China Satellite Research Articles

Parameters determination and sensor correction method based on virtual CMOS with distortion for the GaoFen6 WFV camera

The GaoFen6 (GF6) satellite is an optical remote sensing satellite in China’s GaoFen series that combines the imaging abilities of high resolution and wide field of view (WFV). The WFV camera, equipped on the GF6, can obtain an image 800 km in width and with a resolution of 16 m, which represents the highest level of observation swath width among similar satellites in the world. Guaranteeing the high geometric quality of the imagery obtained by the WFV camera for subsequent remote sensing applications is a prominent problem that must be solved. This paper presents the key geometry processing method for the WFV camera, in which the parameters relative determination method based on two selected reference bands and the sensor correction based on virtual collinear complementary metal oxide semiconductor (CMOS) with distortion are proposed. To overcome the challenge of matching the tie points between different bands with large radiation differences, two bands—rather than traditional one band—were chosen as the reference bands for the WFV camera to perform the parameters relative determination. The geometric influence of the virtual CMOS is fully analyzed in the along- and across-track directions. Then, the virtual CMOS with a real calibrated distortion was designed; based on this design, a sensor correction was applied to stitch the whole image generated by single images in each band. This experiment verified the effectiveness of the proposed parameters relative determination and sensor correction methods in improving the internal relative accuracy and the registration accuracy; moreover, the mosaicking accuracy is also guaranteed.

Marine Floating Raft Aquaculture Detection of GF-3 PolSAR Images Based on Collective Multikernel Fuzzy Clustering

Marine floating raft aquaculture (MFRA) is a primary human activity on oceanic resources utilization and development, the physical structure of which is so particular that conventional optical remote sensing images cannot discover thoroughly. GaoFen-3 (GF-3), as the first launched full polarimetric synthetic aperture radar satellite of China, can offer a unique opportunity to perform long-term precise monitoring of the nearshore MFRA. In this paper, GF-3 product description is discussed to choose applicable modes to analyze MFRA imaging characters. Moreover, the structure of MFRA is decomposed into different parts to reveal its specific polarimetric scattering mechanism for the first time via the electromagnetic wave propagation theory. Based on the above-mentioned physical analysis, appropriate target decomposition methods are selected to combine other source features of MFRA to integrate into initial distinct features. After that, a novel unsupervised collective multikernel fuzzy C-means (CMKFCM) algorithm with collective neurodynamic optimization approach is proposed to exploit these multisource features of MFRA. The effective feature combination based on proposed CMKFCM can take full advantage of various features through establishing a unified framework, which can optimize local and global parameters hierarchically and improve the entire clustering accuracy in the end. The experiments substantiate the efficacy and superiority of GF-3 MFRA detection based on our proposed algorithm compared with other state-of-the-art unsupervised approaches.

Potentiality of Using Luojia1-01 Night-Time Light Imagery to Estimate Urban Community Housing Price-A Case Study in Wuhan, China.

The first professional night-time light remote sensing satellite in China, Luojia1-01, has raised the resolution of night-time light data to 130 m, which provides a possibility for the study of small-scale night-time light. This paper is the first research on spatial analysis and quantitative modeling between night-time light intensity (NTLI) and community housing price (CHP) on a small scale by using the Luojia1-01 night-time light imagery. This paper takes Wuhan as the research area, CHP data obtained by web-crawler technology as the research object, combines Luojia1-01 data, and carries out spatial correlation analysis and quantitative modeling on a small scale for them. The experimental results show that there is a strong linear positive correlation between the NTLI and CHP based on geographically weighted regression (GWR), and the CHP data in Wuhan have obvious spatial non-stationarity. Moreover, the coupling mechanism between the NTLI and CHP is also revealed. We can conclude that there is potential for estimating the CHP by using Luojia1-01 night-time light imagery.

On-Orbit MTF Estimation for GF-4 Satellite Using Spatial Multisampling on a New Target

GF-(Gaofen- means high resolution in Chinese) satellite launched in 2015 is the first geosynchronous orbit remote sensing satellite in China. To evaluate the on-orbit modulation transfer function (MTF) of the space-borne panchromatic camera in GF-4, a modified pulse target is proposed. This new type target is laid in the uniform low-reflection background region whose center region is a high-reflection square with the size of 3 ~ 4 ground sampled distance (GSD), and then two low-reflection rectangle stripes are extended along the center square toward both ends with the length over 4 GSD. In consideration of the staring imaging mechanism of GF-4, the target image sequences are accessed with relative random distance taken by the space-borne camera in a short period to reduce accidental error of one sampling and enhance the accuracy of estimation. Based on the multi-sampling calibrating images, maximum a posteriori estimation model and gradient descent solution with grid searching method are used to fit the pulse response function (PRF) when taking the target center positions as references. MTF is then calculated from PRF via Fourier transformation. Numerical simulation results reveal our method can keep the accuracy stable despite of different kinds of noise. Actual calibration result using this method shows that on-orbit MTF of GF-4 camera at Nyquist frequency is 0.1473.

