Geosynchronous Earth Orbit Satellite Research Articles

Stingray Sensor System for Persistent Survey of the GEO Belt.

The Stingray sensor system is a 15-camera optical array dedicated to the nightly astrometric and photometric survey of the geosynchronous Earth orbit (GEO) belt visible above Tucson, Arizona. The primary scientific goal is to characterize GEO and near-GEO satellites based on their observable properties. This system is completely autonomous in both data acquisition and processing, with human oversight reserved for data quality assurance and system maintenance. The 15 ZWO ASI1600MM Pro cameras are mated to Sigma 135 mm f/1.8 lenses and are controlled simultaneously by four separate computers. Each camera is fixed in position and observes a 7.6-by-5.8-degree portion of the GEO belt, for a total of a 114-by-5.8-degree field of regard. The GAIA DR2 star catalog is used for image astrometric plate solution and photometric calibration to GAIA G magnitudes. There are approximately 200 near-GEO satellites on any given night that fall within the Stingray field of regard, and all those with a GAIA G magnitude brighter than approximately 15.5 are measured by the automated data reduction pipeline. Results from an initial one-month survey show an aggregate photometric uncertainty of 0.062 ± 0.008 magnitudes and astrometric accuracy consistent with theoretical sub-pixel centroid limits. Provided in this work is a discussion of the design and function of the system, along with verification of the initial survey results.

Range-Doppler-Time Tensor Processing for Deep-Space Satellite Characterization Using Narrowband Radar

There is growing demand for the high-fidelity characterization of satellites in Geosynchronous Earth Orbit (GEO) to support Space Domain Awareness (SDA). This is particularly true for newly launched satellites, where it is necessary for satellite providers to ascertain whether components have deployed properly. Conventional wideband radar systems are capable of imaging satellites provided that (i) they have sufficient power aperture and bandwidth, and (ii) they observe enough target aspect change to generate a resolved image. While wideband radars are used routinely for characterizing satellites in Low-Earth Orbit (LEO), powerful radars with sensitivity sufficient for large GEO ranges (36,000 km or greater) are lacking. Thus, researchers often rely on more widely available high-power narrowband tracking radars for GEO characterization. In this paper, we present a novel range-Doppler-time (RDT) tensor processing technique for GEO characterization with narrowband radar. This technique encapsulates the strengths of previously proposed methods for narrowband-radar characterization at GEO, providing a generalized approach that can be applied in a variety of settings. The technique generates fully resolved 2D images of rotating GEO satellites in low-bandwidth scenarios. In cases where aspect change is limited, the technique provides detailed Doppler information for enhanced satellite status monitoring. This work presents a comprehensive quantitative analysis of the technique that considers the impact of key parameters on characterization performance. Simulated radar data, and radar data collected in a compact range on a scaled satellite model, are used to evaluate the technique.

FlexBeamOpt: Hybrid solution methodologies for high‐throughput GEO satellite beam laydown and resource allocation

SummaryModern satellite communication systems are required to serve heterogeneous and geographically dispersed user demands with limited resources. In this paper, we investigate methodologies for dynamic resource allocation in Geosynchronous Earth Orbit (GEO) High‐throughput Satellite (HTS) systems. We designed three solution approaches FlexBeamOpt v1, FlexBeamOpt v2, and FlexBeamOpt v3, each as a hybridization of custom heuristics, integer linear programming, and/or constraint programming. We test the performance of the three approaches on 12 test instances that vary in user distribution (realistic, random, and clustered), user numbers (500 vs. 5000 users), and demand distribution (uniform vs. random). We observed that FlexBeamOpt v1 consistently outperformed FlexBeamOpt v2 and FlexBeamOpt v3 in terms of demand coverage and number of users covered for realistic and random user distribution test instances but at the cost of computation time. FlexBeamOpt v3 is the fastest in these instances. For clustered user distribution instances, FlexBeamOpt v3 performed better in terms of demand coverage and number of users covered, at the cost of using more beams. For these test instances, FlexBeamOpt v2 is the fastest in terms of computation time while providing a comparable solution quality.

