Vivien Enjolras

Re-tracking of SAR altimeter ocean power-waveforms and related accuracies of the retrieved sea surface height, significant wave height and wind speed

This paper describes the simulation of the power- waveforms acquired in by a radar altimeter operating in SAR mode over ocean surfaces, including the effects of the radar transfer function and of the geophysical ocean parameters, namely the sea surface height( SSH), the sea surface wave height (SWH) and the surface wind speed (WS). The performances of the SAR mode with respect to the retrieval of the ocean geophysical variables (SSH, SWH, WS) are then computed using the Cramer-Rao estimation bounds. It is shown that the radar to ocean range estimation provided by the SAR mode is improved by more than a factor of two compared with conventional Ku band altimeters. Improvements on SWH and wind speed are also discussed.

Directional wave spectrum estimation by SWIM instrument on CFOSAT

SWIM is a Ku-band radar designed for wave directional spectrum estimation. This radar operates at six incidence angles (from 0° to 10°) with a complete azimuth scanning. SWIM is currently in Phase B (concept and design phase). In [1, 2], the preliminary design and associated performance analysis have been published taking into account the end of Phase A design. This paper is focused on the performance assessment of the SWIM instrument based on the new developments which occur during Phase B. In addition, major reviews have been carried out on the performance analysis.

SWIM: A state of the art multi-incidence beams Ku-band waves scatterometer to go beyond current radar systems

The instrument SWIM (Surface Waves Investigation and Monitoring) on the CFOSAT program (Chinese French Oceanographic Satellite) is a state of the art radar for several reasons. At first, SWIM is the first ever space radar concept that is mainly dedicated to the measurement of ocean waves directional spectra and surface wind velocities through multi-azimuth multi-incidence observations. Orbiting on a 500 km sun-synchronous orbit, its multiple Ku-band (13, 575 GHz) beams illuminating from nadir to 10° incidence and scanning the whole azimuth angles (0-360°) provide with a 180 km wide swath and a quasi global coverage of the world between -80 and 80°. Secondly, such a wide range of observations requiring high range resolution (about 20 m on the ground) have led to design an instrument whose architecture and technology goes beyond what has been done on altimeter and scatterometer systems. The global coverage and the reduction of telemetry budgets have required to perform onboard range compression. The variety of signals at different incidences, the impact of the complex moving geometry of observation and the required real-time signal processing have led to propose onboard complete digital range compression on backscattered 320 MHz bandwidth signals. Finally, multi-azimuth multi-incidence observations requirements have led to design a complex antenna subsystem that rotates at 6 rounds per minute while transmitting high power RF signals towards tunable directions.

Using Altimetry Waveform Data and Ancillary Information From SRTM, Landsat, and MODIS to Retrieve River Characteristics

In this paper, spaceborne radar altimeters are shown to have the potential for monitoring the height of large rivers with accuracies of approximately 1 m. However, the need for a better height accuracy and the observations of smaller continental basins have led to studies on how to improve and extend the use of nadir-altimeter data. Conventional retracking techniques over land are limited to the examination of altimeter waveforms on a case-by-case basis. Due to the arbitrary geometry which may be present at altimeter river crossings, this approach may be limited to large rivers, which approximate ocean crossings. To overcome this limitation, we introduce a waveform-fitting method which uses the entire set of waveforms associated with a water crossing, rather than individual waveforms. By using ancillary data, such as digital elevation model (obtained from Shuttle Radar Topography Mission and Gtopo30) and classification maps (obtained from Landsat and MODIS), it is possible to recast the retracking problems as a maximum-likelihood-estimation problem. Theoretical power returns based on the a priori knowledge of the observed scenes are generated, resulting in a parametric library of waveform histories, which is then used to constrain the estimation. For demonstration, we concentrate on the river Meuse in northern western Europe and on the river Lena in Russia. The Meuse has important social impact, since it has flooded in the past and better real-time predictions of its changing stage may improve flood-forecasting skill. Furthermore, it presents a challenge to conventional nadir-altimeter waveform retracking. We will present both theoretical performance results and demonstrate the feasibility based on real altimeter data.

An Assessment of a Ka-Band Radar Interferometer Mission Accuracy Over Eurasian Rivers

The Water Elevation Recovery satellite mission is dedicated to the determination of land surface water extent, elevation, and slope using a Ka-band radar interferometer (KaRIn) as its primary instrument. Determining these parameters to the accuracy desired for hydrologic applications is challenging. The scientific objectives of the mission have been set up to 10 cm for the height budget and 10 murad (1 cm/1 km) for the slope budget. In this paper, we implement a Virtual Mission simulation and use it to examine the measurement performances for three case studies in Europe: a relatively small river such as the Meuse in Northern Western Europe, the Lena river in Russia, one of the major Siberian rivers, and Lake Leman in Western Europe. We simulate KaRIn data with the associated instrument and geophysical error sources and implement ground processing techniques to go from the original raw data to science data. We examine the impact of external errors in detail and implement calibration techniques that rely on the use of ancillary topographic data, such as the Shuttle Radar Topography Mission digital elevation model (DEM). We find that the impact of external errors can be reduced to a few centimeters. The random error budget can also be reduced below 10 cm by means of appropriate processing. The scientific requirements of the mission are shown to be met for all cases.

