Juliette Lambin

The OSTM/Jason-2 Mission

The Ocean Surface Topography Mission/Jason-2 (OSTM/Jason-2) satellite altimetry mission was successfully launched on June 20, 2008, as a cooperative mission between CNES, EUMETSAT, NASA, and NOAA. OSTM/Jason-2 will continue to precisely measure the surface topography of the oceans and continental surface waters, following on the same orbit as its predecessors, TOPEX/Poseidon and Jason-1. To maintain the high-accuracy measurements, the mission carries a dual-frequency altimeter, a three-frequency microwave radiometer, and three precise positioning systems. The objectives of the mission are both operational and scientific. The mission will provide near-real time high-precision altimetric measurements for integration into ocean forecasting models and other products. The mission will also extend the precise surface topography time series started by TOPEX/Poseidon in 1992 over two decades in order to study long-term ocean variations such as mean sea level variations and interannual and decadal oscillations. The measurement system has been adapted to provide quality data nearer to the coasts, and over lakes and rivers. This paper provides an overview of the OSTM/Jason-2 mission in terms of the system design and a brief introduction to the science objectives.

The SARAL/AltiKa Altimetry Satellite Mission

The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedi-cated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. Secondary objectives include ice sheet and inland waters monitoring. One year after launch, the results widely confirm the nominal expectations in terms of accuracy, data quality and data availability in general. Todays performances are compliant with specifications with an overall observed performance for the Sea Surface Height RMS of 3.4 cm to be compared to a 4 cm requirement. Some scientific examples are provided that illustrate some salient features of todays SARAL/AltiKa data with regard to standard altimetry: data availability, data accuracy at the mesoscales, data usefulness in costal area, over ice sheet, and for inland waters.

Poseidon-3 Radar Altimeter: New Modes and In-Flight Performances

On June 20, 2008, the altimetry satellite Jason-2 was launched from the Vandenberg site in California. Dedicated to the measure of ocean surface topography, one of the main instruments on-board is a radar altimeter, Poseidon-3, which essentially measures the altimetric range between the spacecraft and the surface. Poseidon-3 is a dual frequency radar altimeter operating in Ku and C bands, very similar to its predecessor Poseidon-2 on-board Jason-1. However some significant improvements have been implemented to improve its tracking capabilities over coastal and inland waters, that is, its capacity to maintain data acquisition over land or mixed land-sea terrain. The performance assessment is excellent: the range measurement accuracy is close to 1.5 cm for 1s averaging and the significant wave height (SWH) noise is less than 12 cm (for a 2m SWH at 1σ).In terms of range, the short-term drift (along an orbit) is around 1 mm, and the long-term drift is negligible so far. The tracking success is close to 100% over oceans and 80% over land surfaces, the new acquisition and tracking modes inducing significantly higher data availability in comparison with Poseidon-2. We assess Poseidon-3 main improvements, with the presentation of the new modes of echo acquisition and tracking: the median tracking algorithm, DIODE/DORIS acquisition, and the coupling between DIODE and digital elevation model (DEM) information. The median tracking algorithm is shown to reinforce the robustness of the altimetry echoes outside the standard Brown conditions. DIODE acquisition mode increases data availability in land-to-water transitions, providing up to 5 km of extra measurements along track, which constitutes an asset for coastal and small water areas (lakes, rivers) observations. Both are now implemented as the default mode on Jason-2. DIODE/DEM mode remains experimental and requires further adjustments but shows promising features such as acquisition of water surfaces in rough terrain.

Finding Desirable Orbit Options for the “Extension of Life” Phase of Jason-1

The ageing of Jason-1, the risk of losing control of the satellite, and the collision risk with TOPEX/Poseidon (still in orbit and no longer maneuverable) initiated a reflection on a so-called “extension of life phase” (EoL) phase that would involve moving Jason-1 to a new orbit to mitigate collision risks while optimizing its science return. This paper describes three practical consequences of any such EoL phase: 1) the ability to build an unprecedented low inclination and high precision geodetic dataset, 2) the loss of coordination with Jason-2 and the associated mesoscale (and sea state) sampling degradation, and 3) the increased topography height error budget stemming from the use of a gridded mean sea surface in place of the classical repeat track analysis that operational systems have been using and improving for almost two decades. More than 17,000 potential orbits were analyzed to identify desirable altitude ranges that could host a Jason-1 EoL phase. The objective was to minimize the sampling degradation of ocean observations (primary objective of Jason-1) while securing a good geodetic EoL dataset (secondary objective of Jason-1). After a first automated screening and scoring process, the final orbit candidates are analyzed through an end-to-end Observing System Simulation Experiment (OSSE) protocol, assessing the multimission observational capability of the EoL phase in a DUACS/AVISO-like system. All EoL orbits are shown to be largely inferior to the interleaved orbit as far as oceanography is concerned. Yet some EoL options are shown to be more desirable than others because their sampling patterns blend well with Jason-2. Good geodetic orbit options could provide a unique bathymetry-oriented dataset and help improve gridded mean sea surfaces (MSS), while repetitive options with a short cycle could cancel some additional EoL errors if a conservative repeat track strategy is preferred.