Anomalous Disturbance of the Ionosphere in the East Asia Region During the Geomagnetically Quiet Day on 1 November 2016

AbstractIn this study, multiple data sets from Beidou geosynchronous Earth orbit (GEO) satellite, ionosonde, magnetometer, Swarm satellite, and meteor radar were used to investigate an anomalous disturbance of the ionosphere in East Asia on 1 November 2016. During 03:00–08:00 universal time (UT), the total electron content (TEC) observed by the Beidou GEO satellite decreased by up to 8.17% at low latitudes and increased by up to 140.19% at middle to low latitudes in East Asia compared to the average TEC on the reference days. The F2 peak electron density (NmF2) measured by ionosondes decreased by 16.00% at Sanya (109.4°E, 18.3°N) and increased by 51.96% and 53.97% at Wuhan (114.4°E, 30.5°N) and Shaoyang (111.3°E, 27.1°N), respectively. The equatorial electric field (EEF) observed by the Swarm satellite at 06:59 UT on 1 November was approximately 77.57% higher than on 4 November. The variations of equatorial electrojet near 100°E and 120°E on 1 November presented an east‐west asymmetry in which there was a significant enhancement in the east while there was no obvious increase in the west. The TEC enhancement derived by the Center for Orbit Determination in Europe also showed the same east‐west asymmetry in the two longitudes. Based on the comprehensive analysis of multiple observational data, the increase in EEF caused the ionospheric TEC and NmF2 to decrease at low latitudes and increase at middle to low latitudes in East Asia during 03:00–08:00 UT on 1 November 2016. The lower atmospheric tidal forcing could contribute to the enhancement of the EEF.

Analysis on navigation accuracy of high orbit spacecraft based on BDS GEO and IGSO satellites

To make up for the insufficiency of earth-based TT&C systems, the use of GNSS technology for high-orbit spacecraft navigation and orbit determination has become a new technology. It is of great value to applying Geosynchronous Earth Orbit (GEO) and Inclined GeoStationary Orbit (IGSO) navigation satellites for supporting the navigation of high-orbit spacecraft since there are three different types of navigation satellites in BeiDou Navigation Satellite System (BDS): Medium Earth Orbit (MEO), GEO and IGSO. This paper conducts simulation experiments based on Two-Line Orbital Element (TLE) data to analyze and demonstrate the role of these satellites in the navigation of high-orbit spacecraft. Firstly, the spacecraft in GEO was used as the target satellite to conduct navigation experiments. Experiments show that for the spacecraft on the GEO orbit, after adding GEO and IGSO respectively on the basis of receiving MEO navigation satellite signals, the accuracies were improved by 7.22 % and 6.06 % respectively. When adding both GEO and IGSO navigation satellites at the same time, the accuracy can reach 16 m. In the second place, navigation and positioning experiments were carried out on three high elliptical orbit (HEO) satellites with different semimajor axis (32037.2 km, 42385.9 km, 67509.6 km). The experiments show that the number of visible satellites has been improved significantly after adding GEO and IGSO navigation satellites at the same time. The visible satellites in these three orbits were improved by 32.84 %, 41.12 % and 37.68 %, respectively compared with only observing MEO satellites.The RMS values of the navigation positioning errors of these three orbits are 25.59 m, 87.58 m and 712.48 m, respectively.

Improving SINR via joint beam and power management for GEO and LEO spectrum-sharing satellite communication systems

In this paper, we investigate a geosynchronous earth orbit (GEO) and low earth orbit (LEO) coexisting satellite communication system. To decrease the interference imposed on the GEO user caused by LEO satellites, we propose a joint beam-management and power-allocation (JBMPA) scheme to maximize signal-to-interference plus noise ratio (SINR) at the GEO user, whilst maintaining the ongoing wireless links spanning from LEO satellites to their corresponding users. Specifically, we first analyze the overlapping coverage among GEO and LEO satellites, to obtain the LEO-satellite set in which their beams impose interference on the GEO user. Then, considering the traffic of LEO satellites in the obtained set, we design a beam-management method to turn off and switch interference beams of LEO satellites. Finally, we further propose a deep Q-network (DQN) aided power allocation algorithm to allocate the transmit power for the ongoing LEO satellites in the obtained set, whose beams are unable to be managed. Numerical results show that comparing with the traditional fixed beam with power allocation (FBPA) scheme, the proposed JBMPA can achieve a higher SINR and a lower outage probability, whilst guaranteeing the ongoing wireless transmissions of LEO satellites.

