Altimetry Measurements Research Articles

Estimating the Atlantic overturning at 26°N using satellite altimetry and cable measurements

AbstractClimate simulations predict a slowing of the Atlantic meridional overturning circulation (MOC), a key oceanic component of the climate system, while continuous observations of the MOC from boundary arrays demonstrate substantial variability on weekly to interannual time scales. These arrays are necessarily limited to individual latitudes. A potential proxy for the MOC covering longer time scales and larger spatial scales is desirable. Here we use sea surface height data from satellites to estimate the interannual variability of transbasin ocean transports at 26°N. Combining this estimate with surface Ekman transport and cable measurements of the Florida Current, we construct a time series of the MOC from 1993 to 2014. This satellite‐based estimate recovers over 90% of the interannual variability of the MOC measured by the RAPID 26°N array. This analysis complements in situ observational efforts to measure the MOC at multiple latitudes and opens the door to a broader spatial understanding of the Atlantic circulation variability.

Intra-seasonal variability of Pacific-origin sea level anomalies around the Philippine Archipelago

Intra-seasonal variability of sea level anomalies (SLAs) originated in the Pacific Ocean around the Philippine Archipelago was investigated using merged altimetry SLA measurements and eddy-resolving ocean model outputs. The results suggest the SLA signals from the tropical North Pacific propagate westward as baroclinic Rossby waves on an intra-seasonal time scale. Upon impinging the east coast of the Philippines, these Rossby wave signals transform into coastal trapped waves (CTWs), propagate clockwise along the coast of the Philippine Archipelago and enter into the eastern South China Sea (SCS) through the Sibutu Passage and Mindoro Strait. The SLA signals, however, cannot propagate anticlockwise and enter into the eastern SCS through the Luzon Strait. The intra-seasonal oceanic wave processes are clearly identified by the eddy-resolving model. The effect of along-shore wind forcing on the SLA signals appears insignificant when compared with the remote signals coming from the interior Pacific. While identified in the model simulation, future observations are needed to verify the intra-seasonal CTWs encircling the Philippine Archipelago.

Satellite radar altimetry for monitoring small rivers and lakes in Indonesia

Abstract. Remote sensing and satellite geodetic observations are capable of hydrologic monitoring of freshwater resources. Although satellite radar altimetry has been used in monitoring water level or discharge, its use is often limited to monitoring large rivers (>1 km) with longer interval periods (>1 week) because of its low temporal and spatial resolutions (i.e., satellite revisit period). Several studies have reported successful retrieval of water levels for small rivers as narrow as 40 m. However, processing current satellite altimetry signals for such small water bodies to retrieve water levels accurately remains challenging. Physically, the radar signal returned by water bodies smaller than the satellite footprint is most likely contaminated by non-water surfaces, which may degrade the measurement quality. In order to address this scientific challenge, we carefully selected the waveform shapes corresponding to the range measurement resulting from standard retrackers for the European Space Agency's (ESA's) Envisat (Environmental Satellite) radar altimetry. We applied this approach to small (40–200 m in width) and medium-sized (200–800 m in width) rivers and small lakes (extent <1000 km2) in the humid tropics of Southeast Asia, specifically in Indonesia. This is the first study that explored the ability of satellite altimetry to monitor small water bodies in Indonesia. The major challenges in this study include the size of the water bodies that are much smaller than the nominal extent of the Envisat satellite footprint (e.g., ~250 m compared to ~1.7 km, respectively) and slightly smaller than the along-track distance (i.e., ~370 m). We addressed this challenge by optimally using geospatial information and optical remote sensing data to define the water bodies accurately, thus minimizing the probability of non-water contamination in the altimetry measurement. Considering that satellite altimetry processing may vary with different geographical regions, meteorological conditions, or hydrologic dynamic, we further evaluated the performance of all four Envisat standard retracking procedures. We found that satellite altimetry provided a good alternative or the only means in some regions of measuring the water level of medium-sized rivers and small lakes with high accuracy (root mean square error (RMSE) of 0.21–0.69 m and a correlation coefficient of 0.94–0.97). In contrast to previous studies, we found that the commonly used Ice-1 retracking algorithm was not necessarily the best retracker among the four standard waveform retracking algorithms for Envisat radar altimetry observing inland water bodies. As a recommendation, we propose to include the identification and selection of standard waveform shapes to complete the use of standard waveform retracking algorithms for Envisat radar altimetry data over small and medium-sized rivers and small lakes.

