Ocean Future Missions

Synoptic maps of total ocean surface currents (TSCV) from space are needed to improve the parameterisations of oceanic mesoscale and sub-mesoscale dynamics and represent their impact on global ocean circulation, air-sea exchanges and ocean biogeochemistry.  An Ocean Surface Current Mission candidate for the Earth Explorer 9 programme is presented in response to these relevant scientific needs.  The mission makes use of a Ku-band SAR instrument with squinted beams to synoptically measure, in a single pass, the total ocean surface current vector (speed and direction), surface wind vector and spectral wave information at high spatial resolution from a single spaceborne platform.

Earlier studies funded by ESA and the UK Space Agency considered a primary mission product consisting of the total ocean surface current vector over 2 x 100 km swaths with challenging requirements on spatial resolution (1 km or finer) and accuracy (5 cm/s), resulting in a single satellite along-track interferometric (ATI) mission that unfortunately exceedsthe current envisaged EE9 cost envelope and was only marginal with respect to the VEGA launcher.  A further evolution of the mission concept for measuring total ocean surface currents is presented that fits within both the envisaged cost envelope and a VEGA launch fairing – making such a mission a credible Earth Explorer 9 candidate.

A key challenge has been to demonstrate how the mission can be feasibly launched within the tight confines of the VEGA launch vehicle, whilst still providing useful scientific return.  This has been accomplished through developing a strong understanding of the relation between the scientific requirements, instrument requirements and the system design, and iterating these to trade-off key parameters of interest to identify potential solutions.  A thorough review of the driving science requirements was performed in conjunction with the National Oceanography Centre (NOC).  Trade-offs were conducted between mass, power, altitude, antenna area and size in order to better understand exactly what is capable in the context of the VEGA launcher limits.

Airbus DS and NOC have established an innovative and technically mature Earth Explorer 9 candidate mission that directly addresses some of the Ocean Challenges for ESA’s Living Planet Programme in the context of the new Earth Observation Science Strategy for ESA.  The mission concept is directly relevant to the updated Ocean Challenges 1, 2, 3 and 4, while also contributing to the broader objectives of the ESA Strategy of delivering innovative observation capability relevant to Earth Observation for Society Challenges and the development of integrated sustained observing systems.

The Chinese and French Space Agencies, CNES and CNSA, propose to jointly carry out an innovative mission, CFOSAT (China France Oceanography Satellite project) devoted to the monitoring of the ocean surface and its related science and applications. CFOSAT will embark both a wind and a wave scatterometers, enabling, for the very first time, a simultaneous measurement of the wind and the wave vectors with a global coverage. The launch is planned for mid-2018.

The satellite embarks two payloads; both are Ku-band radar scanning around the vertical axis:

-       the wave scatterometer SWIM, a rotating 6-beams radar at small incidence (0° to 10°) [1, 2],

-       the wind scatterometer SCAT, a fan-beam radar at larger incidence angles (30° to 50°) [3].

The paper focuses on the SWIM instrument and its performance budgets. SWIM is manufactured by Thalès Alenia Space.

1. Design of SWIM

The design of SWIM will be detailed and the chronogram of the 6 beams presented.

2. Instrument budgets

a. From analysis

The main performance budgets will be presented (SNR, pointing budget, coverage) and their compliance with the scientific requirements will be discussed.

 b. From tests on qualification model, HYM

A qualification model of the instrument has been built and extensively tested. All the functional tests have been performed to prove that the instrument can work properly in every mode. The instrument bench enables also to test some scenarii of sea conditions and positions on orbit. Some results will be shown to assess the performance analysis.

 3. Science performances budgets

 a. Method

The science performance budgets are checked through an end-to-end simulator, SimuSWIM. It simulates the sea surface, taking into account spectral properties of the sea and the backscattering power is computed taking into account the performance budget of the instrument. The data are then inversed through prototypes of the ground segment algorithms.

The results are compared with the inputs to determine the accuracy of the measure and inversion.

 b. Results

Results on reference sea states will be shown and discussed.Fig.2 is an example of the kind of 1D results which can be obtained for performance evaluation.

