Walter Smith | NOAA Center for Satellite Applications and Research (original) (raw)

Papers by Walter Smith

AGU Fall Meeting Abstracts, Dec 1, 2019

AGU Fall Meeting Abstracts, Dec 1, 2017

AGU Fall Meeting Abstracts, Dec 1, 2018

Remote Sensing

In this study, we present an extension to existing numerical retrackers of synthetic-aperture rad... more 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 the standard deviation of vertical wave-particle velocities σv and a new parameter ux, linked to a residual Doppler in the returned radar echoes, which can be related to wind speed and direction. Including this new parameter into the SAR stack retracker mitigates sea surface height estimation errors by up to two centimeters for Sentinel-6MF SAR mode results. Additionally, we found a closed-form equation to describe ux as a function of eastward and northward wind variables, which allows mitigating the effects of this parameter on a SAR stack within level 1B processing and generating a lookup table to correct sea surface height estimates in S...

Sub-ice shelf circulation and freezing/melting rates in ocean general circulation models depend c... more 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 into a global data set. We use the S-2004 global 1-minute bathymetry as the backbone and add an improved version of the BEDMAP topography for an area that roughly coincides with the Antarctic continental shelf. Locations of the merging line have been carefully adjusted in order to get the best out of each data set. High-resolution gridded data for upper and lower ice surface topography and cavity geometry of the Amery, Fimbul, Filchner-Ronne, Larsen C and George VI Ice Shelves, and for Pine Island Glacier have been carefully merged into the ambient ice and ocean topographies. Multibeam survey data for bathymetry in the former Larsen B cavity and the southeastern Bellingshausen Sea have been obtained from the data centers of Alfred Wegener Institute (AWI), British Antarctic Survey (BAS) and Lamont-Doherty Earth Observatory (LDEO), gridded, and again carefully merged into the existing bathymetry map. The global 1-minute dataset (RTopo-1 Version 1.0.5) has been split into two NetCDF files. The first contains digital maps for global bedrock topography, ice bottom topography, and surface elevation. The second contains the auxiliary maps for data sources and the surface type mask. A regional subset that covers all variables for the region south of 50 deg S is also available in NetCDF format. Datasets for the locations of grounding and coast lines are provided in ASCII format.

Earth and Space Science, 2018

The SARAL AltiKa radar altimeter measured sea surface height along ground tracks that were regula... more 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 each repeat profile, and calculates the median height profile. This procedure reinforces geoid signals while reducing measurement noise and height signals produced by ocean dynamics. A seamount detection filter applied to the median profiles reveals 75,208 possible small seamounts along AltiKa ground tracks globally. Of these, 4824 are located over multibeam surveys. Seamount heights estimated by subtracting regional depths from the multibeam depths follow a Poisson statistical distribution that suggests at least 84% are less than 2 km tall. A 1 km along-track sampling of the stacked repeat cycles is available from the National Centers for Environmental Information (NCEI) data repository.

IGARSS 2003. 2003 IEEE International Geoscience and Remote Sensing Symposium. Proceedings (IEEE Cat. No.03CH37477)

... to develop an original spacecraft design within the Johns Hopkins University Applied Physics ... more ... to develop an original spacecraft design within the Johns Hopkins University Applied Physics Laboratory, based ... [7] DT Sandwell, "A detailed view of the South Pacific from satellite altimetry,"J ... Smith, "Marine gravity anomaly from Geosat and ERS-1 satellite altimetry," J. Geophys ...

ABSTRACT Our current understanding of the topography and tectonics of the ocean basins is largely... more ABSTRACT 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 retracking the raw altimeter waveforms using methods optimized for range precision. However, there are three broad areas of earth science that require an additional factor of 5 improvement in gravity accuracy that can only be ach ieved with next generation altimeters: (1) resolving the fine-scale tectonic structure of the deep ocean floor (e.g., abyssal hills, microplates, propagating rifts, seamounts, meteorite impacts); (2) measuring the roughness spectra of the seafloor on a global basis to better constrain models of tidal dissipation, vertical mixing, and mesoscale circulation of the oceans; and (3) resolving the fine-scale gravity field for research, exploration and navigational needs.

ABSTRACT Marine gravity anomalies derived from radar altimeter measurements of ocean surface slop... more ABSTRACT 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 DNSC08), are based on the non-repeat data collected by Geosat (18 mo.) and ERS-1 (12 mo.) which use altimeter technology from the 70's and 80's, respectively. Over the next 3 to 5 years, a wealth of new marine gravity data will be provided by three currently operating satellite altimeters CryoSat, Jason-1, and Envisat. With careful processing of the data, in combination with data from past Geosat and ERS-1/GM altimeter missions, we expect to improve the accuracy of the global marine gravity field by at least a factor of two and in some areas a factor of four. In addition to track coverage, the accuracy of the recovered gravity field depends on the accuracy of the arrival time parameter. We have developed an optimized retracking algorithm for CryoSat in the LRM mode and show that the arrival time estimated from CryoSat is 1.4 times better than Geosat and ERS-1. This is consistent with a 2 times higher pulse repetition frequency for CryoSat. We are currently developing a retracking algorithm for CryoSat in the SAR mode and expect a factor of 2 or more improvement in the accuracy of the arrival time. CryoSat has been collecting ocean data for 1.5 years and the ground track density is now adequate to make significant improvements in the marine gravity field. We will construct an improved global gravity model based on these data and provide an evaluation at the meeting.