Top Cloud Motion Field of Typhoon Megi-2016 Revealed by GF-4 Images

Gaofen-4 (GF-4) is the first high-resolution geostationary satellite of China, launched on December 29, 2015. Its visible-near infrared optical sensor is capable of imaging the earth at 50 m resolution, covering a 512 km $\times512$ km area with a minimum imaging interval of 20 s. More than 300 GF-4 images, taken in four sequences with imaging rates of 36 and 69 s per scene, captured the development of Typhoon Megi-2016—from its peak in the afternoon of September 26, 2016 to its dispersion two days later on September 28. These consecutive images recorded the motion field of the typhoon’s top clouds nearly continuously. By using advanced image matching technology, the motion has been estimated for every pixel, at subpixel accuracy from image pairs with 179 and 206 s intervals. The process has generated time series of atmospheric motion vector (AMV) fields at 50 m spatial resolution for the whole imaged area. It is the first time, to our knowledge, that such high-resolution and nearly continuous AMV data of a typhoon system have been produced. The data provide accurate measurements of the typhoon top cloud motion speed and direction, and reveal quantitative details of the motion field spatiotemporal evolution at high altitude. The finding that the high altitude top cloud motion speed is significantly lower than that recorded at low altitude below the planetary boundary layer is of scientific value and further exploitation of the motion data can lead to a better understanding of typhoon dynamics and thus improve cyclone modeling.

Daytime Evolution of Equatorial Plasma Bubbles Observed by the First Republic of China Satellite

AbstractPlasma bubbles in the equatorial F region are thought of as nighttime phenomena because they develop at night and are assumed to vanish after sunrise. However, bubbles occasionally persist throughout the night and into the day. This study investigates the origin of daytime irregularities and their evolution using data from the first Republic of China satellite. Our results show that daytime irregularities occur in the longitudes where bubbles have developed on previous nights. A newly reported feature is the observation of the temporal variation of the locations of daytime irregularities; daytime irregularities are concentrated near the magnetic equator early in the morning, but the location gradually shifts to higher latitudes with time. This phenomenon is explained in terms of the latitudinal redistribution of fossil bubbles by the ionospheric fountain effect.

Preliminary validation of in situ electron density measurements onboard CSES using observations from Swarm Satellites

A preliminary validation of the in situ electron density data obtained by CSES, a newly launched electromagnetic satellite in China on Feb. 2, 2018, using the Swarm satellites LP measurements, is carried out in this study to assess the consistency and reliability of the LAP observations onboard CSES. Three ways are implemented to achieve the goal: (1) comparison of the global distribution patterns for both the daytime and nighttime data, (2) comparison of the data between CSES and Swarm at intersecting ground tracks with similar local time, and (3) comparison of the relative relationship between nighttime and daytime data using the nighttime enhancement space climatology phenomenon. The results show that the in situ electron densities estimated by LAP onboard CSES are in good agreement with data from the Swarm satellites in terms of global distributions, local details and relationship between nighttime and daytime data. A systematic bias between them is also found: LAP density estimates are much lower than that of Swarm satellites. However, the highly consistent relative variations between the two datasets proves the electron density dataset of LAP onboard CSES for now is applicable for the relative data variation applications on ionosphere-related studies. Further calibration of the LAP dataset is recommended using electron density data obtained by different measurements.

Land use and land cover classification using Chinese GF-2 multispectral data in a region of the North China Plain

The newly launched GF-2 satellite is now the most advanced civil satellite in China to collect high spatial resolution remote sensing data. This study investigated the capability and strategy of GF-2 multispectral data for land use and land cover (LULC) classification in a region of the North China Plain. The pixel-based and object-based classifications using maximum likelihood (MLC) and support vector machine (SVM) classifiers were evaluated to determine the classification strategy that was suitable for GF-2 multispectral data. The validation results indicated that GF-2 multispectral data achieved satisfactory LULC classification performance, and object-based classification using the SVM classifier achieved the best classification accuracy with an overall classification accuracy of 94.33% and kappa coefficient of 0.911. Therefore, considering the LULC classification performance and data characteristics, GF-2 satellite data could serve as a valuable and reliable high-resolution data source for land surface monitoring. Future works should focus on improving LULC classification accuracy by exploring more classification features and exploring the potential applications of GF-2 data in related applications.