Heterogeneous change patterns of water level for inland lakes in High Mountain Asia derived from multi‐mission satellite altimetry

AbstractHigh‐altitude inland lakes in High Mountain Asia (HMA) are key indicators to climate change and variability as a result of mostly closed watersheds and minimal disturbance by human activities. However, examination of the spatial and temporal pattern of lake changes, especially for water‐level variations, is usually limited by poor accessibility of most lakes. Recently, satellite altimeters have demonstrated their potential to monitor water level changes of terrestrial water bodies including lakes and rivers. By combining multiple satellite altimetry data provided by the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS) and Geoscience Laser Altimeter System (GLAS) instrument on the NASA Ice, Cloud and land Elevation satellite (ICESat), this study examined water level changes of typical lakes in HMA at a longer timescale (in the 1990s and 2000s) compared with earlier studies on Tibetan lakes. Cross‐evaluation of the radar altimetry data from LEGOS and laser altimetry data from ICESat/GLAS shows that they were in good agreement in depicting inter‐annual, seasonal and abrupt changes of lake level. The long‐term altimetry measurements reveal that water‐level changes of the 18 lakes showed remarkable spatial and temporal patterns that were characterized by different trends, onsets of rapid rises and magnitudes of inter‐annual variations for different lakes. During the study period, lakes in the central and northern HMA (15 lakes) showed a general growth tendency, while lakes in South Tibet (three lakes) showed significant shrinking tendency. Lakes in Central Tibet experienced rapid and stable water‐level rises around mid‐1990s followed by slowing growth rates after 2006. In contrast, the water‐level rises of lakes in the northern and north‐eastern Tibetan Plateau were characterized by abrupt increases in specific years rather than gradual growth. Meteorological data based on station observations indicate that the annual changes of water level showed strongly correlated with precipitation and evaporation but may not evidently related to the glacier melting induced by global warming. Copyright © 2014 John Wiley & Sons, Ltd.

Due diligence

Satellite altimetry measurements from SARAL-AltiKa are analysed in the coastal ocean using the results of the G-AltiKa mission (1–5 August 2013), which combined altimeter, HF radar and glider data coincident with the satellite track, south-west of Ibiza Island, in the Western Mediterranean Sea.The absolute dynamic topography, computed from 1 Hz and 40 Hz altimetry data, and the dynamic height, computed from glider temperature and salinity, both exhibited a weak signal with amplitudes of order 2 cm. The resulting geostrophic and HF radar velocities along the track depicted a north-westward coastal current with a maximal velocity larger than 20 cm s−1.This demonstrates that the AltiKa altimeter is able to resolve SLA signals of more than 2 cm, and gradients in those signals over several tens of kilometres. After filtering, the 40 Hz data depicted a signal consistent with the other platforms, up to a distance of order 10 km from the coast.

