The interferometric microwave radiometer (IMR) utilizes an interferometric synthetic aperture technique to achieve high spatial resolution in low-frequency microwave remote sensing, addressing the challenges of deploying large-scale passive sensors in space. IMR measures spatial harmonics of scene brightness temperature, known as visibility, which are then used in inversion algorithms to reconstruct the target brightness temperature. In 1-D IMR, the interferometric synthetic aperture technique is applied only in the cross-track direction, resulting in higher resolution compared with the coarser along-track direction determined by the real antenna aperture. Current research focuses on cross-track inversion, which has yielded promising results; however, the low along-track resolution remains a significant limitation for its overall application. This article introduces the Backus-Gilbert (BG)-inspired 1-D IMR resolution enhancement method, inspired by real aperture microwave radiometer techniques, to address along-track resolution limitation. The study utilizes the L-band 1-D IMR of the Microwave Imager Combined Active and Passive (MICAP) aboard the Chinese Ocean Salinity Satellite as an example. Results from synthetic test images and hardware-in-the-loop simulation demonstrate that the proposed method enhances along-track resolution and provides the flexibility to optimize for either higher image quality or radiometric resolution comparable to the traditional 1-D IMR inversion method. Additionally, it improves the accuracy of salinity measurements in coastal areas.

Radio Frequency Interference (RFI) is a significant limiting factor in the retrieval of geophysical parameters measured by microwave radiometers. Microwave Imager Combined Active and Passive (MICAP), which is a package of active and passive microwave instruments including L/C/K-band Microwave Interferometric Radiometer (MIR) and L-band scatterometer, has been approved to be taken on-board in the Chinese Ocean Salinity Mission. RFI is a serious threat to MICAP, especially to the L-band MIR. In the first part of this paper, on-orbit RFI mitigation method in MICAP L-band MIR is proposed, which is detected in time domain, frequency domain, probability spectrum, polarization domain, and RFI is removed in time domain and frequency domain. Secondly, desktop experiment of this RFI mitigation method is proposed based on MICAP L-band MIR. Thirdly, a flight experiment of this RFI mitigation method is demonstrated before MICAP launch. As shown in this paper, MICAP L-band MIR will be the first MIR with on-orbit RFI mitigation capacity.

In this study, an Atmospheric Motion Vector (AMV)-based method for retrieving wind vectors using 183.31-GHz water vapor absorption channels is introduced. The effectiveness of this approach is validated through a comparative analysis with the ERA5 reanalysis data and the wind product derived from the Visible Infrared Imaging Radiometer Suite (VIIRS). Furthermore, this methodology demonstrates a complementary performance with the VIIRS wind product, suggesting its potential for enhancing the diversity of wind field data.

The Chinese Ocean Salinity Mission was officially initiated in 2020. In July 2023, a flight campaign was conducted to assess the system design, data processing methodologies, and performance criteria. This paper discusses the system architecture of the L/C/K-band one-dimensional microwave interferometric radiometers (MIR) tested in the flight campaign and presents the performance evaluations derived from imaging results.

The Chinese Ocean Salinity Mission (COSM) is dedicated to the global sea surface salinity (SSS) using the interferometric aperture synthesis radiometry. The airborne COSM was conducted during July to August 2023 at Qixia of Shandong Province. The introduction of airborne COSM experiment and the pre-simulation results based on the design of airborne instruments, which are the scaled-down model of spaceborne payloads, are presented. The preliminary sea surface salinity (SSS) retrieval results show that, the best accuracy of SSS is achieved when considering all the available measurements from two payloads together in a combined retrieval. The results from the airborne COSM experiment verify the interferometric synthetic aperture payloads performance and will also help to improve the data processing technique and the key error correction methods in the future in-orbit operational phase.

Microwave interferometric radiometers are widely used in measuring soil moisture and sea salinity. However, achieving high-accuracy measurements of geophysical parameters requires complex correction and inversion processes, which existing MIR cannot image in real time. This study using three methods for fast estimation of the quantization threshold, enabling real-time computation of correlation coefficients and simplified calibration for real-time imaging. In this paper, we demonstrate real-time imaging through a one-dimensional prototype.

The concept of a new type of high-resolution microwave interferometric radiometer (MIR) using beamforming antennas is proposed in this paper. Using large sampling intervals can effectively reduce the system complexity of large-scale arrays, but aliasing effect is also magnified. Through dedicated simulations, the aliasing effect can be suppressed when the interferometric element size exceeds the sampling interval, thus a dual-arm Y-shaped array is recommended for the overall system design. A ground-based demonstrator is currently under development, accompanied by preliminary experimental results.

A cross beam correlation microwave radiometer (CBCR) using two orthogonal subarrays is introduced. It consists of two spatially orthogonal subarrays and a correlation receiver, where a pencil beam is realized by the coherence of two orthogonal fan beams. However, the differences between the two orthogonal fan beam patterns lead to gain loss and negative sidelobes. The gain loss (effective aperture reduction) of cross arrays has been proven to lead to deterioration in radiometric resolution. But the properties of negative sidelobes and their impact on observations have not been fully investigated. In this article, the performance of the cross array is comprehensively analyzed, and the systematic expressions of beam characteristics and radiometric resolution are proposed. The main beam efficiency (MBE) is redefined to prevent it from exceeding unity. Two novel factors, namely the spatial sparsity factor and phase influence factor, are introduced to quantitatively assess the influence of the cross array on radiometric resolution. In addition, a planar cross antenna array is proposed to remedy the shortcomings of Mills cross in small radiometer applications, and the performance of the two cross arrays is evaluated based on the above methods. Finally, the numerical simulations and hardware experiments with an L-band prototype are conducted to verify the theoretical results.