4. References

[1] Enjolras V., L. Rey, T. Amiot, C. Tison, P. Castillan, SWIM, a state of the art multi-incidence beams Ku-band waves scatterometer to go beyond current radar systems, in IGARSS’09, July 2009

[2] Hauser, D., Soussi, E., Thouvenot, E., and Rey, L.,“SWIMSAT: a real aperture radar to measure directional spectra of ocean waves from space  main characteristics and performance simulation”. Jour. Atmos. Oceanic Tech., 18, 2001

[3] Zhu J., X. Dong, W. Lin, X. Xu, Calibration and estimation of attitude errors for a rotating fan-beam scatterometer using calibration ground stations, IEEE JSTARS, PP(9), 2014

We are presenting results from a phase 0 study dedicated to a new “ocean surface current” measurement mission concept. This study is conducted by CNES in order to assess how heritage from SWIM/CFOSAT instrument could contribute to the OSC measurement requirement identified in the ESA Globcurrent initiative, and the Living Planet Program Future Challenges.

Ocean Surface Currents (OSC) are fundamental to our understanding of ocean circulation at all time and space scales. They remain one of the last physical ocean variables yet to be directly measured by a space mission at high fidelity and mesoscale resolution. Such measurements are required to drive scientific understanding of ocean/atmosphere surface dynamics, constrain and validate ocean forecasting systems, support marine industries, and provide government agencies sufficient information to develop and monitor marine policies.

There is a the lack of direct satellite derived OSC measurements for synoptically mapping ocean current systems (e.g. Loop Current, Gulf Stream, Agulhas, and others): in most areas no direct surface ocean current measurements have ever been made. A variety of satellite missions and measurement techniques with complementary characteristics are however available. Emerging technologies and novel mission concepts may open the door in the future to enhanced OSC observations at unprecedented resolutions.

In this context, several OSC space missions have been proposed based on Along Track Interferometry (ATI) or Doppler Centroid Anomalies (DCA). These techniques leads to SAR  systems and rather high budgets missions. Others solutions, based on smaller RAR (Real Aperture Radar), are possible. They use high incidence Doppler scatterometers or low incidence wave spectrometers. Scatterometers are more sensible to global surface Doppler and wave spectrometers can retrieve surface currents by the mean of the waves dispersion relationship.

In all cases, it is necessary to measure all the directional surface parameters: wind, waves and current to accurately separate the variables and it is very difficult to get all of them from one satellite (multi-incidences and azimuths measurements), so one satellite can specialize on some of them and the data will be combined with others external data. The compromise to get high spatial and temporal resolution data will be between a few big satellites or more light satellites.

CNES is involved in the future CFOSAT (China-France Oceanography satellite) satellite mission. CFOSAT will carry two instruments: SWIM (designed and manufactured by Thales Alenia Space), which is a Ku-Band wave spectrometer (incidence angles between 0 to 10°) aimed at measuring the ocean directional wave spectra, and SCAT (designed and manufactured by China), which is a Ku-Band wind scatterometer (incidence angles around 40° with a wide swath) which will provide the surface wind vector. As a demonstrator, a new airborne radar, called KuROS (Ku-band Radar for Observation of Surfaces) has been developed. In addition to measuring the normalized radar cross section,  KuROS has the ability to measure the Doppler velocity of the radar echo.  

From this experience, CNES has begun a phase 0 study to address OSC measurements, named VASCO (Velocity Assessment of Surface Currents in Oceans). We choose to focus on the potential to re-use the SWIM instrument based system, differently configured (PRF, observation geometry,…) and with enhanced functions (multi-frequency, Doppler capabilities, on-board processing). These SWIM evolutions will deliver new types of OSC measurements by monitoring, for different spatial scales, the waves dispersion relationship. Preliminary study results, such as instrument configuration and expected contribution to OSC measurements, will be presented.

The requirements of the user community (oceanography or Numerical Weather Prediction centres) for Sea Surface Temperature (SST) are particularly demanding in terms of accuracy, spatial resolution, and revisiting time. A 10 km spatial resolution, with an accuracy of 0.3 K globally, under clear and cloudy conditions is desired. To satisfy these requirements, a new satellite mission needs to be designed.

