Risheng Yun

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.

NWP ocean calibration of Ku-band scatterometers

Calibration procedures over the ocean using forecast winds have the advantage that they may be applied over a large portion of the globe and consequently may provide accurate results over a relatively short period. Ocean calibration has been applied successfully for the European Remote-Sensing Satellite (ERS) and Advanced SCATterometer (ASCAT) C-band fan-beam scatterometer wind products at KNMI. The OceanSat-2 rotating pencil-beam scatterometer (OSCAT) uses a Ku-band radar wavelength (13.515 GHz) which is strongly affected by the presence of rain within a scatterometer wind vector cell (WVC). In this paper, The Indian Space Research Organisation (ISRO) L2A data and 50km resolution L2B data processed by the OSCAT Wind Data Processor (OWDP) software, which is being developed by the KNMI scatterometer group, and include European Centre for Medium-range Weather Forecasts (ECMWF) Numerical Weather Prediction (NWP) equivalent-neutral winds, are successfully used in OSCAT NWP ocean calibration (NOC). With the NOC results obtained, the backscatter is corrected and wind retrieval statistics between ECWMF NWP winds and OSCAT NOC-corrected winds are computed and shown to be improved w.r.t. the OSCAT winds without NOC corrections applied. Based on the KNMI QC(Quality Control) flag, the rain effects to Ku-band NWP ocean calibration are investigated and NOC corrections based on WVCs that pass the QC are about 0.1dB lower than the NOC corrections without the QC applied. The best wind retrieval statistics are thus effectively obtained after that the QC-ed NOC corrections are applied on Ku-band OSCAT scatterometer backscatter 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.

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.

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.

Data and processing of RFSCAT onboard CFOSAT

The Rotating fan-beam scatterometer (RFSCAT) onboard the Chinese-French Oceanography Satellite (CFOSAT) is a novel kind of scatterometer whose performance had been preliminarily verified by airborne campaign. In this paper the whole data processing chain that can achieve wind field derivation from original data set transmitted from the scatterometer had been proposed with definition of corresponding data levels that would be applied for RFSCAT in the CFOSAT mission. The pre-processing part in the processing chain was validated for it was derived from the same system verified by the airborne campaign. While for the post-processing prime algorithms, validity was concluded from processing some of the data gained in that experiment as well. Further researches needed had been explored as well.

Simulation and retrieval of wind of CFOSAT rotating-Fan beam SCATterometer

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, an end-to-end data simulation system of CFOSAT RFSCAT is developed for processing algorithm validation and performance analysis. Using the simulation system, the slice and WVC geometries are determined and the measurement performance is analyzed. Based on the simulated data, the wind is retrieved and the performance of wind retrieval is estimated. Meanwhile, according to the simulation and wind retrieval of CFOSAT RFSCAT, some considerations of the actual data processing are introduced.

Data pre-processing of MICAP (microwave imager combined active and passive) scatterometer

The MICAP(microwave imager combined active and passive), which has been selected to be a candidate payload for future Chinese ocean salinity mission, contains an L-band digital beam forming(DBF) scatterometer for the purpose of elimination of ocean surface roughness and wind retrieving. In this paper, data pre-processing flow for this scatterometer to obtain data ready for sea surface salinity and ocean surface wind retrieval was presented after corresponding data level definitions in this flow had been introduced. The processing flow was verified by processing data resulted from a data set simulated in a simple progress. Future work to perfect the pre-processing progress was also discussed.

Dual Frequency Polarized Scatterometer for global snow observation

Dual Frequency Polarized Scatterometer (DFPSCAT) is one of the three payloads onboard the satellite of Water Cycle Observation Mission (WCOM). DFPSCAT is an X/Ku band rotating pencil beam scatterometer with 2-5 km resolution and 1000km swath for mapping of snow water equivalent (SWE) and freeze-thaw process [1]. DFPSCAT achieves fine resolution by linear frequency modulation pulse compression along the elevation direction, and by unfocused synthetic aperture processing (a technique where the Doppler effect is exploited to synthesize a longer aperture to achieve an improved resolution), as well as super-resolution reconstruction by oversampling in the direction of the azimuth.