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As a special microwave detection system, multistatic radar has obvious advantages in covert operation, anti-jamming, and anti-stealth due to its configuration of spatial diversity. As a high-orbit irradiation source, a geosynchronous orbit satellite (GEO) has the advantages of a low revisit period, large beam coverage area, and stable power of ground beam compared with traditional passive radar irradiation sources. This paper focuses on the key technologies of flight target detection in multistatic radar based on geosynchronous orbit satellite irradiation with one transmitter and multiple receivers. We carry out the following work: Firstly, we aim to address the problems of low signal-to-noise ratio (SNR) and range cell migration of high-speed cruise targets. The Radon–Fourier transform constant false alarm rate detector-range cell migration correction (RFT-CFAR-RCMC) is adopted to realize the coherent integration of echoes with range cell migration correction (RCM) and Doppler phase compensation. It significantly improves the SNR. Furthermore, we utilize the staggered PRF to solve the ambiguity and obtain multi-view data. Secondly, based on the aforementioned target multi-view detection data, the linear least square (LLS) multistatic positioning method combining bistatic range positioning (BR) and time difference of arrival positioning (TDOA) is used, which constructs the BR and TDOA measurement equations and linearizes by mathematical transformation. The measurement equations are solved by the LLS method, and the target positioning and velocity inversion are realized by the fusion of multistatic data. Finally, using target positioning data as observation values of radar, the Kalman filter (KF) is used to achieve flight trajectory tracking. Numerical simulation verifies the effectiveness of the proposed process.

In geosynchronous orbit satellite synthetic aperture radar (GEO SAR) maritime surveillance, imaging of ship targets with complex motion is a hard task. The difficulty lies in how to process the received signal with an extremely low signal-to-noise ratio (SNR), long synthetic aperture time, high-order phase term and range migration. To address the issue, this paper proposes a GEO SAR refocusing algorithm for ship targets with complex motion via ISAR technique, which is based on complex fast-time slow-time frequency distribution (CFSFD). First, the received signal of ship targets with complex motion is derived and modeled as a multicomponent 2-D joint sine-series-polynomial phase signal (2-D JSPS), where 2-D signal is used to describe the signal with range migration induced by complex motion. To deal with the signal, a joint envelope-phase estimation named CFSFD is proposed. Even under low SNR and long synthetic aperture time, CFSFD can achieve directly instantaneous frequency analysis for 2-D JSPS accurately. Finally, a hybrid SAR/ISAR refocusing algorithm is proposed, in which CFSFD-based ISAR technique replaces the range migration correction followed by time–frequency transform approach, yielding clear refocused results of ship targets with complex motion. Simulation and real data experiments validate the effectiveness of the proposed algorithm.

This study proposes a dedicated closed-form solution and satellite phasing rules for designing a geosynchronous orbit (GSO) constellation. Trajectories of GSO satellites have a characteristic figure-eight shape because their rotation speed is the same as that of the Earth. The GSO has the advantage of providing good coverage performance for local areas. Recently, several countries have begun developing local navigation systems based on the GSO. Various GSO constellation designs are available for an effective regional navigation performance analysis; however, no dedicated GSO constellation solution exists. This study provides a solution for such constellations and proves its practicability through a comparative analysis and evaluation of the geometric dilution-of-precision performance for several cases.

Solar wind is an intermediary in energy transfer from the Sun into the Earth's magnetosphere, and is considered as a decisive driver of energetic electron dynamics at the geosynchronous orbit (GEO). Based on machine learning technology, several models driven by solar wind parameters have been established to predict GEO electron fluxes. However, the relative contributions of different solar wind parameters on GEO electron fluxes are still unclear. Recently, a feature attribution method, Deep SHapley Additive exPlanations (Deep SHAP) is proposed to open black boxes of machine learning models. In this study, we use the Deep SHAP method to quantify contributions of different solar wind parameters with an artificial neural network (ANN) model. Backpropagating the prediction results of this ANN model from 2011 to 2020, SHAP values for four solar wind parameters (interplanetary magnetic field (IMF) BZ, solar wind speed, solar wind dynamic pressure, and proton density) are calculated and comprehensively analyzed. The results suggest that solar wind speed with a lag of 1 day is the most important driver. We further investigate relative roles of different parameters in three specific electron fluxes variation events (corresponding to electron fluxes reaching a local maximum, a local minimum, and unchanged, respectively). The results suggest that high solar wind speed and southward IMF BZ (high dynamic pressures) facilitate increases (decreases) of electron fluxes. These findings help reveal the underlying physical mechanisms of GEO electron dynamics and help develop more accurate prediction models for GEO electron fluxes.