OSTM/Jason-2: Assessment of the System Performances (Ocean Surface Topography Mission: OSTM)

On June 20, 2008, Jason-2 was successfully launched by a Boeing Delta II rocket from the Vandenberg site, California. The OSTM/Jason-2 project is a cooperation among NASA, NOAA, EUMETSAT, and CNES. The first two months of the OSTM/Jason-2 mission have been dedicated to the assessment of the overall system. The goal of this assessment phase was: i. to assess the behavior of the spacecraft, at the platform and payload levels; ii. to verify that platform performance requirements are met with respect to Jason-2 requirements; iii. to verify that payload instruments performance requirements evaluated at instrument level are met; and iv. to assess the performance of the Jason-2 Ground System. The paper 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.

Using Objective Analysis of Scanning Radiometer Measurements to Compute the Water Vapor Path Delay for Altimetry

An objective analysis (OA) method is implemented to compute the water vapor path delay (PD) correction of the altimeter range using total precipitable water measurements from scanning microwave radiometers (Advanced Microwave Sounding Unit A, Advanced Microwave Scanning Radiometer-Earth Observing System, Tropical Rain Measuring Mission Microwave Imager, and Special Sensor Microwave Imager). The European Centre for Medium Range Weather Forecasts (ECMWF) model-derived water vapor PD correction given in the altimeter products is used as the first-guess field. The calculation of the statistical variables required by the OA is presented: These include the variance and correlation function of the radiometer observations minus its first guess, as well as the observation error variance. The performance of the OA-derived water vapor PD correction is assessed, using four months of Jason-1 altimeter data. It is shown that the OA-derived correction is more accurate than the ECMWF-derived correction but remains less accurate than the one derived from the Jason microwave radiometer.

Empirical Cross-Calibration of Coherent SWOT Errors Using External References and the Altimetry Constellation

This paper gives an overview of an empirical cross-calibration technique developed for the Surface Water Ocean Topography mission (SWOT). The method is here used to detect and to mitigate two spatially coherent errors in SWOT topography data: the baseline roll error whose signature is linear across track, and the baseline length error whose signature is quadratic across track. Assuming that topography data are corrupted by coherent error signatures that we can model, we extract the signatures, and we empirically use the error estimates to correct SWOT data. The cross-calibration is tackled with a two-step scheme. The first step is to get local estimates over cross-calibration zones, and the second step is to perform a global interpolation of local error estimates and to mitigate the error everywhere. Three methods are used to get local error estimates: 1) we remove a static first guess reference such as a digital elevation model, 2) we exploit overlapping diamonds between SWOT swaths, and 3) we exploit overlapping segments with traditional pulse-limited altimetry sensors. Then, the along-track propagation is performed taking the local estimates as an input, and an optimal interpolator (1-D objective analysis) constrained with a priori statistical knowledge of the problem. The rationale of this paper is to assume that SWOTs scientific requirements are met on all errors but the ones being cross-calibrated. In other words, the algorithms presented in this paper are not needed at this stage of the mission definition, and they are able to deal with higher error levels (e.g., if hardware constraints are relaxed and replaced by additional ground processing). Even in our most pessimistic theoretical scenarios of baseline roll and baseline length errors (up to 70 cm RMS of uncorrected topography error), the cross-calibration algorithm reduces coherent errors to less than 2 cm (outer edges of the swath). Residual errors are subcentimetric for very low-frequency errors (e.g., orbital revolution). Sensitivity tests highlight the benefits of using additional pulse-limited altimeters and optimal inversion schemes when the problem is more difficult to solve (e.g., wavelengths of less than 1000 km), but also to provide a geographically homogeneous correction that cannot be obtained with SWOTs sampling alone.

Measuring Ocean Waves From Space: Objectives and Characteristics of the China-France Oceanography SATellite (CFOSAT)

The Chinese and French Space Agencies are jointly preparing a satellite mission devoted to the monitoring of the ocean surface and related science and applications. This is the so-called “China France Oceanography SATellite” (CFOSAT), to be launched around 2013. This mission will provide simultaneous and collocated observations of wind at the ocean surface and spectral properties of surface ocean waves using two scatterometers, both in Ku-Band: SWIM for measurements of directional wave spectra and SCAT for wind vector measurements. The SWIM instrument will use a real aperture observation technique so as to avoid limitations encountered with SAR systems. This paper describes the main objectives and characteristics of the mission with a focus on the SWIM instrument designed and developed under French responsibility to measure directional spectra of ocean waves.Copyright

Combining all flying microwave radiometers to compute the water vapor path delay for altimeter missions

An objective analysis (OA) method is implemented, to compute the altimeter path delay (PD) induced by atmospheric water vapor. Input PD observations are derived from total precipitable water measurements from all currently flying scanning microwave radiometers (AMSU-A, AMSR-E, TMI and SSMI). ECMWF model derived PD is used as first-guess field. Calculation of the statistical variables required by the OA are presented : these include the variance and correlation function of the radiometer path delay minus its first-guess, as well as the observations error variance. The performance of the OA-derived water vapor path delay is assessed, using 4 months of Jason-1 altimeter data. It is shown that the OA-derived PD estimate is more accurate than the ECMWF model derived one, but remains less accurate than the one derived from the Jason microwave radiometer.

A Spaceborne Radar for Directional Wave Spectrum Estimation: First Performance Simulations

SWIM is a Ku-band radar designed for wave directional spectrums estimation. This radar operates at six incidence angles (0deg to 10deg) with complete azimuth scanning. This paper presents the simulation tool developed for defining SWIM design and evaluating SWIM performance. The simulation tool is an end-to-end simulator, i.e. from the sea surface to the estimated wave spectrum. Some simulations are discussed, showing that the preliminary design of SWIM will fulfill the scientific requirements.