Improving BDS broadcast ephemeris accuracy using ground-satellite-link observations

The BeiDou Navigation Satellite System (BDS) employs a hybrid constellation including GEO (Geosynchronous Earth Orbit), IGSO (Inclined Geosynchronous Orbit), and MEO (Medium Earth Orbit) satellites, where the GEO and IGSO satellites are critical to providing continuous and reliable Positioning, Navigation, and Timing (PNT) services in the Asia–Pacific region. To handle the inconsistency between the satellite orbits and clocks in the broadcast ephemeris, which are determined by the Orbit Determination and Time Synchronization (ODTS) and the Two-way Satellite Time Frequency Transfer (TWSTFT) technique, respectively, we present the strategies using ground-satellite-link observations to improve the accuracy of broadcast ephemeris. The clock differences between the ODTS and TWSTFT techniques are used for correcting the radial orbit component to derive the refined orbits, which are used to generate the refined broadcast ephemeris. The test results show the precision of the refined orbits is improved by 50–60% in the 3-h to 12-h predicted arcs for the GEO satellites, and by 40–50% for the IGSO satellites. Moreover, the validation using satellite laser ranging observations shows the mean precision of the refined broadcast ephemeris is improved by 27% compared to the original one. Applying the proposed strategies in the BDS Operational Control Segment (OCS), the time evolution of BDS Single Point Positioning (SPP) in the period from Jan. 2016 to April 2021 is evaluated. The SPP accuracy is improved from 1.94, 2.06 and 3.29 m to 1.39, 1.85, and 2.39 m in the north, east, and up components, respectively. Further update with the inclusion of BDS-3 satellites improve the corresponding SPP precision to 0.68, 0.70 and 1.91 m.

Resonance-Averaged Solar Torque Dynamics for Tumbling Satellites

Observations indicate that the spin states of retired and otherwise defunct satellites are diverse and can change significantly over time. Understanding defunct satellite spin state evolution is important for solar radiation pressure modeling, active debris removal, satellite servicing, and space situational awareness. Research has shown that many defunct satellites in geosynchronous earth orbit (GEO) are primarily driven by solar radiation torques via the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. Recent exploration of YORP-driven nonprincipal axis tumbling has uncovered rich dynamic behavior, including tumbling cycles, spin–orbit coupling, and tumbling period resonances. Radar and optical observations of the defunct Geostationary Operational Environmental Satellite (GOES) 8 strongly suggest that it has been captured in a tumbling resonance at least once since 2018. Motivated by these findings, we develop a numerical resonance-averaged dynamic model to understand YORP-driven resonance capture, building on our earlier nonresonant averaging framework. This resonance-averaged model illuminates resonance capture mechanisms. Also, by averaging over the satellite’s rotation, this model is roughly 20 times faster to propagate than the full (Euler) dynamics. Overall, this allows for rapid, broad exploration to determine resonance capture durations, spin states changes, and the influence of initial spin rate and resonance order on resonance strength. This resonance-averaged model can be easily applied to other environmental perturbations.

Transmission of photonic polarization states from geosynchronous Earth orbit satellite to the ground

Geosynchronous Earth orbit (GEO) satellites can extend the transmission distance of the quantum states and the intercrossing time between the satellites and the ground, which will greatly promote the application spatial scale of quantum communications. In this work we report the first experimental test of transmission of photonic polarization states from a GEO SJ-20 satellite at more than 36,000 km distance to the Li-jiang ground station at Yun-nan Observatory. The test equipment carried on the satellite can send four kinds of photonic polarization states at 50 MHz pulse frequency. A 1.76 m telescope and related optical system in the ground station are used to receive the photonic polarization states, and then these polarization states are probed by the superconducting nanowire single photon detector. The test results show that the optical attenuation between the GEO satellite and the ground station is 100 ˜ 110 dB and the channel bit error rate is 2%–8%, which meet the security criteria of quantum key distribution and quantum secure direct communication.