The Moon is an ideal target for cross-calibration of radiometric measurements due to its stable surface geophysical properties. To explore the lunar calibration capability for the space-borne microwave radiometer missions, a ground-based interferometric radiometry concept for lunar observation is proposed in this paper, the observation model, site environment and array design are discussed. A large antenna spacing design is considered to achieve satisfactory spatial resolution while maintaining a reasonable system size. The resulting aliasing error is evaluated through imaging simulations.

Digital correlator is the central signal processing unit for an interferometric radiometer system. The core function of the digital correlator is to quantitatively measure the correlation coefficients of all the receiver output combination pairs, and then generate the visibility functions together with the system noise temperatures obtained by single-channel total-power measurements. In this letter, a comprehensive quantitative characterization method is proposed for a 30-channel, three-level quantized digital correlator unit (DCU), which will be used for the microwave imager combined active and passive (MICAP) C- and K-bands radiometers, on-board Chinese ocean salinity mission (COSM). First, several key performance characteristics of DCU, including correlation offset and correlation efficiency, are defined and discussed based on a segmented error model. Second, a test platform based on an arbitrary waveform generator (AWG) is established and applied for the test and evaluation of the DCU engineering model (EM). Orthogonal in-phase and quadrature bandlimited noise signals are generated via a newly proposed approach based on Hilbert transform, facilitating the joint tests of correlation efficiency and phase error. Moreover, the influences of the correlator’s input power level on correlation offset and correlation efficiency are also investigated during the test of DCU. DCU test results show that the correlation offset is smaller than −40 dB, the correlation efficiency is better than 0.996, and the phase error is less than 5°. The result can provide an important reference for the digital correlator test in COSM.

Radio Frequency Interference (RFI) is a significant limiting factor in the retrieval of geophysical parameters measured by microwave radiometers. Microwave Imager Combined Active and Passive (MICAP), which is a package of active and passive microwave instruments including L/C/K-band Microwave Interferometric Radiometer (MIR) and L-band scatterometer, has been approved to be taken on-board in the Chinese Ocean Salinity Mission. RFI is a serious threat to MICAP, especially to the L-band MIR. In the first part of this paper, RFI mitigation strategy combined on-board processing and on-ground processing is proposed based on MICAP structure. Secondly, RFI side lobe blanking criteria is proposed to clarify the requirements for on-board RFI detection capabilities, which indicate a trade-off between detection sensitivity and false alarm rate. Thirdly, a verification experiment of RFI mitigation method is demonstrated for on-board RFI detection.

The calibration strategy for synthetic aperture interferometric radiometers based on noise injection has been successfully demonstrated in the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA). With the emerging demands of increasing both array size and frequency bands, the internal calibration hardware becomes more difficult to implement. Although several external calibration methods have been proposed, the so-called phase ambiguity problem was not carefully considered. In this article, a novel calibration strategy for both phase and amplitude calibrations based on redundant space calibration is proposed, which utilizes the unique structure of equispaced circular array and rotational sampling strategy. The original irregular phase ambiguity degenerates to the fixed$\pi $-ambiguity, where only two possible solutions to phases are introduced. Both phase and amplitude of visibility can be simultaneously calibrated by the proposed strategy, and no cooperative calibration target is needed.

In the theoretical part of this work, a calibration strategy for both phase and amplitude calibration of rotating equispaced circular array based on redundant space calibration was introduced. It avoids the challenging implementation of conventional noise-injection networks, especially for a large-scale array, by using the strong self-consistency accomplished by array rotation. This study presents comprehensive simulation results of the proposed calibration strategy, where calibration residuals and image reconstruction errors are evaluated. The phase calibration shows outstanding performance, while the amplitude calibration is found to be biased due to the logarithms used to linearize the system of calibration equations, and the resulting image reconstruction bias is scene-dependent. A bias correction method is proposed accordingly, which requires ana prioriregional brightness temperature within the observation scene. The performance of both phase and amplitude calibration take advantage of increasing the number of elements, which promises the system scalability for high spatial resolution applications.

Microwave Imager Combined Active and Passive (MICAP), which is a package of active and passive microwave instruments including L/C/K-band radiometers and L-band scatterometer, has been chosen as one of the primary payloads for the Chinese Ocean Salinity Mission (COSM). The L-band one-dimensional synthetic aperture radiometer (L-Rad) is the key part of MICAP to measure sea surface salinity (SSS). As radio frequency interference (RFI) has been reported as a severe threat to L-band radiometry, it can be foreseen that MICAP L-Rad will encounter RFI contaminations unavoidably. Due to the conflict between the huge transmission data volume and the limited downlink budget, it is challenging for MICAP L-Rad to transmit all rapidly sampled sub-band data products and conduct on-ground RFI processing like Soil Moisture Active Passive (SMAP) microwave radiometer. The onboard real-time RFI detection and mitigation strategy is proposed accordingly for MICAP L-Rad. It is designed based on the distributed digital processing structure and the synthetic-aperture feature. The proposed RFI processing strategy is evaluated and verified by the system simulations and hardware experiment, demonstrating its RFI detection capability and low false alarm rate performance.

The Chinese Ocean Salinity Satellite is dedicated to global sea surface salinity (SSS) mapping with two payloads onboard, which are the interferometric microwave radiometer (IMR) and the microwave imager combined active and passive (MICAP), both using the interferometric aperture synthesis radiometry. One of the payloads is an L-band interferometric radiometer system with a Y-shaped antenna array. The other one, MICAP, is a 1-D passive/active combined system, where the L-, C-, and K-band interferometric radiometers’ antenna arrays are arranged in a line, and the active device is an L-band scatterometer. Based on the payload configurations, a series of simulations is applied to analyze the payloads performance, including the brightness temperature (TB) characteristic, the SSS accuracy, and the effects of Sun and land contamination. The TB simulation results show that the IMR possesses a finer spatial resolution compared to the MICAP L-band radiometer, whereas the latter achieves a better TB radiometric resolution. With the assistance of the C- and K-band radiometers and the L-band scatterometer, the combined retrieval accuracy improves compared with the SSS retrieved using only L-band TB and auxiliary parameters. The best accuracy of SSS is achieved when considering all the available measurements from two payloads together in a combined retrieval. Finally, the results related to Sun and land contamination that endanger the interferometric radiometer measurements are presented and discussed. The performance simulation in this article has been used as a reference for the design phase of two payloads and will be updated with the development of payload manufacture.

