Seasat Altimeter Research Articles

Generation of DEMs over A Part of Antarctica Using Altimetry Data and their Implications

Recent advancements in remote sensing techniques have made it possible to monitor and understand the activities of polar ice sheets, which exert profound influence in controlling global climate. Techniques such as laser altimetry, radar altimetry, InSAR, GPS etc. have been used for generation of DEMs, calculation of ice‐sheet mass balance, retrieval of snow‐water equivalent and other snow/ice related studies. In the present study, DEMs were generated using SEASAT altimeter data over parts of Antarctica and the maximum height variations in the studied area have been observed as high as 3200 m. From the analysis of the surface slopes, an attempt was made to qualitatively estimate the magnitude and direction of the ice‐sheet movement vectors. Coupled with temporal data, both field and satellite derived, the results obtained can be used for modelling of ice‐sheet mass balance over Antarctica.

Elevation change measurement of the East Antarctic Ice Sheet, 1978 to 1988, from satellite radar altimetry

Seasat and Geosat satellite radar altimeter measurements over the East Antarctic Ice Sheet (EAIS) are analyzed to determine surface elevation change over the period from 1978 to 1988. The results show that the average elevation change over 1.7 /spl times/10/sup 6/ km/sup 2/ of the EAIS is -0.4/spl plusmn/1.8 cm/yr and is therefore not significantly different than zero. This result is in very good agreement with ERS-1/2 radar altimeter estimates of EAIS elevation change from 1992 to 1996. The combined results suggest that the EAIS has been close to a state of balance for two decades. This further reduces the likelihood that recent changes in snow accumulation have masked a long-term mass imbalance in the EAIS.

Introduction to special section: Studies of the ocean surface from the Spaceborne Imaging Radar‐C/X‐Band SAR experiments

Nearly 20 years have now passed since Seasat briefly sampled the ocean surface with its suite of then extraordinarily advanced active microwave instruments. The tantalizing 3‐month snapshots from Seasat's altimeter and scatterometer were analyzed and reanalyzed and used to justify follow‐on missions that were finally implemented within the last decade, including the ERS missions, TOPEX/POSEIDON, NSCAT, and Geosat.The third active microwave instrument carried on Seasat, the synthetic aperture radar (SAR), has had more of a mixed success in its contribution to physical oceanography, despite its being flown on four satellites since 1991, the European Space Agency's ERS‐I and ERS‐2, the Japanese Space Agency's JERS‐I, and Canada's RADARSAT. The recent special issue on oceanographic results from ERS‐I and ERS‐2 (special section on Advances in Oceanography and Sea Ice Research Using ERS Observations, Journal of Geophysical Research, 103(C4), 1998) has numerous papers that incorporate SAR for both ocean and polar sea ice research. The finescale twodimensional view of the ocean surface seen in SAR imagery provides unparalleled detail of the short wave field and its interactions with longer waves and currents. However, the detailed view comes at an expense. Using the imagery for oceanographic applications has required the unraveling of the data's often baffling signatures, arising not only from the complicated wave‐wave and wave‐current interactions but also from the varying local wind field. Once understood, these signatures provide new and unique oceanographic and atmospheric information at higher spatial resolution than is available in the other microwave instruments. One approach to untangle this challenging interaction of ocean and radar physics is to look at the ocean surface simultaneously with multiple radar frequencies and polarizations. Most commonly, this is done with aircraft and surface‐based instrumentation. This suite of multiple channels provides a more complete understanding of the radar scattering from the ocean surface, since it is scale‐dependent, which can then lead to improved understanding of the ocean physics.

What are the limitations of satellite altimetry?

Radar altimeter measurements of the marine geoid collected during the Seasat altimeter mission gave geophysicists hope of uncovering the gravity field over all the ocean basins. However, because of insufficient track density, it has taken 16 years for the full potential of the satellite altimeter to be realized. The high‐density coverage obtained by ERS-1 during its geodetic mapping phase (April 1994–March 1995) prompted the U.S. Navy to declassify all of the Geosat altimeter data in June 1995. The combination of these two high‐density data sets provided the first global view of all the ocean basins at a wavelength resolution of 20–30 km.

