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.

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.

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 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 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.

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.