Two field experiments named WISE (WInd and Salinity Experiment) were sponsored by the European Space Agency (ESA) to better understand the wind and sea state effects on the L-band brightness temperatures. They took place at the Casablanca oil rig located in the North Mediterranean Sea, 40 km off shore the Ebro river delta: WISE 2000 from November 25 to December 18, 2000, and continued during the January 9 to 16, 2001, and WISE 2001 from October 23 to November 22, 2001. During the spring of 2003, under Spanish National funds, a third field experiment named FROG (Foam, Rain, Oil slicks and GPS reflectometry) took place at the Ebro river delta, to measure the phenomena that were not completely understood during the WISE field experiments, mainly the effect of foam and rain. In order to achieve the objectives of the WISE field experiments the LAURA L-band fully polarimetric radiometer from the Technical University of Catalonia (UPC) was mounted on the Casablanca oil-rig at the 32 meters deck above the sea surface, pointing to the North and North-West, in the direction of the dominant winds. In this paper we present the results of the first study to determine the relationship between the brightness temperature and the sea state.
Sea surface salinity (SSS) measurement is one of the objectives of ESA’s SMOS (Soil Moisture and Ocean Salinity) Earth Explorer Opportunity mission. SMOS’s objective is to provide global soil moisture and sea salinity maps using the MIRAS L-band aperture synthesis interferometric radiometer. Since the sea salinity signature exhibits a very small brightness temperature dynamic margin, it can only be accurately retrieved if the sea surface emissivity at L-band is properly modeled. In addition to the sea salinity signature, other factors influencing the emissivity are the sea surface temperature, and the sea surface roughness induced by wind, the large scale roughness created by swell, and the foam
emissivity. This article is focused on the estimation of the sea surface spectrum, which describes sea roughness, training a neural network with wind and roughness data obtained during WISE 2000/2001 (WInd and Salinity Experiment).
The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission will be launched in 2007. Its goal is the global and frequent measurement of soil moisture over the land and surface salinity over the sea, two key parameters governing the complex global climate. SMOS’ single payload is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), the first space-borne interferometric radiometer. SMOS will provide brightness temperature data over a wide range of incidence angles at vertical and horizontal polarizations (dual-polarimetric mode) or the full Stokes emission vector (full-polarimetric mode), from which the geophysical parameters will be derived. This paper focuses on the soil moisture retrieval problem using dual or full-polarimetric information. In this case, the brightness temperatures, as measured by the radiometer, depend mainly on five parameters descriptive of the surface under study: vegetation opacity and albedo, and soil surface temperature, roughness and moisture. Some of these parameters can be derived from other sensors or can be inferred from the multi-angular brightness temperatures themselves. Simulation results using the SMOS End-to-end Performance Simulator (SEPS) developed at the Universitat Politecnica de Catalunya (UPC) will be presented and discussed.
Improved weather forecast and climate monitoring rely on a better quantification of soil moisture (SM) and sea surface salinity (SSS). The most accurate way to measure them from remote sensors is by using radiometers at L band, where the sensitivity of the brightness temperature is maximum and the atmosphere is almost transparent. In order to provide a global coverage of these basic parameters within a revisit time of 3 days and 30-50 Km spatial resolution, the SMOS mission was recently selected by ESA in the frame of the Earth Explorer Opportunity Missions. The sensor being considered for this mission (called MIRAS) uses the concept of 2D interferometric radiometry1'2'3, and is scheduled to be launched in 2005. A demonstrator consisting of a number of receivers is now under development by CASA (E) by contract with ESA. This work presents the block diagram of a single receiver chain in the SMOS instrument and its main requirements in terms of mission tradeoffs.
The ESA's SMOS (Soil Moisture and Ocean Salinity) Earth Explorer Opportunity Mission will be launched by 2005. Its baseline payload is a microwave L-band (21 cm, 1.4 GHz) 2D interferometric radiometer, Y shaped, with three arms 4.5 m long. This frequency allows the measurement of brightness temperature (Tb) under the best conditions to retrieve soil moisture and sea surface salinity (SSS). Unlike other oceanographic variables, until now it has not been possible to measure salinity from space. However, large ocean areas lack significant salinity measurements. The 2D interferometer will measure Tb at large and different incidence angles, for two polarizations. It is possible to obtain SSS from L-band passive microwave measurements if the other factors influencing Tb (SST, surface roughness, foam, sun glint, rain, ionospheric effects and galactic/cosmic background radiation) can be accounted for. Since the radiometric sensitivity is low, SSS cannot be recovered to the required accuracy from a single measurement as the error is about 1-2 psu. If the errors contributing to the uncertainty in Tb are random, averaging the independent data and views along the track, and considering a 200 km square, allow the error to be reduced to 0.1-0.2 pus, assuming all ancillary errors are budgeted.