Coastal Altimetry Using Interferometric Phase From GEO Satellite in Quasi-Zenith Satellite System

Global navigation satellite system reflectometry (GNSS-R) altimetry has great potential to provide high spatial–temporal resolution sea surface heights (SSHs) at low cost. Interferometric phase measurements between direct and reflected signals can be used for altimetry retrieval to achieve high-precision solutions. The motions of the medium Earth orbit (MEO) satellites cause interferometric phase change rapidly, which would increase the probability of occurrence of the phase unwrapping errors than the case of the geosynchronous Earth orbit (GEO) satellite. In order to overcome this problem, we propose a coastal GNSS-R altimetry algorithm using the signals from Quasi-Zenith Satellite System (QZSS) GEO satellite. Precise SSH variations can be achieved using the interferometric phase measurements without ambiguity fixed. We also perform coastal experiments on a trestle using a specialized GNSS-R setup to verify our algorithm. It is composed of an intermediate frequency (IF) data collector and two antennas. The up-looking antenna is used to receive direct signals, while the down-looking antenna receives the signals reflected from the sea surface. Raw IF data sampled at 62 MHz are collected and processed to derive interferometric carrier phase delay measurements using a self-developed software-defined receiver. Approximately 7 h of reflector heights are retrieved at 1-min intervals and the solutions are evaluated via comparison with measurements provided by a 26-GHz altimetry radar located near the GNSS-R setups. The results show that the root mean square error (RMSE) of sea level estimation is about 1.4 cm by using the QZSS GEO data.

The improvement of BDS Observation Geometry with LEO constellations in Orbit Determination

The ability of low earth orbit (LEO) satellites with space-borne global navigation satellite system (GNSS) to improve the accuracy of GNSS satellite orbit determinations has been demonstrated. In this paper, the ability of LEO satellites to improve the observation geometry in the process of the determination of BeiDou Navigation Satellite System (BDS) satellite orbits is the focus. First, the visible BDS satellite model with ground stations and LEO satellites is derived, and the definition of the position dilution of precision (PDOP) is proposed for assessing the observation geometries of BDS satellite orbit determination. Then, the observation geometry of BDS geosynchronous earth orbit (GEO), inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites are individually analysed by regional ground stations. By designing LEO satellites with different inclinations, altitudes and right ascensions of ascending nodes (RAANs), the PDOP improvements achieved through combinations with different orbital LEO satellites are calculated. Finally, the PDOP improvements caused by incorporating different numbers of LEO constellations are compared. The results show that LEO satellites can improve the observation geometries of BDS satellites, especially when the monitoring of observation configurations by only ground stations is very poor. The PDOP improvements of BDS GEO satellites caused by LEO satellites are more significant than those of IGSO or MEO satellites; i.e., the improvement percentage of C04 is better than 60%. Generally, LEO satellites with high altitudes are more suitable for PDOP improvements to BDS satellites than are LEO satellites with low altitudes; moreover, LEO satellites with high inclinations are more suitable for PDOP improvements to BDS GEO satellites than LEO satellites with low inclinations are. The influence of the RAAN on the PDOP improvements of IGSO and MEO satellites is relatively larger than its influence on the PDOP improvement of GEO satellites. The PDOP improvement percentage and stability of BDS satellites, especially GEO satellites, increase with the number of incorporated LEO satellites. When incorporating the LEO constellation 16/4/1:1000 km, 70°, the maximum improvements of the GEO and IGSO satellites are above 84% and 77%. The improvements of the GEO, IGSO and MEO satellites are above 81%, 42% and 58% respectively.

Averaged Solar Torque Rotational Dynamics for Defunct Satellites

Spin state predictions for defunct satellites in geosynchronous Earth orbit (GEO) are valuable for active debris removal and servicing missions as well as material shedding studies and attitude-dependent solar radiation pressure (SRP) modeling. Previous studies have shown that solar radiation torques can explain the observed spin state evolution of some GEO objects via the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. These studies have focused primarily on uniform rotation. Nevertheless, many objects are in nonprincipal axis rotation (i.e., tumbling). Recent exploration of the tumbling regime for the family of retired Geostationary Operational Environmental Satellite (GOES) 8–12 satellites has shown intriguing YORP-driven behavior including spin-orbit coupling, tumbling cycles, and tumbling period resonances. To better explore and understand the tumbling regime, a semi-analytical tumbling-averaged rotational dynamics model has been developed. The derivation requires analytically averaging over the satellite’s torque-free rotation, defined by Jacobi elliptic functions. Averaging is facilitated by a second-order Fourier series approximation of the facet illumination function. The averaged model is found to capture and explain the general long-term behavior of the full dynamics while reducing computation time by roughly three orders of magnitude. This improved computation efficiency promises to enable rapid exploration of general long-term rotational dynamics for defunct satellites and rocket bodies.

