Xiaolong Dong

WCOM: The science scenario and objectives of a global water cycle observation mission

Earth observation satellites play a critical role in providing information for understanding the global water cycle, which dominates the Earth-climate system. However, limitations in observations will restrict our current ability to reduce the uncertainties in the information used to make decisions regarding to water use and management. Under the support of “Strategic Priority Research Program for Space Sciences” of the Chinese Academy of Sciences, a new satellite concept of global Water Cycle Observation Mission (WCOM) is proposed, aiming to provide higher accuracy and consistent measurements of key elements of water cycle from space, including soil moisture, ocean salinity, freeze-thaw, snow water equivalent and etc. The expected more consistent and accurate datasets would be used to refine existing long-time series of satellite measurements, to constrain hydrological model projections and to detect the trends necessary for global change studies.

WCOM: THE MISSION CONCEPT AND PAYLOADS OF A GLOBAL WATER CYCLE OBSERVATION MISSION

WCOM, the Water Cycle Observation Mission, is proposed to improve the capability of synergetic observation of key water cycle variables. By developing innovative active-passive and multi-frequency combined sensors and retrieval models and techniques, the scientific objectives of this mission is to deepen the understanding on global water distribution, transportation and phase conversion by synergistic observations; and based on the improved model and data, to rebuilt long-term data series for revealing of the responses and feedbacks of water cycle to global changes.

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.

Research Activity on Synthetic Aperture Radiometry in CSSAR/CAS

Interferometric synthetic aperture radiometry is a relative new technique in the area of microwave earth observation to measure the brightness temperature distribution of the earth. It can enhance the spatial resolution of the passive microwave remote sensing effectively. Steady progress of this technology have been achieved in both one dimensional and two dimensional cases since 1990’s. The typical instruments are ESTAR and MIRAS, developed by NASA (and umass) and ESA respectively. Relative research has also been conducted in China, mainly by National Microwave Remote Sensing Laboratory (NMRS Lab), Center for Space Science and Applied Research (CSSAR), Chinese Academy of Sciences (CAS), since the middle of 1990’s. A C-band and an X-band instrument has been developed. In this paper, research activities on synthetic aperture radiometry in CSSAR/CAS will be reviewed and summarized, including the development of the instruments. Finally, further plans in synthetic aperture radiometry in CSSAR/CAS will also be prospected.

A Ku-band rotating fan-beam scatterometer: Design and performance simulations

This paper introduces the design and simulation results of a Ku-band rotating fan-beam radar scatterometer for ocean surface wind vector measurement. It will be flown on a small satellite dedicated to provide data for investigation of the ocean wave and ocean surface wind vector interactions, along with another payload for measurement of directional ocean wave spectra by a real-aperture radar with multiple scanned pencil beams. Key issues about the design of a Ku-band rotating fan-beam radar scatterometer, and results of performance simulations are provided as well. The performance of the system is simulated by the absolute and relative specifications. For the absolute specifications, retrieval performances for both wind speed and wind direction are evaluated, with the maximum likelihood method being employed. For the relative specifications, the figures of merit (FOM) is simulated, for comparison with other Ku-band scatterometers and optimization of system parameters. Simulation results of both the σ° precision and wind retrieval accuracies for different wind speed from 4m/s to 24m/s will be provided, which shows that SCAT can satisfy the performance requirements within most part of the swath.

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.

In-orbit performance of Microwave Humidity Sounder (MWHS) of the Chinese FY-3 meteorological satellite

FY-3A (Feng Yun means wind and cloud in Chinese) meteorological satellite of China was successfully launched on May 27, 2008. MWHS (MicroWave Humidity Sounder) is a very important payload for measuring global atmospheric water vapour profiles. In this paper, the authors will describe the system configuration and parameters of MWHS. Moreover it gives the testing results in-orbit and comparing with AMSU-B which has been installed on NOAA-17.

The water cycle observation mission (WCOM): Overview

Earth observation satellites play a critical role in providing information for understanding the global water cycle, which dominates the Earth-climate system. However, limitations in observations will restrict our current ability to reduce the uncertainties in the information used to make decisions regarding to water use and management. Under the support of “Strategic Priority Research Program for Space Sciences” of the Chinese Academy of Sciences, a new satellite concept of global Water Cycle Observation Mission (WCOM) is proposed, aiming to provide higher accuracy and consistent measurements of key elements of water cycle from space, including soil moisture, ocean salinity, freeze-thaw, snow water equivalent and etc. The expected more consistent and accurate datasets would be used to refine existing long-time series of satellite measurements, to constrain hydrological model projections and to detect the trends necessary for global change studies. The WCOM is expected to be implemented during the 13th five-year-plan period (2016-2020).

Doppler effect and compensation in a Rotating Fanbeam Spaceborne Scatterometer

Spaceborne Rotating Fanbeam Scatterometer, RFSCAT, has a wider continuous swath that can provide a large number of independent samples of Sigma0 for wind speed and direction retrieving. But the large swath and footprint result in a wideband Doppler frequency shift. For a low earth orbiting satellite, the maxim Doppler bandwidth between forward and backward echoes will be about 500 KHz. Even in a single echo of RFSCAT, the Doppler bandwidth is about 90 KHz, while in a pencil beam scatterometer the Doppler bandwidth is almost a single tone. A method of Doppler frequency compensation both on center frequency of transmitted pulses and signal processing section of the echoes are carried out and evaluated.

Accuracy and resolution analysis of the pencil beam radar scatterometer onboard China’s HY-2 satellite

The Ku-band scatterometer onboard HY-2 (SCAT/HY-2) is a pencil-beam radar employing pulse compression techniques to ensure number of the independent measurement samples and the resulted precision of the backscattering coefficient for each resolution cell. In this paper, the system specifications of SCAT/HY-2 are introduced. The signal processing scheme, including pulse compression, resolution cell regroup and the area weighted incoherent average of the backscattered power, is presented and analyzed. Based on the analysis of the resolution cell after pulse compression processing, the radiometric accuracy and surface resolution of the SCAT/HY-2 for all the azimuth angles will be simulated. Simulation and analysis results shows that SCAT/HY-2 can satisfy the specification requirements of HY-2 satellite.