Geosat Exact Repeat Mission Research Articles

Analysis of collinear passes of satellite altimeter data

The analysis of altimetry measurements along repeated satellite ground tracks, as in the Geosat Exact Repeat Mission, provides an excellent way of determining temporal variability in the sea surface height. This collinear analysis is usually carried out in two steps: first, by determining a mean sea level profile and then by removing a radial orbit error. We propose a new unified analysis method for the simultaneous estimation of orbit error, mean sea surface height, and residual sea surface heights (variability) with respect to each pass. All passes are treated in exactly the same way; the consequent rank or datum defect in the model is overcome by appending a minimal set of conditions, chosen either to give no overall distortion of the orbits or otherwise a best fit to a reference profile, such as the geoid. Gaps in the altimeter data are easily accommodated. Numerical results of the method are shown using data from the first year of the Geosat Exact Repeat Mission. Some consideration is given to the choice of radial orbit error model. The results indicate that for orbit error removal, a sinusoid model is the most appropriate over a one‐revolution cycle. For passes less than one revolution in length, a component of the longer wavelength ocean signal is lost if using the sinusoid model. If passes longer than about two revolutions (some 200 mins) are analyzed, additional terms are needed to eliminate nongravitational perturbations.

Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian‐shaped and by successively fitting this synthetic current's height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi‐stationary meanders, and its decrease thereafter is mainly due to large‐scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large‐scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low‐latitude Pacific Ocean.

The mean sea surface height and geoid along the Geosat subtrack from Bermuda to Cape Cod

Measurements of near‐surface velocity and concurrent sea level along an ascending Geosat subtrack were used to estimate the mean sea surface height and the Earth's gravitational geoid. Velocity measurements were made on three traverses of a Geosat subtrack within 10 days, using an acoustic Doppler current profiler (ADCP). A small bias in the ADCP velocity was removed by considering a mass balance for two pairs of triangles for which expendable bathythermograph measurements were also made. Because of the large curvature of the Gulf Stream, the gradient wind balance was used to estimate the cross‐track component of geostrophic velocity from the ADCP vectors; this component was then integrated to obtain the sea surface height profile. The mean sea surface height was estimated as the difference between the instantaneous sea surface height from ADCP and the Geosat residual sea level, with mesoscale errors reduced by low‐pass filtering. The error estimates were divided into a bias, tilt, and mesoscale residual; the bias was ignored because profiles were only determined within a constant of integration. The calculated mean sea surface height estimate agreed with an independent estimate of the mean sea surface height from Geosat, obtained by modeling the Gulf Stream as a Gaussian jet, within the expected errors in the estimates: the tilt error was 0.10 m, and the mesoscale error was 0.044 m. To minimize mesoscale errors in the estimate, the alongtrack geoid estimate was computed as the difference between the mean sea level from the Geosat Exact Repeat Mission and an estimate of the mean sea surface height, rather than as the difference between instantaneous profiles of sea level and sea surface height. In the critical region near the Gulf Stream the estimated error reduction using this method was about 0.07 m. Differences between the geoid estimate and a gravimetric geoid were not within the expected errors: the rms mesoscale difference was 0.24 m rms.

On the Sea-State Bias of the Geosat Altimeter

The sea-state bias in a satellite altimeter's range measurement is caused by the influence of ocean waves on the radar return pulse; it results in an estimate of sea level that is too low according to some function of the wave height. This bias is here estimated for Geosat by correlating collinear differences of altimetric sea-surface heights with collinear differences of significant wave heights (H1/3). Corrections for satellite orbit error are estimated simultaneously with the sea-state bias. Based on twenty 17-day repeat cycles of the Geosat Exact Repeat Mission, the solution for the sea-state bias is 2.6 + or - 0.2 percent of H1/3. The least-squares residuals, however, show a correlation with wind speed U, so the traditional model of the bias has been supplemented with a second term: H1/3 + alpha-2H1/3U. This second term produces a small, but statistically significant, reduction in variance of the residuals. Both systematic and random errors in H1/3 and U tend to bias the estimates of alpha-1 and alpha-2, which complicates comparisons of the results with ground-based measurements of the sea-state bias.

