Origins of perturbations in dayside equatorial ground magnetograms

To determine the cause(s)of perturbations seen in dayside equatorial ground magnetograms,we conducted a systematic survey of simultaneous ground-based and geosynchronous satellite-based observations during the 90-day period from December 1,2020 to February 28,2021.We examined Huancayo ground magnetometer observations from 14:00:00 to 20:00:00 UT each day,during which Huancayo passed through local noon.From those data we chose perturbation events selected on the basis of large(>20 nT)event amplitude and classified the selected events as responding primarily to solar wind pressure,or to variations in the north/south component of the interplanetary magnetic field(IMF Bz),or perhaps in part to both.The results show that an equivalent number of events were identified for each model during this 90-day period.Variations in the lagged solar wind dynamic pressure routinely correspond to nearly simultaneous sudden impulses recorded at both geosynchronous orbit and on the ground.Variations in IMF Bz produce erosion signatures at geosynchronous orbit and can correspond to ground events if lag times for reconnection to enhance convection in the magnetosphere are taken into account.

Proposal for a realtime Einstein-synchronization-defined satellite virtual clock

Realization of high performance satellite onboard clock is vital for various positioning, navigation, and timing applications. For further improvement of the synchronization-based satellite time and frequency references, we propose a geosynchronous(GEO) satellite virtual clock concept based on ground–satellite synchronization and present a beacon transponder structure for its implementation(scheduled for launch in 2025), which does not require atomic clocks to be mounted on the satellite. Its high performance relies only on minor modifications to the existing transponder structure of GEO satellites. We carefully model the carrier phase link and analyze the factors causing link asymmetry within the special relativity. Considering that performance of such synchronization-based satellite clocks is primarily limited by the link's random phase noise, which cannot be adequately modeled, we design a closed-loop experiment based on commercial GEO satellites for pre-evaluation. This experiment aims at extracting the zero-means random part of the ground-satellite Ku-band carrier phase via a feedback loop. Ultimately, we obtain a 1σ value of 0.633 ps(two-way link), following the Gaussian distribution. From this result, we conclude that the proposed real-time Einstein-synchronization-defined satellite virtual clock can achieve picosecond-level replication of onboard time and frequency.

A Deep-Learning and Transfer-Learning Hybrid Aerosol Retrieval Algorithm for FY4-AGRI:Development and Verification over Asia

The Advanced Geosynchronous Radiation Imager(AGRI)is a mission-critical instrument for the Fengyun series of satellites.AGRI acquires full-disk images every 15 min and views East Asia every 5 min through 14 spectral bands,enabling the detection of highly variable aerosol optical depth(AOD).Quantitative retrieval of AOD has hitherto been challenging,especially over land.In this study,an AOD retrieval algorithm is proposed that combines deep learning and transfer learning.The algorithm uses core concepts from both the Dark Target(DT)and Deep Blue(DB)algorithms to select features for the machinelearning(ML)algorithm,allowing for AOD retrieval at 550 nm over both dark and bright surfaces.The algorithm consists of two steps:①A baseline deep neural network(DNN)with skip connections is developed using 10 min Advanced Himawari Imager(AHI)AODs as the target variable,and②sunphotometer AODs from 89 ground-based stations are used to fine-tune the DNN parameters.Out-of-station validation shows that the retrieved AOD attains high accuracy,characterized by a coefficient of determination(R2)of 0.70,a mean bias error(MBE)of 0.03,and a percentage of data within the expected error(EE)of 70.7%.A sensitivity study reveals that the top-of-atmosphere reflectance at 650 and 470 nm,as well as the surface reflectance at 650 nm,are the two largest sources of uncertainty impacting the retrieval.In a case study of monitoring an extreme aerosol event,the AGRI AOD is found to be able to capture the detailed temporal evolution of the event.This work demonstrates the superiority of the transfer-learning technique in satellite AOD retrievals and the applicability of the retrieved AGRI AOD in monitoring extreme pollution events.

SIMULATION AND ANALYSIS OF PERFORMANCE ON THE COMPASSTM ENHANCED BY GEO AND IGSO

Positioning accuracy of the Global Navigation Satellite System(GNSS) can be analyzed by Positioning Dilution Of Precision(PDOP).In order to enhance the navigating performance of Asia and the Pacific areas,this paper analyzes the next generation BeidouTM navigation satellite system(CompassTM) enhanced by Geostationary Earth Orbit(GEO) and Inclining GeoSynchronized Orbit(IGSO).As global navigation satellite system,CompassTM must be robust enough to avoid system layoff,when some nodes are not available.So,the CompassTM enhanced by GEO and IGSO constellation is proposed and analyzed its PDOP proformance,this paper shows some exciting results of performance of CompassTM enhanced by GEO and IGSO.From the simulation results,we can found that:when more than fifteen satellites are invalid,the enhanced system could be operating normally.

