Sun effects in 2-D aperture synthesis radiometry imaging and their cancelation

The Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) is the single payload of the European Space Agency's (ESA) Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity mission. MIRAS will be the first two-dimensional aperture synthesis radiometer for Earth observation. Two-dimensional aperture synthesis radiometers can generate brightness temperature images by a Fourier synthesis process without mechanical antenna steering. To do so and have the necessary wide swath for Earth observation, the array is formed by small and low directive antennas, which do not attenuate enough bright noise sources that may interfere with the measurements. This study analyzes the impact of the radio-frequency emission from the Sun in the SMOS mission, reviews the basic image reconstruction algorithms, and proposes a technique to minimize Sun effects.     

Improved Image Reconstruction Algorithms for Aperture Synthesis Radiometers

The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission aims at producing global and frequent maps of SMOS and will be launched in 2008. SMOS' single payload is a new type of radiometer called Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) operating at L-band in which brightness temperature images are formed by a Fourier synthesis technique. However, in the alias-free field of view where the brightness temperature images are reconstructed, a bias is present which has been found to be higher for high-contrast brightness temperature scenes (coastlines) and lower for homogeneous scenes (all oceans or lands). This scene-dependent bias will ultimately limit the achievable accuracy of the retrieved geophysical parameters, and it is particularly critical for the retrieval of sea surface salinity. This paper presents a general analysis of the origin of this bias, which is found to be actually due to the different measurement errors in the instrument observables (visibility samples). An improvement of the image reconstruction algorithm is then presented to mitigate it. As compared with the previous algorithm versions, the proposed improved reconstruction algorithm further decomposes the visibility samples into some new terms: ocean and land/iced sea, instead of just the Earth's disk over the sky background. This decomposition aims at further reducing the contrast (high-frequency components) in the differential image and, therefore, minimizes the impact of multiplicative errors, improving the radiometric accuracy. In addition, this approach proves to perform the image reconstruction in part of the alias regions and improves the quality of the reconstruction close to the coastlines.     

MIRAS reference radiometer: a fully polarimetric noise injection radiometer

A prototype reference radiometer for the Microwave Imaging Radiometer Using Aperture Synthesis (MIRAS) instrument of the Soil Moisture and Ocean Salinity satellite has been developed. The reference radiometer is an L-band fully polarimetric noise injection radiometer (NIR). The main purposes of the NIR are: 1) to provide precise measurement of the average fully polarimetric brightness temperature scene for absolute calibration of the MIRAS image map and 2) to measure the noise temperature level of the noise distribution network of the MIRAS for individual receiver calibration. The performance of the NIR is a decisive factor of the MIRAS performance. In this paper we present the operation principles and calibration procedures of the NIR, a measurement technique called blind correlation making measurements of full Stokes vector possible with the noise injection method, and finally experimental results verifying certain aspects of the design.     

Earth-Viewing L-Band Radiometer Sensing of Sea Surface Scattered Celestial Sky Radiation—Part II: Application to SMOS

We examine how the rough sea surface scattering of L-band celestial sky radiation might affect the measurements of the future European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission. For this purpose, we combined data from several surveys to build a comprehensive all-sky L-band celestial sky brightness temperature map for the SMOS mission that includes the continuum radiation and the hydrogen line emission rescaled for the SMOS bandwidth. We also constructed a separate map of strong and very localized sources that may exhibit L-band brightness temperatures exceeding 1000 K. Scattering by the roughened ocean surface of radiation from even the strongest localized sources is found to reduce the contributions from these localized strong sources to negligible levels, and rough surface scattering solutions may be obtained with a map much coarser than the original continuum maps. In rough ocean surface conditions, the contribution of the scattered celestial noise to the reconstructed brightness temperatures is not significantly modified by the synthetic antenna weighting function, which makes integration over the synthetic beam unnecessary. The contamination of the reconstructed brightness temperatures by celestial noise exhibits a strong annual cycle with the largest contamination occurring in the descending swaths in September and October, when the specular projection of the field of view is aligned with the Galactic equator. Ocean surface roughness may alter the contamination by over 0.1 K in 30% of the SMOS measurements. Given this potentially large impact of surface roughness, an operational method is proposed to account for it in the SMOS level 2 sea surface salinity algorithm.     

