Accuracy Assessment of Jason-1 and TOPEX/POSEIDON Along-Track Sea Surface Slope

Sea surface slope computed from along-track Jason-1 and TOPEX/POSEIDON (T/P) altimeter data at ocean mesoscale wavelengths are compared to determine the equivalent 1 Hz instrument height noise of the Poseidon-2 and TOPEX altimeters. This geophysical evaluation shows that the Ku-band 1-Hz range noise for both instruments is better than 1.7 cm at 2 m significant wave heights (H_1/3), exceeding error budget requirements for both missions. Furthermore, we show that the quality of these instruments allows optimal filtering of the 1-Hz along-track sea surface height data for sea surface slopes that can be used to calculate cross track geostrophic velocity anomalies at the baroclinic Rossby radius of deformation to better than 5 cm/sec precision along 87.5% of the satellite ground track between 2 and 60 degrees absolute latitude over the deep abyssal ocean (depths greater than 1000 m). This level of precision will facilitate scientific studies of surface geostrophic velocity variability using data from the Jason-1 and T/P Tandem Mission.     

The Harvest Experiment: Monitoring Jason-1 and TOPEX/POSEIDON from a California Offshore Platform Special Issue: Jason-1 Calibration/Validation

We present calibration results from Jason-1 (2001–) and TOPEX/POSEIDON (1992–) overflights of a California offshore oil platform (Harvest). Data from Harvest indicate that current Jason-1 sea-surface height (SSH) measurements are high by 138 ± 18 mm. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from the overflights. In orbit for over 10 years, the T/P measurement system is well calibrated, and the SSH bias is statistically indistinguishable from zero. Also reviewed are over 10 years of geodetic results from the Harvest experiment.     

Calibration and Verification of Jason-1 Using Global Along-Track Residuals with TOPEX Special Issue: Jason-1 Calibration/Validation

It is demonstrated that the Jason-1 measurements of sea surface height (SSH), wet path delay, and ionosphere path delay are within required accuracies, via a global cross-calibration with similar measurements made by TOPEX/Poseidon (T/P) over a 6-month period. Since the two satellites were on the same groundtrack separated in time by only 70 s, measurements were recorded at approximately the same location and time. The variations in the wet path delay measured by Jason-1 compared to T/P are only 5 mm RMS, well within the required performance of 1.2 cm RMS. The RMS of the ionosphere differences is also well within the expected values, with a mean RMS of 1.2 cm. The largest difference is that the Jason-1 SSH is biased high relative to T/P SSH by 144 mm after the T/P and Jason-1 data are both corrected with improved sea state bias (SSB) models. However, the bias will change if a different SSB model is used, so the user should be cautious that the bias used matches the SSB models. The bias is generally constant within ± 10 mm in the open ocean, but appears to be higher or lower in some regions. Additionally, the SSH has been verified by comparison with 36 island tide gauges over the same period. After removing the global relative bias, the Jason-1 SSH data agree with tide gauges within 3.7 cm RMS and with T/P data within about 3.5 cm RMS on average for 1-s measurements, meeting the required accuracy of 4.2 cm RMS.     

Jason-1: Assessment of the System Performances Special Issue: Jason-1 Calibration/Validation