COVERAGE PERFORMANCES ANALYSIS ON COMBINED-GEO-IGSO SATELLITE CONSTELLATION

The Combined-GEO-IGSO constellation is the combination of Geostationary Earth Orbit(GEO) satellite and Inclining GeoSynchronous Orbit(IGSO) satellite.The Combined-GEO-IGSO constellation can integrate the advantages of GEO and IGSO to achieve regional coverage.In order to discuss the performances of the Combined-GEO-IGSO constellation,the performances of coverage,elevation,diversity,and transmission are simulated in China and surrounding regions by Satellite Tool Kit(STK).The simulation results show that:the combined constellation can reach higher multi-satellite coverage and higher communication elevation in China and surrounding areas;the Doppler shift,delay,and propagation loss of this constellation have little impact on the system.As regional coverage constellation,the Combined-GEO-IGSO is feasible.

High Resolution Radar Real-Time Signal and Information Processing

Radar is an electronic device that uses radio waves to determine the range, angle, or velocity of objects. Real-time signal and information processor is an important module for real-time positioning, imaging, detection and recognition of targets. With the development of ultra-wideband technology, synthetic aperture technology, signal and information processing technology, the radar coverage, detection accuracy and resolution have been greatly improved, especially in terms of one-dimensional(1D) high-resolution radar detection, tracking, recognition, and two-dimensional(2D) synthetic aperture radar imaging technology. Meanwhile, for the application of radar detection and remote sensing with high resolution and wide swath, the amount of data has been greatly increased. Therefore, the radar is required to have low-latency and real-time processing capability under the constraints of size, weight and power consumption. This paper systematically introduces the new technology of high resolution radar and real-time signal and information processing. The key problems and solutions are discussed, including the detection and tracking of 1D high-resolution radar, the accurate signal modeling and wide-swath imaging for geosynchronous orbit synthetic aperture radar, and real-time signal and information processing architecture and efficient algorithms. Finally, the latest research progress and representative results are presented, and the development trends are prospected.

Analysis of the Quality of Daily DEM Generation with Geosynchronous InSAR

Up-to-date digital elevation model(DEM)products are essential in many fields such as hazards mitigation and urban management.Airborne and low-earth-orbit(LEO)space-borne interferometric synthetic aperture radar(InSAR)has been proven to be a valuable tool for DEM generation.However,given the limitations of cost and satellite repeat cycles,it is difficult to generate or update DEMs very frequently(e.g.,on a daily basis)for a very large area(e.g.,continental scale or greater).Geosynchronous synthetic aperture radar(GEOSAR)satellites fly in geostationary earth orbits,allowing them to observe the same ground area with a very short revisit time(daily or shorter).This offers great potential for the daily DEM generation that is desirable yet thus far impossible with space-borne sensors.In this work,we systematically analyze the quality of daily GEOSAR DEM.The results indicate that the accuracy of a daily GEOSAR DEM is generally much lower than what can be achieved with typical LEO synthetic aperture radar(SAR)sensors;therefore,it is important to develop techniques to mitigate the effects of errors in GEOSAR DEM generation.

An Improved Double r-iteration IOD Method for GEO UCTs Based on SBSS System

The purpose of initial orbit determination,especially in the case of angles-only data for observation,is to obtain an initial estimate that is close enough to the true orbit to enable subsequent precision orbit determination processing to be successful.However,the classical angles-only initial orbit determination methods cannot deal with the observation data whose Earth-central angle is larger than 360°.In this paper,an improved double r-iteration initial orbit determination method to deal with the above case is presented to monitor geosynchronous Earth orbit objects for a spacebased surveillance system.Simulation results indicate that the improved double r-iteration method is feasible,and the accuracy of the obtained initial orbit meets the requirements of re-acquiring the object.

Effect of change in large and fast solar wind dynamic pressure on geosynchronous magnetic field

We present a comparison of changes in large and sharp solar wind dynamic pressure, observed by several spacecraft, with fast disturbances in the magnetospheric magnetic field, measured by the geosynchronous satellites. More than 260 changes in solar wind pressure during the period 1996-2003 are selected for this study. Large statistics show that an increase （a decrease） in dynamic pressure always results in an increase （a decrease） in the magnitude of geosynchronous magnetic field. The amplitude of response to the geomagnetic field strongly depends on the location of observer relative to the noon meridian, the value of pressure before disturbance, and the change in amplitude of pressure.