Studies of lava flows in the Tharsis region of Mars using SHARAD

AbstractThe Tharsis region of Mars is covered in volcanic flows that can stretch for tens to hundreds of kilometers. Radar measurements of the dielectric properties of these flows can provide information regarding their composition and density. SHARAD (shallow radar), a sounding radar on the Mars Reconnaissance Orbiter, detects basal interfaces beneath flows in some areas of Tharsis northwest and west of Ascraeus Mons, with additional detections south of Pavonis Mons. Comparisons with 12.6 cm ground‐based radar images suggest that SHARAD detects basal interfaces primarily in dust or regolith‐mantled regions. We use SHARAD data to estimate the real relative permittivity of the flows by comparing the measured time delay of returns from the subsurface with altimetry measurements of the flow heights relative to the surrounding plains. In cases where the subsurface interface is visible at different depths, spanning tens of meters, it is also possible to measure the loss tangent (tan δ) of the material. The permittivity values calculated range from 7.6 to 11.6, with an average of 9.6, while the mean loss tangent values range from 7.8 × 10−3 to 2.9 × 10−2 with an average of 1.0 × 10−2. These permittivity and loss tangent estimates for the flows northwest of Ascraeus Mons, west of Ascraeus Mons, and south of Pavonis Mons are consistent with the lab‐measured values for dense, low‐titanium basalt.

Monitoring the Fluctuation of Lake Qinghai Using Multi-Source Remote Sensing Data

The knowledge of water storage variations in ungauged lakes is of fundamental importance to understanding the water balance on the Tibetan Plateau. In this paper, a simple framework was presented to monitor the fluctuation of inland water bodies by the combination of satellite altimetry measurements and optical satellite imagery without any in situ measurements. The fluctuation of water level, surface area, and water storage variations in Lake Qinghai were estimated to demonstrate this framework. Water levels retrieved from ICESat (Ice, Cloud, and and Elevation Satellite) elevation data and lake surface area derived from MODIS (Moderate Resolution Imaging Spectroradiometer) product were fitted by linear regression during the period from 2003 to 2009 when the overpass time for both of them was coincident. Based on this relationship, the time series of water levels from 1999 to 2002 were extended by using the water surface area extracted from Landsat TM/ETM+ images as inputs, and finally the variations of water volume in Lake Qinghai were estimated from 1999 to 2009. The overall errors of water levels retrieved by the simple method in our work were comparable with other globally available test results with r = 0.93, MAE = 0.07 m, and RMSE = 0.09 m. The annual average rate of increase was 0.11 m/yr, which was very close to the results obtained from in situ measurements. High accuracy was obtained in the estimation of surface areas. The MAE and RMSE were only 6 km2, and 8 km2, respectively, which were even lower than the MAE and RMAE of surface area extracted from Landsat TM images. The estimated water volume variations effectively captured the trend of annual variation of Lake Qinghai. Good agreement was achieved between the estimated and measured water volume variations with MAE = 0.4 billion m3, and RMSE = 0.5 billion m3, which only account for 0.7% of the total water volume of Lake Qinghai. This study demonstrates that it is feasible to monitor comprehensively the fluctuation of large water bodies based entirely on remote sensing data.

New processing approaches on the retrieval of water levels in Envisat and SARAL radar altimetry over rivers: A case study of the São Francisco River, Brazil

Radar-based satellite altimetry is well recognized for oceanographic applications. For continental hydrology, its use is complicated by a number of environmental factors such as river width and shape, land cover type in the vicinity of the river banks, and the topography of the relief. These factors make precision vary significantly. Locations where the satellites cross the river can be used as “virtual gauging stations” that can complement the existing network of in situ stations. This article describes processing techniques that take some of these environmental factors (river shape and width) into account to improve the precision of altimetry measurements of the water level. These techniques are based on some a priori information about the river banks and on modeling a phenomenon called “off-nadir hooking”. This approach is tested on the São Francisco River in Brazil, which for most of its path is considered narrow for satellite altimetry applied to hydrology. Data from Envisat cover the period 2002–2010 while the recently launched SARAL satellite provided data for 2013. The results show that the accuracy varies significantly depending on a number of environmental factors some of which are discussed in depth. In about one-half of the 16 satellite water gauging stations, the RMS errors are lower than 60cm and in some cases better than 30cm. These variations could not be directly related to the river width, but appear to be mostly related to the land cover and to the processing chain that often extracts altimetry points from an off-nadir location. All processing is fully described and the results are presented for both the Envisat/RA-2 and SARAL/Altika altimeters.