Application of the GEM‐T2 gravity field to altimetric satellite orbit computation

As part of a continuing effort to provide improved orbits for use with existing altimeter data, we have recomputed ephemerides for both the Seasat and Geosat Exact Repeat altimeter missions. The orbits were computed in a consistent fashion, using the Goddard Earth Model T2 gravity field along with available ground‐based tracking data. Such an approach allows direct comparisons of sea level between the two altimeter systems. Evaluation of the resulting ephemerides indicates that root‐mean‐square accuracies of 30–50 cm have been achieved for the radial component of the orbits for both satellites. An exception occurs for the last year of the Geosat Exact Repeat Mission, when the rms radial orbit accuracy degrades to the 1‐m level at times owing to the inability to adequately model the drag force arising from the increased solar activity.

Ice sheet altimeter processing scheme

Abstract The analysis of altimeter data over non-ocean surfaces is complicated by the effects of topography and the dielectric properties of the surface. The philosophy behind the processing (for the U.K.-Earth Observation Data Centre) of altimeter data over ice sheets and shelves is explained and a description of the function and performance of the algorithms used is provided. The methods available for waveform retracking and slope correction, which represent the primary difference between ocean and land altimeter data analysis, are discussed in detail. Results from their operation with Seasat altimeter data and a simulated surface profile are provided to indicate their likely performance with ERS-1 data. The contents and structure of the ice sheet data product is described and its intended applications discussed. Future modifications and calibration/validation of the data are also considered.

Grim4-C1, -C2p: combination solutions of the global earth gravity field

On the basis of the GRIM4-S1 satellite-only Earth gravity model, being accomplished in a common effort by DGFI and GRGS, a combination solution, called GRIM4-C1, has been derivcd using 1° × 1° mean gravity anomalies and 1° × 1° Seasat altimeter derived mean geoid undulations. In the meantime improvements could be achieved by incorporating more tracking data (GEOSAT, SPOT2-DORIS) into the solution, resulting in the two new parallel versions, the satellite-only gravity model GRIM4-S2 and the combined solution GRIM4-C2p (preliminary). All GRIM4 Earth gravity models cover the spectral gravitational constituents complete up to degree and order 50.

Collinear and cross-over adjustment of Geosat ERM and Seasat altimeter data in the Mediterranean Sea

The computation of mean sea surface heights from a set of collinear Geosat ERM altimeter data tracks was carried out in a collinear adjustment, where 1 cy/rev cosine and sine coefficients for each track are estimated, so the differences between the collinear tracks are minimized. Then bias/tilt cross-over adjustments of stacked Geosat and Seasat altimetry were carried out. The problems with the free surface in the cross-over adjusted altimetric surface were treated using the absolute sea surface heights relative to the geoid model OSU89B. In a combined adjustment of the two altimeter data sets using cross-over and height informations simultaneously a RMS of the cross-overs of 0.080 m and a RMS of the sea surface heights relative to OSU89B of 0.611 m were obtained.

Deformation of the oceanic crust between the North American and South American Plates