SWIM: A Multi-Incidence Beams Ku-band Real Aperture Radar for the Observation of the Ocean Wave Field Spectra

The instrument SWIM (Surface Waves Investigation and Monitoring) on the CFOSAT (Chinese French Oceanographic Satellite) project aims at measuring the directional spectra of ocean waves on a 180 km wide swath. Orbiting on a 500 km sun-synchronous orbit, it provides with a global full coverage of the oceans between -80 and 80deg. It consists in a real aperture radar (RAR) operating in Ku-band (13.575 GHz) on 6 distinct beams pointing at incidence angles from 0deg (nadir) to 10deg, while scanning the whole azimuth angles (0-360deg). The nadir beam is similar to the one provided by a standard altimeter. Its main objective is to measure the significant wave height and wind speed (from the measurement of the radar cross section). Radar cross-sections are also estimated from 2 to 10deg off-nadir observations, then ensuring the access to profiles dependence with incidence and azimuth angles. Appropriate processes of observations at 4 to 10deg incidence provide with an estimate of the spectral properties of the local wave field.

Space Altimetry from Nano-Satellites : Payload Feasibility, Missions and System Performances

Thale graves Alenia Space is an industry world leader in high performance altimeters for ocean topography from space (Poseidon1 on TOPEX-Poseidon, Poseidon2 on Jason1, Poseidon3 on Jason2 and AltiKa on AltiKa/SARAL), and for ice topography from space (SIRAL1 and SIRAL2 on Cryosat). Thale graves Alenia Space is also involved on advanced altimeter concepts such as wide swath and high resolution altimetry (SWOT) for hydrology. CLS is dedicated to satellite environmental monitoring and security services. In this framework, CLS is involved in algorithms and products definition, data processing and CalVal for the altimetry missions developed in cooperation between US and France (TOPEX-Poseidon, Jason1/2), in US (GFO) and in Europe (ERS1-2, Envisat, Cryosat) and is currently involved in the definition and development of future missions (AltiKa, Sentinel3) and future wide swath altimetry concepts (SWOT). Altimetry satellites for space oceanography have seen continuous and tremendous reduction in size, mass, and program cost : TOPEX-Poseidon satellite mass is 2500 kg. Jason1/2 satellites mass is 500 kg. AltiKa microsatellite mass shall be around 150 kg. Is a further step in reduction of satellite mass (and cost) achievable? What kind of altimetry payload? Which (system) performances? Which payload and satellite technologies? For which (new) mission(s)? This paper addresses these points and shows the way of an additional step in innovative solutions opening to low cost constellation of altimetry satellites with new applications and services.

Estimation of wave spectra with swim on cfosat - illustration on a real case

SWIM (Surface Wave Investigation and Monitoring) is a spaceborne radar pointing at nadir and small incidence angles, scanning in azimuth. It is designed for the measurement of directional ocean wave spectra and will be embarked on the CFOSAT (China France Oceanography SATellite) mission to be launched in 2014. The CFOSAT project is now in the C/D phase (manufacturing phase). Taking into account the very last definition of the instrument, we present here the expected performances for estimates of significant wave height and spectral parameters of long ocean waves. These performances have been obtained using numerical simulations taking into account instrument specifications and realistic sea surface conditions, in particular those corresponding to the Atlantic storm of November 2002, which caused the Prestige ship sinking and its terrible oil slick.

Next generation of multi beam rotating antenna on SWIM scatterometer

In the frame of the development of the instrument SWIM (Surface Waves Investigation and Monitoring) on the CFOSAT program (Chinese French Oceanographic Satellite) funded by CNES, Thales Alenia Space is currently developing a new multi beam rotating antenna in Ku Band. This single reflector offset antenna includes a rotating feed comprising 6 beams.

An innovative spaceborne radar concept for global maritime surveillance: Description and performance demonstration

The paper describes an alternative concept to conventional SAR instruments for ship detection over all ocean surfaces. The concept is specifically oriented for ship detection, and not for land or sea imaging. It allows wide swath coverage (as high as 1000 km). It exhibits high detection performances of small ships even in adverse sea states conditions. Its power consumption is reduced allowing a permanent operation all along the orbit. At least, it uses already developed and low cost technologies.