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.

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 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.

As a notable sparse array, low redundant linear array (LRLA) plays an important role in lots of areas, especially in the areas of passive synthetic aperture imaging and direction-of-arrival (DOA) estimation. The existing LRLAs have a problem that they generally contain some dense segments of unit-spaced elements, which restricts the element aperture and hence the radiometric sensitivity and simultaneously causes serious mutual coupling effect that deteriorates the array performance. To solve this problem, a new kind of sparse array named super LRLA (S-LRLA) is proposed in this article. The key characteristic of S-LRLA is that it has no unit-spaced sensor pairs but has the largest continuous difference coarray spanning from 2 to$N - 2$with a redundancy of less than 2. Lots of S-LRLAs are obtained by using a differential evolution (DE) method. On this basis, two types of S-LRLAs, including seven closed-formed patterns and two semiclosed-formed patterns, are discovered. The property and designing factors of S-LRLA are analyzed. Furthermore, the adapted S-LRLA (AS-LRLA) with full difference coarray is also discussed. Comparisons with other prominent arrays are finally presented to demonstrate the superiority of AS-LRLA.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.