Chinese ocean salinity satellite is a mission designed for global ocean salinity monitoring. Two main payloads equipped onboard the satellite are the Interferometric Microwave Radiometer (IMR) and the Microwave Imager Combined Active and Passive (MICAP) which both have L-band radiometer and adopt aperture synthesis technology. A simulation is designed according to the payloads' configurations and consists of three parts: the brightness temperature generator, the radiometer system and the parameters retrieval. Preliminary results, which contain IMR and MICAP's brightness temperature resolution, spatial resolution, and parameter retrieval accuracy, are presented.

Water Cycle Observation Mission (WCOM) is an earth science mission focused on the research of water cycle under global change with three dedicated designed payloads. The main payload in WCOM named Interferometric Microwave Imager (IMI) is an L/S/C tri-frequency one-dimensional aperture synthesis radiometer aiming at measuring the key parameters of soil moisture and ocean salinity. In this paper, first the instrument concept and system design of WCOM/IMI is briefly introduced. Then an end-to-end simulation of ocean salinity using the L/S/C tri-frequency radiometer IMI is presented including the simulation procedure and algorithms. The simulation consists of three primary modules: forward TB generator, radiometer system and salinity retrieval. At last, the preliminary results of the simulation are illustrated.

The dream of launching a microwave radiometer into geostationary orbit (GEO) for atmosphere sounding has lasted for over four decades. Several concepts have been proposed, but none of them has come into reality by now. All these concepts can be divided into two different ways, using real aperture microwave radiometer (RA-MWR) and using synthetic aperture microwave radiometer (SA-MWR). This paper is dedicated to make a preliminary comparison between the imaging performances of the RA-MWR and SA-MWR in GEO applications. In the comparison, a similar concept with the Geostationary Observatory for Microwave Atmospheric Sounding (GOMAS) with a Cassegrain antenna proposed by ESA is taken as the RA-MWR example, while the concept of Geostationary Interferometric Microwave Sounder (GIMS) with a circular antenna array pursued in China is taken as the example of SA-MWR. The numerical simulation results indicate that, with the benefits of using a predesigned Earch model, or even using a simple disc target, SA-MWR can greatly reduce the imaging error especially in the areas near to the Earth contour, and thus achieve a higher imaging accuracy than RA-MWR.

The interferometric synthetic aperture microwave imager (IMI) on WCOM is a one-dimensional L/S/C tri-frequency microwave radiometer aiming to improve the measurement capability on soil moisture and ocean salinity. The IMI antenna is mainly consisted with the deployable parabolic cylinder mesh reflector and the tri-frequency linear patch feed array. The distribution of antenna feeds is very important to the imaging quality of microwave imager. Therefore, requirement quantitative evaluation was accomplished through proper simulation and comparing the influence of different antenna feed arrays on imaging accuracy, sensitivity and spatial resolution. This paper provides a simulation system that is consistent with the real data processing. The efficient and optimal feeds array arrangement is also selected for IMI system after simulation and evaluation of the imaging results of several different feed arrays.

The Geostationary Interferometric Microwave Sounder (GIMS) is millimeter wave imaging sounder concept proposed for China's next generation geostationary meteorological satellite (FY-4M). GIMS is supposed to utilize a rotating circular thinned array, instead of a stationary Y-shape array, to reduce the required number of the antenna elements. A proof-of-concept 53GHz ground-based GIMS demonstrator with 28 elements has been successfully developed and tested during 2009~2012. A new GIMS demonstrator development plan has been approved for further risk mitigation. A 70 element demonstrator is foreseen to be finished before the end of first quarter of 2017. In this paper, recent progresses on the ongoing demonstration activities will be summarized. The preliminary imaging and performance evaluation results of the GIMS-II demonstrator will also be presented.

The aim of the paper is to describe a ground-based microwave scatterometer demonstrator dedicated for the functional verification of MICAP (Microwave Imager Combined Active and Passive). MICAP has been selected to be one of the main payloads for future Chinese ocean salinity mission. The MICAP scatterometer is for estimating ocean surface roughness which measurement will enable the removal of the wind effect from the ocean surface brightness temperature measurement being used to retrieval ocean salinity. The primary technical challenge is its end to end system stability, on the order of 0.1dB. A demonstrator was developed to verify functionality and provide support to trouble shooting in the following project phases. The demonstrator's development and integration, as well as the stability calibration are discussed in the paper.

In this paper, we present the experimental results of a DBF (digital beam forming) scatterometer demonstrator performance on the purpose of development and verification of innovative DBF concepts and advanced operational modes for the future space borne MICAP (microwave imager combined active and passive), China's ocean salinity mission. The hardware configuration of the demonstrator is introduced. The ground-based experiments, verifying the DBF function on receiving and transmitting separately are carried out on the open field using the demonstrator. Analysis of the experimental results verifies the validity of the DBF technique on the receiving and transmitting. At last, the problems caused by the unbalance of the amplitude and phase between channels are discussed. The function verification experiment is the very first step towards the future space borne MICAP, and further researches are in progress.

