Li Qingxia

A Multichannel Passive Imaging Radiometer Using DBF Technique

In this letter, a 16-element imaging radiometer using the digital beamforming technique is described. The system operates in the Ka-band in order to have high spatial resolution. The principle of an image reconstructing system is described. A series of experimental results is reported to demonstrate the effect of the system architecture.

The performance analysis of direct sampling microwave radiometer

L-band direct sampling digital total power microwave radiometer (DSDTPMR) is a novel radiometer, which trades analog complexity for digital ditto. This article considered the low noise amplifier (LNA) gain fluctuation, analog-to-digital converter (ADC) sampling rates and quantization resolution, to derive a general mathematical expression of sensitivity of the DSDTPMR; Analyzed the influence of sampling rate, quantization on sensitivity of DSDTPMR; The simulation result indicating, when the sampling frequency is bigger than two times of signal bandwidth, the multi-bit quantization resolution has very small effect on the sensitivity; when the gain fluctuation obeys the Gaussian distribution, the gain fluctuation may reduce to radicN times.

Enhanced ultraviolet photoresponse based on ZnO nanocrystals/Pt bilayer nanostructure

The development of solution strategies for Zinc oxide (ZnO) quantum dots provides a pathway to utilizing ZnO nanocrystal thin films in optoelectronic devices. In this work, quasi-spherical ZnO quantum dots with a diameter of 5 nm are synthesized by using ethanol as a solvent. ZnO nanocrystal thin film is obtained by spin-coating ZnO quantum dots on a Au interdigital electrode (IDE)/Al2O3 substrate and annealing at different temperatures in order to yield the optimal photosensitive on/off ratio of ZnO. For further enhancing the responsivity, ion sputtering is utilized to deposit Pt nanoparticles on the surface of ZnO nanocrystal thin film, the responsivity of the ZnO/Pt bilayer nanostructure increases from 0.07 A/W to 54 A/W, showing that the metal/inorganic nanocrystal bilayer nanostructure can be used to improve the performance of optoelectronic devices. The excellent properties of ZnO/Pt bilayer nanostructure have important applications in future electronic and optoelectronic devices.

A method for correcting regional bias in SMOS global salinity products

Soil Moisture and Ocean Salinity (SMOS) Level 3 (L3) sea surface salinity (SSS) products are provided by the Barcelona Expert Centre (BEC). Strong biases were observed on the SMOS SSS products, thus the data from the Centre Aval de Traitement des Données SMOS (CATDS) were adjusted for biases using a large-scale correction derived from observed differences between the SMOS SSS and World Ocean Atlas (WOA) climatology data. However, this large-scale correction method is not suitable for correcting the large gradient of salinity biases. Here, we present a method for the correction of SSS regional bias of the monthly L3 products. Based on the stable characteristics of the large SSS biases from month to month in some regions, corrected SMOS SSS maps can be obtained from the monthly mean values after removing the regional biases. The accuracy of the SMOS SSS measurements is greatly improved, especially near the coastline, at high latitudes, and in some open ocean regions. The SMOS and ISAS SSS data are also compared with Aquarius SSS to verify the corrected SMOS SSS data. The correction method presented here only corrects annual mean biases. The measurement accuracy of the SSS may be improved by considering the influence of atmospheric and ocean circulation in different seasons and years.

Correction of SMOS data in coastal area of South China Sea based on land contamination analysis

For SMOS data, significant errors exist in coastal areas because of the contamination by the nearby land. In this paper, the analysis of SMOS brightness temperature (TB) data in coastal area of South China Sea (SCS) reveals a decreasing trend with the increase of the distance to coast. A SMOS TB measurement model according to geophysical parameters and SMOS/MIRAS antenna array is established. The simulation shows that TB is large within 40 km since land is much warmer than ocean and is observed by the mainlobe of the antenna array. Then the simulation result of land contamination is used for correction of SMOS measured TB data in coastal area of SCS. Then the corrected TB is applied for sea surface salinity (SSS) retrieval, which reduces SSS outliers and improves the accuracy of SSS in coastal area of SCS.

Corrections to “A Multichannel Passive Imaging Radiometer Using DBF Technique” [Apr 10 329-332]

In the above titled paper (ibid., vol. 7, no. 2, pp. 329-332, Apr. 10), we omitted the sources of funding for our work. That is provided here. We also add an additional author, Xiaohu Ge, who assumes now the role of corresponding author.

Phase and amplitude calibration of the HUST Ku-band aperture synthesis radiometer using external source

The HUST Ku-band Aperture Synthesis Radiometer (ASR) is a 16-channel thinned array MMW imaging radiometer. Phase and amplitude calibration of MMW ASR is a crucial problem. A simple and effective calibration approach, which makes use of single external source in a known location, is proposed to tackle this problem in this paper. By the external noise source, the approach allows online estimation of the correct values of the ASR array phase and amplitude, without cost of special calibration hardware. Initial experiment images suitable for demonstration of calibration are present. All results show that phase and amplitude calibration is sufficiently accurate to satisfy for the MMW aperture synthesis radiometer requirements.

A statistical inversion algorithm for aperture synthesis radiometers

In order to deal with the inverse problem in aperture synthesis radiometers (ASRs) imaging, some deterministic inversion algorithms are proposed. However, these inversion algorithms are deficient in utilizing the statistical properties of visibility samples and reconstructed image. In this paper, the probability density functions of visibility samples and pixels of reconstructed image are investigated. A statistical inversion algorithm is presented, in which the statistical properties are introduced to constrain solution of the inverse problem. Simulation and experiment results show the statistical inversion algorithm can greatly improve the image quality of ASRs.

Near-field imaging of one-dimensional aperture synthesis radiometers

For synthetic aperture radiometers under far-field (FF) condition, the relationship between the visibility and the brightness temperature is a Fourier Transform. However, it is no longer valid when synthetic aperture radiometers are applied at short distances. A novel method based on a reference position for near-field (NF) imaging of one-dimensional aperture synthesis radiometers (ASR) is proposed. By subtracting the phase of NF visibility samples of a point source at the reference position from the phase of NF visibility samples of the imaged scene, the relation between the brightness temperature and the NF visibility can be represented by another form of Fourier Transform. Then direct DFT inversion algorithm is applied. Simulations and NF experimental results demonstrate the validity of the method for both point and extended sources. The method can be used not only in those applications at NF region of ASR, but also indoor performances test of spaceborne ASR.

Digital Multi-Beam Microwave Radiometer: system configuration and experimental result

A digital multi-beam microwave radiometer is presented in this paper. The system consists of antenna array, receiving channel array, analog to digital convertor and PC. It is the multiply beamforming technique that is used to process the signal which is obtained from the scene. An error correction algorithm is used to improve image quality. Experimental result demonstrates the image ability of this system.