The volume ratio effect on flow patterns and transition processes of thermocapillary convection

Thermocapillary convection has always been one of the most important research topics in microgravity fluid physics. A space experimental study on the thermocapillary convection in an open annular liquid pool – a typical thermocapillary flow system – has been conducted on the SJ-10 satellite of China. This space experiment has observed the spatial temperature distribution of the liquid free surface using an infrared thermal imager, obtained the flow pattern transition process, analysed the oscillation characteristics and revealed the instability mechanism of themocapillary convection. The shape effects on the flow instability are researched by changing the volume ratio, Vr, which denotes the ratio of the liquid volume to the volume of the cylindrical gap between the walls. The volume ratio effect has been focused on for the first time. For a certain volume ratio, the flow pattern would transform from the steady state to the oscillation state accompanied by directional propagating hydrothermal waves with increasing temperature difference. In addition, the significant influences of the volume ratio on the critical conditions and wavenumber selection have been analysed in detail.

Microgravity diffusion flame spread over a thick solid in step-changed low-velocity opposed flows

We report results from a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows. The width of the PMMA sample is chosen to be as large as possible in order to minimize the side diffusion effects of oxygen, and for each of the four oxygen concentration cases considered, four decrementally changing gas flow velocities are imposed such that a wide range of parameter values are spanned near the quenching limit. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subcritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonically with the increase in the flame Damköhler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that the variation of the heat absorbed by the solid for vaporization is sub-linear with respect to the flame spread rate, thereby implying that the fuel regression depth has a tendency to increase with decreasing flame spread rate. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate.

Improving integrated precise orbit determination of GPS, GLONASS, BDS and Galileo through integer ambiguity resolution

The China Satellite Navigation Office announced on December 27, 2018 that the BDS-3 preliminary system had been completed to provide global services. Before this, GPS and GLONASS were the only two global navigation satellite systems (GNSS) supporting global positioning service, and have totally more than 50 satellites in normal operation. Furthermore, Galileo is intending to reach its full constellation around 2020. By that time, the number of available GNSS satellites will increase to more than 100, which brings both opportunities and challenges for high-precision positioning and orbit determination. The precision of orbits could be significantly improved through integer ambiguity resolution (AR), while AR is particularly difficult to achieve especially for GLONASS and BDS due to inter-frequency biases and satellite-induced code biases. Therefore, to address this limitation and further enhance the precision of multi-GNSS orbit determination, we try to fix the double-differenced intra-system ambiguities to integers and propose an integer AR method for multi-GNSS POD. To verify the contribution of AR, an experiment of 141 sites with global coverage is conducted. The results imply that the approach realizes an average fixing rate of 98.1%, 96.4%, 84.6% and 92.6% for GPS, GLONASS, BDS and Galileo over a whole year. The GNSS orbits are further improved with AR in terms of the precision compared with the International GNSS Service (IGS) final orbits, the discontinuity at overlapping day boundaries, and satellite laser ranging residuals. Thus, the integer AR improves the precision of multi-GNSS precise orbit determination, which can enhance integrated data processing of multi-GNSS and their applications in the future.

An Assessment of Anthropogenic CO₂ Emissions by Satellite-Based Observations in China.

Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas and its concentration in atmosphere has been increasing rapidly due to the increase of anthropogenic CO2 emissions. Quantifying anthropogenic CO2 emissions is essential to evaluate the measures for mitigating climate change. Satellite-based measurements of greenhouse gases greatly advance the way of monitoring atmospheric CO2 concentration. In this study, we propose an approach for estimating anthropogenic CO2 emissions by an artificial neural network using column-average dry air mole fraction of CO2 (XCO2) derived from observations of Greenhouse gases Observing SATellite (GOSAT) in China. First, we use annual XCO2 anomalies (dXCO2) derived from XCO2 and anthropogenic emission data during 2010–2014 as the training dataset to build a General Regression Neural Network (GRNN) model. Second, applying the built model to annual dXCO2 in 2015, we estimate the corresponding emission and verify them using ODIAC emission. As a results, the estimated emissions significantly demonstrate positive correlation with that of ODIAC CO2 emissions especially in the areas with high anthropogenic CO2 emissions. Our results indicate that XCO2 data from satellite observations can be applied in estimating anthropogenic CO2 emissions at regional scale by the machine learning. This developed method can estimate carbon emission inventory in a data-driven way. In particular, it is expected that the estimation accuracy can be further improved when combined with other data sources, related CO2 uptake and emissions, from satellite observations.