This paper presents the design and preliminary performance tests of an L-band synthetic aperture interferometic radiometer using the new sampling concept of clock scan. With the advantages of simple and deployable array structure, the concept of Clock Scanning Microwave Interferometric Radiometer (CS-MIR) has much potential to apply for the future Solar Polar Orbit (SPO) and Geostationary Earth Orbit (GEO) missions, where large antenna array with high angular resolution is the prime requirement. This L band demonstrator is developed to validate the concept of CS-MIR and solve the related technical problems.

In the synthetic aperture radiometer system, the imperfection of the antenna pattern and the inconsistency between the elements can cause systematic spatial error on the retrieved image. Antenna pattern error analysis and calibration methods of an L-band 1-D synthetic aperture radiometer are presented in this paper. An improved calibration algorithm proposed here uses approximate observation target modeled brightness temperature to replace the reference brightness temperature defined as the mean value of the field of view in the flat target transformation(FTT) method. This improved method is verified through a simulation system that simulates L-band 1-D synthetic aperture radiometer's observation process in viewing a series of near real case brightness temperature maps. This is submitted for the special session of “New Developments of Chinese Oceanographic and Meteorological Satellites”.

The Geostationary Interferometric Microwave Sounder (GIMS) is a synthetic aperture microwave sounder working in time-sharing sampling mode with a rotating circular antenna array, thus having a relatively long imaging period. Since target motion in the imaging period will pose a problem to imaging effect, observing dynamic target such as fast changing weather of tropical cyclones will introduce imaging error to the retrieved image of GIMS. A method that combines Kalman filter and interpolation to reduce imaging error are presented, which utilizes a time sequence of visibilities collected by time-sharing sampling method. Simulations of GIMS's observation process in viewing a series of near real case brightness temperature maps modeled by the National Center for Environmental Prediction Final Operational Global Analysis data/Weather Research and Forecasting Model/Radiative Transfer for Television Infrared Observation Satellite Operational Vertical Sounder method, together with postprocessing of dynamic observation error-correction processing and image retrieval are conducted for the method. Results show that the method can reduce imaging error even if there are random noises in visibilities.

The microwave interferometric radiometer (MIR) is a passive sensor using synthetic aperture technique for microwave remote sensing. It uses a thinned antenna array to replace the traditional large real aperture antenna, so as to reduce the antenna size, weight, and manufacturing difficulty, and further increase the spatial resolution of the instrument. The antenna array configuration is the first concern for the MIR application. Various array configurations have been proposed. Among them the arrays with hexagonal sampling grid such as the Y-shaped and hexagonal arrays have drawn more attention because of the higher sampling efficiency. However, they do not have the best performance in radiometric sensitivity and radio frequency interference (RFI) mitigation. This paper presents the detailed analysis of the circular array performance and compares it with the Y-shaped and hexagonal arrays. Based on the early developed theory about the radiometric resolution, two new factors named as sampling sensitivity factor and uncertainty factor are first introduced and defined in this paper, which can be used to quantitatively characterize the array performance. The main beam efficiency of the MIR is also redefined to prevent it being uncomfortably greater than unity but still keep a physical significance. Numerical simulations with analysis are finally implemented to verify the theoretical results and also to demonstrate the superiority of the circular array in terms of radiometric sensitivity and RFI mitigation.

Earth observation satellites play a critical role in providing information for understanding the global water cycle, which dominates the Earth-climate system. However, limitations in observations will restrict our current ability to reduce the uncertainties in the information used to make decisions regarding to water use and management. Under the support of “Strategic Priority Research Program for Space Sciences” of the Chinese Academy of Sciences, a new satellite concept of global Water Cycle Observation Mission (WCOM) is proposed, aiming to provide higher accuracy and consistent measurements of key elements of water cycle from space, including soil moisture, ocean salinity, freeze-thaw, snow water equivalent and etc. The expected more consistent and accurate datasets would be used to refine existing long-time series of satellite measurements, to constrain hydrological model projections and to detect the trends necessary for global change studies. The WCOM is expected to be implemented during the 13thfive-year-plan period (2016–2020).

Water Cycle Observation Mission (WCOM) is a mission dedicated to synergetic observations of global water cycle parameters, with emphasis on soil moisture, ocean surface salinity, snow water equivalent and frozen/thaw. WCOM implements its observation requirements by measurement of microwave emission/scattering of both the frequencies sensitive to the key parameters and also the auxiliary frequencies providing necessary atmospheric and surface roughness corrections. In order to satisfy this measurement requirements, WCOM is equipped with payloads with combination of active and passive microwave sounding capabilities of frequency from L-band to W-band. In addition to the multiple frequency requirement, the requirement for spatial resolutions and swath width leads to large aperture antennas and scanning of antenna beams, the accommodation of the three payloads becomes the major challenge for the design of the WCOM satellite. In order to satisfy the temporal resolution requirement, the design of data downlink and ground segment are also discussed. In this presentation, a preliminary design for WCOM satellite is provided, some technical issues need further investigation are also discussed.

The inter-element phase calibration plays an important role in the overall calibration scheme for synthetic aperture radiometer. The traditional relative phase calibration approach is to use correlated noise injection network, which will be difficult to be implemented in millimeter wave band due to the complexity of the waveguide divider network. A novel self-calibration method for interferometric radiometers with rotating thinned array, especially for the geostationary interferometric microwave sounder(GIMS), has been proposed in this paper. By using this approach, neither dedicated hardware nor dedicated calibration working model is needed to achieved the relative phase calibration. The self-calibration approach is inherently merged with the nominal observation working model of GIMS, thanks to the continuous array rotating of GIMS instrument. A running average scheme has been introduced into the self-calibration approach to enhance the SNR of the calibration data, which is normally very low with the natural earth scene. The method is demonstrated by both simulation and field imaging experiment.