Illustration of the emerging capabilities of SARAL/AltiKa in the coastal zone using a multi-platform approach

Satellite altimetry measurements from SARAL-AltiKa are analysed in the coastal ocean using the results of the G-AltiKa mission (1–5 August 2013), which combined altimeter, HF radar and glider data coincident with the satellite track, south-west of Ibiza Island, in the Western Mediterranean Sea.The absolute dynamic topography, computed from 1Hz and 40Hz altimetry data, and the dynamic height, computed from glider temperature and salinity, both exhibited a weak signal with amplitudes of order 2cm. The resulting geostrophic and HF radar velocities along the track depicted a north-westward coastal current with a maximal velocity larger than 20cms−1.This demonstrates that the AltiKa altimeter is able to resolve SLA signals of more than 2cm, and gradients in those signals over several tens of kilometres. After filtering, the 40Hz data depicted a signal consistent with the other platforms, up to a distance of order 10km from the coast.

Satellite radar altimetry waveform retracking over the Caspian Sea

Post-processing of complex altimetry-derived echo shapes is necessary over the Caspian Sea, in which the altimetry-inferred level fluctuations, derived from on-board ‘retrackers’, represent errors within a wide boundary close to the coastlines, i.e. up to ~10–30 km. Four different techniques were implemented to retrack 10 Hz radar waveforms of the TOPEX/Poseidon (T/P) mission and 20 Hz waveforms of its follow-on mission Jason-1. These include three previously introduced algorithms of the Off Center of Gravity, the standard NASA β-5, and Threshold Retracking, after some improvements. We also introduced a new method called ‘Extrema Retracking (ExtR)’, which works based on edge detection and extracting extremum points of the returned waveforms. Before implementing each technique, a two-step preprocessing was applied to exclude unwanted waveforms. Then, an unsupervised classification approach was applied to classify the remaining waveforms into multiple groups, according to their shape and geometrical properties. Finally, the waveforms of each group were post-processed using an appropriate retracking algorithm, and subsequently, the derived corrected ranges were converted to sea surface height (SSH) values. Our post-processed results were evaluated (1) by comparing the time series of SSHs with in situ tide gauge measurements as well as (2) by comparing the along-track retracked SSHs to their corresponding geoid profiles, computed from the EGM2008 model. For each evaluation, in situ measurements or geoid profiles were subtracted from the retracked SSH values to compute residuals. Thereafter, the mean and root mean squares of the residuals were calculated and interpreted, respectively, as a measure of bias and noise level of the processed SSHs. Our results indicated a reduction in noise and bias after the application of retrackers. Specifically, the bias and noise of the ExtR-derived SSH time series were decreased at locations close to the Anzali, Noshahr and Neka tide-gauge stations (along the southern coast). For example, the correlation between altimetry observations and tide gauge measurements close to the Anzali Port over the T/P era was increased from 0.31 (using original altimetry data) to 0.83 (using ExtR), which is equivalent to a noise reduction of 0.58 to 0.39 m. Comparisons of the along-track SSHs with geoid profiles indicated a significant mitigation of land contamination over the coastal areas.