Evaluation of HJ-1A/B CCD Surface Reflectance Products Using the VNIR and MODIS-Based Atmospheric Correction Approaches

The growing development of medium- to high-resolution satellites in China has led to a considerable increase in the quantitative applications using the data; therefore, it is important to produce standard surface reflectance (SR) products operationally from such data. However, there is still a lack of relevant SR systems and SR products. We applied two atmospheric correction algorithms, adaptive to most multispectral satellites with visible and near-infrared (VNIR) bands, to HJ-1A/B charge-coupled device (CCD) instrument data, namely, the VNIR method and the MODIS-based method, with both methods being based on the Second Simulation of the Satellite Signal in the Solar Spectrum, Vector (6SV) code, and the look-up tables. We evaluated the accuracy of the SR by these two approaches for HJ-1A/B CCD images compared with the AErosol RObotic NETwork (AERONET) corrected reflectance for the period July 2011 to June 2012 over China and surrounding regions. We assessed more than 12 million pixels for the 49 spatial circular subsets, with a radius of 5 km, centered at 12 AERONET sites. The evaluation results indicated that both methods were suitable for operational flow, and that the MODIS-based method had better accuracy than the VNIR method, except for the near-infrared band. This conclusion was also validated by comparison with the normalized difference vegetation index products derived from the MODIS-based SR, VNIR SR, and AERONET SR. Additionally, the MODIS-based method showed superior accuracy when the overpass time of HJ-1A/B was more approximate to that of Terra MODIS.

On-Orbit Geometric Calibration and Validation of Luojia 1-01 Night-Light Satellite

The Luojia 1-01 Satellite (LJ1-01) is the first professional night-light remote-sensing satellite in China, and thus, it is of pioneering significance for the development of night-light remote sensing satellites in China and the application of remote sensing in the social and economic fields. To ensure the application of night-light remote-sensing data, several studies concerning on-orbit geometric calibration and accuracy verification have been carried out for the complementary metal oxide semiconductor (CMOS) rolling shutter camera of LJ1-01 since the launch of the satellite. Owing to the lack of high-precision nightlight geometric reference at home and abroad, it is difficult to directly calibrate the nighttime light image of LJ1-01. Based on the principle of rolling shutter dynamic imaging, a rigorous geometric imaging model of the time-sharing exposure of the rolling shutter of LJ1-01 is established, and a geometric calibration method for daytime imaging calibration and compensated nighttime light data is proposed. The global public Landsat digital orthophoto image (DOM) with a 15-m resolution and 90-m Shuttle Radar Topography Mission digital elevation model (SRTM-DEM) are used as control data. The images obtained in England, Venezuela, Caracas, Damascus, and Torreon (Mexico) were selected as experimental data. The on-orbit calibration and accuracy verification of LJ1-01 were carried out. Experiments show that after on-orbit geometric calibration, the daytime calibration parameters can effectively compensate for the systematic errors of night-light images. After compensation, the positioning accuracy of night-light images without geometric control points (GCPs) is improved from nearly 20 km to less than 0.65 km. The internal accuracy of the calibrated night-light images is better than 0.3 pixels, which satisfies the requirement of subsequent applications.

Polarimetric Calibration of the GaoFen-3 Mission Using Active Radar Calibrators and the Applicable Conditions of System Model for Radar Polarimeters

GaoFen-3, the first polarimetric SAR satellite of China, carried out polarimetric calibration experiments using C-band polarimetric active radar calibrators (PARCs), trihedral corner reflectors (TCRs), and dihedral corner reflectors (DCRs). The calibration data were firstly processed referring to the classic 2 × 2 receive R and transmit T model for radar polarimeter systems, first proposed by Zebker, Zyl, and Held, and Freeman’s method based on PARCs, but the results were not good enough. After detailed analysis about the GaoFen-3 polarimetric system, we found that the system had some nonlinearity, then a new imbalance parameter was introduced to the classic model, which is equivalent to the γ proposed in Freeman’s paper about a general polarimetric system model. Then, we proposed the calibration data processing algorithm for GaoFen-3 based on the improved model and obtained better results. The algorithm proposed here is verified to be suitable for GaoFen-3 and can be applied to other spaceborne and airborne fully-polarimetric SAR systems.

Influence of micro-vibration on the image quality of a GEO remote sensor

GF-4 satellite is the first geosynchronous orbit high-resolution optical remote sensing satellite in China. Given that a geosynchronous orbit is 70 times as high as a sun-synchronous orbit, the geosynchronous optical remote sensing satellite has a stringent requirement of micro-vibration, which is almost two orders of magnitude smaller than that a of low-orbit satellite. On the basis of research on the characteristics of on-board disturbance sources, such as the momentum wheel and cryocooler, an integrated model of opto-mechanical and micro-vibration is first developed for geosynchronous visible and infrared integrated satellite in this paper. A micro-vibration suppression method based on structural optimization, vibration reduction, and vibration isolation is proposed, which has been proved to effectively suppress the in-orbit micro-vibration of the geosynchronous high-resolution optical remote sensor and lays the foundation for GF-4 to obtain excellent images.

