Walter HF Smith

National Oceanic and Atmospheric Administration | NOAA · Satellite Oceanography and Climatology Division

In this study, we present an extension to existing numerical retrackers of synthetic-aperture radar (SAR) altimetry signals. To our knowledge at the time of writing this manuscript, it offers the most consistent retrieval of geophysical parameters compared to low-resolution mode (LRM) retracking results. We achieve this by additionally estimating t...

In this study we present an analytical formulation of synthetic-aperture radar (SAR) altimetry signals including narrow banded nonlinear wave fields and conditional statistics between wave elevation displacements, horizontal wave slopes and vertical wave particle velocities. Considering the wave elevation displacements coskewness with respect to ho...

In this paper, we characterize the sea-ice elevation distribution by using NASA’s Operation IceBridge (OIB) Airborne Topographic Mapper (ATM) L1B data over the Arctic Ocean during 94 Spring campaigns between 2009 and 2019. The ultimate objective of this analysis is to better understand sea-ice topography to improve the estimation of the sea-ice fre...

Sentinel‐3 A&B radar altimeters yield sea surface height measurements in both a high‐precision Synthetic Aperture Radar Mode (SARM), and a Pseudo‐Low Resolution Mode (PLRM). We stacked repeat cycles from both missions and in both modes to compare their resolution of small seamounts. Stacking entailed removing non‐geoidal heights and height errors,...

To date, ∼20% of the ocean floor has been surveyed by ships at a spatial resolution of 400 m or better. The remaining 80% has depth predicted from satellite altimeter‐derived gravity measurements at a relatively low resolution. There are many remote ocean areas in the southern hemisphere that will not be completely mapped at 400 m resolution during...

Given the considerable range of applications within the European Union Copernicus system, sustained satellite altimetry missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to provide precision sea level, sea surface height, significant wave height, inlan...

The resolutions of current global altimetric gravity models and mean sea surface models are around 12 km wavelength resolving 6km features, and for many years it has been difficult to improve the resolution further in a systematic way. For both Jason 1 and 2, a Geodetic Mission (GM) has been carried out as a part of the Extension-of-Life phase. The...

The CNES/ISRO altimetric satellite SARAL/AltiKa was launched in February 2013 and since then has provided useful data for various scientific and operational applications in oceanography, hydrology, cryospheric sciences and geodesy. However, a Reaction Wheel problem forced relaxation of the repeatability constraint on the satellite’s orbit, which ha...

The Generic Mapping Tools (GMT) software is ubiquitous in the Earth and Ocean sciences. As a cross‐platform tool producing high quality maps and figures, it is used by tens of thousands of scientists around the world. The basic syntax of GMT scripts has evolved very slowly since the 1990s, despite the fact that GMT is generally perceived to have a...

An updated global bathymetry and topography grid is presented using a spatial sampling interval of 15 arc seconds. The bathymetry is produced using a combination of shipboard soundings and depths predicted using satellite altimetry. New data consists of >33.6 million multi and singlebeam measurements collated by several institutions, namely, the Na...

Satellite radar altimetry collected during a number of geodetic missions has provided a new understanding of the topography and tectonics of the deep oceans. As altimeter performance and coverage improves, smaller structures are revealed. Here we investigate the contribution of six altimeter missions that have been placed into geodetic mapping phas...

Two critical parts of establishing long-term datasets for climate studies are reducing the errors in individual measurements ansd in understanding any biases between different instruments or techniques. The tandem mission of Sentinel-3A and Sentinel-3B, when they followed the same track 30 seconds apart, enables us to examine the repeatability of m...

Presents corrections to the paper, “Removing intra-1-Hz covariant error to improve altimetric profiles of σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> and sea surface height,” (Quartly, G.D., et al), IEEE Trans. Geosci. Remote Sens., vol. 57, no. 6, pp. 3741–3752, Jun. 2019. doi: 10.1109/T...

In this paper, we revisit the pulse-to-pulse correlation properties of nadir-looking pulse-limited altimeters, with the objective of determining the effect of the partial correlation of radar echoes transmitted at much higher rate than the conventional pulse repetition frequency (PRF). This is particularly relevant for the Sentinel-6/Jason-CS missi...

The SARAL AltiKa radar altimeter measured sea surface height along ground tracks that were regularly revisited by repeating cycles. We devised an automated method of “stacking” the repeat cycles that aligns them to common positions along a model track, selects segments that pass quality criteria, removes the non-geoidal height and height error from...