Evolution of geoids in recent years and its impact on oceanography

Mean surface geostrophic ocean currents may be calculated from the Mean Dynamic Topography ( MDT ), estimated as the difference between a mean sea surface height ( MSS ) calculated from radar altimeters and a reference geoid height. A review of the most widely used geoids is presented. The difference between the third release of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) geoid and three earlier geoids (the Earth Geopotential Model 1996 [EGM96], one of the geoids obtained by the Gravity Recovery and Climate Experiment [GRACE05], and the Earth Gravitational Model 2008 [EGM2008]) is computed and interpreted as an ‘artefact’ MDT , i.e. a misfit when non-accurate geoid models are used to calculate the ocean MDT and related geostrophic currents. These results are contrasted with the MDT computed by comparing the GOCE geoid with the MSS distributed by Collecte Localisation Satellites in 2001 (CLS01). The comparison shows that there was a strong influence of altimetry measurements in the construction of the EGM96 geoid, i.e. the artefact MDT calculated using EGM96 shows a high resemblance to the MDT computed using the MSS CLS01 field, both considering GOCE as the reference geoid. The correlation disappears largely, but not completely, for the two most recent geoids; in particular, the MSS has greater global influence on GRACE05 than on EGM2008 although the latter does better at latitudes of less than 60° and is more useful for reproducing the intense western boundary currents. The results show that EGM96 may lead to significant errors in the spatial gradients of MDT (for latitudes of less than 60° the global root mean square is 0.2422 m) and therefore in the geostrophic surface velocities. When the spatially averaged GRACE and EGM2008 geoids are used for latitudes of less than 60°, the global MDT root mean square is substantially reduced.

The Effect of Atmospheric Water Vapor Content on the Performance of Future Wide-Swath Ocean Altimetry Measurement

Abstract Measurement of sea surface height (SSH) over a finite swath along satellite tracks has been planned for future space missions. The effect of water vapor in the troposphere on the delay of radar signal must be corrected for in the SSH measurement. The efficacy of a nadir-looking radiometer that has been the approach for conventional altimetry is examined in the study. The focus is placed on the cross-track variability of water vapor that is not measured by the nadir-looking radiometer. Simulations of the 2D field of water vapor were performed by spectral analysis of existing radiometer data. The residual error from the application of the correction made by a nadir-looking radiometer was computed over the global ocean and compared to the SSH signal estimated from satellite altimeter data. Global maps of the signal-to-error ratio (the square root of spectral variance at wavelengths shorter than 500 km) were created, showing values of 20–50 in the regions of high SSH variability of the boundary currents and the Antarctic Circumpolar Current, and 3–5 in the regions of low SSH variability in the tropics. Improvement in the correction by using a two-beam radiometer looking off nadir for measuring the slope of the cross-track variability was also explored, leading to a reduction of the error to below 1 cm at wavelengths of 10–500 km.

Adaptive Retracking of Jason-1, 2 Satellite Altimetry Data for the Volga River Reservoirs

The problem of minimization of errors in water level retrieval for the Volga reservoirs from altimetry measurements can be resolved by retracking Jason-1, 2 satellite altimetry data. For justification of the optimal retracking algorithm average impulse response of a statistically inhomogeneous surface was calculated theoretically for a model of the terrain in the neighbourhood of the reservoirs. The modeled waveforms are in good agreement with Jason-1, 2 waveforms for the same area. Comparison of the data with in situ measurements shows that retracking significantly improves measurement of the water level. General principles of the retracking algorithms for complex areas (land, coastal zone, inland waters, etc.) based on calculations of the waveforms taking into account statistical inhomogeneity of the reflecting surface adapted to a certain geographic region, are discussed.

Seasonal variations of the Arabian Sea level anomaly fields and circulation

The changes in the field of the Arabian Sea level anomaly and the geostrophic currents are analyzed based on the data of satellite altimetry measurements in 1993–2008 within the framework of the AVISO project. On the intra-annual scale, the current field generally agrees with the circulation schemes published in a number earlier works. Their differences are due to the occurrence of the mesoscale eddies and jet currents revealed by the authors.

Long-Life Europa Geodesy Orbits Accounting for Navigation Uncertainties

This work investigates the properties of phase space near a periodic orbit and their applications to orbit design and orbit determination. A low-altitude, near-polar periodic orbit is computed and an error covariance is generated by processing range-rate and altimetry measurements over seven days. The resulting covariance is used to disperse the orbit initial conditions in a Monte Carlo simulation. The distribution of the Monte Carlo run is biased toward longer orbit lifetimes, due to the stable and unstable manifolds associated with the periodic orbit. The orbit determination covariance is mapped into manifold coordinates and long lifetime orbits are shown to be aligned with the stable manifold. Periodic orbit continuation is used to generate a family of orbits with similar phase-space characteristics to understand the variation of manifold structure with orbit elements and Jacobi energy. The effect of Europa eccentricity on the phase-space location of long lifetime orbits is discussed and conclusions are given regarding the connection between orbit lifetime, design, and determination.

