Jintai Zhu

Some considerations about the in-orbit calibration of spaceborne rotating fan beam radar scatterometer

Rotating fan-beam scatterometer (RFSCAT) is a new kind of spaceborne scatterometer for ocean surface vector wind (OSVW) measurement. In this paper, some consideration about the calibration of a Ku-band RFSCAT is presented. Both internal calibration and external calibration are described. Preliminary analyses about the internal calibration precision and external calibration accuracy are presented. Based on the method of Long, etal, external calibration method for RFSCAT is proposed. Some simulation results for the external calibration with natural extended targets, such as Amazon forest, are presented.

An Improved Adaptive Regularization Method for Forward Looking Azimuth Super-Resolution of a Dual-Frequency Polarized Scatterometer

Dual-frequency polarized scatterometer (DFPSCAT) is a pencil-beam rotating scatterometer which is designed for snow water equivalent (SWE) measurement, and Doppler beam sharpening (DBS) technique is proposed for DFPSCAT to achieve the azimuth resolution. However, the DBS technique is inapplicable for the forward-looking and afterward-looking regions. Based on an approximate aperiodic model of scatterometer echo signal, an improved adaptive regularization deconvolution algorithm with gradient histogram preservation (GHP) constraint is implemented to settle the problem. To investigate its performance of resolution enhancement and resulted accuracy, both a synthetic backscattering coefficient (σ0) field reconstruction and SWE σ0 reconstruction are carried out. The results show that the proposed method can recover the truth signal and achieve azimuth resolution of 2 km with the designed scatterometer system, which is required by the SWE retrieval. Moreover, the relative errors of reconstructed σ0 are less than 0.5 dB that satisfy the accuracy requirement for SWE retrieval, and comparisons with observed results show that the error reduction is more than 0.03 dB. Meanwhile, a comparison between the proposed algorithm and some existing resolution enhancement methods is analyzed, which concludes that the proposed method can obtain a comparable resolution enhancement as L1 method and has less noise. The technique is also verified with advanced scatterometer (ASCAT) scatterometer data.

Calibration and Estimation of Attitude Errors for a Rotating Fan-Beam Scatterometer Using Calibration Ground Stations

The rotating fan-beam scatterometer (RFSCAT) onboard Chinese-French Oceanic SATellite (CFOSAT) due to launch in 2018 is a new type of radar scatterometer system for ocean surface wind vector measurement. It can give observations with more azimuth and incidence angles for a single wind vector cell (WVC) than other available scatterometers. This has been proved effective in bettering the retrieved wind quality by the simulation approach. However, its innovative observing geometry is challenging for the coming in-orbit external calibration. In this paper, CFOSAT attitude errors are estimated, and its antenna gain pattern is monitored and verified based on the external calibration strategy of a Ku-band scatterometer employing calibration ground stations (CGSs). The effects of satellite attitude errors on the measurements are also analyzed, together with simulation results for the external calibration. It is shown that a gain pattern with accuracy of 0.08 dB and attitude errors within 0.025° are achieved.

Calibration and validation of the HY-2 scatterometer backscatter measurements over ocean

The Normalized Radar Cross-Sections (NRCS,σ0) measured by spaceborne scatterometer are used to determine the near-surface wind field using the geophysical model functions (GMF). The accuracy of the retrieved wind field is a sensitive function of the radiometric accuracy of the σ0 measurements. Therefore, in-orbit calibration is essential for the retrieval of optimum quality geophysical products. In this paper, the scatterometer onboard HY-2A satellite (HY-2 SCAT) is calibrated using Numerical Ocean Calibration (NOC) and its winds retrieval after calibration are validated by winds from global buoys and from the European Centre for Medium-Range Weather Foresting (ECMWF) Numerical Weather Prediction model (NWP). The standard deviations for wind speed and wind direction against buoy winds are better than 1.3 m/s and 15°, indicating that the HY-2 SCAT calibration has been successful and that HY-2 SCAT wind products after NOC are of high quality.

Progresses of development of CFOSAT scatterometer

In this paper, the status and progress of the scatterometer (SCAT) of the Chinese-French Oceanography Satellite (CFOSAT) will be reported. SCAT/CFOSAT will be the first rotating fan-beam scatterometer (RFSCAT) ever flown on a satellite for global ocean vector wind measurement. The mission schedule, some design and development status and simulation results of its performances will be presented. The considerations about the calibration of RFSCAT is also presented in this presentation.