Can meddies be detected by satellite altimetry?

Geosat Exact Repeat Mission altimeter data have been used to estimate the sea surface height anomaly relative to a 2‐year mean reference surface in the eastern North Atlantic Ocean. Horizontal maps of the sea surface height anomaly for various periods were constructed by means of an objective analysis procedure. The mean sea level was approximated by the dynamic topography from climatological hydrographic data. Results were compared to the dynamic topography calculated from hydrographic ground truth data sampled during March 1988 on an eddy‐resolving station network in the Iberian Basin. Despite the low signal‐to‐noise ratio of the altimeter data in this part of the ocean, the agreement of the altimeter signal with hydrographic measurements is found to be significant. Individual cyclonic and anticyclonic eddies in the Mediterranean outflow are identified in the altimetric maps. The information visible in the sea surface height is significantly correlated with the dynamic topography of the layer 500/3000 dbar containing the core of the Mediterranean outflow and maximum velocities of the baroclinic eddies. The successive development of the identified eddies as studied from a sequence of sea surface height maps indicates a slight westward drift of the most pronounced feature with approximately 1.9 cm/s propagation velocity. The inferred path of that feature is consistent with additional information from moored current meter data.

Loop current eddy shedding estimated using Geosat altimeter data

A method for extracting periodic signals from altimeter data is applied in the Gulf of Mexico using data from the first two years of the Geosat Exact Repeat Mission. The least squares fitting of selected frequencies of sine and cosine functions to the time series of sea surface height at each point of the Geosat ground track is used to estimate the dominant frequency in the gulf for this time period. Optimal interpolation of the sine and cosine coefficients at the dominant frequency to a uniformly spaced grid allows reconstruction of the time series in the gulf and shows eddies shedding from the Loop Current with a period of approximately 10.5 months, consistent with shedding periods determined from numerical models and analysis of satellite infrared data. This technique is a novel method for investigation of periodic signals in Geosat altimetry and is not severely affected by the data outages present in the time series.

Eddy Variability in the Kuroshio Extension as Revealed by Geosat Altimetry: Energy Propagation Away from the Jet, Reynolds Stress, and Seasonal Cycle

Altimetric sea level data around the Kuroshio Extension, collected during the first year of the Geosat Exact Repeat Mission (ERM) (starting in November 1986), have been analyzed for energy propagation away from the jet, the Reynolds stress pattern, and the seasonal cycle of the eddy field. Space-time diagrams and wavenumber-frequency spectra are employed to discern the dominant direction of phase propagation, revealing westward and inward phase propagation towards the jet (i.e., energy propagation away from the jet), which is consistent with the notion that the widespread eddy variability from the jet is the result of transient Rossby wave response to forcing provided by growing, pulsating, and decaying meanders of the jet. It can be shown that any theory based on the linear instability cannot normally account for the energy propagation, because any linear instability that is wavelike away from the jet is by necessity not the fastest growing linear instability. Rather, a nonlinear instability mechanism is needed, whose nonstationary finite amplitude behaviors can excite Rossby waves away from the jet. The computed Reynolds stress pattern is consistent with previous observation indicating that, on the whole, the jet is reinforced, rather than weakened, thus favoring the baroclinic instability as the dominant source for meandering growth. A seasonal cycle in the intensity of both the eddy field and the surface transport of the Kuroshio Extension is found, which is weakest during winter and progressively strengthens from spring to fall.