Analysis of BDS GEO satellite multipath effect for GNSS integrity monitoring in civil aviation

With the great development of Global Navigation Satellite System (GNSS), multi-GNSS constellations (GPS, BDS, GLONASS, and Galileo) are able to provide users with more accurate positioning result. For civil aviation, to guarantee user’s safety, multi-constellation GNSS needs to meet the integrity requirement. Using conservative error models, Multiple Hypothesis Solution Separation (MHSS) Advanced Receiver Autonomous Integrity Monitoring (ARAIM) is proposed to evaluate GNSS integrity. Current multipath error model in ARAIM algorithm is based on data of GPS Medium Earth Orbit (MEO) satellites. But BDS is a hybrid constellation. For BDS II, there has 5 Geosynchronous Earth Orbit (GEO) satellites. Previous studies have shown that the multipath effect of GEO satellites has statistical characteristics different from MEO satellites. Meanwhile, the multipath magnitude of GEO satellites is larger than that of MEO satellites. This paper mainly focuses on validating whether the multipath error model in ARAIM algorithm is conservative enough for GEO satellites. In this paper, Code Minus Carrier (CMC) residuals are calculated for BDS GEO dual-frequency signals. Then the Standard Deviation (STD) of CMC residuals can be conservatively estimated by bounded Cumulative Distribution Function (CDF). After eliminating interference from receiver noise, STD of GEO multipath can be obtained. Comparing the STD of GEO multipath effect with ARAIM multipath error model, a conclusion could be drawn that current multipath error model in ARAIM algorithm is no longer able to conservatively estimate the statistical characteristics of GEO satellites.

Design of Laser Ranging Device Using the Improved Photodetector and Its Usage in Geosynchronous Earth Orbit Navigation Satellite Orbit Determination

The geosynchronous earth orbit (GEO) satellites have good coverage performance and are widely used in WAAS, BDS, CAPS and other regional augmentation and regional navigation systems. At the same time, the precise orbit determination and prediction of such satellites play a significant role in high-precision navigation and user real-time positioning. In order to obtain higher accuracy of orbit determination, the laser ranging device is improved by equipping with a silicon-substrate germanium MSM photodetector in this study. In addition, the surface plasmon resonance augmentation effect is further studied to further enhance the photoelectric performance of the silicon-substrate germanium MSM photodetector. The detector is connected to the OPA657. The corresponding pre-amplified circuit is further designed in this study so that the laser ranging device can be used for the orbit determination application of GEO navigation satellites. In the experiment, the designed silicon-substrate germanium MSM photodetector is tested firstly, the finite-different time-domain (FDTD) method is used to analyze the structure of the photodetector. Then, the effects of the structural parameters such as the grating period on the resonance augmentation of the designed photodetector are analyzed. The results reveal that the photodetector has the best performance at 1500 nm with the absorption enhancement factor of higher than 7. The GNSS combined with the laser ranging is used for comparing the orbit determination errors of GEO satellites. 10 laser observation stations are selected, some of which are equipped with the laser ranging device designed in this study and supply to various GEO satellites for information collection. The results show that GEO satellites have to be introduced to the system deviation when adding the laser ranging data, otherwise they will deviate from the orbit. In addition, the laser ranging device designed in this study can significantly reduce the deviation caused by the introduction of laser ranging data from GEO satellites compared with traditional laser ranging devices.

