J. Bara

Impact of antenna errors on the radiometric accuracy of large aperture synthesis radiometers

A classification of system errors in aperture synthesis radiometry applied to Earth observation is presented. A general procedure to quantify the impact of antenna errors on the radiometric accuracy is developed and is then particularized to an L-band Y-shaped interferometer called MIRAS (microwave imaging radiometer by aperture synthesis) currently under study at the European Space Agency. This work analyzes in detail the impact of antenna errors on the radiometric accuracy of the instrument. These antenna errors are grouped into amplitude and phase antenna pattern errors, antenna position errors and antenna cross polarization errors. Special attention is paid to antenna coupling effects because of their importance in the selection of a suitable inversion algorithm for large aperture synthesis interferometers: the G-matrix techniques or the Fourier techniques proposed for MIRAS.

Angular resolution of two-dimensional, hexagonally sampled interferometric radiometers

A theoretical analysis of the angular resolution of two-dimensional interferometric radiometers for Earth observation from low-orbit satellites and its degradation due to spatial decorrelation effects is presented. The analysis extends basic effects known in the context of radio astronomy (application with narrow field of view, very few baselines) and in one dimension (ESTAR L band, few baselines interferometric radiometer) to the wide-field-of-view, many-baseline, high-resolution two-dimensional system required by Earth observation applications and computes beam width, encircled energy (or main beam efficiency), and side lobe level as a function of windowing (apodization) to allow for an optimum angular versus radiometric resolution trade-off. It is found that the extension of the Barlett window (which has a poor performance in one dimensional signal processing) to two dimensions produces high-quality results, comparable or better than those of Gaussian and Blackmann windows. Theory is extended to hexagonally sampled systems based on a ? or Y-shaped instrument, with hexagonal- and star-shaped support regions in the visibility space, respectively. The superior performance of the latter over the former for the same number of antennas and correlators is quantified and details of the angular resolution of one instrument of this kind, MIRAS, under development by the European Space Agency, are presented. For this radiometer Gaussian or Barlett windows should be used for good radiometric sensitivity or spatial resolution, respectively. In both cases the effects of decorrelation within the small alias-free field of view are negligible. It is also found that the impact of hardware imperfections which exist within the strict requirements of the specifications have a negligible effect on the angular resolution. Finally, experimental angular resolution results with a laboratory breadboard in a focused near-field setup are presented and compared to the theoretical predictions.

Impact of receiver errors on the radiometric resolution of large two-dimensional aperture synthesis radiometers

The specifications of the subsystems that compound a radiometer interferometer devoted to Earth observation are of main concern because they set the viability and final performance of such an instrument. The importance of these errors is related to the exact way they are generated, since this determines if a particular calibration procedure is capable of removing them or if they remain as residual errors. This paper presents a general method to analyze system errors. This method is used to analyze in detail the amplitude and phase errors of the receivers and their impact on the radiometric resolution. Special attention has been paid to nonseparable errors, since foreseen calibration procedures can only deal with separable phase and amplitude terms. Finally, the results have been used to set the receiver requirements of the instrument called MIRAS (microwave imaging radiometer by aperture synthesis), which is currently being developed by the European Space Agency (ESA).

Analysis of noise-injection networks for interferometric-radiometer calibration

The spatial resolution of current space-borne Earth observation radiometers is limited by the physical antenna aperture. This is especially critical at L-band, which exhibits high sensitivity to soil moisture and sea surface salinity. Interferometric radiometers (InRs) are currently being studied by several space agencies as a feasible alternative to overcome this problem. However, their calibration is a crucial issue since most techniques inherited from radio astronomy cannot be directly applied. Due to the large number of receiving channels, calibration techniques based on centralized noise injection from a single noise source will require a large and stable distribution network, which is technically very complex and unacceptable from the point-of-view of mass and volume. Procedures based on distributed noise injection from a set of noise sources through smaller distribution networks have been recently proposed by the authors as an alternative to alleviate these technological problems. In this paper, the analysis of these networks, the impact of the noise generated by the network losses on the calibration, and the impact of front-end reradiated noise are analyzed. Finally, the optimum circuit topologies and tolerances to which these networks have to be characterized in order to achieve the required calibration are derived. These configurations are formed by cascading basic 1:2 and 1:3 isolated power splitters. Isolators at receiver inputs have to be included in order to minimize offsets originating from the correlation of reradiation of receiver noise. It has been found that, in order to satisfy the calibration requirements of InRs, the S-parameters of the ensemble noise-injection network plus isolators have to be known to within 0.025-0.050 dB in amplitude and 0.5/spl deg/ in phase, and their physical temperature known to within 0.5/spl deg/C.

Atmospheric-turbulence-induced power-fade statistics for a multiaperture optical receiver

To estimate the probability distributions of power fades, we consider two basic types of disturbance in electromagnetic wave propagation through atmospheric turbulence: wave-front intensity fluctuations and wave-front distortion. We assess the reduction in the cumulative probability of losses caused by these two effects through spatial diversity by using a multiaperture receiver configuration. Degradations in receiver performance are determined with fractal techniques used to simulate the turbulence-induced wave-front phase distortion, and a log normal model is assumed for the collected power fluctuations.