Simulation and Analysis of the Impacts of Anisotropic Ionospheric Scintallition on Geosynchronous Synthetic Aperture Radar

The impacts of ionospheric scintillation on geosynchronous synthetic aperture radar（GEO SAR）focusing is studied based on the multiple phase screen（MPS）theory.The power spectrum density of electron irregularities is first modified according to the ionospheric anisotropy.Then propagation wave equations in random medium are deduced in the case of oblique incidence in GEO SAR.The amplitude and phase errors induced by the random electron fluctuations are generated by the iterated MPS simulations and are superimposed into the GEO SAR signals.Through the following imaging and evaluation,the effects of the anisotropic ionospheric scintallition on GEO SAR are assessed.At last,the optimized integration time under different ionospheric scintillation conditions are recommended through Monte Carlo experiments.It is concluded that,greater ionospheric fluctuations and longer integration time will result in more severe deterioration,even no focus at all in the worst case.

First results of the low energy ion spectrometer onboard a Chinese geosynchronous satellite

A Chinese geosynchronous satellite was launched on June 23,2020.It carried a plasma detection package to monitor the space environment around the orbit.Here we report the inflight performance of a low energy ion spectrometer(LEIS),one of the primary instruments in the plasma detection package,and its initial observations in flight.Benefiting from the state-of-the-art design of a top-hat electrostatic analyzer cooperated with angular scanning deflectors,three-dimensional measurement of ions in space with a large field of view of 360°×90°and a wide energy range from 50 eV to 25 keV per charge has been achieved.The differential energy flux spectra of ions around the orbit have shown clear signatures of surface charging and storm/substorm ion injections.The occurrence of surface charging could be caused by the lack of photoemission at the Earth's eclipse(near the midnight)or the storm energetic electron injection at the dawn sector.The present results demonstrated a good performance of the LEIS payload in flight for monitoring the space ion environment around the orbit.In situ measurements of the LEIS payload provide us an opportunity to understand the magnetospheric ion dynamics and forecast the associate space weather impacts.

A method of correlation analysis for space-based GEO object surveillance

A method of correlation analysis to determine the correlation between two too-short arcs is presented to monitor geosynchro- nous earth orbit objects on a space-based surveillance system. The necessary condition for a geosynchronous earth orbit object to be observed by a space-based visible sensor in two successive orbital periods is derived, and the principle for the selection of search fence parameters is proposed. The processing procedure of the correlation analysis for the two too-short arcs as well as two correlation judgment methods is given and the selection of the threshold value for the judgment is analyzed. Finally, a simulation is performed to demonstrate the applicability of the presented methods. Simulation results ir^dicate that a surveil- lance system can acquire two observation arcs in two successive orbital periods by selecting the suitable search fence parame- ters, and that the two correlation judgment methods and the selected threshold are valid. The initial orbit of the uncorrelated objects can be determined by two observation arcs, and the accuracy of the initial orbit meets the requirements of re-acquiring the object.

Effect of orbital errors on the geosynchronous circular synthetic aperture radar imaging and interferometric processing

The geosynchronous circular synthetic aperture radar (GEOCSAR) is an innovative SAR system,which can produce high resolution three-dimensional (3D) images and has the potential to provide 3D deformation measurement.With an orbit altitude of approximately 36 000 km,the orbit motion and orbit disturbance effects of GEOCSAR behave differently from those of the conventional spaceborne SAR.In this paper,we analyze the effects of orbit errors on GEOCSAR imaging and interferometric processing.First,we present the GEOCSAR imaging geometry and the orbit errors model based on perturbation analysis.Then,we give the GEOCSAR signal formulation based on imaging geometry,and analyze the effect of the orbit error on the output focused signal.By interferometric processing on the 3D reconstructed images,the relationship between satellite orbit errors and the interferometric phase is deduced.Simulations demonstrate the effects of orbit errors on the GEOCSAR images,interferograms,and the deformations.The conclusions are that the required relative accuracy of orbit estimation should be at centimeter level for GEOCSAR imaging at L-band,and that millimeter-scale accuracy is needed for GEOCSAR interferometric processing.

