Geosat Altimetry Research Articles

The Determination of Large‐Scale Sea Surface Topography and its Variations Using Geosat Altimetry

The Determination of Large‐Scale Sea Surface Topography and its Variations Using Geosat Altimetry

Estimation of the Sea Surface Topography off Mid‐Norway Using Geosat Altimetry and Sea‐Gravity Data

Estimation of the Sea Surface Topography off Mid‐Norway Using Geosat Altimetry and Sea‐Gravity Data

Seasat and Geosat Revisited: Using Sea Level Measurements to Improve Satellite Altimeter Orbits

Global maps of sea surface height can be reconstructed by fitting patterns of satellite altimeter-derived sea level variability to historical tide gauge measurements. The accuracy of these maps is uncertain because of the limited sampling available in the historical record. Reconstructed maps from August 9, 1978 and August 9, 1987 are composed with maps derived from 17-day repeat orbit sampling provided by Seasat and GEOSAT altimetry, respectively, to evaluate the fidelity of the sea level reconstructions. Given sufficiently accurate reconstructed time series, it is shown that it is possible to improve orbit error filtering procedures to better account for oceanographic signals. This could improve the accuracy of the Seasat and GEOSAT orbits and the utility of the altimeter measurements for ocean and ice studies relying on these unique historical altimetric records.

Gravity Anomalies from Retracked ERS and Geosat Altimetry over the Great Lakes: Accuracy Assessment and Problems

Lake waters are a useful area for investigating the response of the Earth's gravity field to topographical density variations, since lake beds represent large density contrasts between water and rock. However, our ability to investigate relations between lake bathymetry and gravity field effects is limited because of relatively sparse gravity data coverage over most lake surfaces, and the inability of satellite-derived gravity fields to capture the high frequency response to density effects. Satellite altimetry may provide an alternate method to determine a dense gravity field at surfaces of large inland lakes. We calculate satellite altimetry derived free-air (FA) anomalies over the Great Lakes in Canada, and evaluate them in terms of resolution and accuracy. The anomalies are compared to those calculated from the GRACE-derived EIGEN-GL04C gravity field to test for bias, and from shipborne or airborne results (where available) to test the accuracy of their determination of the high frequency component of the gravity field. Our results show that accuracies may reach less than 10 mgal, both in terms of bias and higher frequency effects, but are often tens of milligals. Furthermore, we find that altimetry does provide a higher resolution gravity field than satellite-derived gravity fields, and so if the accuracy can be improved in later efforts, it will be a useful tool for investigations of topographical density effects of lake waters.

Evolution of the Lofoten‐Vesterålen margin inferred from gravity and crustal modeling

The Lofoten‐Vesterålen continental margin in Norway shows pronounced elongated positive and negative gravity anomalies. The source of these anomalies cannot be explained by the flat bathymetry, but can be explained by the crustal density anomalies that are observed. We propose a boudinage mechanism to explain the generation of these density anomalies and respective crustal layer undulations. Free‐air gravity anomalies have been derived using ERS‐1 and Geosat altimetry data and have been validated by comparison with shipborne data. Structural models of the margin are available from extensive seismic surveys, which have been used to carry out a gravity inversion to cross‐validate the observed gravity anomalies with seismic data. The crustal structure data reveal two wavelengths of crustal undulations. The first is related to crustal blocks separated by listric faults, and the second wavelength is related to an undulated Moho. The undulations were likely generated during the opening of the North Atlantic, 56 Myr ago, and were particularly formed during the transition phase between rifting and spreading. The dominating wavelengths of the undulations are 46 and 109 km, respectively. To explain the evolution of crustal undulations in an extensional stress regime, we use a two‐dimensional finite element model. A viscoelastoplastic power law rheology is used to study the growth of instabilities developing in a three‐layer lithosphere. Prescribed random vertical undulations show wavelength‐dependent growth under extension, and the dominating wavelengths are 18–65 and 129 km, respectively. These results are in good agreement with the gravity observations and provide a strong indication that the proposed boudinage mechanism influences the evolution of the passive margin segment.

Long-Term Thinning of the Southeast Greenland Ice Sheet From Seasat, Geosat, and GFO Satellite Radar Altimetry

Geosat Follow On (GFO) radar altimeter data are processed with previous altimeter datasets to measure long-term elevation change of higher elevation portions of the southern Greenland ice sheet over time periods from 1978-1988, 1985-2002, and 1978-2002. Average elevation change results indicate approximately zero overall elevation change for all time periods. The results also indicate that upper-elevation thinning in southeast Greenland has been widespread and has existed for several decades.

