Yves Menard

Exploiting the potential of an improved multimission altimetric data set over the coastal ocean

Until now, most satellite altimetry studies of the coastal ocean have been based on along-track data from a single mission, whereas up to four missions were operative in 2002–2005. Here, to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast. The resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern Mediterranean. The mean rms difference between this data set and the sea level stations is 2.9 cm against 3.7 cm when using the AVISO altimetric product. Comparison of altimeter-derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced. The agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal-band.

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.

Offshore Absolute Calibration of Space-Borne Radar Altimeters

Absolute calibration of sea level measurements collected from space-borne radar altimeters is usually performed with respect to collocated sea level in situ records from tide gauges or GPS buoys (Ménard et al. 1994; Haines et al. 1996; Bonnefond et al. 2003; Haines et al. 2003; Schum et al. 2003; Watson et al. 2003; Watson et al. 2004). Such a method allows regular and long-term control of altimetric systems with independent records. However, this approach is based on a single, geographically dependent point. In order to obtain more significant and accurate bias and drift estimates, there is a strong interest in multiplying the number of calibration opportunities. This article describes a method, called the “offshore method” that was developed to extend the single-point approach to a wider regional scale. The principle is to compare altimeter and tide gauge sea level data not only at the point of closest approach of an overflying pass, but also at distant points along adjacent satellite passes. However, connecting sea level satellite measurements with more distant in situ data requires a more accurate determination of the geoid and mean ocean dynamic topography slopes, and also of the ocean dynamical changes. In this demonstration experiment, 10 years of averaged TOPEX/Poseidon mean sea level profiles are used to precisely determine the geoid and the mean ocean circulation slope. The Mog2d barotropic ocean model (Carerre et Lyard 2003) is used to improve our estimate of the ocean dynamics term. The method is first validated with Jason-1 data, off Corsica, where the dedicated calibration site of Senetosa provides independent reference data. The method is then applied to TOPEX/Poseidon on its new orbit and to Geosat Follow On. The results demonstrate that it is feasible to make altimeter calibrations a few tens to hundreds of kilometers away from a dedicated site, as long as accurate mean sea level altimeter profiles can be used to ensure the connection with reference tide gauges.

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.

Calibrating the jason-1 measurement system: Initial initial results from the corsica and harvest verification experiments

Abstract We present calibration results from Jason-1 (2002-) and TOPEX/Poseidon (1992-) overflights of dedicated verification sites on the Mediterranean island of Corsica and on a California offshore oil platform (Harvest). Harvest served for a decade (1992–2002) as a calibration site for the TOPEX/Poseidon (T/P) mission, and is serving in a similar capacity for Jason-1. Initiated in 1996, the Corsica experiment features a fiducial reference station near Aspretto, and a primary sub-satellite tide-gauge deployment site 40 km south at Cape Senetosa. Both Corsica and Harvest feature carefully designed collocations of space-geodetic and tide-gauge systems to support the absolute calibration of the altimetric sea-surface height (SSH). Early estimates of the SSH bias from Harvest and Corsica are in excellent agreement, indicating that interim SSH measurements from Jason-1 were too high by +5 ± 1 cm (one standard error). By incorporating improved estimates of the Jason-1 sea-state bias and columnar atmospheric wet path delay, we observe a significant increase—to about 12 cm—in the SSH. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from early overflights. In addition to providing important insight on the accuracy of the science data products during the validation phase of the mission, the estimates of the SSH bias and stability from Corsica, Harvest and other calibration programs will be used to link the T/P and Jason-1 sea-level records.

Impact of the DORIS precise orbit determination system on climate change studies

Abstract The goal of the paper is to demonstrate that DORIS helped opening a new era in altimetry since TOPEX/POSEIDON (T/P), and that DORIS will insure that long time series of altimeter measurements may be used in a consistent manner for the highly accurate quantification of the tiny ocean signals impacting climate change at multi-year time scales.

Ocean and climate: A quantitative answer, TOPEX/POSEIDON☆

Abstract Since B. Franklin and F. Arago, the Oceans are known for their regulator role in the climate evolution. Beyond this simple ascertainment, it is now necessary and possible to quantify more precisely the impact of the Oceans in climate global changes. Accurate observations at a global scale and over several decades, jointly used with dynamic models, are needed to solve this problem. One dedicated technique is radar altimetry on-board satellites. Sea surface slopes and derived surface currents can thus be measured with a remarkable accuracy. The TOPEX/POSEIDON altimetry mission was optimized to fulfil this objective, this project was jointly conducted by NASA and CNES with support of the associated scientific community involved in the design, the evaluation and the analysis of the system. Two years after the launch (10 August 1992), the results are even better than expected. Sea surface heights and variations are measured at a basin-scale, with an accuracy of 1–2 centimeters over 10 days. Such performances give access to the seasonal changes due to the thermal expansion between the two hemispheres, the interannual variability and the detection and propagation of Kelvin and Rossby waves in the tropical regions. These observations can then be assimilated in numerical models to describe and predict the ocean circulation in four dimensions.

Advances in Large-Scale Ocean Dynamics From a Decade of Satellite Altimetric Measurement of Ocean Surface Topography

The past decade has seen the most intensive observations of the global ocean surface topography from satellite altimeters. The Joint U.S./France TOPEX/POSEJDON (T/P) Mission has become the longest radar mission ever flown in space, providing the most accurate measurements for the study of ocean dynamics since October, 1992. The European Space Agency’s ERS-1 and -2 Mission also provided altimetric observations from 1991–2000. The combined data from T/P and ERS have higher spatial resolution and greater coverage than the individual missions. Major advances in large-scale ocean dynamics from these observations are reviewed in the paper, including the ocean general circulation and its variability, the evolution of the El Nino Southern Oscillation cycles as well as the emerging decadal variability, the response of the ocean to wind forcing, assimilation of altimeter data by ocean general circulation models and the estimation of deep ocean circulation, global sea level rise, and tidal models and mixing.