Error Propagation in Calibration Networks of Synthetic Aperture Radiometers

During the last two decades, the development of synthetic aperture radiometers for remote sensing has been studied intensively. One of the proposed methods for the calibration of such an instrument is the application of a distributed noise injection network. This paper focuses on the origin and effect of errors arising from this methodology. A generalized analytical method to calculate the accumulation of phase and amplitude errors in a distributed noise injection network is presented. This method is then applied to the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), the interferometric radiometer aboard the European Soil Moisture and Ocean Salinity satellite. The effect of the resulting errors to MIRAS' brightness temperature is analyzed. The presented method is applicable also to other interferometric radiometers, whose calibration relies on distributed noise injection.      

Estimates of brightness temperatures from scanning radiometer data

In the analysis of antenna temperature maps of the earth obtained by satellite-borne microwave radiometers, estimates of brightness temperatures or averages of brightness temperatures over areas considerably smaller than the region sensed at a given position of the radiometer antenna are often needed. An application of the Backus-Gilbert methodology is made to obtain an objective criterion of the best resolution (in a least squares sense) obtainable from a given system and to investigate the trade-off between resolution and noise in the derived average brightness temperatures. The mathematically related problem of simultaneously analyzing antenna temperature measurements made at different frequencies by antennas with noncoincident antenna patterns is also considered.     

Two-load radiometer precision and accuracy

The advent of fast computers and digital signal processing permits many aspects of radiometer operation to be decided in real time so that precision and accuracy are maximized. Closed-form expressions for precision, optimal antenna/load dwell times, and maximum chopping period are derived for a class of radiometers that uses two loads to calibrate its measurements of antenna brightness temperature. The importance of load brightness temperature selection emphasizes the need to develop "cold" calibration loads that can operate at ambient temperature. A method of incorporating critical thermistor data into the radiometer brightness temperature calibration can be used to help stabilize measurements against drift and may permit radiometers to operate without thermal control in some low-accuracy applications.     

综合孔径微波辐射计成像观测能力

对综合孔径微波辐射计成像观测原理进行了阐述,分析了综合孔径微波辐射计的灵敏度、积分时间、天线波束宽度、目标亮温、背景噪声温度、系统工作频率和目标等效截面等因素与系统作用距离的关系,对系统的目标观测能力进行仿真验证,探讨了阵列通道不一致等误差的补偿方法。研究成果将有助于综合孔径微波辐射计的实用化推进。     

综合孔径辐射计反演成像算法研究

高效的反演成像算法是综合孔径辐射计成像的关键技术之一。提出一种基于视场细分的改进反演成像算法,将视场细分成多个矩形子区域,利用反演亮温与可视度函数的傅里叶变换关系,得到每个子视场的反演亮温。通过理论证明和仿真计算表明:该算法可使系统中未消除的误差分量对亮温反演的影响最小,与常用的BG算法对比分析:表明了新算法的正确性和有效性,且计算速度快。     

The visibility function in interferometric aperture synthesis radiometry

The fundamental equation of interferometric aperture synthesis radiometry is revised to include full antenna pattern characterization and receivers' interaction. It is shown that the cross correlation between the output signals of a pair of receivers is a Fourier-like integral of the difference between the scene brightness temperature and the physical temperature of the receivers. The derivation is performed using a thermodynamic approach to account for the effects of mutual coupling between antenna elements. The analysis assumes that the receivers include ferrite isolators so that the noise wave passing from the receiver toward the antenna can be modeled as uncorrelated ambient noise. The effect of wide beamwidth antennas on the polarization basis of the retrieved brightness temperature is also discussed.     