The sensitivity of passive microwave observations to SST is carefully analysed, with the objective of designing an optimized satellite instrument, MICROWAT, dedicated to an “all-weather” accurate estimation of the SST, at high spatial resolution. For this purpose, two methodologies were developed. First, an information content analysis involving radiative transfer simulations, realistic instrumental noises and large datasets of surface and atmospheric conditions is used to study the sensitivity of retrievals to the instrumental configurations. It is then followed by a neural network SST-retrieval scheme calibrated at the global scale and tested over one year of data.

The obtained results stress the importance of the low frequency observations around 6 GHz for accurate SST retrieval. Compared to the 11 GHz channel, the 6 GHz channel provides more sensitivity to the low SSTs and offers lower instrument noise, thanks to possibly broader channel bandwidths. However, it requires much larger antenna size for a given spatial resolution. Two instrument concepts have been suggested: one using synthetic interferometric antennas, and the other one a classic real aperture antenna. This first analysis shows that 2D interferometric systems would be very complex, and would not satisfy the user requirements in terms of SST accuracy. A 1D interferometric system can be considered instead, and its feasibility has been investigated in recent ESA-funded projects. A dedicated conical scanner on board a satellite flying in train with Metop with a 6 m antenna and channels at 6.9 and 18.7 GHz (both with V and H polarizations) can provide a SST accuracy of 0.3 K with a 15 km spatial resolution, with today's technology (Prigent et al. 2013)

In addition to retrieve SST and ocean vector winds, such instruments could be used to obtain Land Surface Temperature (LST) (Aires et al. 2001; Catherinot et al. 2011) regardless to cloud coverage, to supplement infrared observations.

Aires, F., C. Prigent, W.B. Rossow, and M. Rothstein, A new neural network approach including first-guess for retrieval of atmospheric water vapor, cloud liquid water path, surface temperature and emissivities over land from satellite microwave observations, J. Geophys. Res.,106, No. D14, 14,887-14,907, 2001.

Catherinot, J., C. Prigent, R. Maurer, F. Papa, C. Jimenez, F. Aires, and W. B. Rossow, Evaluation of ‘all weather’ microwave-derived land surface temperatures with in situ CEOP measurements, J. Geophys. Res., 116, D23, doi:10.1029/2011JD016439, 2011.

Prigent, C., F. Aires, F. Bernardo, J.-C. Orlhac, J.-M. Goutoule, H. Roquet, and C. Donlon, Analysis of the potential and limitation of microwave radiometry for the retrieval of Sea Surface Temperature: Definition of new mission concepts, J. Geophys. Res., 118, 6, 3074-3086, 2013.

Observing Ocean Currents globally at (sub-)mesoscale resolutions remains one of the key scientific gaps to study ocean dymanics and global transport of heat, salt, momentum and nutrients. Processes at this scale are also important to understand the interaction between oceans and the atmosphere. Several mission concepts have been proposed to fill this gap in our knowledge.

As part of the STSE programme EO Sentinel Convoy study on the Ocean & Ice theme, a passive SAR companion satellite flying in formation with Sentinel 1 was investigated. The primary applications of this concept were to address across-track single pass interferometric measurements of the surface topography and topographic change of glaciers, ice streams and permafrost areas. The concept also included along-track interferometry for measurement of ocean surface currents.

Building on this work, a mission and instrument definition study is in progress to evolve the concept with a primary focus on the retrieval of mesoscale ocean surface current (OSC) measurements. An overview of the work carried out in the study will be presented including the new mission objectives and requirements, and a comparison between the use of a short baseline bi-static along-track interferometry based observation approach and a long baseline multi-static observations based on the Doppler Centroid Anomaly (DC-A) technique. The preliminary instrument and mission architecture are presented together with some of the challenges of flying in formation with Sentinel 1 (e.g. phase synchronisation).

This paper discusses the opportunity that is presented by a Sentinel 1 Companion Satellite concept which supports the scientific objective of delivering OSC measurements within a modest mission cost envelope. The potential offered by this mission and instrument as a candidate Earth Explorer is described identifying the key performance parameters and technical design drivers.