Identifying flowlines and limitations of flux analyses in the interior of Thwaites Glacier, Antarctica

AbstractPatterns in radar-detected internal layers in glaciers and ice streams can be tracked hundreds of kilometers downstream. We use distinctive patterns to delineate flowbands of Thwaites Glacier in the Amundsen Sea sector of West Antarctica. Flowbands contain information for the past century to millennium, the approximate time for ice to flow through the study region. GPS-detected flow directions (acquired in 2007/08) agree within uncertainty (~4°) with the radar-detected flowlines, indicating that the flow direction has not changed significantly in recent centuries. In contrast, InSAR-detected directions (from 1996) differ from the radar- and GPS-detected flowlines in all but the middle tributary, indicating caution is needed when using InSAR velocities to define flow directions. There is agreement between all three datasets in the middle tributary. We use two radar-detected flowlines to define a 95 km long flowband and perform a flux balance analysis using InSAR-derived velocities, radar-detected ice thickness, and estimates of the accumulation rate. Inferred thinning of 0.49 ± 0.34 m a–1 is consistent with satellite altimetry measurements, but has higher uncertainty due mainly to the velocity uncertainty. The uncertainty is underestimated because InSAR velocities often differ from GPS velocities by more than the stated uncertainties.

21st-century increase in glacier mass loss in the Wrangell Mountains, Alaska, USA, from airborne laser altimetry and satellite stereo imagery

AbstractAlaskan glaciers are among the largest regional contributors to sea-level rise in the latter half of the 20th century. Earlier studies have documented extensive and accelerated ice wastage in most regions of Alaska. Here we study five decades of mass loss on high-elevation, land-terminating glaciers of the Wrangell Mountains (~ 4900 km2) in central Alaska based on airborne center-line laser altimetry data from 2000 and 2007, a digital elevation model (DEM) from ASTER and SPOT5, and US Geological Survey topographic maps from 1957. The regional mass-balance estimates derived from center-line laser altimetry profiles using two regional extrapolation techniques agree well with that from DEM differencing. Repeat altimetry measurements reveal accelerated mass loss over the Wrangell Mountains, with the regional mass-balance rate evolving from –0.07 ± 0.19 m w.e. a–1 during 1957–2000 to –0.24 ± 0.16 m w.e. a–1 during 2000–07. Nabesna, the largest glacier in this region (˜1056 km2), lost mass four times faster during 2000–07 than during 1957–2000. Although accelerated, the mass change over this region is slower than in other glacierized regions of Alaska, particularly those with tidewater glaciers. Together, our laser altimetry and satellite DEM analyses demonstrate increased wastage of these glaciers during the last 50 years.

Glacier mass changes on the Tibetan Plateau 2003–2009 derived from ICESat laser altimetry measurements

Glacier mass changes are a valuable indicator of climate variability and monsoon oscillation on the underexplored Tibetan Plateau. In this study data from the Ice Cloud and Elevation Satellite (ICESat) is employed to estimate elevation and mass changes of glaciers on the Tibetan Plateau between 2003 and 2009. In order to get a representative sample size of ICESat measurements, glaciers on the Tibetan Plateau were grouped into eight climatically homogeneous sub-regions. Most negative mass budgets of − 0.77 ± 0.35 m w.e. a−1 were found for the Qilian Mountains and eastern Kunlun Mountains while a mass gain of + 0.37 ± 0.25 m w.e. a−1 was found in the westerly-dominated north-central part of the Tibetan Plateau. A total annual mass budget of − 15.6 ± 10.1 Gt a−1 was estimated for the eight sub-regions sufficiently covered by ICESat data which represents ∼80% of the glacier area on the Tibetan Plateau. 13.9 ± 8.9 Gt a−1 (or 0.04 ± 0.02 mm a−1 sea-level equivalent) of the total mass budget contributed ‘directly’ to the global sea-level rise while 1.7 ± 1.9 Gt a−1 drained into endorheic basins on the plateau.