Cross-calibration of TOPEX/Poseidon, ERS-2 and GEOSAT altimetry

Long-term monitoring of the NASA (TOPEX) altimeter bias drift onboard TOPEX/Poseidon has been undertaken by comparison against in-situ tide gauge data and used as a benchmark for cross-calibration of the concurrent altimeter mission ERS-2 and of the earlier GEOSAT mission. Dual crossovers between ERS-2 and TOPEX/Poseidon provide a measure of the relative bias along with other parameters defining differences between the altimetric data sets. Attempts to link GEOSAT with the missions in the 1990s have relied on tide gauge time series with the assumption that geographically correlated errors in GEOSAT have zero effect. In this study we utilise GEOSAT crossovers and tide gauge enhanced altimetry between TOPEX/Poseidon and GEOSAT to estimate a 1 cy/rev correction simultaneously with the relative altimeter bias. Evidence of improvement is shown by comparison of the GEOSAT altimetry against the EGM96 mean sea-level.

Global ocean tides from Geosat altimetry by Quasi-Harmonic analysis

Global ocean tides data were derived from Geosat altimeter data by means of the Quasi-Harmonic Constituent Method (QHCM). Tidal solutions with resolution of 1°/3 in longitude and latitude were obtained for constituentsM 2,S 2,O 1,K 1,M 4 andMS 4. The means sea heights above the reference ellipsoid were also obtained consequently. The obtained tidal constants were compared with those from deepsea and island tide gauge data. The rms differences between the harmonic constants derived from Geosat altimetry and deep-sea tide gauges forM 2,S 2,O 1 andK 1 ranged from 1.4 cm to 2.6 cm, although the GM altimeter data have significant errors due to instrument malfunction and other reasons.M 2 tide obtained was the most accurate one among all the tides. Comparison also showed that island tidal constants cannot represent well the tidal distribution in the ocean near the island, because of the significant local effect on tides.

Ocean tides from T/P, ERS-1, and GEOSAT altimetry

Aliasing of the diurnal and semi-diurnal tides is a major problem when estimating the ocean tides from satellite altimetry. As a result of aliasing, the tides become correlated and many years of altimeter observations may be needed to seperate them. For the three major satellite altimetry missions to date i.e., GEOSAT, ERS-1, and TOPEX/POSEIDON (T/P), the alias periods as well as the Rayleigh periods over which the tides decorrelate can be identified. Especially in case of GEOSAT and ERS-1, severe correlation problems arise. However, it is shown by means of covariance analyses that the tidal phase advance differences on crossing satellite groundtracks can significantly reduce the correlations among the diurnal and semi-diurnal tides and among these tides and the seasonal cycles of ocean variability. Therefore, it has been attempted to solve a multi-satellite response tidal solution for the diurnal and semi-diurnal bands from a total of 7 years of altimetry. Unfortunately, it could be shown that the GEOSAT and ERS-1 orbit errors are too large to improve a 3-year T/P tidal solution with about 2 years of GEOSAT and 2 years of ERS-1 altimeter observations. However, these results are preliminary and it is expected that more accurate orbits, which have become available recently for ERS-1, and additional altimeter data from ERS-2 and the GEOSAT Follow-On (GFO) should lead to an improved T/P tidal model.

Orthogonal intersections of megatrends in the Western Pacific ocean basin: a case study of the Mid-Pacific mountains

Present-day tectonic concepts of events on the western paleo-Pacific lithosphere must be assessed relative to new data. Data collected by the newer techniques of geophysical surveying reveal leaky fracture zones, trending NNW–SSE and WSW–ENE; non-sequential in-line ages on most seamount chains; and orthogonal intersections of fracture zones. The fracture zones meander, braid, merge, splay, start and stop at any place, and are generally aligned with, or contain, linear chains of seamount. The combination of these in-line features is called megatrends. When the GEOSAT data are compared to the available bathymetry, this seemingly jumbled tectonic structure is verified. As the pole of rotation changes, the stress field changes alignment to agree with the Chandler wobble of Earth. Younger megatrends, propagating ever eastward, cross the older, already imprinted megatrends. During the Cretaceous, the voluminous outpouring of igneous rock created the large Pacific plateaus and rises where the megatrends, active and inactive, orthogonally intersected. The magma floods at the intersections flowed outwardly, and the outward flooding accounts for the fanning magnetic lineations around the Manihiki, Magellan, Shatskiy, and Hess rises. A case study of the Mid-Pacific Mountains (MPM), lying in the north-central Pacific Ocean basin, shows that the MPM formed about 125–110-Ma by overprinting the orthogonal intersections of fracture zones at the Molokai and Easter/Krusenstern–Emperor megatrends and the Murray and Tubai/Mamua megatrends. The MPM have been undergoing distortion into a vortex structure, a feature which has been confirmed by updated bathymetry, GEOSAT altimetry data, and drillsite information.