GaoFen-4(GF-4) is the highest spatial resolution Earth observation satellite operating in geosynchronous orbit. Its fixed Earth observation location, rapid responsiveness, and wide observation range make it popular in disaster and emergency monitoring. To evaluate the GF-4 image quality in detail on a long-term basis, this study analyzes the image quality after the commissioning phase by focusing on ground sample distance (GSD) and geometric and radiometric quality. The theoretical calculation, geometric and radiometric measurements, and on-site experiments results show that (1) the GSD of the GF-4 image is ~50 m at the nadir point and increases gradually with the distance away from the nadir point, (2) most external geometric errors are within the design requirements of 4 km despite some exceeding the limit, and the internal geometric errors are tested within 1 pixel, and (3) image sharpness is generally stable but varies with the atmosphere condition and imaging time, and the radiometric response gradually degrades at the rate of less than 5.5% per year.

The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is scheduled to be launched to geostationary orbit in 2027. It will carry a telescope with an unprecedentedly large field of view (204 deg2) and near-ultraviolet (NUV; 230–290 nm) sensitivity (22.5 mag, 5σ, at 900 s). ULTRASAT will conduct the first wide-field survey of transient and variable NUV sources and will revolutionize our ability to study the hot transient Universe. It will explore a new parameter space in energy and timescale (months-long light curves with minutes cadence), with an extragalactic volume accessible for the discovery of transient sources that is >300 times larger than that of the Galaxy Evolution Explorer (GALEX) and comparable to that of the Vera Rubin Observatory’s Legacy Survey of Space and Time. ULTRASAT data will be transmitted to the ground in real time, and transient alerts will be distributed to the community in <15 minutes, enabling vigorous ground-based follow up of ULTRASAT sources. ULTRASAT will also provide an all-sky NUV image to >23.5 AB mag, over 10 times deeper than the GALEX map. Two key science goals of ULTRASAT are the study of mergers of binaries involving neutron stars, and supernovae. With a large fraction (>50%) of the sky instantaneously accessible, fast (minutes) slewing capability, and a field of view that covers the error ellipses expected from gravitational-wave (GW) detectors beyond 2026, ULTRASAT will rapidly detect the electromagnetic emission following binary neutron star/neutron star–black hole mergers identified by GW detectors, and will provide continuous NUV light curves of the events. ULTRASAT will provide early (hour) detection and continuous high-cadence (minutes) NUV light curves for hundreds of core-collapse supernovae, including for rarer supernova progenitor types.

This study aims to investigate the effects of the solar activity phases on the magnetospheric processes in relation to daily relativistic electron (E>2 MeV) dynamics at geosynchronous orbit (GEO) during Solar Cycles 23–24. GOES observations indicate that in the descending phase the electron fluxes are seasonally dependent with largest flux in equinoctial periods, relatively high, and 27-day recurrent, while in the maximum phases they are relatively low and nonrecurrent. The electron fluxes are relatively low and partially recurrent in the ascending phases. The cross correlation coefficients (c.c.s) of daily Kp–Vsw, Kp–AE, Kp–AL evolve in the similar trend for both solar cycles: highest during the descending phases, lower in the ascending phases, and lowest in the maximum and minimum phases. The correlation of Kp–viscous term is strongest during descending phases and weakest around the maximum phases. The correlation of Kp–merging term slightly varies in a chaotic way from one solar phase to another, but drops to its lowest value in the solar minimum. The correlation analysis results signify the role of the solar activity in controlling the solar wind–magnetosphere couplings. Two case studies during maximum (2000) and descending (2017) phases indicate that the solar activity dependence of the magnetospheric processes and relativistic electrons is characterized by substorm activity and solar wind drivers. Most of the substorms induced by coronal mass ejections in the maximum phase exhibit strong but short and non-repetitive features that associate with low electron fluxes at GEO. In contrast, high intensity, prolonged, and repetitive substorm activities induced by high-speed solar winds are the main cause of the relativistic electron enhancements during the descending phases. The enhancements strongly depend on Vsw and ΣKp in which only appropriate Vsw and ΣKp are required. In the descending phase, the requirements for the flux enhancements to ≥103 cm−2 sr−1 s−1 are the simultaneity of Vsw>500 km/s and the prolongation of ΣKp>220 and to ≥104 cm−2 sr−1 s−1 when Vsw≥630 km/s and ΣKp>227. Furthermore, the correlations of ΣKp and log-electron fluxes are stronger in the descending/ascending phases than in the maximum phase, while the time lag between them is longer in the maximum phase. The remarkable correlations of Vsw–Kp and in the descending phase indicate appropriate magnetospheric convection in association with the repetitive substorms that can effectively trigger the stochastic mechanisms of electron acceleration. The results are extensively discussed in the light of observations and current theories of radiation belt dynamics.