On 7 December 2001, Jason-1 was successfully launched by a Boeing Delta II rocket from the Vandenberg Air Force Base, California. The Jason-1 satellite will maintain the high accuracy altimeter service provided since 1992 by TOPEX/Poseidon (T/P), ensuring the continuity in observing and monitoring the Ocean Dynamics (intraseasonal to interannual changes, mean sea level, tides, etc.). Despite one-fourth the mass and power, the Jason-1 system has been designed to have basically the same performance as T/P, measuring sea surface topography at a centimetric level. This new CNES/NASA mission also provides near real-time data for sea state and ocean forecast. The first two months of the Jason-1 mission have been dedicated to the assessment of the overall system. The goals of this assessment phase were: 1. To assess the behavior of the spacecraft at the platform and payload levels (Jason-1 being the first program to call on the PROTEUS versatile multimission platform for Low and Medium Earth Orbit Missions developed in partnership between Alcatel Space and CNES); 2. To verify that platform performance requirements are met with respect to Jason-1 requirements; 3. To verify that payload instruments performance requirements evaluated at instrument level are met; 4. To assess the performance of the Jason-1 Ground System. This article will display the main outputs of the assessment of the system. It will demonstrate that all the elements of the onboard and ground systems are within the specifications. Provision of data to the Jason-1 Science Working Team started at the end of March 2002. This is the goal of a six-month phase after closure of the initial assessment phase to derive the error budget of the system in terms of altimetry user products.     

The Jason-1 Mission Special Issue: Jason-1 Calibration/Validation

On December 7, 2001, the Jason-1 satellite was successfully launched by a Boeing Delta II rocket from the Vandenberg site in California, USA. Its main mission was to maintain the high accuracy altimeter measurements, provided since 1992 by TOPEX/Poseidon (T/P), ensuring continuity in observing and monitoring the ocean for intraseasonal to interannual changes, mean sea level, tides, and so forth. Despite four times less mass and power, the Jason-1 system has been designed to have the same performances as T/P, measuring sea surface topography at the centimeter level. This new Centre National d'Etudes Spatiales/National Aeronautics and Space Administration (CNES/NASA) mission also provides near real-time data for sea state and ocean forecast. The first 10 months of the Jason mission were dedicated to the verification of the system performance and cross-calibration with T/P measurements. A complete CALVAL plan was conducted by the Science and Project Teams of the mission based on in situ and regional experiments, global statistical approaches, and multisatellite comparisons, taking advantage of the T/P-Jason overlap during the first months of the mission. CALVAL and first science results showed that the Jason-1 performances were compliant with prelaunch specifications. This was a needed preamble before starting the routine phase of the mission in July 2003 with generation and distribution of validated geophysical data records to the whole user community.     

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica Special Issue: Jason-1 Calibration/Validation

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.     

Calibration of JASON-1 Altimeter over Lake Erie Special Issue: Jason-1 Calibration/Validation

This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992–2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr.     

The 1-Centimeter Orbit: Jason-1 Precision Orbit Determination Using GPS,SLR, DORIS,and Altimeter Data Special Issue: Jason-1 Calibration/Validation

The Jason-1 radar altimeter satellite, launched on December 7, 2001 is the follow on to the highly successful TOPEX/Poseidon (T/P) mission and will continue the time series of centimeter level ocean topography measurements. Orbit error is a major component in the overall error budget of all altimeter satellite missions. Jason-1 is no exception and has set a 1-cm radial orbit accuracy goal, which represents a factor of two improvement over what is currently being achieved for T/P. The challenge to precision orbit determination (POD) is both achieving the 1-cm radial orbit accuracy and evaluating the performance of the 1-cm orbit. There is reason to hope such an improvement is possible. The early years of T/P showed that GPS tracking data collected by an on-board receiver holds great promise for precise orbit determination. In the years following the T/P launch there have been several enhancements to GPS, improving its POD capability. In addition, Jason-1 carries aboard an enhanced GPS receiver and significantly improved SLR and DORIS tracking systems along with the altimeter itself. In this article we demonstrate the 1-cm radial orbit accuracy goal has been achieved using GPS data alone in a reduced dynamic solution. It is also shown that adding SLR data to the GPS-based solutions improves the orbits even further. In order to assess the performance of these orbits it is necessary to process all of the available tracking data (GPS, SLR, DORIS, and altimeter crossover differences) as either dependent or independent of the orbit solutions. It was also necessary to compute orbit solutions using various combinations of the four available tracking data in order to independently assess the orbit performance. Towards this end, we have greatly improved orbits determined solely from SLR+DORIS data by applying the reduced dynamic solution strategy. In addition, we have computed reduced dynamic orbits based on SLR, DORIS, and crossover data that are a significant improvement over the SLR- and DORIS-based dynamic solutions. These solutions provide the best performing orbits for independent validation of the GPS-based reduced dynamic orbits. The application of the 1-cm orbit will significantly improve the resolution of the altimeter measurement, making possible further strides in radar altimeter remote sensing.     

