A Color Restoration Algorithm for Diffractive Optical Images of Membrane Camera.

To achieve the demand of earth observation with high spatiotemporal resolution and large scale, a novel optical imaging system is planning to equip on Chinese next generation geo-stationary earth orbit (GEO) satellite. The primary mirror of camera is made of a set of thin films and adopts the diffraction imaging mechanism. A series of ground experiments have been carried out using a thin-film camera with 80 mm aperture for the technical verification. The inherent chromatic aberration due to diffraction imaging appears in the obtained data. To address the issue, a color restoration algorithm by matching, tailoring and linearly stretching the histograms is proposed in this paper. Experimental results show the proposed approach has good performances in color restoration of the diffractive optical images. The effectiveness and robustness of the algorithm are also assessed with various color deviation indexes.

Image degradation characteristics and restoration based on regularization for diffractive imaging

The higher harmonic terms of Earth’s gravitational potential slowly modify the nominal longitude of geostationary Earth orbit (GEO) satellites, while the third-body presence (Moon and Sun) mainly affects their latitude. For this reason, GEO satellites periodically need to perform station-keeping maneuvers, namely, east–west and north–south maneuvers to compensate for longitudinal and latitudinal variations, respectively. During the operational lifetime of GEO satellites, the thrusters’ response when commanded to perform these maneuvers slowly departs from the original nominal impulsive behavior. This paper addresses the practical problem of how to perform reliable east–west station-keeping maneuvers when thruster response is degraded. The need for contingency intervention from ground-based satellite operators is reduced by breaking apart the scheduled automatic station-keeping maneuvers into smaller maneuvers. Orbital alignment and attitude are tracked on-board during and in between sub-maneuvers, and any off nominal variations are corrected for with subsequent maneuvers. These corrections are particularly important near the end of the lifetime of GEO satellites, where thruster response is farthest from nominal performance.

Frame synchronization time of geostationary Earth orbit (GEO) satellites is faster than that of incline geosynchronous satellite orbit and medium Earth orbit satellites in Beidou navigation satellite system (BDS). So time to first fix can be reduced greatly with GEO satellites (GEOs) to provide emergency positioning service during the cold start process. Meanwhile, GEOs can also be utilized to provide radio determination satellite services. Therefore, it is meaningful to research the emergency positioning algorithm based on GEOs. However, due to the poor geometry of the GEOs, the availability and accuracy degrade distinctly. To solve this problem, a low-cost high resolution MS5534B barometric (baro-) module is applied to BDS. Firstly, the principle of emergency positioning based on a baro-altimeter sensor and its performance such as the accuracy are elaborated. Then the effects of baro-altimeter sensor measurement error on positioning are analyzed. Finally, after analyzing the limitation of the conventional algorithms, a new high-accuracy emergency positioning algorithm with baro-altimeter sensor aiding is proposed, which is not limited by the integration and user's altitude. In comparison to the conventional algorithms, the stimulant and experimental results validate the effectiveness, robustness, and higher performance of the proposed algorithms incorporated with sensors.

The current constellation of the BeiDou Navigation Satellite System (BDS) consists of five geostationary earth orbit (GEO) satellites, five inclined geosynchronous satellite orbit (IGSO) satellites, and four medium earth orbit (MEO) satellites. The advantage of using GEO satellites to monitor the ionosphereis the almost motionless ionospheric pierce point (IPP), which is analyzed in comparison with the MEO and IGSO satellites. The results from the analysis of the observations using eight tracking sites indicate that the ionospheric total electron content (TEC) sequence derived from each GEO satellite at their respective fixed IPPs is always continuous. The precision of calculated vertical TEC (VTEC) using BDS B1/B2, B1/B3, and B2/B3 dual-frequency combinationsis compared and analyzed. The VTEC12 precision based on the B1/B2 dual-frequency measurements using the smoothed code and the raw code combination is 0.69 and 5.54 TECU, respectively, which is slightly higher than VTEC13 and much higher than VTEC23. Furthermore, the ionospheric monitoring results of site JFNG in the northern hemisphere, and CUT0 in the southern hemisphere during the period from 1 January to 31 December 2015 are presented and discussed briefly.