Fracture zone trends and magnetic anomalies in the Atlantic Ocean indicate that the North American plate must have moved with respect to the South American plate during the opening of the Atlantic. A comparison of plate tectonic flow lines with fracture zones identified from Geosat and Seasat altimeter data suggests that the North American‐South American plate boundary migrated northward from die Guinea‐Demarara shear margin to the Vema Fracture Zone before chron 34 (84 Ma), to north of the Doldrums Fracture Zone before chron 22 (51.9 Ma), and to north of the Mercurius Fracture Zone between chron 32 (72.5 Ma) and chron 13 (35.5 Ma). The paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area, indicate that the triple junction may have been located between the Mercurius and the Fifteen‐Twenty fracture zones after 67 Ma (chron 30). Plate reconstructions indicate a Late Cretaceous phase of transtension, followed by transpression in the Tertiary for the Tiburon/Barracuda Ridge area south of the Fifteen‐Twenty Fracture Zone. The ocean floor in this area is characterized by a series of ridges and troughs with large Bouguer gravity anomalies (up to ∼135 mGal). We use smoothing spline estimation to invert Bouguer anomalies for crustal layer structure. Our model results suggest that the Moho is uplifted 2–4 km over short wavelengths (∼70 km) at the Barracuda and Tiburon ridges and imply large anelastic strains. The severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted. We propose that the North‐South American plate boundary migrated to the latitude of the Tiburon Ridge, bounded by the Vema and Marathon fracture zones, before chron 34 (84 Ma). Post‐chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the Vema, Marathon, Mercurius, and Fifteen‐Twenty fracture zone troughs, but reaching the Fifteen‐Twenty Fracture Zone and future Barracuda Ridge area only after chron 32 (72.5 Ma). This interpretation concurs with our crustal structural model, which shows stronger crustal thinning underneath the Tiburon Ridge than at the Barracuda Ridge. Subsequent transpression may have continued along the existing zones of weakness in the Tertiary, creating the presently observed crustal deformation and uplift of the Moho, accompanied by anelastic failure of the crust. Middle‐Eocene‐Upper Oligocene turbidites on the slope of the Tiburon Ridge, now located 800 m above the abyssal plain, suggest that most of its uplift occurred at post‐Oligocene times. The unusually shallow Moho underneath the Tiburon and Barracuda ridges represents an unstable density distribution, which may indicate that compressive stresses are still present to maintain these anomalies, and that the North American‐South American plate boundary may still be located in this area.

Assimilation of altimeter data in a global third‐generation wave model

We investigated the effect of the assimilation of altimeter satellite data in the third‐generation ocean wave model WAM. We used a sequential method, where analyzed significant wave height fields are created by optimum interpolation, and the analyzed values are then used to construct the analyzed wave spectrum. The method provides also an estimate of the surface stress showing the possibility of using the analysis of the wave spectrum to derive an analyzed surface stress field. In a first set of numerical experiments, the data, provided by the Seasat altimeter, have been assimilated in the WAM model for 1½ days. The comparison between model results and satellite data during the continuation of the run shows a positive and persistent impact of the assimilation. In a second set of numerical experiments, Geosat altimeter data were assimilated for 10 days and the resulting analysis was compared with buoy data. Although the assimilation improves the model results, it is not capable of compensating the differences between model and buoys. Some failures are clearly derived from the absence in the satellite data of the high‐wave events that were reported by the buoys. Other failures may be the consequence of an excessive swell attenuation in the WAM model, which compromises the effect of a previous correction. In fact, the comparison of WAM model results with altimeter data suggests that there is a tendency of the model to overevaluate initially the wind sea, and successively to overestimate the decay of the wave energy, when the waves leave the area of the storm.

Validation and assimilation of Seasat altimeter wave heights using the WAM wave model

The mutual consistency of the Seasat global data sets of scatterometer winds and altimeter wave heights is investigated for the complete Seasat period using the third‐generation wave model WAM. The wave model was driven by surface (1000 hPa) wind and surface stress fields constructed by the Goddard Laboratory for Atmospheres (GLA) by assimilation of the scatterometer winds in an atmospheric model. For the 10‐day period September 7–17 the intercomparison was extended to two further scatterometer wind fields: a 1000‐hPa assimilated wind field from the European Centre for Medium‐Range Weather Forecasts and a subjectively analyzed 19.5‐m‐height surface wind field from the Jet Propulsion Laboratory. On the global average, the modeled and observed wave heights agree reasonably well. Regional differences, however, can be large and sometimes exceed 40%. The errors are attributed mainly to deficiencies in the forcing wind fields. Low wind speeds are found to be overestimated and high wind speeds underestimated by the Seasat scatterometer algorithm. The friction velocities of the GLA model are found to be significantly underestimated in the high‐wind belt of the southern hemisphere. The analysis demonstrates the diagnostic advantages of applying a wave model for the quality assessment of satellite wind and wave data. A preliminary wave data assimilation scheme is presented in which the wave field is updated without changing the forcing wind field. A considerable improvement of the computed wave field is achieved, particularly in regions in which the wave energy is dominated by swell. However, a more general assimilation scheme including modifications of the wind field is needed to upgrade wind sea forecasts.

