The remote-sensing satellite ERS-1, launched in 1991 to study the Earth's environment, was placed on a geodetic (168-day repeat) orbit between 1994 April and 1995 March to map, through altimetric measurements, the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along-track and cross-track directions. We have analysed the precise altimeter data of the ge...

Variations of the 10‐day‐averaged sea level extracted from TOPEX/POSEIDON (T/P) altimeter data are examined from October 1992 to February 1995. Data are corrected from instrumental drifts, and recent precise orbit and ocean tide corrections are used. When data from the less accurate first eight cycles are excluded and atmospheric pressure effects are not corrected, the 10‐day mean sea level varies by 5.0 mm rms. Its variations can be described by a 4.1±1 mm/yr drift, a small annual signal of 2.7 mm amplitude and a residual signal of 3.5 mm rms amplitude, and Gaussian statistics close to formal error estimates. The mean sea level rise is difficult to explain by drifts of geophysical corrections, though the case for the wet tropospheric correction and that for the orbit are uncertain. Pressure effects should be corrected relative to the global pressure averaged over the ocean. The latter shows a seasonal signal of 1 cm amplitude and apparent fluctuations of 30‐ to 70‐day periods. These fluctuations could result from errors in the atmospheric pressure fields, but they seem to be mostly related to large regional pressure evolutions. Evidence is also found for a sampling effect by the T/P orbit of pressure signals at shorter than 10‐day periods. Pressure effects on sea level are then assumed to be an inverse barometer relative to 10‐day global atmospheric pressure over the ocean, which allows analysis of the geographical repartition of the mean sea level evolution due to other effects. The mean sea level rise is comparable in the northern (4.2±2.5 mm/yr) and southern (3.7±1.9 mm/yr) hemispheres. It seems mostly related to interannual variations of the tropical ocean (41% of the global signal) and of the northern hemisphere subtropical gyres (30% of the global signal); residual noise on these regional values seems inversely proportional to the square root of the number of data points in each area. The hemispheric seasonal pressure variation relative to the globally averaged pressure over the ocean is less than 3 mm amplitude; in our calculation it is implicitely assumed that it induces a small interhemispheric water mass exchange. The amplitudes of the seasonal hemispheric sea level variations are 27 and 18 mm in the northern and southern hemispheres, respectively. These amplitudes are very close to being inversely proportional to the ocean surface in each hemisphere. The maxima of sea level in these hemispheres are reached in the last weeks of September and March, respectively. At subpolar latitudes, semiannual components are observed in each hemisphere, which are consistent with those of sea surface temperature.

We analyze the Geosat altimeter data off the mouth of the Amazon river. Variations of the sea level of up to 6 m are observed, which are correlated with bathymetry, and dominated by a complex tidal pattern. M2, N2 and O1 altimetric cotidal maps are constructed. Their amplitudes decrease strongly off the river mouth, probably by dissipation in the estuary. Strong maxima are found on both sides of the river. In particular, a M2 amplitude of 2.9 m is found near 2°N–50°W, just north of Cape Norde. These results are in very good agreement with in situ tide gage data as well as with a preliminary analysis of Topex-Poseidon altimeter data. The models of Schwiderski (1980, Review of Geophysics and Space Physics, 18, 243–268) and Cartwright et al. (1991, NASA Technical memorandum, No. 104544) are out of phase with these observations, probably because of their low resolution. The M2 phase signal is rather well reproduced by the high resolution finite element tidal model of the north Atlantic Ocean, developed for the correction of Topex-Poseidon data. However, in this model, amplitude is too low off the river mouth, and the maximum north of Cape Norde is absent. This is probably due to inaccurate bathymetry in the model.

Using the altimeter data from the Geosat Exact Repeat Mission, we have produced yearly averaged mean profiles and a global mean sea surface. The radial error of each 6‐day orbital arc computed with the GEM‐T2 geopotential was first estimated by calculating the amplitude and phase of the nine dominant frequencies of the difference between the altimetric profiles and the mean sea surface obtained when adding the permanent sea surface topography (computed from the Levitus' Climatological Atlas) to the GEM‐T2 geoid. We show that this operation is little affected by the choice of the geoid or by its formal error. The resulting correction has been subtracted from each individual arc. Yearly mean profiles were then obtained by averaging the corrected altimetric data of each repeat cycle on a yearly basis. Their noise level is 1 to 2 cm and their resolution is 20 km, but the differences of the altimetric heights at crossovers of ascending and descending tracks are still 30 cm nns. The latter can be reduced to 7 cm rms by a crossover analysis. In addition to the mean values, standard deviations were computed at each point of the repeat cycle. This “yearly along‐track variability” is of the order of 10 cm rms and is dominated by the ocean mesoscale variability. A global yearly mean sea surface has been derived by bilinear interpolation. Its resolution ranges approximately from 160 km to 80 km, depending on the latitude. It is shown to be much less noisy than those deduced from GEOS 3 and Seasat data.