AGU Fall Meeting Abstracts, Dec 1, 2019

AGU Fall Meeting Abstracts, Dec 1, 2017

AGU Fall Meeting Abstracts, Dec 1, 2018

Remote Sensing

In this study, we present an extension to existing numerical retrackers of synthetic-aperture rad... more 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 the standard deviation of vertical wave-particle velocities σv and a new parameter ux, linked to a residual Doppler in the returned radar echoes, which can be related to wind speed and direction. Including this new parameter into the SAR stack retracker mitigates sea surface height estimation errors by up to two centimeters for Sentinel-6MF SAR mode results. Additionally, we found a closed-form equation to describe ux as a function of eastward and northward wind variables, which allows mitigating the effects of this parameter on a SAR stack within level 1B processing and generating a lookup table to correct sea surface height estimates in S...

Sub-ice shelf circulation and freezing/melting rates in ocean general circulation models depend c... more 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 into a global data set. We use the S-2004 global 1-minute bathymetry as the backbone and add an improved version of the BEDMAP topography for an area that roughly coincides with the Antarctic continental shelf. Locations of the merging line have been carefully adjusted in order to get the best out of each data set. High-resolution gridded data for upper and lower ice surface topography and cavity geometry of the Amery, Fimbul, Filchner-Ronne, Larsen C and George VI Ice Shelves, and for Pine Island Glacier have been carefully merged into the ambient ice and ocean topographies. Multibeam survey data for bathymetry in the former Larsen B cavity and the southeastern Bellingshausen Sea have been obtained from the data centers of Alfred Wegener Institute (AWI), British Antarctic Survey (BAS) and Lamont-Doherty Earth Observatory (LDEO), gridded, and again carefully merged into the existing bathymetry map. The global 1-minute dataset (RTopo-1 Version 1.0.5) has been split into two NetCDF files. The first contains digital maps for global bedrock topography, ice bottom topography, and surface elevation. The second contains the auxiliary maps for data sources and the surface type mask. A regional subset that covers all variables for the region south of 50 deg S is also available in NetCDF format. Datasets for the locations of grounding and coast lines are provided in ASCII format.

Earth and Space Science, 2018

The SARAL AltiKa radar altimeter measured sea surface height along ground tracks that were regula... more 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 each repeat profile, and calculates the median height profile. This procedure reinforces geoid signals while reducing measurement noise and height signals produced by ocean dynamics. A seamount detection filter applied to the median profiles reveals 75,208 possible small seamounts along AltiKa ground tracks globally. Of these, 4824 are located over multibeam surveys. Seamount heights estimated by subtracting regional depths from the multibeam depths follow a Poisson statistical distribution that suggests at least 84% are less than 2 km tall. A 1 km along-track sampling of the stacked repeat cycles is available from the National Centers for Environmental Information (NCEI) data repository.

IGARSS 2003. 2003 IEEE International Geoscience and Remote Sensing Symposium. Proceedings (IEEE Cat. No.03CH37477)

... to develop an original spacecraft design within the Johns Hopkins University Applied Physics ... more ... to develop an original spacecraft design within the Johns Hopkins University Applied Physics Laboratory, based ... [7] DT Sandwell, "A detailed view of the South Pacific from satellite altimetry,"J ... Smith, "Marine gravity anomaly from Geosat and ERS-1 satellite altimetry," J. Geophys ...

ABSTRACT Our current understanding of the topography and tectonics of the ocean basins is largely... more ABSTRACT 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 retracking the raw altimeter waveforms using methods optimized for range precision. However, there are three broad areas of earth science that require an additional factor of 5 improvement in gravity accuracy that can only be ach ieved with next generation altimeters: (1) resolving the fine-scale tectonic structure of the deep ocean floor (e.g., abyssal hills, microplates, propagating rifts, seamounts, meteorite impacts); (2) measuring the roughness spectra of the seafloor on a global basis to better constrain models of tidal dissipation, vertical mixing, and mesoscale circulation of the oceans; and (3) resolving the fine-scale gravity field for research, exploration and navigational needs.

ABSTRACT Marine gravity anomalies derived from radar altimeter measurements of ocean surface slop... more ABSTRACT 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 DNSC08), are based on the non-repeat data collected by Geosat (18 mo.) and ERS-1 (12 mo.) which use altimeter technology from the 70's and 80's, respectively. Over the next 3 to 5 years, a wealth of new marine gravity data will be provided by three currently operating satellite altimeters CryoSat, Jason-1, and Envisat. With careful processing of the data, in combination with data from past Geosat and ERS-1/GM altimeter missions, we expect to improve the accuracy of the global marine gravity field by at least a factor of two and in some areas a factor of four. In addition to track coverage, the accuracy of the recovered gravity field depends on the accuracy of the arrival time parameter. We have developed an optimized retracking algorithm for CryoSat in the LRM mode and show that the arrival time estimated from CryoSat is 1.4 times better than Geosat and ERS-1. This is consistent with a 2 times higher pulse repetition frequency for CryoSat. We are currently developing a retracking algorithm for CryoSat in the SAR mode and expect a factor of 2 or more improvement in the accuracy of the arrival time. CryoSat has been collecting ocean data for 1.5 years and the ground track density is now adequate to make significant improvements in the marine gravity field. We will construct an improved global gravity model based on these data and provide an evaluation at the meeting.