The advantages of reprocessing the data from pulse-limited altimetry with the Adaptive Leading Edge Subwaveform Retracking (ALES) algorithm have been already demonstrated at the coast. We demonstrate in this talk that the same strategy improves the precision of satellite altimetry in the global ocean, presenting the new global ALES dataset, which i...

A satellite altimeter's waveform is a power spectral density (PSD) estimate that displays backscattered power as a function of range, and in the Delay/Doppler or multi-looked SAR (D/D-SAR) algorithm, also of along-track position. Earth surfaces reflect radar power at a continuum of ranges and along-track positions, and so waveforms inevitably suffe...

The India–France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated primarily to oceanography. The mission objectives were firstly the observation of the oceanic mesoscales but also global and regional sea level monitoring, including the coastal zone, data assimilation, and operational oceanography. SARAL/AltiKa proved also to b...

This paper develops a model of the synthetic aperture, interferometric satellite radar altimeter echo power, and echo cross-product. The model uses the smallness of the satellite pitch and roll angles, and the limited range of satellite altitude to provide a semianalytical echo model, whose numerical dimensions are limited to two in the synthetic a...

An assessment of ocean depth knowledge underneath commercial airline routes shows just how much of the seafloor remains "terra incognita."

Global models of seafloor topography have incomplete and inconsistent resolution at horizontal wavelengths less than about 10-20 km, notably due to their inability to resolve abyssal hills in areas unsurveyed by ships (that is, about 90% of the global seafloor). We investigated the impact of this unresolved bottom roughness on global numerical simu...

In this paper, we introduce the concept, develop the theory, and demonstrate the advantages of fully focused coherent processing of pulse echoes from a nadir-looking pulse-limited radar altimeter. This process, similar to synthetic aperture radar (SAR) imaging systems, reduces the along-track resolution down to the theoretical limit equal to half t...

We present a technique of stacking repeat cycles of satellite altimeter sea surface height profiles that lowers the noise and improves the resolution of small seamounts. Our approach differs from other studies because it uses the median (not the mean) of the stacks, which suppresses outliers. Seamounts as small as 720 m tall are easily detected in...

The resolution of seamount geoid anomalies by the SARAL AltiKa Ka-band radar altimeter is compared to that of the Envisat RA-2 Ku-band altimeter using cross-spectral analysis of exact-repeat profiles. Noise spectra show white noise floors at root-mean-square levels around 8 mm per root-Hz for AltiKa and 19 mm per root-Hz for RA2, and are colored at...

the Jason-1, TOPEX, Envisat, and GFO satellites obtained profi les of sea surface height on transects across the Indian Ocean between two and nine hours after the December 26 Sumatra earthquake. The data are received hours to days af-ter ìreal time,? too late to be used in detection and warning of tsunamis. We compared the sea level anomaly profi l...

Visit topex.ucsd.edu/grav_outreach/ for additional information and data download details and portal.gplates.org for global terrain visualisation of the vertical gravity gradient (vgg) data and for plate tectonic raster reconstructions of the vgg from 200 Ma to present. Gravity models are powerful tools for mapping tectonic structures, especially i...

We develop a slope correction model to improve the accuracy of mean sea surface topography models as well as marine gravity models. The correction is greatest above ocean trenches and large seamounts where the slope of the geoid exceeds 100 \(\upmu \) rad. In extreme cases, the correction to the mean sea surface height is 40 mm and the correctio...

On the morning of 8 March 2014, Malaysia Airlines flight MH370, from Kuala Lumpur to Beijing, lost contact with air traffic control shortly after takeoff and vanished. While the world waited for any sign of the missing aircraft and the 239 people on board, authorities and scientists began to investigate what little information was known about the p...

The full deramp pulse compression scheme employed by satellite radar altimeters digitizes each radar echo at a sampling rate matched to the chirp bandwidth. Echo power is under-sampled by a factor of two when the power samples are simply obtained by squaring the magnitude of the echo samples, without first resampling, as is done in all altimeters t...

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 t...

Generic Mapping Tools (GMT) is an open-source software package for the analysis and display of geoscience data, helping scientists to analyze, interpolate, filter, manipulate, project, and plot time series and gridded data sets. The GMT toolbox includes about 80 core and 40 supplemental program modules sharing a common set of command options, file...

Although CryoSat-2 was developed as a cryospheric altimeter and not as an ocean altimeter, oceanographic applications of the CryoSat-2 data have started to emerge. NOAA, working in close partnership with the ESA Cryosat Mission Team, has been producing an ocean product, in netCDF form, similar to the Interim Geophysical Data Record (IGDR) of Jason-...

