The Roles of the S3MPC: Monitoring, Validation and Evolution of Sentinel-3 Altimetry Observations

The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges

Satellite altimetry for monitoring lake level changes

International audienceAccurate and continuous monitoring of lakes and inland seas is possible since 1991 thanks to the recent missions of satellite altimetry (Topex-Poseidon, ERS-1, ERS-2, Jason-1 and Envisat). Global processing of the data of these satellites could provide temporal and spatial times series of lakes water level from 1991 to 2003 on the whole Earth with a decimeter precision. The response of water level to regional hydrology is particularly marked for lakes and inland seas of semi-arid regions. Altimetry data can provide an invaluable source of information in hydrology sciences, but insitu data (rivers runoff, temperature, precipitation etc.) are still strongly needed to study the evolution of water mass balance of each lake. Moreover, sea level variations that result from variation of hydrological parameters such as river discharge, precipitation and evaporation, are very sensitive indicators of regional climate variations. Recent results obtained on Aral Sea and Issykkul Lake are presented here. Inter-annual changes of water level have been obtained over these lakes that must be interpreted in term of hydrological water balance. Since 1960 the Aral sea has been drying and since 1989 it is divided into two lakes that follow different evolution, the Big Aral in the south which continuously dried up the last 10 years, while the so- called Small Aral in the north presented large inter-annual fluctuations related to constructions and destructions of a dam in the Berg’s strait retaining the water from the Syr Darya. For Issykkul, a slow decrease of the level has been observed over the last hundreds years (4cm/year), followed by an abrupt and bigger increase of the level of around 10 cm/yr since 1998. The impact on local populations and infra-structures of these fluctuations are dramatic in the case of Aral, much less in the case of Issykkul, but comparative study of both water bodies may help in the future to understand the respective consequences of human-induced activities from the natural changes. It is also the task of a new project recently submitted and accepted by the NATO with scientists from Uzbekistan and Kyrgyz Republic

Investigation of hydrological and atmospheric loading by space geodetic techniques

Observations of sea level can only be interpreted correctly if land motion in particular in terms of vertical deformation of coastal areas is taken into account. In the last decades space geodetic techniques such as VLBI (Very Long Baseline Interferometry), SLR (Satellite Laser Ranging), the GPS (Global Positioning System), and Doris (Doppler Orbitography and Radio positioning Integrated by Satellite) have proved to be very powerful for determining displacements of points on the solid Earth. These can be modeled by using various geodynamical parameters, e.g. the Love and Shida numbers in the model of the solid Earth tides and site-dependent amplitudes and phases of the ocean loading models. Today, the small deformations associated with the response of the Earth to atmospheric and hydrological loading are of growing interest. These effects cause site-dependent vertical displacements with ranges up to ±30mm due to atmospheric pressure variations and due to mass redistribution in surface fluid envelopes, in particular in continental water reservoirs (soil moisture, snow, and groundwater). Several new global and regional models of soil moisture and snow depths are now available and can be validated by space geodetic techniques. This paper is intended to give a short overview about state-of-the-art of modeling loading effects. A short introduction to the Special Bureau for Loading within the Global Geophysical Fluid Center (GGFC) of the IERS will be given, too. Finally, it will be shown how the effects influence the results of high precision space geodetic measurements. The paper mainly concentrates on vertical crustal motions on seasonal and interannual time scales observed by VLBI and describes also some results obtained from Doris

Evolution of Sea Level of the Big Aral Sea from Satellite Altimetry and Its Implications for Water Balance

The Aral Sea, one of the biggest lakes in the world, started to shrink in the 1960s because water was withdrawn for irrigation. The lowering of the Aral Sea level led to the separation of the lake into two basins – the Small Aral in the north, and the Big Aral in the south. For several decades there were no continuous observations of sea level, and the few data that exist are fragmentary or unavailable. We present observations of the Big Aral Sea level estimated from the TOPEX/Poseidon (T/P) altimetry with high temporal resolution over the last decade (1993- 2004). Since the sea volume is one of the key parameters for the studies of water balance, we use the T/P-derived time series of sea level to reconstruct, using a dedicated digital bathymetry model (DBM), associated changes in the sea surface and volume. We introduce variations of the sea volume as the new constraint for the water budget of the Big Aral Sea. This is an important step forward towards estimating detailed seasonal and interannual changes of the water budget. We assess various existing components of the water budget of the Aral Sea and discuss the quality of the existing data and their applicability for establishing detailed water balance. In particular, large uncertainties in estimating the evaporation and underground water supply are addressed. Desiccation of the Aral Sea resulted in dramatic changes in the salinity regime and, consequently, affected marine ecosystems. We also discuss changes in the aquatic fauna and its possible evolution under continuing desiccation of the Big Aral Sea. Combination of satellite altimetry with other parameters of the water budget show that this approach offers promising potential for the assessment of the temporal evolution of the water budget in arid or semi-arid conditions, even with poor ground monitoring network