Water Cycle Observation Mission (WCOM) is an earth science mission proposed and focused on the research of water cycle under global change. With its three dedicated designed main payloads, WCOM can achieve synchronized observation on a group of global water cycle key parameters, including soil moisture, ocean salinity, snow water equivalent, soil freeze-thaw, atmospheric water vapor, precipitation and other associated parameters. One of the three WCOM main payloads names IMI (Interferometric Microwave Imager), which is a newly designed L/S/C tri-frequency radiometer aiming to provide advanced measurement capability on soil moisture and ocean salinity. In this paper, the instrument concept and preliminary system design of WCOM/IMI will be introduced. Recent progresses on the filed experiments of an L-band demonstrator will also be introduced.

A combined L-band active and passive sensor for space-based ocean salinity observation has been proposed in MIRSlab, CAS [1] [2]. A ground-based prototype has been developed to demonstrate the concept and performance of the instrument. The experimental result of the passive part of the prototype, the L-band 1-D synthetic aperture radiometer, has been introduced in this paper.

Microwave interferometric radiometer (MIR) is a passive sensor using synthetic aperture technique for microwave remote sensing. Antenna array configuration is the first concern for the MIR application. Various array configurations have been proposed. This paper presents the analysis of the circular array performance and compares it with the Y-shaped array and hexagonal array. Based on the early developed theory about the radiometric resolution, two new factors named as sampling sensitivity factor and uncertainty factor are firstly introduced and defined in this paper, which can be used to quantitatively characterize the array performance. Numerical simulations with analysis are finally implemented to verify the theoretical results and also to demonstrate the superiority of the circular array in terms of radiometric sensitivity and RFI mitigation.

The MICAP(microwave imager combined active and passive), which has been selected to be a candidate payload for future Chinese ocean salinity mission, contains an L-band digital beam forming(DBF) scatterometer for the purpose of elimination of ocean surface roughness and wind retrieving. In this paper, data pre-processing flow for this scatterometer to obtain data ready for sea surface salinity and ocean surface wind retrieval was presented after corresponding data level definitions in this flow had been introduced. The processing flow was verified by processing data resulted from a data set simulated in a simple progress. Future work to perfect the pre-processing progress was also discussed.

A 1-D interferometric system at 1.4GHz, 6.9GHz, 18.7 GHz and 23.8GHz combined with a scatterometer at 1.26GHz, called microwave imager combined active/passive (MICAP), has been proposed to retrieve sea surface salinity (SSS) and to reduce geophysical errors due to surface roughness and sea surface temperature (SST). The MICAP will be a candidate payload onboard the Ocean Salinity Satellite of China. The sensitivity of active/passive microwave observations to SSS, SST and wind is analyzed and the stability requirement of the instruments is estimated, with the objective of designing an optimized satellite instrument, dedicated to an “all-weather” estimate of the SSS with high accuracy from space.

Imaging period is an important consideration to geostationary interferometric microwave sounder (GIMS) when mapping fast changing target such as typhoon. GIMS simulation system with near real case observation target can evaluate system performance in different system configurations and thus help determine the optimal imaging period. In this paper, GIMS simulation system using MATLAB and near real case observation modeled by FNL/WRF/RTTOV method has been used to analyze the effect of imaging period on image quality. System simulation results for each frequency channel will be presented and analyses of imaging period's effect on image quality will also be given.

The Geostationary Interferometric Microwave Sounder (GIMS) is a new concept of atmospheric microwave sounder for China's future geostationary Earth orbit meteorological satellite (FY-4). A novel self-calibration method for interferometric radiometers with a rotating thinned array, particularly for GIMS, has been proposed in this letter. Compared with the traditional relative phase calibration approach, neither dedicated hardware nor dedicated calibration working model is needed to achieve the relative phase calibration in this novel interelement phase calibration. The self-calibration approach is inherently merged with the nominal observation working model of GIMS, owing to the continuous array rotating of the GIMS instrument. A running average scheme has been introduced into the self-calibration approach to enhance the signal-to-noise ratio of the calibration data, which is normally very low with the natural earth scene. The method is demonstrated by both simulation and field imaging experiment.

Sea surface salinity (SSS) plays an important role in global water cycle. In recent years, satellite based remote sensing has proven to be a promising approach for global SSS observation. A new payload concept, named MICAP (microwave imager combined active and passive), has been introduced in this paper. MICAP is a suit of active/passive instrument package, which includes L/C/K band one-dimensional MIR (microwave interferometric radiometer) and L-band DBF (digital beamforming) scatterometer, sharing a parabolic cylinder reflector. MICAP has been selected to be a candidate payload for future Chinese ocean salinity mission. In this paper, the MICAP instrument concept, specification and preliminary system design will be introduced.

Target brightness temperature maps can be used in geostationary interferometric microwave sounder (GIMS) system simulation. System simulation with accurate target brightness temperature maps can evaluate system performance in near real case and thus help adjust design parameters for the sensor before it is finally put into use. In this paper, method of simulating target brightness temperature using Weather Research and Forecasting Model and Radiative Transfer for TOVS (RTTOV) has been discussed. Target brightness temperature simulation results at oxygen absorption band and water-vapor absorption band have been presented. Some preliminary analyses of simulated brightness temperature for GIMS' observation have also been given.

Sea surface salinity as an ocean variable plays an important role in various components of the water and energy cycle. Recently a Chinese space mission using a new concept of Microwave Imager Combined Active and Passive (MICAP) at L-band is proposed for the ocean salinity measurement. MICAP uses interferometric techniques for both the radiometer and the scatterometer, which adopt two kinds of linear array feeds sharing one 3×4.5 m parabolic cylinder reflector. This paper is focused on the microwave interferometric radiometer (MIR) aboard the MICAP, and discusses the imaging simulation, imaging algorithm, and instrument performance.