More than 60% of the Earth's land and shallow marine areas are covered by > 2 km of sediments and sedimentary rocks, with the thickest accumulations on rifted continental margins (Figure 1). Free-air marine gravity anomalies derived from Geosat and ERS-1 satellite altimetry (Fairhead et al., 2001; Sandwell and Smith, 2009; Andersen et al., 2009) ou...

Incorporating new altimeter data from CryoSat-2 (30 months), Envisat (18 months), and Jason-1 (7 months) satellites into an updated marine gravity field yields significant reduction in noise and improved resolution. Compared to an older gravity field that did not include the new altimeter data, incoherent power is reduced globally by approximately...

We compute the radially symmetric coherence between multibeam bathymetry and satellite gravity grids in 25 areas distributed around the world. In contrast to previous studies employing one-dimensional analysis of data along profiles, our results cannot be biased by unseen off-track topography. The mean coherence averaged over the 20–160 km waveband...

This paper describes the calibration of the CryoSat-2 interferometer, whose principal purpose is to accurately measure the height of the Antarctic and Greenland ice sheets. A sequence of CryoSat-2 data acquisitions over the tropical and midlatitude oceans were obtained between June and September 2010, from the SIRAL "A" and redundant SIRAL "B" rada...

Marine gravity anomalies derived from radar altimeter measurements of ocean surface slope are the primary data for investigating global tectonics and seafloor bathymetry. The accuracy of the global marine gravity field is limited by the availability of non-repeat altimeter data. Current models, having accuracies of 3-5 milligals (e.g., S&S V18 and...

The SIRAL altimeter on CryoSat, launched in 2010, can operate in three modes: the low-rate mode (LRM) behaves as a conventional altimeter; the SAR mode allows more precise range and more focused footprint through use of synthetic aperture radar (SAR), also known as delay-Doppler, processing; the SARIN mode, or interferometric SAR, also affords acro...

GMT is a well-established, open source collection of tools for manipulating and plotting geographic and Cartesian data sets (including filtering, trend fitting, gridding, spatial analysis, mapping, etc.). The software produces high-quality PostScript illustrations ranging from simple x-y plots via contour maps to artificially illuminated surfaces a...

We investigate the spectrum of recent changes in total sea level measured by the Jason series of satellite altimeters and changes in the components of sea level -- steric sea level derived from Argo profiles and ocean mass from GRACE maps of gravity. The sea level budget is closed when the sum of these independently measured components agrees with...

Sub-ice shelf circulation and freezing/melting rates in ocean general circulation models depend critically on an accurate and consistent representation of cavity geometry. Existing global or pan-Antarctic topography data sets have turned out to contain various inconsistencies and inaccuracies. The goal of this work is to compile independent regiona...

The total electron content (TEC) in the ionosphere is an important factor in the propagation of radio waves. Since 1998 the coverage global positioning system (GPS) observations has been sufficient to monitor the TEC globally. We have used the global ionosphere maps provided by the Jet Propulsion Laboratory to devise a new ionosphere climatology (N...

We analyze errors in the global bathymetry models of Smith and Sandwell that combine satellite altimetry with acoustic soundings and shorelines to estimate depths. Versions of these models have been incorporated into Google Earth and the General Bathymetric Chart of the Oceans (GEBCO). We use Japan Agency for Marine-Earth Science and Technology (JA...

Sub-ice shelf circulation and freezing/melting rates in ocean general circulation models depend critically on an accurate and consistent representation of cavity geometry. Existing global or pan-Antarctic data sets have turned out to contain various inconsistencies and inaccuracies. The goal of this work is to compile independent regional fields in...

Despite playing a significant role in the global water cycle, ocean volume has not been re-examined in over 25 years. The main uncertainty associated with ocean volume is the mean ocean depth. The earliest studies tended to overestimate ocean depth due to undersampling of seamounts and ocean ridges. The advent of the echosounder in the 1920s and su...

The marine geoid defines the hydrostatic equilibrium shape that sea level would take in the absence of tides, currents and winds. The geoid is not directly observable, but its height above a reference ellipsoid may be calculated from a model of the Earth’s gravity field. Satellite altimeters measure the instantaneous sea surface height, which is th...

The U.S. Navy GEOSAT mission provided the first long-term altimetric record for studies of ocean circulation, marine gravity/bathymetry and continental ice, from early 1985 through 1989. The GEOSAT Follow-On spacecraft (GFO), launched in 1998, began continuous radar altimeter coverage of the oceans in 2000 and was terminated in late 2008. By provid...