Calibration and Verification of Jason-1 Using Global Along-Track Residuals with TOPEX

It is demonstrated that the Jason-1 measurements of sea surface height (SSH), wet path delay, and ionosphere path delay are within required accuracies, via a global cross-calibration with similar measurements made by TOPEX/Poseidon (T/P) over a 6-month period. Since the two satellites were on the same groundtrack separated in time by only 70 s, measurements were recorded at approximately the same location and time. The variations in the wet path delay measured by Jason-1 compared to T/P are only 5 mm RMS, well within the required performance of 1.2 cm RMS. The RMS of the ionosphere differences is also well within the expected values, with a mean RMS of 1.2 cm. The largest difference is that the Jason-1 SSH is biased high relative to T/P SSH by 144 mm after the T/P and Jason-1 data are both corrected with improved sea state bias (SSB) models. However, the bias will change if a different SSB model is used, so the user should be cautious that the bias used matches the SSB models. The bias is generally constant within ± 10 mm in the open ocean, but appears to be higher or lower in some regions. Additionally, the SSH has been verified by comparison with 36 island tide gauges over the same period. After removing the global relative bias, the Jason-1 SSH data agree with tide gauges within 3.7 cm RMS and with T/P data within about 3.5 cm RMS on average for 1-s measurements, meeting the required accuracy of 4.2 cm RMS.     

Towards a Seamless Transition from TOPEX/Poseidon to Jason-1

The Jason-1 verification phase has proven to be a unique and successful calibration experiment to quantify the agreement with its predecessor TOPEX/Poseidon. Although both missions have met prescribed error budgets, comparison of the mean and time-varying sea surface height profiles from near simultaneous observations derived from the missions' Geophysical Data Records exhibit significant basin scale differences. Several suspected sources causing this disagreement are identified and improved upon, including (a) replacement of TOPEX and Jason project POE with enhanced orbits computed at GSFC within a consistent ITRF2000 terrestrial reference frame, (b) application of waveform retracking corrections to TOPEX significant wave height and sea surface heights, (c) resultant improved efficacy of the TOPEX sea state bias estimation from the value added sea surface height, and (d) estimation of Jason-1 sea state bias employing dual TOPEX/Jason crossover and collinear sea surface height residuals unique to the validation mission. The resultant mean sea surface height comparison shows improved agreement at better than 60 percent level of variance reduction with a standard deviation less then 0.5 cm.     

Towards a Seamless Transition from TOPEX/Poseidon to Jason-1

The Jason-1 verification phase has proven to be a unique and successful calibration experiment to quantify the agreement with its predecessor TOPEX/Poseidon. Although both missions have met prescribed error budgets, comparison of the mean and time-varying sea surface height profiles from near simultaneous observations derived from the missions' Geophysical Data Records exhibit significant basin scale differences. Several suspected sources causing this disagreement are identified and improved upon, including (a) replacement of TOPEX and Jason project POE with enhanced orbits computed at GSFC within a consistent ITRF2000 terrestrial reference frame, (b) application of waveform retracking corrections to TOPEX significant wave height and sea surface heights, (c) resultant improved efficacy of the TOPEX sea state bias estimation from the value added sea surface height, and (d) estimation of Jason-1 sea state bias employing dual TOPEX/Jason crossover and collinear sea surface height residuals unique to the validation mission. The resultant mean sea surface height comparison shows improved agreement at better than 60 percent level of variance reduction with a standard deviation less then 0.5 cm.     