Annual and Intra-annual Sea Level Variability in the Region of the Kuroshio Extension from TOPEX/POSEIDON and Geosat Altimetry

Abstract Using 4.2 years of sea surface height data collected by the TOPEX/POSEIDON altimeter and 2.3 years of data collected by the Geosat altimeter, the authors study the annual, intra-annual, and linear-trend sea level height variability in the region of the Kuroshio Extension. Both the annual and intra-annual variabilities are separated spatially into large (≥2000 km), intermediate (∼1500 km), and short scales (∼800 km). The intermediate and short-scale annual and intra-annual sea level height fluctuations in both periods have large components with two-dimensional phase propagation. Close correspondence between the characteristic spatial structure of both the intermediate and short-scale annual and intra-annual sea level height fluctuations and major bottom topographic features suggests that bottom topography plays a critical role in the generation of those low-frequency variabilities. On the other hand, large-scale annual variability is primarily a standing oscillation, and bottom topography does not...

Elevation changes on the East Antarctic ice sheet, 1978-93, from satellite radar altimetry: a preliminary assessment

Radar altimeter data from Seasal (1978), Geosat (1985-88) and ERS-1 (1991—93) are employed to estimate multi-year mean changes of the surface height throughout a region on the East Antarctic ice sheet (EAIS) extending to 72.1° S, the southernmost limit of coverage for Seasat and Geosat altimetry, and above 1500 m elevation, using orbit crossover analysis. The changes are estimated on a same-season (austral late-winter (ALW) toALW) basis, where ALW is the 10 July 9 October time-frame of the Seasat altimetry. Altimeter data corrected for slope-induced errors are used. Altimeter data not corrected for slope-induced errors are also used, for comparison. Intersatellite orbit bias, combined with the effect of other radial errors such as instrumental bias, is estimated using crossover differences on the offshore ALW sea ice, which is employed as a geoid-parallcl reference surface. If similar intersatellite radial biases are characteristic of the continental Antarctic ice-sheet altimetry to 72.1° S, the results of all crossover analyses adjusted for this intersatellite bias — suggest that the mean rate-of-change of the surface height between Seasat and Geosat for ALWs 1978 to 1986-88 was with in the range +11 to -11 mm a−1. The bias-adjusted results of all crossover analyses between Seasat and ERS-1 suggest that the mean rate-of-change of the surface height between ALWs 1978 and 1991-93 was with in the range-17 to-55mma−1 (maximum intersatellitc bias estimate) or 0 to -40 mm a−1 (minimum bias estimate), suggesting that the surface may have lowered slightly during this time interval. The inconsistency of the adjusted Seasat to Geosat vs Seasat to ERS-1 results, however, may be an indication that orbits more accurate than JGM-2 are needed for estimation of regional multi-year mean changes of elevation on the EAIS. Alternatively, it may be a reflection of the differing orbit inclinations of Seasal and ERS-1.

Elevation changes on the East Antarctic ice sheet, 1978-93, from satellite radar altimetry: a preliminary assessment