Within the next couple of years, the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) will start the deployment of its next-generation geostationary meteorological satellites. The Meteosat Third Generation (MTG) is composed of four imaging (MTG-I) and two sounding (MTG-S) platforms. The satellites are three-axis stabilized, unlike the two previous generations of Meteosat that were spin stabilized, and carry two sets of remote sensing instruments each. Hence, in addition to providing continuity, the new system will provide an unprecedented capability from geostationary orbit. The payload on the MTG-I satellites are the 16-channel Flexible Combined Imager (FCI) and the Lightning Imager (LI). The payloads on the MTG-S satellites are the hyperspectral Infrared Sounder (IRS) and a high-resolution Ultraviolet–Visible–Near-Infrared (UVN) sounder Sentinel-4/UVN, provided by the European Commission. Today, hyperspectral sounding from geostationary orbit is provided by the Chinese Fengyun-4A (FY-4A) satellite Geostationary Interferometric Infrared Sounder (GIIRS) instrument, and lightning mappers are available on FY-4A and on the National Oceanic and Atmospheric Administration (NOAA) GOES-16 and GOES-17 satellites. Consequently, the development of science and applications for these types of instruments have a solid foundation. However, the IRS, LI, and Sentinel-4/UVN are a challenging first for Europe in a geostationary orbit. The four MTG-I and two MTG-S satellites are designed to provide 20 and 15.5 years of operational service, respectively. The launch of the first MTG-I is expected at the end of 2022 and the first MTG-S roughly a year later. This article describes the four instruments, outlines products and services, and addresses the evolution of the further applications.

We have studied the solar cycle variation of equatorial plasma mass density ρeq in the plasma trough at geosynchronous altitude. The density was indirectly determined from the frequency, fT3, of the third harmonic of toroidal standing Alfvén waves detected over a 12 year period from 1980 to 1991 with magnetometers on five Geostationary Operational Environmental Satellites (GOES). Realistic models of the ambient magnetic field and field line mass distribution were used in numerically solving the wave equation to relate fT3 to ρeq. Scanning the magnetometer data in a 30 min time window that moved forward in 10 min steps, we obtained 228,382 fT3 samples equivalent to 1586 days of data. The detection rate of fT3 is highest (∼50%) in the prenoon sector, and fT3 and ρeq samples from this sector were used to examine their dependence on F10.7, Kp, and Dst. Overall, F10.7 exhibits the highest correlation with fT3 and ρeq, implying that the solar UV/EUV control of ion production at the ionospheric height is strongly reflected in mass density variations at geosynchronous orbit. Using 27 day medians computed excluding periods of plasmasphere expansion to geosynchronous orbit and geomagnetic storm, we obtained the empirical formula fT3 (mHz) = 38 − 0.097F10.7 and logρeq (amu cm−3) = 0.42 + 0.0039F10.7, where F10.7 is given in the solar flux units 10−22 W · m−2 · Hz−1. This last formula means that with the 27 day F10.7 in the range of 68–255 in the selected solar cycle, the mass density varied by a factor of ∼5 from ∼5 to ∼26 amu cm−3. During extremely quiet times (Kp averaged using a 3 day time scale <1), for which the plasmasphere may extend out to geosynchronous orbit, and during storm periods (Dst < −50 nT), the mass density may be enhanced beyond these values.

A model and a software module were developed to analyze the shape of the trajectory described by the subsatellite point on the Earth’s surface depending on eccentricity, perigee angle, and inclination of the geosynchronous orbit (GEO). For various parameters of the satellite’s orbit and synthetic aperture radar (SAR) equipment, we built the dependences of the geometric resolution in the longitudinal (azimuthal) and transverse (relative to the trajectory of the subsatellite point) direction, as well as the radiometric sensitivity of a SAR operating in GEO in the bistatic oblique quasi-specular mode of wave reflection from the Earth’s surface. The optimal time shifts of the orbital positions of two satellites constituting a bistatic pair were found, at which the specified restrictions on the SAR parameters are fulfilled for observation points in a wide area of ​​the Earth’s surface for a long time interval. A generalized assessment of a set of SAR parameters (transmitter and receiver antenna areas, size of the resolution element, and average transmitter power) that provides a given level of radiometric sensitivity is performed.