Adaptive regularization method for forward looking Azimuth super-resolution of a Dual-Frequency Polarized Scatterometer

Dual-Frequency Polarized Scatterometer (DFPSCAT) is a pencil-beam rotating scatterometer which is used to measure snow water equivalence (SWE). Respecting the low azimuth resolution of its forward-looking region, an adaptive regularization deconvolution super-resolution method, based on the scatterometer echo signal model, is proposed. Compared with the classical SIR and MAP algorithms, the proposed method can better reconstruct the original signal, and has less noise amplification. The algorithm processing accuracy with different Kpc is also studied, and the results show that when the value of Kpc is less than 0.1, nearly the entire restored data can satisfy the requirement of 0.5dB accuracy.

Calibration of the Ku-band Rotating Fan-beam Scatterometer Using Land Extended-area Targets

Rotating Fan-beam SCATterometer (RFSCAT) is a new radar scatterometer system for ocean surface vector wind measurement. Compared with other available scatterometers, RFSCAT can provide more combination of azimuth and incidence angles for a single surface resolution cell. To achieve the required wind vector accuracy, radar scatterometry measurement of backscattering coefficient 0 ( ) σ must be calibrated within a few tenths of a decibel. In this paper, the method for external calibration of RFSCAT is proposed, based on the system parameters of the scatterometer onboard the Chinese French Oceanography SATellite (CFOSAT), and is verified by simulations. Then QuikSCAT L2A data and SIR of several large homogenous areas are analyzed to check the stability and azimuthal dependence of the 0 σ over these areas. A new calibration mask is generated and will be used as a reference for the calibration of RFSCAT.

Spatial Resolution and Precision Properties of Scatterometer Reconstruction Algorithms

Various reconstruction methods have been used to enhance the spatial resolution of scatterometer data. Most of the image reconstructions are two-dimensional problems, which combine multiple passes of overlapping data over the temporally homogeneous surface, and thus are only suitable for land and ice applications. This paper attempts to address the one-dimensional reconstruction to enhance the azimuth resolution of scatterometer data using a single pass of observations. Since the range resolution determined by the on-board dechirping technique is generally up to several hundred meters, the one-dimensional reconstruction is adequate for certain near real-time ocean applications, such as the development of coastal scatterometer winds. Three well-known reconstruction algorithms, including additive algebraic reconstruction technique (AART), multiplicative algebraic reconstruction technique (MART), and scatterometer image reconstruction (SIR), are evaluated. The spatial resolution and the reconstruction precision resolved by each algorithm are separately analyzed using the local impulse response and Monte Carlo methods. The dependence of the spatial resolution and the reconstruction precision on a variety of parameters, such as the mean backscatter coefficient and its variance, the beamwidth of spatial response function (SRF), and the SRF function type, is evaluated using a simulation framework. In particular, the tradeoff between the spatial resolution and the reconstruction precision is examined for three algorithms. The results show that SIR offers the quickest convergence and lowest noise.

A Preliminary Study of the Calibration for the Rotating Fan-Beam Scatterometer on CFOSAT

The first rotating fan-beam scatterometer (RFSCAT) will be launched onboard the Chinese-French Oceanic Satellite (CFOSAT) in 2018. It provides a set of radar cross-section (σ0) measurements at different azimuth/incidence angles over a wind vector cell (WVC), in order to determine the near-surface wind field using the backscatter model, i.e., the so-called geophysical model function (GMF). The accuracy of the retrieved wind vector is a sensitive function of the radiometric accuracy of the σ0 measurements. Therefore, in-flight calibration, including the loop-back (internal) calibration and the external calibration performed with natural extended-area targets, is studied in this paper. Several homogeneous areas over land are first analyzed to check the stability and azimuthal dependence of the σ0 over these areas. A new calibration mask of the homogeneous land areas is generated and will be used by RFSCAT calibration. Then a simple method of external calibration is proposed to eliminate the azimuthal-dependent σ0 errors induced by the insertion loss of the rotating joint, which can be applied to both the rotating pencil-beam scatterometers and the coming RFSCAT. The “observed” σ0 of RFSCAT is simulated using the SeasatA scatterometer (SASS) measurements and the “perturbed” azimuthal-dependent σ0 errors. The latter is then tracked by the proposed external calibration. The results show that the accuracy of gain corrections is up to 0.2 dB, ensuring consistency between different azimuthal measurements.

Analysis of Backus-Gilbert approach on resolution enhancement of dual-frequency polarized scatterometer

Dual-Frequency Polarized Scatterometer (DPS) is a new system utilizing Doppler Beam Sharpening (DBS) technology for resolution enhancement. Respecting the low azimuth resolution over nadir swath, the Backus-Gilbert method is investigated to improve the resolution. According to the precision definition of scatterometer measurement, a Backus-Gilbert method for scatterometer is derived. Using the method, the tradeoff of resolution and precision of resolution-enhanced backscatter (normalized radar cross section, σ0) is established in a quantitative manner. The results show that resolution enhancement is achieved by the price of greatly decreased precision of reconstructed backscatter.