Radar altimeter data from Seasal (1978), Geosat (1985-88) and ERS-1 (1991—93) are employed to estimate multi-year mean changes of the surface height throughout a region on the East Antarctic ice sheet (EAIS) extending to 72.1° S, the southernmost limit of coverage for Seasat and Geosat altimetry, and above 1500 m elevation, using orbit crossover analysis. The changes are estimated on a same-season (austral late-winter (ALW) toALW) basis, where ALW is the 10 July 9 October time-frame of the Seasat altimetry. Altimeter data corrected for slope-induced errors are used. Altimeter data not corrected for slope-induced errors are also used, for comparison. Intersatellite orbit bias, combined with the effect of other radial errors such as instrumental bias, is estimated using crossover differences on the offshore ALW sea ice, which is employed as a geoid-parallcl reference surface. If similar intersatellite radial biases are characteristic of the continental Antarctic ice-sheet altimetry to 72.1° S, the results of all crossover analyses adjusted for this intersatellite bias — suggest that the mean rate-of-change of the surface height between Seasat and Geosat for ALWs 1978 to 1986-88 was with in the range +11 to -11 mm a−1. The bias-adjusted results of all crossover analyses between Seasat and ERS-1 suggest that the mean rate-of-change of the surface height between ALWs 1978 and 1991-93 was with in the range-17 to-55mma−1 (maximum intersatellitc bias estimate) or 0 to -40 mm a−1 (minimum bias estimate), suggesting that the surface may have lowered slightly during this time interval. The inconsistency of the adjusted Seasat to Geosat vs Seasat to ERS-1 results, however, may be an indication that orbits more accurate than JGM-2 are needed for estimation of regional multi-year mean changes of elevation on the EAIS. Alternatively, it may be a reflection of the differing orbit inclinations of Seasal and ERS-1.

Gulf Stream and Ring Feature Analyses for Forecast Model Validation*

Abstract A series of Gulf Stream forecast model test cases were developed for the Data Assimilation and Model Evaluation Experiment (DAMEE). The model initialization and verification procedure relies heavily on a series of accurate synoptic snapshots of the Gulf Stream north wall and ring locations. Satellite infrared imagery, Geosat altimetry, and numerous in situ temperature profiles were combined using a geographic information system to construct Gulf Stream and ring location analyses at approximately weekly intervals during a 6-week, data-rich time period. To improve the accuracy of the feature analyses, a new image compositing technique called patching was developed to decrease the spatial smearing experienced with standard warmest pixel composites. During the 6-week test period, three ring formations, two ring absorptions, and one ring merger event were observed. The average difference between the weekly north wall positions ranged from 20 to 34 km (average 27 km). When the DAMEE GSR north wall posi...

The hydrography and circulation of the upper 1200 meters in the tropical North Atlantic during 1982–91

We assemble a collection of 7591 conductivity-temperature-depth stations in the tropical Atlantic between 5S and 20N for the period 1982-1991 using data from the Soviet SECTIONS program enhanced by contributions primarily from the WESTRAX and FOCAL/SEQUAL programs. Most of the stations are west of 30W, forming a series of 21 surveys. In addition there were five multi-ship basinwide surveys, each taking 1.5-3 months to complete. The quality of the SECTIONS data is discussed and comparisons between the data sets are shown. Within the pycnocline, southern water is distinguished by salinity that is 0.3 psu lower than its northern counterpart at the same density. This difference allows us to distinguish the origin of pycnocline water. Based on this information together with geostrophic analysis we confirm that much of the water transported across the equator in the North Brazil Current retroflects eastward south of 8N. In summer and fall the water of the North Brazil Current merges with the water of the North Equatorial Current to form the North Equatorial Countercurrent, whose axis shifts southward with depth. East of 35W part of the mass transport in the Countercurrent is lost to the equatorial zone, while the rest continues eastward. During winter and spring eastward currents are found in two latitude bands, a surface current between 5-10N and a weaker current south of 5N confined to the thermocline. This latter North Equatorial Undercurrent has no surface expression in winter. During the summer and fall the northern boundary of Southern Hemisphere water at pycnocline depths is given by the North Equatorial Countercurrent. During the rest of the year the northern boundary of Southern Hemisphere water only penetrates to the edge of the weaker North Equatorial Undercurrent south of 5N. The availability of ten years of data allows us to examine aspects of year-to-year variability. Among these results the data set reveals strong meandering of the North Equatorial Countercurrent between 42W and 35W during the summer of 1987. The meandering also appears in contemporaneous Geosat altimetry. Another unusual feature occurred in the summer 1986 when there was a strong thermocline current transporting water northward at the rate of 26 Sv. Water mass analysis shows that this current was the result of a pressure gradient set up by an intrusion of warm low-salinity water from the Southern Hemisphere. The fact that this high transport occurred in the interior ocean suggests that interior flows must be monitored during any attempt to observe meridional transports of mass or heat.