Mutual coupling effects on antenna radiation pattern: An experimental study applied to interferometric radiometers

Antenna pattern mismatches are one of the most important error sources in planned Earth-observation interferometric radiometers. From a low Earth orbit, the wide field of view, about ±30°, leads to the use of antennas with a large beam. In addition, antennas must be closely spaced to avoid, or at least minimize, aliasing effects in the formation of the synthetic brightness temperature images. The accuracy demanded of these systems requires the precise knowledge of all the antenna radiation voltage patterns (amplitude and phase), which may differ from their theoretical values due to mechanical and electrical tolerances in the manufacturing process and which can change due to the proximity of other structures, i.e., other antennas of the array or the mechanical support. Two approaches are found in the literature to interpret the impact of antenna mutual coupling on the performance of an interferometric radiometer: (1) a modification of the antenna voltage pattern and (2) a mixing of the cross correlations measured between the signals collected by the antennas. The main contribution of the present work is a detailed theoretical analysis of the impact of mutual coupling effects showing the equivalence between both approaches. Theoretical results are corroborated with a set of experimental measurements with two kinds of antennas. Theoretical and experimental results can be used in the design of the antennas of interferometric radiometers in order to predict the impact of mutual coupling on the systems performance and point out the importance of an accurate antenna pattern characterization.

Calibration and experimental results of a two‐dimensional interferometric radiometer laboratory prototype

In recent years, Earth observation by means of aperture synthesis radiometry has received special attention by some space agencies as a possible solution to achieve high radiometric accuracy and spatial resolution at low microwave frequencies (L band), where the apparent brightness temperature is much more sensitive to soil moisture and sea surface salinity. This paper presents the characterization and calibration procedure, as well as some synthetic images measured with an X band experimental Y-shaped Synthetic Aperture Interferometric Radiometer prototype developed at the Polytechnic University of Catalonia. The instrument is composed of a single pair of antennas that can be moved along the arms of an Y structure to synthesize a set of baselines. An experimental procedure is proposed to evaluate and then calibrate offset, inphase, quadrature, and amplitude errors generated by receivers and correlators.

L-band aperture synthesis radiometry: hardware requirements and system performance

Aperture synthesis radiometry is becoming a feasible concept for imaging applications, especially at low microwave frequencies where it takes clear advantage of the absence of mechanical antenna motion. A 2D interferometric radiometer consists of a large number of receivers with small antennas distributed along a 2D structure, and the brightness temperature image is formed by inversion of the measured cross-correlation between all pairs of antennas. This is the concept of MIRAS (MIcrowave Radiometer by Aperture Synthesis), the core instrument of the SMOS (Soil Moisture and Ocean Salinity) mission selected by the European Space Agency (ESA) and planned to be launched in 2005. In its preliminary design, MIRAS receivers are uniformly distributed along a Y-shape structure and work at L-band. This approach, however, poses a challenge in the specifications required for the receivers: a) The short integration time due to the platform motion strongly limits the achievable sensitivity, b) the spatial resolution is determined by the structure dimensions which cannot be made arbitrarily large and c) the radiometric accuracy depends on the non ideal behavior of the receivers, although, to some extent can be corrected by internal calibration. This paper contributes to define the main trade-off between hardware requirements and system performance of this complex instrument.

New radiometers: SMOS-a dual pol L-band 2D aperture synthesis radiometer

Since the mid 1980s, aperture synthesis interferometric radiometers have received increased attention to monitor the Earth at low microwave frequencies (L-band), where there is a maximum sensitivity to soil moisture and ocean salinity. At L-band, classic radiometers require large steerable antennas to meet the spatial resolution requirements (30-50 km at most, 10-20 km wished for), from a low polar orbit platform. During the 1990s, technological studies were conducted by the ESA with an eye to design a 2D synthetic aperture L-Band radiometer (the Microwave Imaging Radiometer by Aperture Synthesis project: MIRAS). In 1998, in answer to a call for Earth Explorer Opportunity Missions issued by ESA, the Soil Moisture and Ocean Salinity Mission proposal (SMOS), based upon a radiometer concept derived from the MIRAS studies, was submitted, In 1999, following a selection procedure, ESA approved the SMOS mission for an extended phase. This paper summarize part of the work carried out on the interferometric radiometry concept and the optimization of the instrument configuration.

Integration of MIRAS breadboard and future activities

The European Space Agency (ESA) is conducting a feasibility study on MIRAS, a Microwave Imaging Radiometer using two dimensional aperture synthesis, for the mapping of soil moisture and ocean salinity from space. An aircraft breadboard of this instrument is discussed. This paper concentrates on the results achieved following the integration of the prototype. The breadboard consists of an 11 element Y-shaped bi-dimensional L-band interferometer which can be switched to measure the two polarizations. In addition the future activities related to MIRAS are presented. A technology demonstrator of MIRAS is proposed as a first major step towards a future spaceborne sensor.