The Geostationary Interferometric Microwave Sounder (GIMS) is a new concept of atmospheric microwave sounder for China's future geostationary Earth orbit meteorological satellite (FY-4). It is a microwave interferometric radiometer (MIR) using aperture synthesis and working in rotating time-sharing mode with a circular antenna array. A GIMS proof-of-concept demonstrator operating in the temperature sounding bands of 50-56 GHz has been successfully developed. The instrument uses a circular array with 28 elements (including one in the center of the array). It is capable of imaging a scene with an angular resolution of about 0.08° and a radiometric resolution of less than 1 K with 5-m integration time within a 5° field of view (FOV). Some theoretical aspects of the imaging characteristics of GIMS are discussed, such as the alias-free FOV, angular resolution, radiometric resolution, and imaging algorithm. Some early tests and preliminary imaging experiments of the GIMS demonstrator are also presented.

WCOM, the Water Cycle Observation Mission, is proposed to improve the capability of synergetic observation of key water cycle variables. By developing innovative active-passive and multi-frequency combined sensors and retrieval models and techniques, the scientific objectives of this mission is to deepen the understanding on global water distribution, transportation and phase conversion by synergistic observations; and based on the improved model and data, to rebuilt long-term data series for revealing of the responses and feedbacks of water cycle to global changes.

A new ocean salinity mission concept has been proposed and a ground-based prototype is being developed in MIRSlab, CAS. The passive part of the combined instrument is a one-dimension synthetic aperture radiometer. A critical technical challenge is that the radiometer needs to have very high sensitivity and high calibration stability. In this paper, theoretical analysis is given to high stability radiometer, and a high stability L-band radiometer has been developed to demonstrate its performance.

Earth observation satellites play a critical role in providing information for understanding the global water cycle, which dominates the Earth-climate system. However, limitations in observations will restrict our current ability to reduce the uncertainties in the information used to make decisions regarding to water use and management. Under the support of “Strategic Priority Research Program for Space Sciences” of the Chinese Academy of Sciences, a new satellite concept of global Water Cycle Observation Mission (WCOM) is proposed, aiming to provide higher accuracy and consistent measurements of key elements of water cycle from space, including soil moisture, ocean salinity, freeze-thaw, snow water equivalent and etc. The expected more consistent and accurate datasets would be used to refine existing long-time series of satellite measurements, to constrain hydrological model projections and to detect the trends necessary for global change studies.

In recent years, a new scanning type, called rotating scanning, is introduced to synthetic aperture radiometer. In the literatures, synthetic aperture radiometer obtained spatial frequency samplings usually by 360° rotating-scanning circular antenna array. As the rotating-scanning angle of the antenna array is not 360°, how to obtain the even sampling points on the spatial frequency domain and decrease the system complexity is very important. In this paper, we focus on design of x° rotating-scanning circular antenna array, where x is in the range of 0 to 180. Firstly, the background is introduced. Secondly, the design requirements are analyzed, and the design strategy is proposed subsequently. Thirdly, 180°, 90° and 45° rotating-scanning circle antenna array are designed as examples via numerical simulating. The simulation results prove the validity of the design strategy. The design method can be used to optimize other similar rotating-scanning circular antenna array that with different rotating-scanning angle.

The recent progress of the potential main payload of the Solar Polar ORbit Telescope (SPORT) mission, a space weather mission proposed by the National Space Science Center, is introduced in this paper. The main payload of SPORT is essentially an interferometric radiometer using synthetic aperture technique, and the time sharing clock rotation to implement the visibility sampling. The progress on system design, including frequency selection, payload configuration, rotation scan schemes and image retrieval are presented in this paper.

Before the program of new type combined active and passive L-band sensor for ocean salinity measurement embarks upon, the behavior of both brightness temperature and backscattering coefficient with sea surface salinity and roughness is premeditated. It is shown that sea surface brightness temperature is sensitive to salinity, which makes the reconstruction of sea surface salinity by brightness temperature possible. In order to refine the measurement and for better reconstruction, the ”excess” brightness temperature engendered by sea surface roughness need to be eliminated. Through illustrating the dramatic change of backscattering coefficient with roughness, the elimination can be practicable.

System study and development of an L-band one-dimensional synthetic aperture radiometer, or microwave interferometric radiometer (MIR), has been introduced in this paper. This radiometer is the passive part of a combined active/passive instrument for space-based ocean salinity observation. Recent progresses on radiometer hardware development and system simulation have been introduced.

Geostationary interferometric microwave sounder (GIMS), which is developed by National Space Science Center of CAS, will be the main payload on board of FY-4M satellite to monitor and measure internal constitution and precipitation parameters of atmosphere cloud cluster. GIMS is an interferometric aperture synthesis radiometer. Antenna array of GIMS can obtain visibility functions of observation objects. It is very important for improving the system sensitivity and decreasing the complexity of the system. Design of the antenna array is the key technique of GIMS, and position error analysis and adjustment of antenna array is also very important for improving the quality of the retrieved image of GIMS. According to the system performance requirements of GIMS, theory and method for design, position error analysis and adjustment of the antenna array have been investigated, and simulated results is also be presented in this paper.

Full Polarization Interferometric Radiometer (FPIR) is concept of one dimensional synthetic radiometer. This paper introduces the design of the FPIR antenna. A novel linear array antenna is described with emphasis on the simulation of the antenna pattern, design of the stripline feeding network and simulation of the polarimetric isolation.

Active and passive combining observation microwave instrument combines L band one dimension push-broom synthetic aperture radiometer and L band array mode microwave scatterometer to make accurate measurement for sea surface salinity. In this system, microwave scatterometer provides the information about the sea surface roughness, one of the main sources of error for salinity measurement. Phased array mode is adopted in scatterometer and the array elements are arranged between two elements with the largest baseline of radiometer. The number of array elements is set to 7 and digital beam forming technique is used to get scanning in cross azimuth direction. Then same footprints with radiometer can be obtained. Narrow band chirp signal is used to get range resolution. In this way, accurate backscattering information corresponds to observing area of radiometer can be achieved and then sea surface roughness can be derived. Finally, the error coming from surface roughness can be corrected.

