, thus enhancing tidal energy dissipation and ocean mixing. Mapping the complete global distribution will help constrain models of seamount formation as well as aid in understanding marine habitats and deep ocean circulation. Two approaches have been used to map the global seamount distribution. Depth soundings from single- and multibeam echosounders can provide the most detailed maps with up to 200-m horizontal resolution. However, soundings from the > 5000 publicly available cruises sample only a small fraction of the ocean floor. Satellite altimetry can detect seamounts taller than similar to 1.5 km, and. studies using altimetry have produced seamount catalogues holding almost 13,000 seamounts. Based on the size-frequency relationship for larger seamounts, we predict over 100,000 seamounts > 1 km in height remain uncharted, and speculatively 25 million > 100 m in height. Future altimetry missions could improve on resolution and significantly decrease noise levels, allowing for an even larger number of intermediate (1-1.5-km height) seamounts to be detected. Recent retracking of the radar altimeter waveforms to improve the accuracy of the gravity field has resulted in a twofold increase in resolution. Thus, improved analyses of existing altimetry with better calibration from multibeam bathymetry could also increase census estimates.

Sandwell, DT, Smith WHF.  2005.  Retracking ERS-1 altimeter waveforms for optimal gravity field recovery. Geophysical Journal International. 163:79-89.   10.1111/j.1365-246X.2005.02724.x   AbstractWebsite

We have reprocessed ERS-1 radar altimeter waveforms using an algorithm designed to minimize sea surface slope error and decouple it from significant wave height (SWH) error. Standard waveform retracking estimates three parameters-arrival time, SWH and amplitude. We show that errors in retracked estimates of arrival time and SWH are inherently correlated because of the noise characteristics of the returned waveform. This suggests that some of what is called 'sea state bias' in the literature may be caused by correlated errors rather than true electromagnetic or skewness bias. We have developed a retracking algorithm that reduces this error correlation and makes the resolution of sea surface slope signals independent of sea state. The main assumption is that the SWH varies smoothly along the satellite track over wavelengths of 90 km. This approach reduces the rms error in sea surface slope to only 62 per cent of that of standard retracking methods. While our method is optimized for gravity field recovery, it may also improve the resolution of sea surface height signals of interest to physical oceanographers.

Yale, MM, Sandwell DT.  1999.  Stacked global satellite gravity profiles. Geophysics. 64:1748-1755.   10.1190/1.1444680   AbstractWebsite

Gravity field recovery from satellite altimetry provides global marine coverage but lacks the accuracy and resolution needed for many exploration geophysics studies. The repeating ground tracks of the ERS-1/2, Geosat, and Topex/Poseidon altimeters offer the possibility of improving the accuracy and resolution of gravity anomalies along widely spaced (similar to 40-km spacing) tracks. However, complete ocean coverage is usually needed to convert the sea-surface height (br along-track slope) measurements into gravity anomalies. Here we develop and test a method for constructing stacked gravity profiles by using a published global gravity grid (Sandwell and Smith, 1997), V7.2, as a reference model for the slope-to-gravity anomaly conversion. The method is applied to stacks (averages) of Geosat/ERM (up to 62 cycles), ERS-1/2 (up to 43 cycles), and Topex (up to 142 cycles) satellite altimeter profiles. We assess the accuracies of the ERS-1/2 profiles through a comparison with a gravity model of the northern Gulf of Mexico (profiles provided by EDCON Inc.). The 40 ERS profiles evaluated have a mean rms difference of 3.77 mGal and full wavelength resolution (0.5 coherence) of 24 km. Our processing retains wavelengths as short as 10 km so smaller, large-amplitude features can be resolved, especially in shallow ocean areas (<1000 m deep). We provide an example of combining these higher resolution profiles with lower resolution gravity data in the Caspian Sea.

Sandwell, DT, Price EJ.  1998.  Phase gradient approach to stacking interferograms. Journal of Geophysical Research-Solid Earth. 103:30183-30204.   10.1029/1998jb900008   AbstractWebsite

The phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping. Our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms. The standard approach requires phase unwrapping, scaling the phase according to the ratio of the perpendicular baseline, and finally forming the average or difference; however, unique phase unwrapping is usually not possible. Since the phase gradient due to topography is proportional to the perpendicular baseline, phase unwrapping is unnecessary prior to averaging or differencing. Phase unwrapping may be needed to interpret the results, but it is delayed until all of the largest topographic signals are removed. We demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m. Cross-spectral analysis of the difference between two Tandem interferograms provides estimates of spatial resolution, which are used to design prestack filters. A wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage. Outside of mountainous areas the topography has a relative accuracy of better than 2 m. Residual interferograms (single interferogram minus stack) have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth, reflecting both orbit error and atmospheric delay. Smaller-scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves. A few Global Positioning System (GPS) control points within a Game could increase the precision to similar to 20 mm for a single interferogram; further improvements may be achieved by stacking residual interferograms.

Smith, WHF, Sandwell DT.  1997.  Global sea floor topography from satellite altimetry and ship depth soundings. Science. 277:1956-1962.   10.1126/science.277.5334.1956   AbstractWebsite

A digital bathymetric map of the oceans with a horizontal resolution of 1 to 12 kilometers was derived by combining available depth soundings with high-resolution marine gravity information from the Geosat and ERS-1 spacecraft. Previous global bathymetric maps lacked features such as the 1600-kilometer-long Foundation Seamounts chain in the South Pacific. This map shows relations among the distributions of depth, sea floor area, and sea floor age that do not fit the predictions of deterministic models of subsidence due to lithosphere cooling but may be explained by a stochastic model in which randomly distributed reheating events warm the lithosphere and raise the ocean floor.

Sandwell, DT, Smith WHF.  1997.  Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research-Solid Earth. 102:10039-10054.   10.1029/96jb03223   AbstractWebsite

Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (similar to 6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random skip tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.