Evaluation of BDS-2 real-time orbit and clock corrections from four IGS analysis centers

Real-time precise orbit/clock product is fundamental to BeiDou Navigation Satellite System (BDS) real-time precise point positioning (PPP). In order to explore achievable performance of BDS real-time orbit/clock products and thus offer usage recommendations for BDS real-time PPP users, this paper studies BDS real-time orbit/clock corrections provided by four International GNSS Service (IGS) Analysis Centers (ACs), i.e. Wuhan University (WHU), Deutsches Zentrum für Luft- und Raumfahrt (DLR), GeoForschungsZentrum Potsdam (GFZ) and Centre National d’ Etudes Spatiales (CNE) for seven consecutive days staring from day of year (DOY) 324, 2019. Firstly, the availability of BDS-2 real-time orbit/clock corrections is studied. Larger than 85% availabilities are identified for all ACs. During the test period, DLR does not provide all Geosynchronous Earth Orbit (GEO) satellites’ corrections, whereas CNE does not provide C14/C16′s corrections. Secondly, the BDS real-time orbit accuracy is analyzed. Results indicate that the real-time orbit accuracy of GEO satellites is at meter level, and those of Inclined Geosynchronous Satellite Orbit (IGSO)/Medium Earth Orbit (MEO) satellites are at decimeter level. Among all ACs, the accuracy of CNE’s three-dimensional (3D) orbit correction is the best, which is 2.391/0.278/0.271 m for GEO/IGSO/MEO satellites, respectively. DLR has the largest orbit errors of 1.116/0.504 m for IGSO/MEO satellites. Thirdly, the BDS real-time clock offset precision is investigated. On the one hand, CNE’s real-time corrections still perform best with 1.19/0.28/0.32 ns clock offset precision for GEO/IGSO/MEO satellites, respectively. On the other hand, DLR’s real-time clock is the worst with 2.61/1.91 ns IGSO/MEO clock offset precision. Finally, all corrections are applied to estimate 47 Multi-GNSS Experiment (MGEX) station’s BDS-2 real-time PPP on DOY 330, 2019. Results show an average of 0.53–1.46 h convergence time and 8.8–10.9 cm positioning accuracy in the static mode. In the meantime, the average convergence time in the kinematic mode is 2.11–9.84 h, much longer than the static counterpart. The corresponding converged positioning accuracy is 30.7–68.0 cm. To sum up, WHU/GFZ/CNE’s real-time corrections are proven with satisfied orbit/clock performance, thus are recommended for BDS-2 real-time PPP.

GOES spin state diversity and the implications for GEO debris mitigation

Many defunct satellites in geosynchronous earth orbit (GEO) spin rapidly or have highly evolving spin states, with some transitioning between uniform and non-principal axis rotation (tumbling). It is hypothesized that the observed evolution of some defunct GEO satellites is caused by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect and internal energy dissipation. YORP torques are generated by the absorption, reflection, and thermal re-emission of solar radiation and are known to cause secular changes in asteroid spin rates and obliquities. The defunct GOES 8 satellite is particularly notable. This satellite's uniform spin rate rapidly decreased in 2014 and it began tumbling. In this paper, newly analyzed light curve observations of the five nearly identical defunct GOES 8-12 satellites obtained between 2014 and 2018 are presented. The observations show large diversity in evolutionary time histories, with several satellites in consistent slow tumbling, GOES 10 in fast uniform rotation, and GOES 8 transitioning between both. To better understand this diversity, YORP dynamical models are investigated. They reveal that YORP driven spin state evolution is strongly dictated by end of life appendage orientations, which differ among the five satellites. The known end of life configurations are consistent with the observed evolution of GOES 8 and GOES 10. This provides a plausible explanation for the observed spin state diversity. Implications of the observed and simulated evolution for GEO debris mitigation are discussed.

Accuracy Analysis of Ionospheric Prediction Models for Repairing Cycle Slips for BeiDou Triple-Frequency Observations

Both the code–phase combination and the Geometry-Free (GF) phase combination are widely employed to detect and repair cycle slips for BeiDou Navigation Satellite System (BDS) triple-frequency observations. However, the effect of residual ionospheric delay on Narrow-Lane (NL) or GF observations must be considered to avoid incorrect cycle–slip estimation. To improve the accuracy in repairing cycle slips, a corrective ionospheric delay value predicted from the previous ionosphere sequence is used to amend the NL or GF observations at the current epoch. The main purpose of the work reported here is to evaluate the efficacy of a three-step method proposed to detect and repair cycle slip using two extra-wide-lane code–phase and one GF phase combination observations. BDS triple-frequency data were processed in two stages: separate processing of geosynchronous Earth orbit satellites, and the division of inclined geosynchronous satellite orbit and medium Earth orbit satellites into two groups for processing at 30° elevation thresholds. Results revealed that using the prediction models to correct NL or GF observations could ensure a rounding success rate of cycle slip close to 100%, even under high ionospheric activity.