Accuracy Assessment of the TOPEX/Poseidon Ionosphere Measurements

We conducted an assessment of the TOPEX dual-frequency nadir ionosphere observations in the TOPEX/Poseidon (T/P) GDR by comparing TOPEX with the Center for Orbit Determination in Europe (CODE) Global Ionosphere Map (GIM), the climatological model IRI2001, and the DORIS (onboard T/P) relative ionosphere delays. We investigated the TOPEX (TOPEX Side A and TOPEX Side B altimeters, TSA and TSB, respectively) ionosphere observations for the time period 1995–2001, covering periods of low, intermediate, and high solar activity. Here, we use absolute path delays (at Ku-band frequency of the TOPEX altimeter and with positive signs) rather than Total Electron Content (TEC). We found significant biases between GIM and TOPEX (GIM–TOPEX) nadir ionosphere path delays: ?8.1 ± 0.4 {mm} formal uncertainties and equivalent to 3.7 TECu) and ?9.0 ± 0.7 {mm} (4.1 TECu) for TSA and TSB, respectively, indicating that the TOPEX path delay is longer (or with higher TECu) than GIM. The estimated relative biases vary with latitude and with daytime or nighttime passes. The estimated biases in the path delays (DORIS–TOPEX) are: ?10.9 ± 0.4 {mm} (5.0 TECu) and ?14.8 ± 0.6 {mm} (6.7 TECu), for TSA and TSB, respectively. There is a distinct jump of the DORIS path delays (?3.9 ± 0.7 {mm}, TSA delays longer than TSB delays) at the TSB altimeter switch in February 1999, presumably due to inconsistent DORIS processing. The origin of the bias between GIM (GPS, L-band) and TOPEX (radar altimeter, Ku-band) is currently unknown and warrants further investigation. Finally, the estimated drift rates between GIM and TSA, DORIS and TSA ionosphere path delays for the 6-year study span are ?0.4 mm/yr and ?0.8 mm/yr, respectively, providing a possible error bound for the TOPEX/Poseidon sea level observations during periods of low and intermediate solar activity.     

Velocity Statistics Inferred from the TOPEX/Poseidon-Jason-1 Tandem Mission Data

Using a parallel-track approach to estimate geostrophic surface velocities, an estimate of the statistics of ocean geostrophic surface currents and momentum stresses is provided on a 10 km along-track resolution from the first 49 repeat cycles (16 months) of the Jason-TOPEX/Poseidon tandem altimetric sea surface height (SSH) data. Results are compared with estimates obtained in a traditional way from along-track SSH data at crossover points and with in situ, Acoustic Doppler Current Profiler (ADCP) measurements obtained on board the VOS Oleander along a nominal path connecting Bermuda with the U.S. mainland. Agreements with the Oleander data are reasonable when simultaneous (in space and time) sampling is available. However, amplitudes of parallel-track geostrophic velocity variances are about 25% lower as compared to Oleander measurements which represent geostrophic and ageostrophic flow components. Estimates of velocity variances show clear signs of an anisotropic eddy field in the vicinity of all major current systems. At the same time estimates of Reynolds stresses and eddy momentum fluxes show a convergence of eddy momentum in all those regions, suggesting a forcing of the mean flow by the eddy field there.     

Velocity Statistics Inferred from the TOPEX/Poseidon-Jason-1 Tandem Mission Data

Using a parallel-track approach to estimate geostrophic surface velocities, an estimate of the statistics of ocean geostrophic surface currents and momentum stresses is provided on a 10 km along-track resolution from the first 49 repeat cycles (16 months) of the Jason-TOPEX/Poseidon tandem altimetric sea surface height (SSH) data. Results are compared with estimates obtained in a traditional way from along-track SSH data at crossover points and with in situ, Acoustic Doppler Current Profiler (ADCP) measurements obtained on board the VOS Oleander along a nominal path connecting Bermuda with the U.S. mainland. Agreements with the Oleander data are reasonable when simultaneous (in space and time) sampling is available. However, amplitudes of parallel-track geostrophic velocity variances are about 25% lower as compared to Oleander measurements which represent geostrophic and ageostrophic flow components. Estimates of velocity variances show clear signs of an anisotropic eddy field in the vicinity of all major current systems. At the same time estimates of Reynolds stresses and eddy momentum fluxes show a convergence of eddy momentum in all those regions, suggesting a forcing of the mean flow by the eddy field there.     