A new 30-arc second resolution global topography/bathymetry grid (SRTM30_PLUS) has been developed from a wide variety of data sources. Land and ice topography comes from the SRTM30 and ICESat topography, respectively. Ocean bathymetry is based on a new satellite-gravity model where the gravity-to-topography ratio is calibrated using 298 million edi...

The Altimetric Bathymetry from Surface Slopes (ABYSS), which is the proposed science payload on the International Space Station (ISS), is a Johns Hopkins University Applied Physics Laboratory-developed flight-proved delay-Doppler phase-monopulse radar altimeter capable of measuring ocean surface slope in the 6-200-km half-wavelength frequency band...

We compare sea surface height anomalies observed by satellite altimeters in exact repeat mission orbits to anomalies observed by altimeters in geodetic or drifting orbits. One experiment compares data from the drift of TOPEX between its Tandem and Interleaved missions to repeat-orbit data from Jason-1, ERS-2 and GFO. A second experiment compares da...

Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to...

Comparing single beam and multibeam echo sounder data where surveys overlap we find that: 95% of multibeam measurements are repeatable to within 0.47% of depth; older single beam data can be at least as accurate as multibeam; single beam and multibeam profiles show excellent agreement at full-wavelengths longer than 4km; archival sounding errors ar...

Seafloor roughness varies considerably across the world’s ocean basins and is fundamental to controlling the circulation and mix- ing of heat in the ocean1 and dissipating eddy kinetic energy2. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates3, with rougher basement formi...

Seafloor roughness varies considerably across the world's ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming be...

We have developed global gravity and bathymetry grids at 1-minute resolution. Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have use...

A new version of the "Smith and Sandwell" global marine topography model is available in two formats. A one-arc-minute Mercator projected grid covering latitudes to +/- 80.738 degrees is available in the "img" file format. Also available is a 30-arc-second version in latitude and longitude coordinates from pole to pole, supplied as tiles covering t...

Satellite radar altimeter measurements of sea surface height (SSH), significant wave height, and wind speed have many potential applications in coastal zones, despite the common perception that altimetry does not “work” near the coast. The altimeter's primary measurement, the radar travel time from the spacecraft to the sea surface, is reliable sea...

The Altimetric Bathymetry from Surface Slopes, ABYSS, the proposed science payload on the International Space Station (ISS), is a Johns Hopkins University Applied Physics Laboratory (JHU-APL) developed flight-proved delay-Doppler phase monopulse (D2P) radar altimeter capable of measuring ocean surface slope in the 6-200 km half-wavelength frequency...

We examine peak-to-trough gravity anomaly amplitudes over seamounts, comparing gravity derived from satellite altimetry with gravity measured by ships. We also compare amplitudes from linear and higher-order (non-linear) computations, and conclude that two-term forward modeling of seamounts should be adequate. Altimetric amplitudes are within 90% o...

We have examined three factors influencing the use of satellite altimeter data to map seamounts and guyots in the deep ocean: (1) the resolution of seamount and guyot gravity anomalies by altimetry; (2) the non-linearity of the relationship between gravity and bathymetry; and (3) the homogeneity of the mass density within the seamount or guyot. Whe...

Hypsometry is a basic analysis of ocean depth data: frequency distribution of ocean depths. Within that distribution lies evidence of primary earth processes, e.g. the bimodal distribution of depths manifests the different thicknesses of continental and oceanic crust. Menard and Smith (1966) used American and Russian contour bathymetry, to compile...

Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Applications as diverse as tsunami hazard assessment, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable...

The seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin. Relatively few of these features (

The U.S. N avy GEO SAT mission provided the first long-term altimetr ic record for studies of ocean circu lation , mar ine grav ity /bathymetry and continen tal ice. The Geodetic Mission (G M) data were declassified by the Navy in 1995 and released by NOAA togeth er with the Ex act Rep eat Mission data in the 1997 JG M-3 Geophysical Data Records (G...

Our current understanding of the topography and tectonics of the ocean basins is largely derived from dense satellite altimeter measurements of the marine gravity field combined with sparse geophysical measurements from research vessels. Research efforts over the past few years have provided a 30-40% improvement in gravity field accuracy by retrack...

In their comment on our Eos article [ Scharroo et al. , 2005], Sun et al. [this issue] conclude that sea surface temperature (SST) had a significant impact on the rapid intensification of Hurricane Katrina. Although SST may have played some role, we want to stress that dynamic topography is a more reliable proxy than SST for upper ocean heat conten...