Medium Earth Orbit Research Articles

TID evaluation based on variabilities of space radiation and device failure dose in typical navigation satellite orbits

Navigation satellites mainly operate in Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and Inclined Geosynchronous Satellite Orbit (IGSO). These orbits lie in or repeatedly cross the outer Van Allen belt of Earth. With large uncertainty, the radiation in these orbits mainly comes from electrons trapped in the outer radiation belt and solar energetic protons. The accurate design of the radiation shielding structure plays an important role in improving the efficiency of satellites. We introduce a method that incorporates the radiation variability in space and failure dose uncertainty of devices to evaluate the survival probability of devices caused by the Total Ionizing Dose (TID) effect. After fitting the failure dose data of operational amplifier TL084 by the Weibull distribution, we calculate the Confidence Levels (CLs) of all possible parameters of the distribution. Based on these parameters and the cumulative probability distribution of TID from space, we evaluate the effects of shielding thickness and mission duration on TL084's failure possibility at different CLs in MEO/GEO/IGSO. Furthermore, the required shielding thickness and the expected survival duration to ensure that TL084 could operate safely at specific CL are also investigated. By using this method, we could approach a balance among the device's ability, the shielding thickness, the expected duration of safe operation, and the confidence level of the design.

Quasi‐Periodic Whistler Mode Emission in the Plasmasphere as Observed by the DSX Spacecraft

AbstractWe describe the quasi‐periodic (QP) whistler‐mode emissions found in the plasmasphere as detected by electric and magnetic instrumentation onboard the Demonstration and Science Experiments (DSX) spacecraft in medium Earth orbit. Over the course of the nearly 2‐year mission, at least 45 episodes of whistler mode QP emissions were detected by the Broad Band Receiver (BBR) onboard DSX. Episodes of QP emissions were identified by discrete events having a clear unambiguous periodic nature as detected by both the electric antennae and search coil magnetic sensors in the BBR survey data at 30 s temporal resolution. Most of the QP episodes occurred in a frequency range between 1 and 4 kHz, in a band previously identified by Van Allen Probes and Cluster investigators. However, episodes were also detected by DSX at higher frequencies ‐ events in these episodes extending all the way to 15 kHz. We present our findings on these unusual high frequency events in the presentation herein. Specifically, these high frequency QP episodes tended to be observed near dawn/dusk when the spacecraft was at relatively high magnetic latitudes and on magnetic L‐shells between 3 and 5. Another unusual feature of these episodes is that individual up‐drifting events making up the episode were found to sometimes occur concurrently in time: The high frequency portion of one up‐drifting “polliwog‐shaped” event overlapped in time with the low frequency portion of the subsequent event. This behavior of the QP emissions has not been previously emphasized and we consider how this temporal concurrence relates to the source processes.

Multifrequency Observations of Postmidnight Ionospheric Irregularities From an Anomaly Crest Location

AbstractThe paper reports occurrences of postmidnight to early morning ionospheric depletions in total electron content (TEC) observed using the Low Earth Orbiting satellite beacon from Calcutta (22.57°N, 88.36°E geographic, and 32°N magnetic dip) situated around the northern crest of the equatorial ionization anomaly (EIA) in the Indian longitude sector for the period 2015–2016. These results are novel and contrary to the usual scenario of equatorial ionospheric irregularities being dominant during the early evening to pre‐midnight hours of equinoctial months. The TEC depletions, commonly called “bite‐outs,” exhibit amplitudes of 1–2.5 TECU, with maximum depletion found near the northern crest of the EIA around subionospheric coordinates 25.8°N and 82.2°E. These bite‐outs exhibited seasonal variabilities, with maximum occurrences found during local summer months. The observed TEC depletions during postmidnight hours point toward irregularities, which are not generated over the magnetic equator and drifted along magnetic field lines. A case study of the observation of these ionospheric disturbances has also been presented across a spectrum of frequencies, namely 150 and 400 MHz (Low Earth Orbit satellites), 250 MHz (Geostationary satellites), and 1.5 GHz (GPS—Medium Earth Orbit satellites) to establish the presence of irregularities of different scale sizes even during postmidnight hours.