Free-space optical (FSO) communication importance has increased during the last two decades, presenting its unique features like license-free spectrum, high data rate, ease of deployability, and low power consumption. Moreover, In the future, communication between different space orbits like Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO) satellites will be widely established by utilizing Inter-Satellite Optical Wireless Communication (IsOWC) systems that allow GEO satellites to relay information to and from LEO satellites and fixed Earth stations, otherwise, information will not be permanently relayed. IsOWC systems are the most important application of the FSO and they will be preferred shortly because of their features. In this paper, we test the IsOWC system by varying some parameters that have a significant influence on the IsOWC link such as wavelength, Pulse generator at the transmitter side, and photodetector type at the receiver side to obtain the maximum Quality factor at minimum allowable bit error rate (BER) utilizing IsOWC between LEO and GEO satellites up to 45000 km link.

Enabling completely universal coverage, the space-ground communication system integration is one of the most important aspects in the fifth generation (5G) or even the next 6G wireless communications, which significantly benefits railways whose transportation lines always cross diverse environments. Based on this observation, to achieve seamless coverage for environment-diverse high-speed railways (HSRs), by leveraging the control/user-plane (C/U-plane) decoupling and cloud radio access network (C-RAN) technologies, we propose a space-ground integrated cloud railway network consisting of space and ground cloud layers, where in the space, baseband units (BBUs) of low earth orbit (LEO) satellites are collected and centrally-managed by geostationary earth orbit (GEO) satellites. To improve the mobility support and take advantage of the stable and ultra-wide terrestrial coverage of GEO satellites, we establish an additional backup space C-plane (BS-C-plane) connection between trains and GEO satellites. Under this architecture with diverse network resources, we develop a safety-oriented resource allocation scheme based on both the resource allocation priority of safety services and the network handover costs to deliver the safety-oriented services. Simulation results demonstrate that the proposed scheme can always meet the transmission requirements for safety services in HSRs.

This paper proposes a double-layered satellite network based on the rateless code, which can globally distribute the information with a few satellites. The satellite network is composed of Geostationary Earth Orbit (GEO) and Low Earth Orbit (LEO) satellites. The GEO satellites which cover the low and middle latitudes serve as the backbone network, whereas the LEO satellites are considered the enhanced network to make up for the shortage of the backbone network. Due to the interruption tolerance of the rateless code, only a few satellites are employed to achieve worldwide information distribution in the design of the satellite network. The coverage performance and the average elevation angle of the user are simulated, and results demonstrate that the network can achieve globally information distribution.

In this paper, a collision avoidance maneuver was sought for low Earth orbit (LEO) and geostationary Earth orbit (GEO) satellites maintained in a keeping area. A genetic algorithm was used to obtain both the maneuver start time and the delta-V to reduce the probability of collision with uncontrolled space objects or debris. Numerical simulations demonstrated the feasibility of the proposed algorithm for both LEO satellites and GEO satellites.

This paper investigates the potential for interference from multipoint video distribution systems (MVDS) transmissions, specifically side lobe radiation in Lebanon, to geostationary Earth orbit (GEO) satellites. Through simulation and analysis of antenna radiation patterns, the impact of varying MVDS power levels on the carrier-to-noise ratio (C/N) at the satellite receiver is quantified. The results demonstrate a significant degradation in signal quality, with the C/N dropping to −2.29 dB at an MVDS power of 0 dBW for the current system. To mitigate this interference, a two-step potential strategy is proposed and evaluated. The study boosts the potential for the coexistence of MVDS and GEO satellite services in the Ku-band within the Lebanese context.