A wet tropospheric correction for the Geosat altimeter data is derived from the special sensor microwave imager (SSM/I) water vapor products by sub-objective mapping. The correction presents wavenumber spectral characteristics close to those of the original SSM/I data in the 200- to 2000-km wave band, and is filtered in the short wavelength domain. In both respects, it behaves significantly better than corrections derived from 4-day averages of the SSM/I data interpolated quadratically under the Geosat track. As an example, the along-track rms mesoscale variability is lowered by 3 cm for some tracks in the northeast Atlantic Ocean when this correction is applied rather than the correction of Wentz [1990]. This suggests that it indeed improves the altimeter data.

The experience gained by analysing the Seasat altimetric observation of the ocean dynamic topography is getting more valuable as almost 10 years of nearly continuous altimeter data may be obtained by Geosat, ERS1 and Topex/Poseidon. Our group worked on all aspects of the question: the accuracy of the measurements and more particularly the improvements of the orbit determination and its error; the extraction of the dynamic topography signal both on the mesoscale and on the large scales and of tides; the analysis of the observations, whether statistically or in the form of synoptic maps, and finally, how to constrain dynamical models from altimetry data. This paper describes these various aspects of our work.

Our group has been very actively involved in promoting satellite altimetry as a unique tool for observing ocean circulation and its variability. TOPEX/POSEIDON is particularly interesting as it is optimized for this purpose. It will probably be the first instrument really capable of observing the seasonal and interannual variability of subtropical and polar gyres and the first to eventually document the corresponding variability of their heat flux transport. The studies of these phenomena require data of the best quality, unbiased extraction of the signal, mixing of these satellite data with in situ measurements, and assimilation of the whole set into a dynamic description of ocean circulation. Our group intends to develop responses to all these requirements. We will concentrate mostly on the circulation of the South Atlantic and Indian Oceans: This will be done in close connection with other groups involved in the study of circulation of the tropical Atlantic Ocean, in the altimetry measurements (in particular, those of the tidal issue), and in the techniques of data assimilation in ocean circulation models.

The objectives of this study are the following: (1) to assess the quality of the sea level observed by altimetry on a global scale, especially in tropical regions where the atmospheric effects are the most critical, and to prepare the altimeter data for their assimilation into tropical Oceanic General Circulation Models; and (2) to validate and calibrate the altimeter-derived sea state parameters, to assimilate these parameters into numerical models, and to estimate the altimeter's sea state and electromagnetic biases.

Altimeter data provide a useful complement to tide gages for monitoring the mean sea level or the continental ice topography. We discuss here how these signals can be separated from measurement errors and from other ocean signals. With existing data, the formal error which can be achieved is of the order of 5 cm rms, for a regional, yearly averaged, mean sea surface, and 50 cm rms for continental ice. These errors are likely to improve further as better orbits and corrections are made available. This suggests that secular trends will be observable when 20 years of coherent data (from SEASAT to the Polar Platforms) will be assembled.

Abstract We analyse, above continental ice, the various factors which affect the power return of the Seasat radar altimeter as measured by its Automatic Gain Control (AGC). Corrections of effects due to the AGC loop control are first applied. AGC is then normalized by positioning the half-power point at the middle of the instrument receiving window. This operation is valid for both surface and volume scattering. Over a part of Antarctica between long. 90° and 150°E., the remaining variations of AGC are of the order of 15 dB. Most of these variations occur on a large scale (>100km) and are correlated with the katabatic wind intensity. This indicates that AGC measures either surface roughness of the ice, which is related to wind intensity, or grain-size which could also be dependent on the wind. In-situ measurements support the evidence that the radar altimeter is more sensitive to surface scattering. These data could therefore provide a measurement of the intensity of katabatic winds over the continental ice.

AbstractSatellite-altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations. However, these data are affected by measurement errors, orbit errors, and slope errors. We develop here a three-step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors: it estimates first a large-scale reference surface, then maps the residuals related to undulations, and finally iteratively corrects the slope error. The method is tested on overlapping small fragments of the Antarctic ice sheet, using a sub-set of Seasat data. Finally, a topographic map of Terre Adélie is produced. Over areas of small slopes, the a posteriori error should be of the order of 0.4 m. Using ERS-I data, it is therefore expected that climatic variations in the ice-sheet topography since the introduction of Seasat will be observable.

One hundred and fifty three Geos 3 altimetry profiles have been obtained in the Indian Ocean around the Kerguelen Islands, during a limited campaign which took place from February to June 1976. A detailed geoid has been computed in this area with a resolution of 0.5°, based on accurate orbits provided by the Defense Mapping Agency (DMA). On this occasion, several algorithms have been developed and compared, which all give the same predicted values (within 1 m) at the no des of a regular grid. Strong correlations with the sea bottom topography are already visible, especially with Crozet, Kerguelen, and Heard Is lands and with the newly discovered seamounts Ob, Lena, and Marion Dufresne. Geophysical interpretation of these correlations are under way.