A-contrario structural inference for space object detection and tracking

Monitoring space assets for safety and sustainability compliance requires active measurement of positional information, commonly done via optical remote sensing in which image-frame coordinates are extracted from a high-noise environment of stellar, atmospheric, and hardware features. This task has been traditionally approached by using a-priori models to differentiate potential Anthropogenic Space Objects from the background noise. Source extraction and track-before-detect methods rely on absolute pixel intensity thresholding and require substantial processing to remove noise (stars, hot pixels, etc.), while machine learning shows promise in reducing processing and improving low-visibility performance but requires context-specific labeled training data. We introduce a new approach based on a-contrario detection, arguing that any space object must be unattributable to noise using a sequence of low-fidelity hypotheses. Through this approach, we aim to relax the dependency on a-priori assumptions of data content and improve performance where high-quality data is sparse, poorly labeled, or challenging to characterize, e.g., in satellite-based applications, and provide detection confidence measures from individual data content to enhance risk evaluation for orbital populations. We present an initial qualitative proof-of-concept for our a-contrario approach using data collected by the ASTRIANet telescope network, showing potentially strong performance on Medium Earth Orbit observations for both rate and sidereal-tracking telescope modes. We also discuss how our approach handles epistemic uncertainties, i.e., a lack of a-priori model information, with implications to Type I and Type II error sources and potential mitigation steps when considering Low Earth Orbit observations with higher tracking noise.

On the potential contribution of BeiDou-3 to the realization of the terrestrial reference frame scale

Since the release of the phase center calibrations for both the receivers and the satellites, the BeiDou Navigation Satellite System (BDS) became a new potential contributor to the realization of the terrestrial reference frame (TRF) scale of future International Terrestrial Reference Frame releases. This study focuses on the evaluation of the potential usage of the BDS-3 Medium Earth Orbit (MEO) constellation to the definition of the TRF scale. To that aim, we used ground calibrated BDS-3 satellite PCOs provided by the China Satellite Navigation Office and multi-GNSS robot calibrations for the ground antennas conducted by Geo++. Two ionosphere-free linear combinations of signals, namely B1I/B3I and B1C/B2a, have been investigated to find out whether using different frequencies may lead to different TRF scale definitions. Differences between the z components of the satellite phase offsets as given by manufacturer calibrations and those estimated based on IGS14 scale amount to 6.55 ± 12.56 cm and − 0.32 ± 10.99 cm for B1I/B3I and B1C/B2a frequency pairs, respectively. On the one hand, the substantial deviation from the mean reflects the disparities in the quality of calibrations for the individual spacecraft, especially those manufactured by the Shanghai Engineering Center for Microsatellites (SECM). On the other hand, the difference between the two frequency pairs arises to a great extent from the doubtful quality of the SECM PCO calibrations, which certainly do not reflect the frequency dependence of the PCOs. Eventually, the mean scale bias with respect to IGS14 equals + 0.546 ± 0.085 ppb and + 0.026 ± 0.085 ppb for B1I/B3I and B1C/B2a solutions, respectively, when using all 24 BDS-3 MEO satellites.

A detailed dynamical model for inclination-only dependent lunisolar resonances. Effect on the “eccentricity growth” mechanism

The focus of this paper is on inclination-only dependent lunisolar resonances, which shape the dynamics of a MEO (Medium Earth Orbit) object over secular time scales (i.e. several decades). Following the formalism of Daquin et al. (2022), we discuss an analytical model yielding the correct form of the separatrices of each one of the major lunisolar resonances in the “action” space (i,e) (inclination, eccentricity) for any given semi-major axis a. We then highlight how our method is able to predict and explain the main structures found numerically in Fast Lyapunov Indicator (FLI) cartography. We focus on explaining the dependence of the FLI maps from the initial phase of the argument of perigee ω and of the longitude of the ascending node Ω of the object and of the moon ΩL. In addition, on the basis of our model, we discuss the role played by the Ω-ΩL and the 2Ω-ΩL resonances, which overlap with the inclination-only dependent ones as they sweep the region for increasing values of a, generating large domains of chaotic motion. Our results provide a framework useful in designing low-cost satellite deployment or space debris mitigation strategies, exploiting the natural dynamics of lunisolar resonances that increase an object’s eccentricity up until it reaches a domain where friction leads to atmospheric re-entry.