An absolute calibration site for radar altimeters in the continental domain : lake Issykkul in Central Asia

Altimetry missions such as Topex/Poseidon, Jason-1, GFO and ENVISAT have been widely used in the continental domain over lakes, rivers and wetland although they were mostly dedicated to oceanic studies. Knowledge of the instrumental biases is a key issue. Numerous sites have been dedicated to calibration purposes, either in the oceanic domain (Harvest offshore platform in California, Corsica, Bass Strait in Australia) or over lakes (Lake Erie in United States). A new site (Lake Issykkul in Kirghizstan) is proposed for calibration in the continental domain. This lake is covered by past (T/P) and current radar altimetry satellites (Jason-1, T/P, GFO, and ENVISAT). Several in situ water levels and local meteorological variables are available at the site. Located in a mountainous area, it offers an opportunity for calibration far away from all other existing sites and very different environment contexts. Two GPS campaigns have been conducted on the lake in 2004 and in 2005. They consisted of cruises with stations installed onboard a boat following the satellite ground tracks, and onshore settings. This enabled estimating a bias for each altimeter and each tracking algorithm available. Biases obtained for Envisat, GFO, T/P and Jason-1 using the default ocean tracker (respectively, 48.1 +/- 6.6, 7.5 +/- 4.0, 0 +/- 4.3 and 7.0 +/- 5.5 cm) agree with biases published at the other calibration sites. For Jason-1, there is a significant disagreement with results obtained in the ocean field (7 cm instead of 13 cm) but is coherent with bias obtained on the Lake Erie site. Erroneous estimates of the sea state bias correction from non-oceanic-like waveforms is discussed as a possible explanation. Errors in the ionospheric, wet and dry tropospheric corrections for the continental domain are also highlighted and quantified

Broadband terahertz emission from two-color femtosecond-laser-induced microplasmas

We investigate terahertz emission from two-color fs-laser-induced microplasmas. Under strongest focusing conditions, microplasmas are shown to act as point-sources for broadband terahertz-to-far-infrared radiation, where the emission bandwidth is determined by the plasma density. Semi-analytical modeling allows us to identify scaling laws with respect to important laser parameters. In particular, we find that the optical-to-THz conversion efficiency crucially depends on the focusing conditions. We use this insight to demonstrate by means of Maxwell-consistent 3D simulations, that for only 10-µJ laser energy a conversion efficiency well above 10 −4 can be achieved.Spectroscopie Terahertz Atmosphérique d'Explosifs par Laser

SOLS: A lake database to monitor in the Near Real Time water level and storage variations from remote sensing data

An accurate and continuous monitoring of lakes and inland seas is available since 1993 thanks to the satellite altimetry missions (Topex-Poseidon, GFO, ERS-2, Jason-1, Jason-2 and Envisat). Global data processing of these satellites provides temporal and spatial time series of lakes surface height with a decimetre precision on the whole Earth. The response of water level to regional hydrology is particularly marked for lakes and inland seas in semi-arid regions. A lake data centre is under development at by LEGOS (Laboratoire d'Etude en Geophysique et Oceanographie Spatiale) in Toulouse, in coordination with the HYDROLARE project (Headed by SHI: State Hydrological Institute of the Russian Academy of Science). It already provides level variations for about 150 lakes and reservoirs, freely available on the web site (HYDROWEB: http://www.LEGOS.obs-mip.fr/soa/hydrologie/HYDROWEB), and surface-volume variations of about 50 big lakes arc also calculated through a combination of various satellite images (Modis, Asar, Landsat, Cbers) and radar altimetry. The final objective is to achieve in 2011 a fully operating data centre based on remote sensing technique and controlled by the in situ infrastructure for the Global Terrestrial Network for Lakes (GTN-L) under the supervision of WMO (World Meteorological Organization) and GCOS (Global Climate Observing System)