Analysis of Precise Orbit Predictions for a Hy-2A Satellite with Three Atmospheric Density Models Based on Dynamic Method

HY-2A (Haiyang 2A) is the first altimetry satellite in China, and it was designed to be in a repeated ground track orbit to achieve the mission targets. Maneuvers are necessary to keep the satellite on the designed orbit according to the dynamic precise orbital prediction. Atmospheric density models are essential for predicting the low Earth orbit (LEO) satellites, such as HY-2A. Nevertheless, it is a complex process to determine the optimal atmospheric density model for orbit prediction. In this paper, short-term and long-term orbit predictions based on the dynamic method using three different atmospheric density models are tested. Detailed comparisons and evaluation of the accuracy of the predicted results are performed. Furthermore, to assess the results for the ground tracking of the satellite, the interpolation method especially for a spherical surface is introduced. The results show that among the three models, the Jacchia 1971 model is in the closest agreement with Multi-Mission Ground Segment for Altimetry precise positioning and Orbitography (SSALTO) precise orbits. The root-mean-squares (RMSs) of radial orbit differences between the predicted and precise orbits are 0.016 m, 0.091 m, 0.176 m, 0.573 m, and 1.421 m for predicted 1-h, 12-h, 1-day, 3-day, and 7-day arcs, respectively.

Absorbing Aerosol Sensor on Gao-Fen 5B satellite

Abstract The Absorbing Aerosol Sensor (AAS) will be launched aboard the GaoFen-5B satellite in China. The main purpose of AAS is to monitor absorbing aerosols by measuring the solar backscatter radiation. AAS is an ultraviolet-visible imaging spectrometer that uses a single charge coupled device to capture both the spectrum and the cross-track direction with a 114° wide swath. The large field of view enables daily global coverage with 4-km spatial resolution. The spectral range of the instrument extends from 340 to 550 nm with spectral resolution (full width at half maximum) of 2 nm. This paper provides details of the instrument design, including system design, optical design, and mechanical design, as well as detector and calibration unit on orbit. The numerous simulations show that all design results satisfy the specification and vibration requirements of the instrument.

Error Source Analysis and Correction of GF-3 Polarimetric Data

The GaoFen-3 (GF-3) satellite is the first polarimetric synthetic aperture radar (PolSAR) satellite in China. With a designed in-orbit life of 8 years, it will provide large amounts of PolSAR data for ocean monitoring, disaster reduction, and many other applications. The polarimetric data quality is essential for all these applications, so the analysis and calibration of the polarimetric error sources are very important for GF-3. In this study, we established a full-link error model for GF-3 PolSAR system. Based on this model, we comprehensively analyzed the quantitative effects of the main error sources including the composition, figured out characteristics of the phase imbalance introduced by the antenna, and pointed out the error sources which have to be corrected. Furthermore, the polarimetric correction method for GF-3 PolSAR system is proposed. Finally, assisted by several external calibration experiments, polarimetric errors of GF-3 data are efficiently corrected during in-orbit-test phase.

A Comparative Study of Near-Limit Flame Spread Over a Thick Solid in Space- and Ground-Based Experiments

Microgravity experiments have been performed aboard the SJ-10 satellite of China to investigate flame spread behaviors over a thick PMMA in low-velocity opposed flow. Two variables are considered: opposed-flow velocity in a range of 0 to 9 cm/s, and ambient oxygen concentration in a range of 25% to 50%. It is found that, when the flow velocity is reduced, the initial extended flame may breaks into separate flamelets after a dynamic transition process. This is the first observation of the flamelets spreading over a thick solid fuel in microgravity. Flame and flamelet propagate with a steady spread rate, which increases with the increasing flow velocity and oxygen concentration. A flammability map using oxygen concentration and flow velocity as coordinates is established, which delineates the uniform regime, the flamelet regime, and extinguished regime. The flammability boundary was extended to lower oxygen concentrations and lower flow velocities by the flamelet regime. The microgravity results are compared with the counterparts in ground-based narrow channel apparatus (NCA) experiments. Results showed that although the NCA tests overestimate the flame spread rate and flammable area, also exhibit differences in detailed flamelet formation process, flame and flamelet behaviors agree well with that in microgravity in a qualitative manner.