The estimation of geoid profiles in the northwest Atlantic from simultaneous satellite altimetry and airborne expendable bathythermograph sections

In the NW Atlantic, the temporal mean circulation results in large amplitude surface topography precluding the reasonable use of mean altimetric surfaces or profiles for precise estimation of the geoid. We examine a procedure which (1) uses airborne expendable bathythermograph (AXBT) sections collected along altimeter ground tracks to estimate the single‐pass or instantaneous surface dynamic topography, (2) employs concurrent or simultaneous satellite altimeter overflights to provide a measure of instantaneous sea level, and (3) arrives at a precise estimate of the alongtrack geoid profile by differencing the instantaneous altimetric sea level and the AXBT‐derived dynamic topography. This technique is applied to several Geosat Exact Repeat Mission (ERM) ground tracks in the area of the NW Atlantic Regional Energetics Experiment (REX) using AXBT survey data collected in April and July 1987. Geoid profile estimates are repeatable between these two independent data sets to within 10–20 cm rms. This is near the estimated noise level due to barotropic variability in the Gulf Stream region. These AXBT/Geosat‐derived profiles provide a more consistent estimate of the alongtrack geoid gradient than do available gravimetric geoids. Analysis of the statistics of the ensemble of collinear passes provides an estimate of the geographically uncorrelated orbit error of approximately 1–2 m. The total surface topography derived as the difference between adjusted instantaneous sea level profiles and the AXBT/Geosat‐derived geoid profiles does not suffer significant contamination by either geographically uncorrelated or correlated orbit error.

Variations of global mesoscale eddy energy observed from Geosat

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2‐year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty‐four 17‐day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm2/s2 for most of the western boundary currents; however, it only reaches approximately 500 cm2/s2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm2/s2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.

Annual Rossby Wave Characteristics in the California Current Region from the GEOSAT Exact Repeat Mission

Abstract Examination of sea level variability in the California Current region, from 20° to 50°N and from the coast of North America to 140°W, was made possible with altimetric observations obtained by the GEOSAT Exact Repeat Mission (ERM) for the period November 1986-October 1987. During this period, a time sequence of twenty-one 17-day maps of altimetric sea level residuals over the Califomia Current region was constructed. The rms distribution of these altimetric sea level residuals displayed local maxima at three locations adjacent to the coast of California; i.e., off Point Eugenia at 27°N, southwest of Point Conception at 32°N, and between Monterey and Cape Mendicino (i.e., 37°–40°N). Complex empirical orthogonal function (CEOF) analysis reveals that 53% of the variance over this one-year period was associated with the annual cycle, with amplitude maximum in summer and minimum in winter. The spatial distribution of amplitude displayed local maxima adjacent to the coast, again at 27°N, 32°N, and from...

Geodetic and oceanographic results from the analysis of 1 year of Geosat data

The first year of the Geosat Exact Repeat Mission has been analyzed to obtain improved orbits, gravity model, and representation of sea surface topography. The starting point for this study were the original geophysical data records merged with orbits by Haines et. al. (1990) computed with the GEM‐T1 gravity model. A radial orbit error model following the development of Engelis (1987a) was modified to account for resonant effects above degree 36. Twenty‐four 17‐day arcs were analyzed. Separate 17‐day and a single 1‐year solution for a gravity model and a spherical harmonic representation of the sea surface topography to degree 10 were found. The root‐mean‐square crossover discrepancy with the a priori (i.e., GEM‐T1) model was ±1.1 m, which was reduced to ±0.22 m with the 1‐year solution. Comparison of overlaps in the arcs showed discrepancies of ±1.3 m with the a priori field which were reduced to ±0.07 m with the 1‐year solution. The determination of the degree (1, 0) term in the sea surface topography was not possible in this analysis because of the high correlation of this term with an orbit error term. In addition, the (1, 1) terms showed serious incompatibility with that implied by historical oceanographic information. The final solution used here incorporated the degree 1 coefficients from oceanographic data. The problem with the (1, 1) coefficients may imply a small center of mass offset in the tracking station positions used in the orbit calculations or may result from the poor distribution of the available tracking stations. Error analysis indicated a ±10 cm sea surface topography commission error to degree 10. The geoid commission error to degree 36 was ±9 cm in the ocean areas. The new potential coefficient model was compared with terrestrial data, in a manner to that done by Engelis and Knudsen (1989) to show the improvement of the new model.