Feasibility of LTE-Based GEO Satellite Mobile Communication Systems to Support VoIP for Handheld Terminals

To facilitate the integration of terrestrial and satellite mobile communication systems in the future, the new generation of satellite mobile communication system should be Long-term evolution (LTE)-based. In LTE, voice is carried by Internet protocol (IP) packets (voice over IP, VoIP). The feasibility of LTE-based Geosynchronous earth orbit (GEO) satellite mobile communication systems for VoIP is discussed. Detailed link budgets are created for typical handheld terminals. The effectiveness of Physical resource block (PRB) bundling and Transmission time interval (TTI) bundling for the capacity improvement of downlink and uplink are evaluated, and further uplink improvement for low level terminals by Narrowband resource block (NRB) scheme is also studied. The analysis results show that the downlink VoIP transmission is mainly power limited, and optimization of PRB or TTI bundling can maximize the capacity. For uplink, only highlevel terminals can support VoIP under the normal LTE frame structure, and TTI bundling and Resource block (RB) block techniques can help the low-level terminals support VoIP transmission.

Six-Year BDS-2 Broadcast Navigation Message Analysis from 2013 to 2018: Availability, Anomaly, and SIS UREs Assessment.

The second-generation of the Beidou Navigation Satellite System (BDS-2) has been officially providing positioning, navigation, and timing (PNT) services within the Asia–Pacific region for six years, starting from 2013. A comprehensive analysis of BDS-2 satellite broadcast navigation message performance during the past six years is highly demanded, not only for the regional service but also for the global service announced in December 2018. Therefore, this study focuses on the performance assessment of six-year BDS-2 broadcast navigation messages from 2013 to 2018 in three aspects: Message availability, anomaly detection, and signal-in-space user range errors (SIS UREs). Firstly, our results, based on International GNSS service (IGS) Multi-GNSS Experiment (MGEX) navigation files, indicate that the BDS-2 Geosynchronous Earth Orbit (GEO) and Inclined Geosynchronous Satellite Orbit (IGSO) satellites have >98.51% broadcast navigation message availability, and the Medium Earth Orbit (MEO) satellites has a ~90.03% availability. Secondly, the comparison between broadcast navigation messages and IGS precise products reveals that the User Range Accuracy Index (URAI) contained in the broadcast message could not reflect satellite performance correctly. Another satellite status indicator, space vehicle (SV) health, can only partially detect a satellite anomaly. The anomaly detection result using IGS precise products for reference shows 20241 anomalies out of 651038 broadcast navigation messages within six years. Finally, compared with the IGSO and MEO satellites, the orbit qualities of GEO satellites are significantly worse due to their large along-track orbit error. The clock performance of all satellites are at the comparable level. The satellite orbit type (GEO/IGSO/MEO) does not impact the orbit-only URE, global-average URE, and worst-case URE.

Satellite Laser Ranging System at Geochang Station

Korea Astronomy and Space Science Institute (KASI) has been developing the space optical and laser tracking (SOLT) system for space geodesy, space situational awareness, and Korean space missions. The SOLT system comprises satellite laser ranging (SLR), adaptive optics (AO), and debris laser tracking (DLT) systems, which share numerous subsystems, such as an optical telescope and tracking mount. It is designed to be capable of laser ranging up to geosynchronous Earth orbit satellites with a laser retro-reflector array, space objects imaging brighter than magnitude 10, and laser tracking low Earth orbit space debris of uncooperative targets. For the realization of multiple functions in a novel configuration, the SOLT system employs a switching mirror that is installed inside the telescope pedestal and feeds the beam path to each system. The SLR and AO systems have already been established at the Geochang station, whereas the DLT system is currently under development and the AO system is being prepared for testing. In this study, the design and development of the SOLT system are addressed and the SLR data quality is evaluated compared to the International Laser Ranging Service (ILRS) tracking stations in terms of single-shot ranging precision. The analysis results indicate that the SLR system has a good ranging performance, to a few millimeters precision. Therefore, it is expected that the SLR system will not only play an important role as a member of the ILRS tracking network, but also contribute to future Korean space missions.