The GAVDOS Mean Sea Level and Altimeter Calibration Facility: Results for Jason-1

The location of the GAVDOS facility is under a crossing point of the original ground-tracks of TOPEX/Poseidon and the present ones for Jason-1, and adjacent to an ENVISAT pass, about 50 km south of Crete, Greece. Ground observations and altimetry comparisons over cycles 70 to 90, indicate that a preliminary estimate of the absolute measurement bias for the Jason-1 altimeter is 144.7 ± 15 mm. Comparison of Jason microwave radiometer data from cycles 37 and 62, with locally collected water vapor radiometer and solar spectrometer observations indicate a 1–2 mm agreement.     

The GAVDOS Mean Sea Level and Altimeter Calibration Facility: Results for Jason-1

The location of the GAVDOS facility is under a crossing point of the original ground-tracks of TOPEX/Poseidon and the present ones for Jason-1, and adjacent to an ENVISAT pass, about 50 km south of Crete, Greece. Ground observations and altimetry comparisons over cycles 70 to 90, indicate that a preliminary estimate of the absolute measurement bias for the Jason-1 altimeter is 144.7 ± 15 mm. Comparison of Jason microwave radiometer data from cycles 37 and 62, with locally collected water vapor radiometer and solar spectrometer observations indicate a 1-2 mm agreement.     

Assessment of Radar Waveform Retracked Jason-2 Altimetry Sea Surface Heights Near Taiwan Coastal Ocean

This study focuses on assessing the accuracy of 20-Hz waveform retracked Jason-2 (J-2) altimetry sea surface heights (SSHs) in the vicinity of Taiwan by comparisons with the TOPEX/Poseidon (T/P) 10-Hz SSHs and sea level data from the Anping tide gauge. The study areas exhibit high, medium, and low amplitudes of ocean tides and contain diverse bathymetries with depths of 0–4000 m. The performance of Offset Center of Gravity (OCOG), threshold, modified threshold, and ice retrackers was examined by comparing the retracked SSHs with Earth Gravitational Model 2008 (EGM08) geoid via the use of the improvement percentages (IMPs). The results indicate that both altimetry measurements are significantly improved by waveform retracking techniques, with a maximum IMP of 46.6% for T/P and 82.0% for J-2, and the optimal achievement of retrackers is influenced by the characteristics of the study areas. In addition, valid retracked J-2 SSHs are much closer to shorelines than T/P. A comparison of retracked J-2 data with Anping tide gauge records reveals that applying the optimal retracking algorithms reduces the root mean squares of differences and increases the number of valid measurements.     

Jason-1: Assessment of the System Performances

On 7 December 2001, Jason-1 was successfully launched by a Boeing Delta II rocket from the Vandenberg Air Force Base, California. The Jason-1 satellite will maintain the high accuracy altimeter service provided since 1992 by TOPEX/Poseidon (T/P), ensuring the continuity in observing and monitoring the Ocean Dynamics (intraseasonal to interannual changes, mean sea level, tides, etc.). Despite one-fourth the mass and power, the Jason-1 system has been designed to have basically the same performance as T/P, measuring sea surface topography at a centimetric level. This new CNES/NASA mission also provides near real-time data for sea state and ocean forecast. The first two months of the Jason-1 mission have been dedicated to the assessment of the overall system. The goals of this assessment phase were:

1. To assess the behavior of the spacecraft at the platform and payload levels (Jason-1 being the first program to call on the PROTEUS versatile multimission platform for Low and Medium Earth Orbit Missions developed in partnership between Alcatel Space and CNES);

2. To verify that platform performance requirements are met with respect to Jason-1 requirements;

3. To verify that payload instruments performance requirements evaluated at instrument level are met;

4. To assess the performance of the Jason-1 Ground System.

This article will display the main outputs of the assessment of the system. It will demonstrate that all the elements of the onboard and ground systems are within the specifications. Provision of data to the Jason-1 Science Working Team started at the end of March 2002. This is the goal of a six-month phase after closure of the initial assessment phase to derive the error budget of the system in terms of altimetry user products.     

Sea Surface Height Variability in the Indian Ocean from TOPEX/POSEIDON Altimetry and Model Simulations

Sea Surface Height (SSH) variability in the Indian Ocean during 1993-1995 is studied using TOPEX/POSEIDON (T/P) altimetry data. Strong interannual variability is seen in the surface circulation of the western Arabian Sea, especially in the Somali eddy structure. During the Southwest (SW) monsoon, a weak monsoon year is characterized by a single eddy system off Somalia, a strong or normal monsoon year by several energetic eddies. The Laccadive High (LH) and Laccadive Low (LL) systems off southwest India are observed in the altimetric SSH record. The variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal, is also detected. Evidence is found for the propagation of Kelvin and Rossby waves across the northern Indian Ocean; these are examined in the context of energy transfer to the western boundary currents, and associated eddies. A simple wind-driven isopycnal model having three active layers is implemented to simulate the seasonal changes of surface and subsurface circulation in the North Indian Ocean and to examine the response to different wind forcing. The wind forcing is derived from the ERS-1 scatterometer wind stress for the same period as the T/P altimeter data, enabling the model response in different (active/weak) monsoon conditions to be tested. The model output is derived in 10-day snapshots to match the time period of the T/P altimeter cycles. Complex Principal Component Analysis (CPCA) is applied to both altimetric and model SSH data. This confirms that long Rossby waves are excited by the remotely forced Kelvin waves off the southwest coast of India and contribute substantially to the variability of the seasonal circulation in the Arabian Sea.     

TOPEX/Poseidon and Jason Equatorial Sea Surface Slope Anomaly in the Atlantic in 2002: Comparison with Wind and Current Measurements at 23W

A time series of velocity profile in the upper 150 m of the equatorial Atlantic was gathered at 23W in 2002 within the PIRATA program. It constitutes the first time series of near surface current measurements simultaneous with altimetric data in the equatorial Atlantic. The surface slope anomaly along the equator is computed from satellite altimetry, and, as a proxy for the pressure gradient along the equator, compared with the wind and near surface current data. In a first step, a time series of the surface slope anomaly along the equator in the Atlantic is computed from the 10-year-long TOPEX/Poseidon sea level anomalies. A sensitivity study establishes the robustness of the calculation. Apart from a 15 cm bias, the equatorial sea surface slope anomalies estimated either from TOPEX/Poseidon or from Jason over the 6-month overlap (Feb.–Aug. 2002) do not reveal drastic differences. We produce two sea surface slope anomaly composite time series for 2002 (one with T/P data, the other with Jason data during the commissioning phase) and compare them to the wind and velocity data at 23W. As expected, the near surface velocity and depth of the upper limit of the equatorial undercurrent (EUC) are extremely well correlated with the surface pressure gradient anomaly. 10-year-long time series of altimetry-derived zonal sea surface slope anomaly and ECMWF ERA40 wind stress are also well correlated. They exhibit similar spectral content and similar anomalous years. This is a first step towards a full analysis of the EUC dynamics using altimetry, PIRATA data (near surface current and subsurface thermohaline structure) and model. These initial comparisons reinforce the utility of Jason measurements for continuing the 10-year and highly accurate TOPEX/Poseidon time series for study of equatorial signals.