Estimating the Surface Transport of Meandering Oceanic Jet Streams from Satellite Altimetry: Surface Transport Estimates for the Gulf Stream and Kuroshio Extension

Abstract A method has been developed to detect the location, width maximum velocity, and suffice transport of a meandering oceanic jet stream from satellite altimetry. Ale method is based on a nonlinear least squares scheme, in which Gaussian-shaped velocity profiles are fitted to explain the observed current variability in the meandering region. The method has been applied to 25 Geosat tracks in the Kuroshio Extension and Gulf Stream for the first you of the Geosat Exact Repeat Mission. A reasonably smooth circulation pattern has emerged for the surface transport for both currents (at opposed to the widely varying direct transport estimates). After leaving the coast, both currents have surface strength of the order of 90 × 103 m2 a−1(equivalent to a 90 cm height jump, i.e., the sea surface height difference across the jet). They continue to gain strength downstream until a maximum of the order of 130 × 10 3 m2> a−1 is reached (near 150°E for the Kuroshio and 63°W for the Gulf Stream). The Gulf Stream the...

Gravity field of the southeast central Pacific from Geosat Exact Repeat Mission Data

Efficient techniques for mapping the marine gravity field with Geosat Exact Repeat Mission (ERM) data are demonstrated. Geosat has completed more than fifty‐two 17‐day, global ERM repeat cycles and has collected roughly 50 million observations of geoid undulations or, more precisely, sea surface topography. Because the global ground track repeats every 17 days, the ERM observations are highly redundant. By averaging the repeat cycles, very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central Pacific. Without any crossing‐arc, orbit adjustments, these profiles are shown to be repeatable to within 10 cm over their entire 3000km length (i.e., to three parts in 108). This southeast central Pacific region contains a portion of the East Pacific Rise as well as some cross‐grain gravity field lineations and is therefore of considerable geophysical interest. In addition to profiles of alongtrack deflections of the vertical, regularly gridded gravity anomalies are derived for this region and portrayed in color. The procedure used for recovering gravity has two stages: in stage 1, alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights, computing alongtrack slopes at standardized latitudes, and then averaging slopes for coincident passes; in stage 2, gridded, band‐limited gravity anomalies are recovered from deflections by using fast Fourier transform techniques to accomplish the necessary inverse Vening Meinesz transformation. High‐precision GEM‐T1 orbits are used and crossover adjustments are not performed. The operational Navy Astronautics Group orbits are shown to have systematic errors which may be as large as 10 m.

Oceanic tides from Geosat altimetry

Direct tidal analysis of the altimetry from the Geosat Exact Repeat Mission's first year is used to derive estimates of the diurnal and semidiurnal oceanic tides. The geoid is removed by collinear differences at 34.1‐day separation, and the orbit error is reduced by subtracting a slowly modulated 1 cycle/revolution term. A sequence of independent analyses at grid areas of 1° latitude×1.5° longitude using “orthotide” functions ensures complete definition of the diurnal and semidiurnal species. Global admittance maps for M2 and S2 within the latitudes 58°N and 59°S compare well with ground truth at 66 open‐ocean sites. Maps of differences between Geosat and the Naval Surface Weapons Center model show important areas of the order of 10–15 cm. The integral for the global power input from the moon gives 2.51 TW (M2), 0.44 TW (S2), lower than many previous estimates, but in agreement with figures deduced from a recent analysis of satellite orbits.

Mean sea surface and variability of the Gulf of Mexico using Geosat altimetry data

Geosat Exact Repeat Mission (ERM) altimetric measurements of the sea surface height, in the Gulf of Mexico are used to determine the mean sea surface height with respect to the ellipsoid and mesoscale variability along Geosat ground tracks in the gulf for the time period from November 8, 1986, to November 25, 1988. The alongtrack mean sea surface is determined using a regional crossover adjustment procedure, in which the tilt and bias of mean arcs are estimated using a least squares technique to minimize the height differences at crossovers points. A mean surface generated using the Geosat ERM alongtrack mean is calculated and contrasted with a previously derived mean surface determined using GEOS 3 and Seasat crossover differences. This provides a first look at the variability in the mean between the time periods of 1987–1988 and 1975–1978. In addition, the alongtrack mesoscale variability time series has been produced from the Geosat ERM data set by using a robust orbit error removal algorithm to determine the variability of the sea surface height with respect to the alongtrack mean. A surface generated using the rms of this alongtrack time series shows good qualitative and quantitative agreement with previous in situ observations in the region. This study demonstrates the potential of satellite altimetry for oceanographic studies of the Gulf of Mexico.