Filtered Geosat altimetry as a tool for global bathymetric chart development

Now that the Geosat altimetry and precise orbit information collected during the 1985–1986 Geodetic mission is declassified, the Naval Oceanographic Office is releasing its chart of Geosat altimetry of the world's oceans (Figure 1). This data set has been used at the Naval Oceanographic Office since 1987.

Oceanic gravity by analytical inversion of Hotine's formula

An analytical inversion of the Hotine formula is developed using fast Fourier transform techniques. Detailed mathematical derivations are used to explain the concepts behind the inverse transformation. Three modifications of the analytical inversion of the Hotine formula are compared and tested using both synthetic data from the OSU91A geopotential model and real GEOSAT altimetry data from the Exact Repeat Mission. The stability of this inverse Hotine approach is investigated using simulated data, and numerical tests are done to quantify the stability of this approach. The approach seems to be numerically stable without employing any stabilization technique. Estimated gravity information from GEOSTAT altimetry data is compared to marine gravity data from shipboard measurements in the Orphan Knoll area. The standard deviations and mean values of the differences between satellite and marine gravity disturbances are 8.2 and 2.9 mGal for the planar approximation, 9.2 and 3.7 mGal for the spherical approximati...

Estimating the sea surface dynamic topography from Geosat altimetry data

Optimal interpolation method is applied to Geosat altimetry data both to remove orbit error and to separate temporal mean sea surface dynamic topography (SSDT) from temporal fluctuations around the mean. Loss of long-wavelength oceanic signals at orbit error reduction procedure is smaller in this method than that in conventional collinear methods, but the areal average height over the study domain is still removed as the orbit error. The fluctuation SSDT is quantitatively evaluated by sea level data from tide gauge stations at Japanese islands. The correlation coefficient of the two sea-level variations is 0.83 when the loss of the areal average is compensated by the seasonal variation of the areal average height determined from the climatological monthly-mean SSDT. In addition, the improvement of the geoid model by combined use of Seasat altimetry data and hydrographic data is validated through the estimated temporal mean SSDT. In a local area where hydrographic data contemporary with the Seasat mission exist, the geoid model has been significantly improved so that the absolute SSDT can be determined from combination of the altimetry data and geoid model; the absolute SSDT describes the onset event of a quasi-stationary large meander of the Kuroshio south of Japan very well. Outside this local area, however, errors of several tens of centimeters still remain in the improved geoid model.

Improved resolution of Geosat altimetry using dense sampling and polynomial adjusted averaging

Abstract The altimeter data processing technique presented here improves the spatial wavelength resolution of Geosat ERM by smoothing the 10 Hz data, using measurements of the wave height and averaging repeated (collinear) altimeter measurements. For each repeated track, the residuals between its height profile and the mean height profile were modelled with a third degree polynomial and then subtracted from the data and input in a new averaging. The 10 Hz sampling rate is shown to contain a greater portion of short-wavelength signal than the standard 1 Hz data format. The effective precision is determined to be 1-5 cm, and the along-track wavelength resolution limit is determined to be 19km.

Near-equatorial eddies off South America

A considerable amount of the Amazon River water that is discharged into the equatorial Atlantic is then advected northward along the shelf by the strong North Brazil Current (NBC). Being relatively fresh, this water remains in the near-surface layer and can serve as an excellent tracer for the complex and variable flow of the offshore mesoscale eddies. Both surface salinity observations and CZCS (Coastal Zone Color Scanner) imagery can be mapped to estimate the circulation patterns of the eddies. Presented here are two sets of XBT (expendable bathythermograph) sections that give the thermal structure of eddies off the Demerara Rise (6–9°N). They were occupied nearly contemporaneously with CZCS imagery obtained during October 1980 and November 1981. Several studies have shown from ship drift data, from CZCS observations, and from Geosat altimetry that, particularly during late summer and fall, the NBC is found to retroflect offshore to the east, supplying the North Equatorial Counter Current (NECC) and is associated with eddies along the coast. Good agreement is shown between the CZCS and a NAVOCEANO AXBT (airborne bathythermograph) survey during this period as well as observations of surface phytoplankton and geopotential anomaly from an earlier 1964 survey. Estimates of volume transport within the eddy structure indicate that at times the offshore retroflection during spring as well as autumn can amount to 10 × 106 m3 s−1. The pronounced eddy variability off South America is shown by comparing a number of past hydrographic surveys. These suggest that considerable spatial variability can occur as can seasonal changes in volume transport of the NBC and the associated eddy circulation.