Evidence of Time-dependent Sverdrup Circulation in the South Pacific from the Seasat Scatterometer and Altimeter

Abstract Seasat scatterometer and altimeter data are analyzed to investigate time-dependent Sverdrup dynamics in the Southern Ocean (40°S to 60°S) over seasonal time scales. Sverdrup dynamics are shown to be inadequate to describe the circulation in the South Atlantic and Indian oceans. The Sverdrup circulation in the South Pacific is reasonable north of 55°S. The changes in Sverdrup circulation from July to September 1978 indicate an eastward acceleration along 55°S and westward acceleration along 40°S, suggesting a southward shift in the subpolar eastward flow. Sea level in the South Pacific is estimated for July and September 1978 from scatterometer vector wind data based on Sverdrup dynamics assuming a flat-bottom ocean with barotropic flow. The changes in Sverdrup sea level are compared with the changes in sea level observed by the altimeter for the same time period. Both estimates indicate a rise in sea level along a zonal band centered at about 50°S. This sea level rise inferred from both the scatt...

A global mean sea surface based upon GEOS 3 and Seasat altimeter data

A mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS‐9012, has been computed on a grid of 1/8° resolution. Each elevation value was calculated by fitting all data within 111 km to a local biquadratic surface using Bayesian least squares. Individual data points were weighted inversely to the square of the distance to the grid location in the gridding process. The surface covers the global ocean between 70°N and 72°S. In the vicinity of sea ice the altimeter heights have been corrected for the on‐board tracker error that occurs over non‐Gaussian surfaces. Comparisons are made between MSS‐9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data.

Satellite altimetry reveals seafloor tectonic fabric

During the 1980s satellite radar altimetry matured as a geophysical technique, quickly becoming the best way to map the gravity field of the world's oceans. Maps derived from SEASAT altimeter observations of sea surface topography (or geoid) revealed much that was previously unknown about the marine gravity field [Haxby, 1987] and underlying seafloor, particularly in the vast areas that are poorly charted by traditional shipboard observations.Seafloor topography, and to a lesser extent, crustal/upper mantle density variations, produce the short wavelength (<200 km) undulations in the marine gravity field. This is simply due to the gravitational attraction of the mass in the seamount or similar structure emplaced on the seafloor. For topographic structures beneath the sea that are 20‐200 km in spatial extent, this attraction causes the gravity anomalies at the sea surface to vary in proportion to the relative height of the underlying structures. (This ignores isostasy or internal deformation of the Earth's crust and mantle, which acts to reduce this gravitational effect particularly over long‐wavelength—greater than 200 km—topography.) The ratio between gravity anomalies and height variations is slightly less than 0.07 milligals per meter (1 mGal equals 10‐5 m/sec2). Therefore, the fine structure in these marine gravity fields reflects the tectonic fabric imprinted in the seafloor. This fabric includes the mid‐ocean ridge axes (where tectonic plates form), fracture zones, which record the directional history of seafloor spreading, seamount chains, rifted continental margins, and other features that chronicle the history of relative motion among the tectonic plates over the past 200 million years.

Ocean modelling studies inspired by the Seasat altimeter

Abstract Some recent ocean modelling studies are reviewed which illustrate the ways in which dynamic ocean circulation models can be used to advise on the optimum deployment of, and help analyse the data from, future satellite altimeters for studies of ocean dynamics. At the time of the launch of Seasat the ocean modelling community were unaware of the promise of altimetry. It is a tribute to the enormous success of the Seasat mission that now, ten years on, the modelling community eagerly awaits the launch of a new generation of altimeters.