Continuous assimilation of simulated Geosat altimetric sea level into an eddy‐resolving numerical ocean model: 1. Sea level differences

Continuous assimilation of altimetric sea level differences from the simulated Geosat exact repeat mission (ERM) into a three‐layer, quasi‐geostrophic, eddy‐resolving numerical ocean box model at mid‐latitude was conducted using the method of optimal interpolation (Lorenc, 1981). In the linear portion of the model domain, the assimilation of simulated Geosat altimetric sea level differences constrained the detectable periods/wavelengths of mesoscale eddy activity, within the limits of accuracy of the initialization, nearly indefinitely. However, in the nonlinear portion of the model domain, the constraint of these otherwise detectable periods/wavelengths degraded with time from the point of initialization, due to the influence of nonlinear processes and aliasing upon the assimilation process. Hence reinitialization at period intervals was indicated. The Nyquist wavelength of mesoscale eddy activity that could be constrained by the simulated Geosat ERM (i.e., 140‐km track separation at mid‐latitudes, 45‐km sampling interval along track) in the nonlinear portion of the model domain was determined to be the track separation, and not the sampling interval along track; hence in both the linear and nonlinear portions of the model domain, wavelengths greater than 280 km were constrained, but wavelengths shorter than 280 km were not. Therefore decreasing the track separation in future satellite altimeter strategies will be the only way that smaller wavelength scales of mesoscale eddy activity can be detected. This should also increase the capability of the model to constrain the nowcast of the mesoscale eddy activity in the nonlinear portions of the model domain beyond initialization. It was established that the constraint of mesoscale eddy activity in the middle and lower layer stream function fields of the model was similar to that in the upper layer stream function field, responding to the same Nyquist criteria.

Validation of the geostrophic method for estimating zonal currents at the equator from Geosat altimeter data

The applicability of satellite altimeter data for estimating zonal current variability at the equator is assessed using the meridionally differenced form of the geostrophic balance. Estimates of geostrophic zonal flow anomalies in the equatorial Pacific have been deduced from 17‐day collinear altimeter data during the first year of the Geosat Exact Repeat Mission, November 1986 to November 1987. Altimeter‐derived geostrophic estimates agree well with in situ zonal current variability. Comparison of low‐frequency, near‐surface zonal current observed from equatorial moorings at 165°E;, 140°W, and 110°W yield correlations of 0.83, 0.85, and 0.51, respectively, with a mean rms difference of 23 cm s−1. The geostrophic currents were calculated from all available ascending and descending Geosat tracks within ±4.5° of longitude from each mooring site. The inclusion of up to 11 ascending and descending Geosat tracks within the 9° band for every 17‐day repeat effectively reduced the temporal sampling interval to 1.5 days at 165°E and 140°W. However, only ascending tracks were available at 110°W. Alongtrack sea surface heights were first smoothed using a combination of linear and nonlinear filters. The 6.8 km alongtrack spacing of the altimeter measurements provides sufficient resolution for the effective filtering of small‐scale meridional noise, both instrumental and oceanic. High‐frequency temporal variability, such as noise and ageostrophic motions, was suppressed with a 31‐day Hanning filter. Sea level and zonal velocity solutions from a tropical Pacific numerical model were used as proxy data sets in order to estimate errors induced into the geostrophic calculation by the Geosat space‐time sampling.