Applying passive microwave sensor in GEO earth observations are of special interest because it has the advantages of temporally continuous and spatially large coverage compared to the LEO systems, which provide relatively narrow swaths with a revisit time of 12 hours or more. Unfortunately the required horizontal resolution leads to a rather large antenna size in GEO which is unpractical for the traditional real aperture radiometers especially in low frequency bands. The new concept of clock scanning microwave interferometric radiometer (CS-MIR) using aperture synthesis provides an opportunity to resolve this problem. This paper describes some approaches of CS-MIR, specifically on the items of array configuration, sampling patterns and potential GEO applications.

The concept of a new type combined L-band active and passive sensor for soil moisture and ocean salinity measurement has been proposed in this paper. The passive part of this combined system is a one-dimensional synthetic aperture radiometer, while the active part is a digital beam forming scatterometer. Preliminary considerations for space-borne system design and performance evaluation has also been introduced in this paper. A ground-based demonstrator is defined and under development.

Up to the present, most earth observation applications of synthetic aperture imaging radiometers (SAIR) are air-born and low earth orbit (LEO) space-born, where snap shot system with high imaging frame rate is necessary. With the increasing demands of microwave Geostationary Earth Orbit (GEO) earth observation for weather forecast, non snap shot type of imaging technology can be considered. While an image can be taken within a certain period of time, the hardware complexity can be dramatically reduced by scanning the sampling system during this period of time. In this paper, two kinds of rotational time shared scan scheme are analyzed, which are single rotation scan and double rotation scan. The recent progress on polarimetric correction methods developed for the two rotation schemes are present.

The concept of Geostationary Interferometric Microwave Sounder (GIMS) has been proposed based on the rotating circular thinned array, aiming for China's next generation geostationary meteorological satellite (FY-4M). The space borne system design has been investigated. The tradeoff analysis between the system performance and system complexity has been studied. A full-scale ground-based 50~56GHz GIMS demonstrator with 28 elements has been defined and developed. Preliminary test results of the demonstrator will also be presented in this paper.

The Solar Polar Orbit Telescope (SPORT) mission is proposed to observe the origination and propagation of the interplanetary coronal mass ejections (ICMEs), monitor its long-term behavior and forecast the near earth space weather. A time shared synthetic aperture interferometric radiometer with clock scan is chosen as the main payload instrument to fulfill the objectives. In this paper, the preliminary results of the scan scheme design and the imaging algorithm of clock scan are present. Two scan schemes of co-rotating scan fits for spin-stable satellite platform and contra-rotating scan fits for three-axis stable platform are proposed. Pseudo-polar FFT method is adapted to perform the image reconstruction.

The Geostationary Interferometric Microwave Sounder (GIMS) is a new concept imaging radiometer proposed by CSSAR, aiming for China's next generation geostationary meteorological satellite (FY-4M). The concept of GIMS is based on aperture synthesis with a rotating circular thinned array. A three-element interferometer has been developed and tested to investigate the feasibility of the GIMS system design. A full-scale ground-based demonstrator with 27 elements is also under development, which is defined as a minimum system intended to fulfill the threshold application requirements. In this paper, the preliminary results of these activities will be reported.

Demonstrator of the Full Polarization Interferometric Radiometer (FPIR) is a light weight high resolution imaging polarimeter at X-band. It is based on techniques of one dimensional aperture synthesis. Differing from ESTAR-like precedents on polarimetric mode, conical antenna beam and 2-point calibration, FPIR employs two interlaced arrays. They are orthogonally polarized, therefore enable polarimetric application. Main subsystems, including the antenna, receiver and digital correlator, are introduced. First brightness image of dual polarizations with FPIR demonstrator are presented and compared with the optical image.

Recently, a new concept of rotation scanning synthetic aperture interferometric radiometer (RS-SAIR) has been receiving more and more attentions for its advantage of much simpler configuration with looser requirements of antenna elements. In this article, we investigate the imaging theory of RS-SAIR, and introduce the pseudo-polar FFT algorithm to deal with the polar u-v samples. It only involves 1D interpolation and 1D FFT routine that guarantee a high accuracy and computation efficiency. The sampling strategy and aliasing effect of RS-SAIR are studied with this imaging algorithm. Numerical simulations are provided to validate the associate theory. Finally we develop a 5-elements RS-SAIR instrument and carry out rotating scanning imaging experiment successfully. The first imaging results consist with the expectation.

By analyzing historical low frequency microwave radiometric observation and ground truth data, it is shown the Taklamakan desert is well suited for calibration of spaceborne microwave sensors such as SMOS MIRAS. The pioneer experiment confirms the assumption in the simulation and defines requirements for further works. Sampling tools for the specified environment as well as an L band ground based radiometer were designed and constructed. Data analysis shows many areas in China are affected by RFI problem. It will be necessary to investigate the problem further in depth both technically and tactically.

Applying synthetic aperture interferometric radiometer (SAIR) in near field imaging has been receiving great interest in the recent years. Because the traditional far-field Fourier imaging theory is not valid in near-field condition, developing the proper near-field imaging techniques is the major desirable objective of near field SAIR applications. This work is devoted to establish an effective near field imaging method still with traditional far-field plane antenna array. The discrete numerical inversion method with focal plane approximation is developed. The regularization method is also introduced to deal with the ill condition and measurement errors.

In this paper, the authors will report some basic considerations on the synthetic aperture imaging radiometer for mm-wave sounding from GEO, especially for the China's next generation geostationary meteorological satellite (FY-4M).