A Geosat altimeter wind speed algorithm and a method for altimeter wind speed algorithm development

A Geosat altimeter wind speed algorithm is derived by cross‐calibrating Geosat and Seasat altimeter estimates of the normalized radar cross section σ0 and modifying an existing Seasat altimeter wind speed model function to obtain a model function appropriate for Geosat observations. It is argued that the σ0 distribution measured by an altimeter is relatively stable over a sufficiently large geographical region and a long enough time period. Systematic differences between σ0 estimates from two altimeters can therefore be identified based on comparisons of their σ0 histograms. Any such systematic differences can then be corrected using independent σ0 estimates. When this method is applied to the Geosat and Seasat altimeters, a systematic difference between the two σ0 histograms is shown to be consistent with differences between Seasat altimeter and nadir Seasat scatterometer estimates of σ0 deduced independently by a previous study. This supports the conclusions that (1) the σ0 distribution is stable, and (2) the Seasat altimeter estimates of σ0 were miscalibrated. After modifying the existing Seasat altimeter wind speed algorithm to account for this apparent σ0 error, the resulting Geosat estimates of wind speed agree with high‐quality buoy observations to within an rms difference of less than 2 m/s.

An assessment of the capability of the satellite radar altimeter for measuring ice sheet topographic change

Abstract The monitoring of surface topography has frequently been cited as the most important application of the satellite radar altimeter over the ice sheets. The potential of the instrument in this respect is examined in detail using Seasat and Geosat altimeter data recorded over the Wilkes plateau in East Antarctica. The precision of the range measurements is optimized by the use of an improved retracking technique. However, although the r.m.s. precision of range measurements indicated at orbit crossovers, after orbit adjustment, is 20-30 cm, tilt and bias orbit adjustment techniques are shown to leave errors in the measurement of surface elevation change, with magnitudes of the order of tens of centimetres, depending on the location. Additional orbit error, which cannot be identified from crossover residuals, is almost certainly present in the difference measurements. Variable mispointing of the Geosat antenna is considered the most likely explanation for an apparent change in surface scattering prope...

Simultaneous Estimation of the Gravity Field and Sea Surface Topography From Satellite Altimeter Data By Least-Squares Collocation

SUMMARY The signal content of Seasat altimeter data is evaluated with respect to magnitude and wavelength. Based on this information covariance functions associated with the different signals/errors have been determined. the covariance function associated with the sea surface heights contains covariances associated with the geoid, the stationary sea surface topography, and the time-varying sea surface topography. These three components have been estimated from Seasat altimeter data in the Faeroe Islands region using least-squares collocation. In the solution error covariances associated with errors in the orbits, corrections for instrumental, atmospherical, and tidal effects, and sea state related bias are taken into account. the accuracies of the solutions are evaluated by computing error estimates and error correlations. A special effort has been made to evaluate the correlations between the estimated geoid and the other signals/errors. the results show that geoid undulations may be estimated with an accuracy of 0.13–0.16m. Approximately 50 per cent of the error variance has a long-wavelength signature, which is mainly correlated with errors in the estimation of the permanent sea surface topography. the short-wavelength part of the error is mainly correlated with errors due to liquid water in rain and clouds. Free air gravity anomalies may be estimated with an accuracy of 8.8–11.7 times 10-5 ms-2. This error has a short-wavelength signature which is mainly correlated with errors due to liquid water in rain and clouds. However, a comparison with gravity observations resulted in an agreement of 7.8 times 10-5 ms-2. Both the permanent and the time-varying sea surface topography may be estimated with accuracies of about 0.21 m.

A comparison between active and passive microwave measurements of the Antarctic ice sheet and their association with the surface katabatic winds

AbstractThe intensity of the Seasat altimeter return power over Antarctica varies in strong correlation with the intensity of model katabatic winds. It is also strongly correlated with the polarization of the passive microwave signal at 37 GHz of the Nimbus-7 SMMR data. It is shown that this is most likely the result of the wind-induced micro-roughness of the ice surface.

A comparison between active and passive microwave measurements of the Antarctic ice sheet and their association with the surface katabatic winds

AbstractThe intensity of the Seasat altimeter return power over Antarctica varies in strong correlation with the intensity of model katabatic winds. It is also strongly correlated with the polarization of the passive microwave signal at 37 GHz of the Nimbus-7 SMMR data. It is shown that this is most likely the result of the wind-induced micro-roughness of the ice surface.