Adriano Camps, Ignasi Corbella, Jordi Font, Jacqueline Etchetto, Nuria Duffo, Merce Vall-llossera, Javier Bara, Francisco Torres, Patrick Wursteisen, Manuel Martin-Neira
Sea surface salinity (SSS) has been recognized as a key parameter in climatological studies. SSS can be measured by passive microwave remote sensing at L band, where the sensitivity of the brightness temperatures shows a maximum and the atmosphere is almost transparent. To provide global coverage of this basic parameter with a 3-day revisit time, the SMOS mission was recently selected by ESA within the frame of the Earth Explorer Opportunity Missions. The SMOS mission will carry the MIRAS instrument, the first 2D L-band aperture synthesis interferometric radiometer. To address new challenges that this mission presents, such as incidence angle variation with pixel, polarization mixing, effect of wind and foam and others, a measurement campaign has been sponsored by ESA under the name of WISE 2000 and it is scheduled for October-November 2000. Two L-band radiometers, a video, a IR and a stereo-camera and four oceanographic and meteorological buoys will be installed in the oil platform 'Casablanca' located at 40 Km off the coast of Tarragona, where the sea conditions are representative of the Mediterranean open sea with periodic influence of the Ebro river fresh water plume.
In the last years two new kinds of microwave radiometers are being studied for Earth observation: aperture synthesis interferometric radiometers and polarimetric radiometers. The first ones are formed by an array of small antennas whose outputs are cross-correlated and then, properly processed to obtain a map of the apparent brightness temperature of the whole scene being imaged. One- and two-dimensional systems have been studied by some space agencies, e.g. ESTAR by NASA, and MIRAS by ESA, as a solution that avoids the implementation of large steerable antennas at low frequencies (L-band), while reaching a relatively high spatial resolution: about 20 - 30 Km. More recently preliminary studies of mm-wave systems have also been studied to improve the spatial resolution achieved by today's radiometers. On the other hand, polarimetric radiometers are formed by a dual-polarization antenna. The real and the imaginary parts of the complex cross-correlation computed from the H/V outputs leads to the third and fourth Stokes parameters of the incoming thermal radiation, which are basically related to roughness state of the surface being imaged. At present, a number of studies are being conducted to establish the relationship with the wind direction over the sea surface. The performance analysis of those systems requires the modeling of the apparent brightness temperature map of the Earth and/or sea surface that would be imaged at the microwave and the mm-wave frequencies, which is the object of this paper.
Millimeter-wave radiometry of the earth's surface from Low Earth Orbit (LEO) with a resolution of a few km requires antenna apertures several meters across and sub-second scanning times. Fulfilling these requirements with a mechanically scanned real-aperture antenna presents formidable mechanical challenges. An attractive alternative described here is to use synthetic aperture techniques employing a sparse-array of antennas that trade the mechanical complexity of real-aperture imaging for the electrical complexity of synthetic aperture imaging. We present results of an ESA- sponsored study aimed at seeking the optimum technique for high performance synthetic aperture mm-wave radiometry from LEO.
This paper presents a new method for receiver calibration of interferometric radiometers. It is based on the distributed noise injection mechanism and it consists of fully calibrating the baseline error terms of the central antennas (which share the same noise source) while keeping only separable error calibration for the distant ones. This improves the accuracy of the shortest baselines, which are the most significant ones due to the smoothness of the brightness temperature to measure. Simulations show that, compared to previously reported methods, the improvement on the radiometric resolution can be as high as 3.9. The robustness against frequency response mismatch between receivers is improved by a factor 2.9.
In the last years aperture synthesis interferometric radiometers have received a special attention by space agencies as a feasible solution to passive monitoring of the Earth at low frequencies (L-band), where classical total power radiometers would require heavy steerable antennas to meet the spatial resolution requirements (10 - 20 Km), from a low polar orbit. While the performance of such instruments is well known in the radioastronomy field, its application to Earth remote sensing is quite new. The study of different array structures, system errors, calibration and inversion methods and instrument global performance requires the implementation of a simulator of a two-dimensional space borne interferometric radiometer. It allows us to analyze not only its snap shot radiometric accuracy, but also its improvement by means of pixel averaging, that is, the averaging of the common pixels recovered in a sequence of consecutive brightness temperature images. The simulations performed use the system parameters of the planned MIRAS (microwave imaging radiometer by aperture synthesis) instrument, a Y-shaped array with 43 antennas per arm spaced 0.89 lambda, currently under study by the European Space Agency.
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