Lagrangian analysis of satellite-derived currents: Application to the North Western Mediterranean coastal dynamics

Optimal interpolation methods for improving the reconstruction of coastal dynamics from along-track satellite altimetry measurements have been recently developed over the North Western Mediterranean Sea. Maps of satellite-derived geostrophic current anomalies are generated using these methods, and added to different mean circulation fields in order to obtained absolute geostrophic currents. The resulting fields are then compared to standard AVISO products, and their accuracies are assessed with Lagrangian diagnostics. The trajectories of virtual particle clusters are simulated with a Lagrangian code either with new current fields or with the AVISO ones. The simulated trajectories are then compared to 16 in situ drifter trajectories to evaluate the performance of the different velocity fields. The comparisons show that the new current fields lead to better results than the AVISO one, especially over the shallow continental shelf of the Gulf of Lion. However, despite the use of innovative strategies, some altimetry limitations still persist in the coastal domain, where small scale processes remain sub-sampled by conventional altimetry coverage but will benefit from technological development in the near future. Some of the limitations of the Lagrangian diagnostics presently used are also analyzed, but dedicated studies will be required for future further investigations.

Effect of the processing methodology on satellite altimetry-based global mean sea level rise over the Jason-1 operating period

Determining how the global mean sea level (GMSL) evolves with time is of primary importance to understand one of the main consequences of global warming and its potential impact on populations living near coasts or in low-lying islands. Five groups are routinely providing satellite altimetry-based estimates of the GMSL over the altimetry era (since late 1992). Because each group developed its own approach to compute the GMSL time series, this leads to some differences in the GMSL interannual variability and linear trend. While over the whole high-precision altimetry time span (1993–2012), good agreement is noticed for the computed GMSL linear trend (of \(3.1\pm 0.4\) mm/year), on shorter time spans (e.g., \({<}10~\hbox {years}\)), trend differences are significantly larger than the 0.4 mm/year uncertainty. Here we investigate the sources of the trend differences, focusing on the averaging methods used to generate the GMSL. For that purpose, we consider outputs from two different groups: the Colorado University (CU) and Archiving, Validation and Interpretation of Satellite Oceanographic Data (AVISO) because associated processing of each group is largely representative of all other groups. For this investigation, we use the high-resolution MERCATOR ocean circulation model with data assimilation (version Glorys2-v1) and compute synthetic sea surface height (SSH) data by interpolating the model grids at the time and location of “true” along-track satellite altimetry measurements, focusing on the Jason-1 operating period (i.e., 2002–2009). These synthetic SSH data are then treated as “real” altimetry measurements, allowing us to test the different averaging methods used by the two processing groups for computing the GMSL: (1) averaging along-track altimetry data (as done by CU) or (2) gridding the along-track data into \(2^{\circ }\times 2^{\circ }\) meshes and then geographical averaging of the gridded data (as done by AVISO). We also investigate the effect of considering or not SSH data at shallow depths \(({<}120~\hbox {m})\) as well as the editing procedure. We find that the main difference comes from the averaging method with significant differences depending on latitude. In the tropics, the \(2^{\circ }\times 2^{\circ }\) gridding method used by AVISO overestimates by 11 % the GMSL trend. At high latitudes (above \(60^{\circ }\hbox {N}/\hbox {S}\)), both methods underestimate the GMSL trend. Our calculation shows that the CU method (along-track averaging) and AVISO gridding process underestimate the trend in high latitudes of the northern hemisphere by 0.9 and 1.2 mm/year, respectively. While we were able to attribute the AVISO trend overestimation in the tropics to grid cells with too few data, the cause of underestimation at high latitudes remains unclear and needs further investigation.