FY-4A/GIIRS Temperature Validation in Winter and Application to Cold Wave Monitoring

In order to improve the operational application ability of the Fengyun-4A(FY-4A)new sounding dataset,in this paper,validation of the FY-4A Geosynchronous Interferometric Infrared Sounder(FY-4A/GIIRS)temperature was carried out using the balloon sounding temperature from meteorological sounding stations.More than 350,000 samples were obtained through time–space matching,and the results show that the FY-4A/GIIRS temperature mean bias(MB)is 0.07°C,the mean absolute error(MAE)is 1.80°C,the root-mean-square error(RMSE)is 2.546°C,and the correlation coefficient(RR)is 0.95.The FY-4A/GIIRS temperature error is relatively larger in the upper and lower troposphere,and relatively smaller in the middle troposphere;that is,the temperature at 500 hPa is better than that at 850 hPa.The temporal variation is smaller in the upper and middle troposphere than in the lower troposphere.The reconstruction of missing data of FY-4A/GIIRS temperature in cloudy areas is also carried out and the results are evaluated.The spatial distribution of reconstructed FY-4A/GIIRS temperature and the fifth generation ECMWF reanalysis(ERA5)data is consistent and completely retains the minimum temperature center with high precision of FY-4A/GIIRS.There are more detailed characteristics of intensity and position at the cold center than that of the reanalysis data.Therefore,an operational satellite retrieval temperature product with time–space continuity and high accuracy is formed.The reconstructed FY-4A/GIIRS temperature is used to monitor a strong cold wave event in November 2021.The results show that the product effectively monitors the movement and intensity of cold air activities,and it also has good indication for the phase transition of rain and snow triggered by cold wave.

Response of magnetic fields at geosynchronous orbit and on the ground to the sudden changes of IMF B_Z

The rapid change in the Earth’s magnetosphere caused by solar wind disturbances has been an important part of the solar wind-magnetosphere interaction.However most of the previous studies focused on the perturbation of the Earth’s magnetic field caused by solar wind dynamic pressure changes.In this paper,we studied the response of geosynchronous magnetic field and the magnetic field to the rapid southward turning of interplanetary magnetic field during the interval 1350 1420 UT on 7May 2007.During this event,BZ component of the interplanetary magnetic field decreased from 15 nT to 10 nT within 3 min(1403 1406 UT).The geosynchronous magnetic field measured by three geosynchronous satellites(GOES 10 12)first increased and then decreased.The variations of magnetic field strength in the morning sector(9 10 LT)were much larger than those in the dawn sector(5 LT).Meanwhile,the H components of geomagnetic field on the ground have similar response features but exhibit latitude and LT dependent variations.Compared with H components,the D components do not have regular variations.Although the solar wind dynamical pressure encounters small variations,the magnetic field both in space and on the ground does not display similar variations.Therefore,the increase of geomagnetic field in the dawn sector is caused by the southward turning of IMF(interplanetary magnetic field)BZ.These results will help to better understand the coupling process of geomagnetic filed and interplanetary magnetic field.

Research progress on geosynchronous synthetic aperture radar

Based on its ability to obtain two-dimensional(2D)high-resolution images in all-time and all-weather conditions,spaceborne synthetic aperture radar(SAR)has become an important remote sensing technique and the study of such systems has entered a period of vigorous development.Advanced imaging modes such as radar interferometry,tomography,and multi-static imaging,have been demonstrated.However,current in-orbit spaceborne SARs,which all operate in low Earth orbits,have relatively long revisit times ranging from several days to dozens of days,restricting their temporal sampling rate.Geosynchronous SAR(GEO SAR)is an active research area because it provides significant new capability,especially its much-improved temporal sampling.This paper reviews the research progress of GEO SAR technologies in detail.Two typical orbit schemes are presented,followed by the corresponding key issues,including system design,echo focusing,main disturbance factors,repeat-track interferometry,etc,inherent to these schemes.Both analysis and solution research of the above key issues are described.GEO SAR concepts involving multiple platforms are described,including the GEO SAR constellation,GEO-LEO/airborne/unmanned aerial vehicle bistatic SAR,and formation flying GEO SAR(FF-GEO SAR).Due to the high potential of FF-GEO SAR for three-dimensional(3D)deformation retrieval and coherence-based SAR tomography(TomoSAR),we have recently carried out some research related to FF-GEO SAR.This research,which is also discussed in this paper,includes developing a formation design method and an improved TomoSAR processing algorithm.It is found that GEO SAR will continue to be an active topic in the aspect of data processing and multi-platform concept in the near future.

Survey of the electrostatic tractor research for reorbiting passive GEO space objects

The number of operational satellites and debris objects in the valuable geosynchronous ring has increased steadily over time such that active debris removal missions are necessary to ensure long-term stability. These objects are very large and tumbling, making any mission scenarios requiring physical contact very challenging. In the last 10 years, the concept of using an electrostatic tractor has been investigated extensively. With the electrostatic tractor concept, active charge emission is employed to simultaneously charge the tug or services vehicle, while aiming the charge exhaust onto the passive space debris object to charge it as well. The resulting electrostatic force has been explored to actuate this debris object to a disposal orbit or to detumble the object, all without physical contact. This paper provides a survey of the related research and reviews the charging concepts, the associated electrostatic force and torque modeling, and the feedback control developments, as well as the charge sensing research.