Using GNSS observable from some stations in the Asia-Pacific area, the carrier-to-noise ratio (CNR) and multipath combinations of BeiDou Navigation Satellite System (BDS), as well as their variations with time and/or elevation were investigated and compared with those of GPS and Galileo. Provided the same elevation, the CNR of B1 observables is the lowest among the three BDS frequencies, while B3 is the highest. The code multipath combinations of BDS inclined geosynchronous orbit (IGSO) and medium Earth orbit (MEO) satellites are remarkably correlated with elevation, and the systematic “V” shape trends could be eliminated through between-station-differencing or modeling correction. Daily periodicity was found in the geometry-free ionosphere-free (GFIF) combinations of both BDS geostationary Earth orbit (GEO) and IGSO satellites. The variation range of carrier phase GFIF combinations of GEO satellites is −2.0 to 2.0 cm. The periodicity of carrier phase GFIF combination could be significantly mitigated through between-station differencing. Carrier phase GFIF combinations of BDS GEO and IGSO satellites might also contain delays related to satellites. Cross-correlation suggests that the GFIF combinations’ time series of some GEO satellites might vary according to their relative geometries with the sun.

Beidou satellites, especially geostationary earth orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites, need to be frequently maneuvered to keep them in position due to various perturbations. The satellite ephemerides are not available during such maneuver periods. Precise estimation of thrust forces acting on satellites would provide continuous ephemerides during maneuver periods and could significantly improve orbit accuracy immediately after the maneuver. This would increase satellite usability for both real-time and post-processing applications. Using 1 year of observations from the Multi-GNSS Experiment network (MGEX), we estimate the precise maneuver periods for all Beidou satellites and the thrust forces. On average, GEO and IGSO satellites in the Beidou constellation are maneuvered 12 and 2 times, respectively, each year. For GEO satellites, the maneuvers are mainly in-plane, while out-of-plane maneuvers are observed for IGSO satellites and a small number of GEO satellites. In most cases, the Beidou satellite maneuver periods last 15–25 min, but can be as much as 2 h for the few out-of-plane maneuvers of GEO satellites. The thrust forces acting on Beidou satellites are normally in the order of 0.1–0.7 mm/s2. This can cause changes in velocity of GEO/IGSO satellites in the order of several decimeters per second. In the extreme cases of GEO out-of-plane maneuvers, very large cross-track velocity changes are observed, namely 28 m/s, induced by 5.4 mm/s2 thrust forces. Also, we demonstrate that by applying the estimated thrust forces in orbit integration, the orbit errors can be estimated at decimeter level in along- and cross-track directions during normal maneuver periods, and 1–2 m in all the orbital directions for the enormous GEO out-of-plane maneuver.

How to explore many priors of the underlying image for diffraction imaging, i.e., recovering the image from recorded diffraction patterns, is an important issue. To address this issue, we present a sparse regularization model called sparse regularization model induced by block-matching and 3-D filtering (SBM3D). The proposed SBM3D exploits the sparsity of the residual image (difference image between the underlying image and its filtered version). The filtered image can be obtained by filtering the estimated image. The proposed regularization model is suitable for any inverse imaging problem. Specially, we incorporate the proposed SBM3D into the diffraction imaging field. Inspired by the effectiveness of the reweighted approach in compressed sensing, we formulate the weighted and regularized diffraction imaging problem. The smooth technique and the accelerated gradient descent approach with an adaptive step size are utilized to solve this challenging problem. Experimental simulations for coded diffraction imaging demonstrate that the proposed diffraction imaging algorithm outperforms the previous algorithms in terms of the reconstruction quality.

Geophysical diffraction tomography

We have conducted a series of low-velocity impact experiments to understand the dispersion properties of fragments newly created by low-velocity impacts possible in space, especially in geostationary Earth orbit. The test results are utilized to establish a mathematical prediction model to be used in debris generation and propagation codes. Since the expected collision velocity between catalogued objects in geostationary Earth orbit shows a peak at a few hundreds meters per second, these impact experiments were conducted at a velocity range lower than 300 m/s. As a typical structure of satellites in geostationary Earth orbit, thin aluminum honeycomb sandwich panels with carbon fiber reinforced plastics face sheets were prepared, while the projectile was a stainless steel ball of 9 mm diameter. The data collected through these impact experiments have been re-analyzed based on the method used in the National Aeronautics and Space Administration (NASA) standard breakup model 1998 revision. The results indicate that the NASA standard breakup model derived from hypervelocity impacts could be applied to low-velocity collision possible in geostationary Earth orbit with some modifications.