Brightness Temperature Map Reconstruction from Dual-Polarimetric Visibilities in Synthetic Aperture Imaging Radiometry

Synthetic aperture imaging radiometers are powerful sensors for high-resolution observations of the Earth at low microwave frequencies. Within this context, the European Space Agency is currently developing the soil moisture and ocean salinity (SMOS) mission devoted to the monitoring of SMOS at global scale from L-band spaceborne radiometric observations obtained with a 2-D interferometer. This paper is concerned with the reconstruction of radiometric brightness temperature maps from interferometric measurements. More exactly, it extends the concept of ldquoband-limited resolving matrixrdquo to the case of the processing of dual-polarimetric data.     

Design and test of the ground-based L-band Radiometer for Estimating Water In Soils (LEWIS)

In the framework of the preparation of the Soil Moisture and Ocean Salinity (SMOS) mission, several field experiments are required so as to address specific modeling issues. The goal is to improve current models and to test retrieval algorithms. However, adequate ground instrumentation is scarce and not readily available "off the shelf". In this context, a high-accuracy L-band radiometer was required for a specific long-term campaign for the preparation of the SMOS mission. For this purpose, a dual-polarized radiometer was designed and built to check algorithms for surface soil moisture retrieval from multiangular dual-polarized brightness temperatures. This radiometer has been tested in the field for 20 months and is operational since end of January 2003. The aim of this paper is to give details of the system architecture, calibration procedures, together with the performances obtained and some preliminary results.     

Interferometric synthetic aperture microwave radiometry for theremote sensing of the Earth

Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the Earth's brightness temperature from low Earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular important are the number and placement of antenna elements in the imaging array     

Correlation Denormalization in Interferometric or Polarimetric Radiometers: A Unified Approach

This paper presents a general analysis of correlation measurements in an interferometer or a radiometer based on noise injection and/or switching and measurement of normalized correlations (e.g., imaging synthetic aperture or polarimetric radiometers). A compact unifying notation for denormalizing the measured normalized correlations in the presence of noise injection in one or both of the receiving channels is presented. Technological limitations are also assessed by evaluating the effect of associated approximations. Finally, the approach is validated by experimental results of the measurement and calibration of related front-end nonidealities, namely, the finite isolation of the front-end switch. The methods presented in this paper are illustrated by a thorough analysis of the so-called mixed baselines of microwave imaging radiometer using aperture synthesis, which refer to those baselines which are formed between the regular receivers (light-weight cost-efficient front-end) and the reference radiometers. These baselines require special attention, since the reference radiometers are noise-injection radiometers, which inject noise to the measured signal, whereas the regular receivers are total power receivers.      

The determination of surface salinity with the European SMOS space mission

The European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission aims at obtaining global maps of soil moisture and sea surface salinity from space for large-scale and climatic studies. It uses an L-band (1400-1427 MHz) Microwave Interferometric Radiometer by Aperture Synthesis to measure brightness temperature of the earth's surface at horizontal and vertical polarizations (T/sub h/ and T/sub v/). These two parameters will be used together to retrieve the geophysical parameters. The retrieval of salinity is a complex process that requires the knowledge of other environmental information and an accurate processing of the radiometer measurements. Here, we present recent results obtained from several studies and field experiments that were part of the SMOS mission, and highlight the issues still to be solved.     

基于截断奇异值的镜像综合孔径亮温重建方法

镜像综合孔径(MAS)辐射计的图像重建是由余弦可见度函数到场景亮温的图像反演过程,余弦可见度函数由转移方程求解得到.而转移方程为不适定方程组,相关输出中一个较小的误差,都可能引起求解的余弦可见度函数存在较大偏离.因此,对这个不适定方程组的求解是整个亮温重建算法成功的关键.该文基于镜像综合孔径的基本原理,分析转移矩阵的病...     

Errors resulting from the reflectivity of calibration targets

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurements are described that enable one to estimate the resulting uncertainty in the measured brightness temperature. The measurement results are presented for several combinations of antenna and calibration target. The resulting uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered.     

遥感微波辐射计逆向辐射噪声对天线温度标定的影响

本文分析了遥感微波辐射计的逆向辐射噪声温度和口面定标源的亮度温度，建立了口面定标源的亮度温度模型。深入分析用口面定标方法反演微波辐射计的天线温度时过向辐射噪声对天线温度标定的影响。