Continuous assimilation of simulated Geosat altimetric sea level into an eddy‐resolving numerical ocean model: 2. Referenced sea level differences

Continuous assimilation of referenced altimetric sea level differences from the simulated Geosat Exact Repeat Mission (ERM) into a three‐layer, wind‐driven, quasi‐geostrophic, eddy‐resolving numerical ocean box model at mid‐latitude was conducted utilizing an improved version of the optimal interpolation procedure found in Part I (White et al., this issue), forming the basis of an operational dynamical interpolation procedure to be used in subsequent gridding of real Geosat ERM altimetric sea level data. The major improvement over the dynamical interpolation method used in Part I was the inclusion of temporal decorrelation in the error covariance matrix and the introduction of noise into the system. As referenced altimetric sea level differences (i.e., differences referenced by the mean of the model) were observed from a control model integration in real model time by the simulated Geosat altimeter, attention focused upon the ability of the dynamical interpolation procedure to reconstruct a mesoscale eddy field resembling that of the control model integration. Results of this dynamical interpolation procedure were compared with those of a statistical (i.e., optimum) interpolation procedure in its ability to reconstruct the mesoscale eddy field in the western boundary extension current of the control model integration. A series of model integrations were made, with initial conditions independent from those of the control model integration (the latter taken to represent the observed mesoscale eddy field), where the dynamical interpolation of simulated referenced Geosat ERM altimetric sea level differences was found capable of reproducing the control model integration (i.e., explaining 80–90% of the control model variance) after only one repeat cycle (i.e., 17 days) of the ERM. This success was attributed to the use of time decorrelation in the error covariance (together with noise) to nudge the first‐guess model upper level stream function field toward the corresponding observation field over a period of a few days, reducing the shock of inserting the observations into the model. The degree of similarity between the upper layer stream function of the updated model and the control model integration after one repeat cycle was, however, only marginally significantly greater than that achieved by statistical interpolation (i.e., explaining 65–90% of the control model variance), both interpolation procedures (i.e., dynamical and statistical) using covariance estimates that were determined a priori in both space and time. Two important advantages of the dynamical interpolation over the statistical interpolation became evident after three cycles (i.e., 51 days) of continuous assimilation of referenced simulated Geosat ERM altimetric sea level differences. First, mesoscale eddy activity in the middle and lower layers of the updated model integration began to resemble that of the model control integration (i.e., explaining 65% and 15% of the control model variance, respectively), without resorting to vertical statistical interpolation. Second, the spectral coherence between the upper layer stream function field of the updated model integration and that of the control model integration was symmetric with respect to direction of propagation, while the spectral coherence between the upper layer stream function field by statistical interpolation and that of the control model integration was not, with the latter robust enough only to detect the dominant westward propagating portion of the spectrum of mesoscale eddy activity and not the subdominant information. An important finding was the fact that both interpolation procedures were able to resolve wavelengths longer than the Nyquist (i.e., 280 km) dictated by the track separation distance, as expected, but both interpolation procedures were able to resolve periods (i.e., 12 days) much shorter than the Nyquist (i.e., 34 days) dictated by the exact repeat period.

A preliminary estimate of geoid‐induced variations in repeat orbit satellite altimeter observations

Altimeter satellites are often maintained in a repeating orbit to facilitate the separation of sea height variations from the geoid. However, atmospheric drag and solar radiation pressure cause a satellite orbit to drift. For Geosat this drift causes the ground track to vary by ±1 km about the nominal repeat path. This misalignment leads to an error in the estimates of sea surface height variations because of the local slope in the geoid. We have estimated this error globally for the Geosat Exact Repeat Mission using a mean sea surface constructed from Geos 3 and Seasat altimeter data. Over most of the ocean the geoid gradient is small, and the repeat track misalignment leads to errors of only 1 to 2 cm. However, in the vicinity of trenches, continental shelves, islands, and seamounts, errors can exceed 20 cm. The estimated error is compared with direct estimates from Geosat altimetry, and a strong correlation is found in the vicinity of the Tonga and Aleutian trenches. This correlation increases as the orbit error is reduced because of the increased signal‐to‐noise ratio.

High‐accuracy, high‐resolution gravity profiles from 2 years of the Geosat Exact Repeat Mission

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17‐day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short‐wavelength altimeter noise and longer‐wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross‐spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal‐to‐noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short‐wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half‐wavelength resolution of 12 km by a 4.5‐year satellite altimeter mapping mission.