Xingou Xu

The Feasibility of Ocean Surface Current Measurement Using Pencil-Beam Rotating Scatterometer

Ocean current is highly related to the interaction between ocean and atmosphere. By measuring the speed and direction of the ocean current from space, we can investigate the ocean- atmosphere interaction on a global scale. The ocean-atmosphere interaction helps to maintain the balance that is essential for planet habitability. However, the conventional scatterometer is unable to measure the ocean current vector. To achieve this, a potentially feasible approach is to use a bigger antenna, a higher PRF, and measure the interferometric phase of two successive echoes. This paper derives four decorrelation factors, and provides the phase error model first. Then, an end-to-end simulation model is established, and it is used to analyze the feasibility of ocean surface current measurement from space. Based on the simulation model, the system parameters are optimized. The simulation results show that the current speed standard deviation (Std), which means the measurement accuracy, in along-track and cross-track direction is smaller than 0.1 m/s when the wind speed is larger than 4 m/s. The swath can be used for current vector inversion that is greater than 70% when the wind speed is larger than 7 m/s. Meanwhile, Kpc of the modified scatterometer is computed and the results show that Kpc is better than the traditional pencil-beam rotating scatterometer when the wind speed is larger than 6 m/s.

Biomass related parameter retrieving from quad-pol images based on Freeman-Durden decomposition

Microwave remote sensing (MRS) has been widely employed for biomass estimation. However, the retrieval of biomass related parameters for fractional areas with both trees and bare ground areas remains a problem unsolved. The Freeman-Durden decomposition (FD) was developed for applications with vegetation and could make efficient use of both amplitude and phase information provided by polarized MRS data. Based on FD, the authors devised a methodology to solve the problematic caused by discontinuous distribution of vegetation. Effectiveness of the proposed method was tested by experiments on RADARSAT-2 quad-pol images and corresponding in situ data.

The processing and simulation of the CFOSAT RFSCAT

The RFSCAT(Rotating Fan-beam SCATterometer) is one of the two payloads of CFOSAT, the China-France Oceanography Satellite. It is a radar scatterometer designed to measure the electromagnetic back-scatter from wind roughened ocean surface. The operating frequency of the scatterometer is 13.256GHz (Ku-band) and has a swath about 1,000 kilometers. In this paper, based on the characteristics of echo signal of RFSCAT, the on-board processing of RFSCAT signal is introduced. A Doppler pre-compensation LUT and A slice division LUT are developed, and the signal processing algorithms are validated and the wind retrieval performances of RFSCAT are analyzed based on the well-developed data simulation system of CFOSAT RFSCAT.

An unfocused SAR design to improve azimuth resolution of dual-frequency full-polarized scatterometer

In order to satisfy a high resolution for the measurement of snow water equivalent, we use the system of using the dual frequency(X-band 9.6GHz and Ku-band 17GHz ) and full polarization. This paper discusses the various system options(scanning pencil-beam, high PRF, high SNR) and tradeoffs considered for improving the azimuth resolution of scatterometer are required.

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.

Airborne experiments validating the spaceborne RFSCAT on CFOSAT

Airborne experiments were carried out to validate the spaceborne scatterometer. Airborne scatterometer use all the instruments of the spaceborne scatterometer to validate the system design and reliability, except the power amplifier and the antenna. The software of the scatterometer is redesigned due to the observation geometry of airborne platform. The wind retrieval results are compared with the HY-2A scatterometer and the real time wind vector measured on the ship at the center of testing region.

Recent advances in developing the CFOSAT scatterometer

The scatterometer onboard the China-France Oceanography Satellite (CFOSAT) will be the first rotating fan-beam scatterometer (RFSCAT) ever flown in space for global ocean vector wind measurement. The status and progress of the scatterometer will be presented, including the mission schedule, payload design and development, validation experiments. The internal calibration strategy, Doppler compensation method and onboard signal processing arithmetic are also reported.

Preliminary performance simulation of microwave imager combined active/passive - a new instrument for Chinese salinity mission

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.

Pre-processing algorithm and airborne campaign result analysis of a rotating fan-beam scatterometer

A Rotating fan-beam scatterometer (RFSCAT), which provides a set of different incidence and azimuth angle combinations and wide continuous swath coverage, will be flown on the Chinese-French Oceanography Satellite (CFOSAT). In this paper, a pre-processing algorithm is developed to estimate the backscatter coefficient (σ0) of RFSCAT, which includes the procedures of system distortion correction, thermal noise removal, internal signal calibration, and elimination of the observing geometry effects. The algorithm is then tested using the experimental data from an airborne campaign. The estimated σ0 are consistent with Nscat-2 geophysical model function (GMF), indicating that the pre-processing chain acts well for RFSCAT.

Wind speed retrieving for combined observations of scatterometer and radiometer onboard HY-2A for typhoons using neural network

In typhoon conditions, air-sea interactions are fierce, the wind speeds are quite high and usually tangled with complex rains. Under such circumstances, the combined observations of scatterometer and radiometer would be able to provide good information although traditional wind filed retrieving methods for them, i.e. MLE based on GMF (Geographical Modelling Function) for scatterometer and RTM(Radiative Transfer Method) which are originated from low or median wind conditions will not be applicable. In this paper, a neural network was employed to combine the observations of the two sensors onboard HY-2A satellite: radiometer (HRAD) and scatterometer (HSCAT) to achieve wind speed retrieving under typhoon conditions. The network was trained by GRAPS (global/regional assimilation and prediction system) data used as true values of wind speeds in typhoons. The established network was verified by data excluded from training data set. Results show that the wind speeds obtained from the network are better than the products of HY-2A compared to GRAPS data. The research of this paper has provided a clue of retrieving typhoon wind speeds for HY-2A products and for the data processing of coming HY satellite series, though further work on wind direction obtain, data comparison between different kinds of assimilation and perdition systems and the test of this method on other scatterometer and radiometer data are to be done in the near future.