This paper presents a novel concept of one dimensional full polarization interferometric radiometer (FPIR). Purpose of FPIR is to implement high resolution polarimetric radiome-try without large antenna and complex electricity. The operation principle of FPIR is introduced. Advantages of the polarimetric measurement of FPIR are achieved by squint antenna beam, interlaced dual polarized array, unique polarization switching and shared receivers. An example of the interlaced array and the particular switching sequence is discussed. The development of FPIR demonstrator is presented. Element waveguide antenna of the demonstrator is measured within a ground plane. A breadboard receiver has been built. A special digital correlator, which is compatible with both the analog and digital demodulation, is designed.

This paper discusses the sensitivities achieved by either real or synthetic aperture radiometers. In the discussions of both kinds of radiometers, sensitivity is normalized to effective integration time, which is defined to relate the sensitivities of different kind of instruments. Derived relations explicitly demonstrate that the sensitivity by synthetic aperture radiometer is comparable to that of real aperture radiometer.

Synthetic aperture radiometers with a rotating thinned array can effectively reduce the number of antenna units required for the applications with very high spatial resolution, like GEO atmospheric sounding. The rotating array results in non-regular distributed visibility samples. A grid-based measurement approach is proposed in this paper to facilitate the image reconstruction and system performance evaluation, especially the radiometric sensitivity computation.

High spatial resolution image of passive microwave brightness temperature can be taken by so called synthetic aperture radiometer or imaging interferometric radiometer. However the future application of this new technology is limited by its large number of element antennas and receiving channels. To overcome this difficulty, a novel time shared scan scheme is presented using the concept of clock arms. Potential applications of this new scan scheme are also presented in this paper.

Using interferometric synthetic aperture radiometer to measure the brightness temperature distribution of the Earth is a relative new technique for the microwave earth observation. Steady progresses of this technology have been achieved in both one-dimensional and two-dimensional cases since 1990's. The typical instruments are ESTAR and MIRAS. The CAS airborne X-band synthetic aperture radiometer is a thinned array microwave imaging radiometer, developed by National Microwave Remote Sensing Laboratory (NMRS Lab), Center for Space Science and Applied Research (CSSAR) and Chinese Academy of Sciences (CAS). The system integration of the instrument was accomplished in the Jan, 2004, and the first flying experiment was held in Apr, 2004. In this paper, both the system configuration and the experimental results of the instrument are described in detail

Calibration for a one-dimensional synthetic aperture microwave radiometer is critical. Phase and amplitude calibration of the visibility samples must be carried out to compensate for system bias before measurement. This bias mainly comes from the un-uniform characteristics and the couplings between the receiving channels. To take into account this problem, a numerical image retrieving method is considered using an intermediate transformation matrix called the gain matrix or simply G-matrix. This matrix takes into account all the couplings and un-uniformed amplitude and phase between the channels. However, measuring the G-matrix is not an easy job. One has to set up a point source in the far field and rotate the system in the fan beam direction. The difficulty is mainly due to the incomplete test environment. A measurement set-up called image simulator is present. It is a circuit network composed of a number of high accurate microwave I-Q vector modulators and a noise generator. It simulates the incoming waves to the thinned antenna array corresponding to any image scene to be measured. While in measurement, the simulator's outputs are connected to the RF inputs of each channel. The incoming wave is then simulated by computer program to drive each of the digital I-Q vector modulators. The design and test of the image simulator are presented followed with test results of G-matrix.

Unlike optical images, the microwave radiometer image contains information of ground brightness temperature. To integrate some features of microwave radiometer images and optical images, some transform schemes, such as intensity-hue-saturation (IHS), principal component analysis (PCA) and wavelet transform (WT), were applied to merge the airborne synthetic aperture radiometer images and Landsat ETM+ images. The experiments show that the wavelet based transform method shows the best potential among the three methods.

We briefly introduce B-splines of continuous expression, discrete expression, and their interesting properties as well as describing the fundamental processing steps of image registration. In virtue of error analysis and image registration, we approve spline interpolation and approximation in resampling and transformation reliable and efficient.

Using synthetic aperture radiometer to retrieve the brightness temperature distribution of the Earth is a relative new technique for the microwave Earth observation. But the side lobe of the synthetic aperture pattern will deteriorate the retrieved image. In the one dimensional synthetic aperture radiometer system, synthetic aperture pattern side lobe will produce the along-track streaks in the retrieved image, together with some other factors. Reducing these streaks is one of the most important tasks in the post data processing and image retrieving. In this paper, the Dolph-Chebyshev Linear Array Method is introduced to reduce the side lobe level (SLL) of the synthetic aperture pattern. Using the Dolph-Chebyshev taper window can achieve the optimal compromise between the main beam width and the SLL of the synthetic aperture pattern: all side lobes would be suppressed to the same specific level, while the main beam width would be as narrow as possible. The method is applied to some experimental data we achieved before. And the retrieved brightness temperature images are improved apparently.

Image correction and preliminary analysis of a field measurement by a C band 6-channel airborne microwave synthetic aperture radiometer is presented. This measurement was carried out along the Yellow River banks, Shaan'Xi, China, during 25/sup th/ to 30/sup th/ April, 2002. The CSSAR one dimensional C-band (6.6 GHz) 6-channel synthetic aperture radiometer was used as the airborne sensor. On board of the helicopter, a GPS receiver is also used to record the geographical positions of the flight pass. The scan was carried in over a square area across the experiment fields covering about 8/spl times/8 km/sup 2/. Ground truth observation also took place at the same time. The ground measurement parameters include: temperature of water body, air temperature, soil temperature, soil moisture at varied depth, soil surface roughness, etc. Geometric correction and registration were done before the analysis of the imagery. Some preliminary results are given.