Local Oscillator Concept for the Event Horizon Imager

The Event Horizon Imager (EHI) is a mission concept conceived to provide a radio image of the surroundings of the event horizons of SgrA* and M87* supermassive black holes with an angular resolution at least an order of magnitude better than that achievable by radio telescopes on ground. The concept is based on performing space-to-space very long baseline interferometry (VLBI) using two satellites in medium Earth orbit at slightly different altitudes. Projected baselines up to about 26,000[Formula: see text]km and down to a few tens of kilometers can be realized. Free of atmospheric perturbations, EHI is observed in three channels: 557[Formula: see text]GHz, the main frequency to achieve the finest angular resolution by minimizing the scattering of the inter-stellar medium towards our galactic center; 230[Formula: see text]GHz, inherited from the event horizon telescope (EHT); finally, 43[Formula: see text]GHz in support of the relative delay determination. The whole EHI concept relies on very precise relative positioning between the two satellites including wavelength bootstrapping and a novel local oscillator concept that is the topic of this paper. It is shown that atomic clocks cannot provide sufficient coherence over the needed integration time. Instead, a system consisting of conventional crystal oscillators and an inter-satellite link (ISL) that can work coherently for extended periods of time, longer than required, and is therefore better suited for EHI is demonstrated.

Research on the Rotational Correction of Distributed Autonomous Orbit Determination in the Satellite Navigation Constellation

The autonomous orbit determination of the navigation constellation uses only bidirectional ranging data of the inter-satellite link for data processing. The lack of space-time benchmark information related to the Earth inevitably causes overall rotational uncertainty in the constellation, leading to a decrease in orbit accuracy and affecting user positioning accuracy. This study (1) introduces a method for rotation correction in distributed autonomous orbit determination based on inter-satellite bidirectional ranging; (2) conducts constellation autonomous orbit determination and time synchronization processing experiments based on inter-satellite ranging data for the 24 medium Earth orbit (MEO) satellites in the Beidou-3 global satellite navigation system (BDS-3); and (3) makes comparative analyses on the accuracy of autonomous orbit determination based on three rotation correction cases, including a no-rotation-correction case, independent satellite constraints case, and global satellite constraints case. The experimental results are described as follows. For the no-rotation-correction case, the prediction error of the orbital inclination angle (iot, i) for the entire constellation on the 30th day was 2.11 × 10−7/rad, the prediction error of the right ascension of the ascending point (Omega, Ω) was 2.25 × 10−7/rad, and the average root mean square (RMS) of the user range error (URE) for the entire constellation orbit was 1.41 m. In the autonomous orbit determination experiment with independent constraints on satellites, the prediction error of i for the entire constellation on the 30th day was 5.43 × 10−7/rad, the prediction error of Ω was 2.03 × 10−7/rad, and the average RMS of the orbital URE for the entire constellation was 1.09 m. In the autonomous orbit determination experiment with global satellite constraints, the prediction error of i for the entire constellation on the 30th day was 5.31 × 10−7/rad, the prediction error of Ω was 1.95 × 10−7/rad, and the RMS of the orbital URE for the entire constellation was 0.94 m. According to the analysis of the above experimental results, compared with the autonomous orbit determination under the no-rotation-correction case, the adoption of an algorithm for independent satellite constraints to correct the overall constellation rotation weakens the constellation rotation influence; however, it may destroy the overall constellation configuration, which affects the stability of autonomous orbit determination. Finally, the algorithm based on global satellite constraints both impairs the influence of constellation rotation and maintains the overall constellation configuration.

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.

Investigation of Equatorial Medium Earth Orbits for Space Solar Power

Most existing space solar power concepts place one or more power stations in geosynchronous Earth orbit (GEO). However, due to the limited availability of GEO orbital slots, it may not be feasible to locate a power station in GEO. To overcome this limitation, this article presents a system analysis for a space solar power system that incorporates a constellation of power stations in a 20184 km altitude equatorial medium Earth orbit (MEO). The orbiting power stations are based on the Caltech Space Solar Power Project architecture. The constellation consists of multiple power stations in a shared equatorial MEO each transmitting to a nonequatorial receiving station. The analysis assumes a one-to-one correspondence between the number of power stations and the number of ground stations. Like a GEO-based system, this constellation architecture enables a MEO-based system to provide near continuous power (outside of eclipse) to each ground station. It is shown that a MEO constellation with three or more power stations provides comparable transmission efficiency to a GEO-based system. The levelized cost of electricity (LCOE) is then computed for MEO systems with three, four, and five power stations and compared to the LCOE for the GEO-based system. Ground station area is identified as a significant contributor to the LCOE for the MEO-based systems. The system analysis shows that a MEO constellation with as few as four power stations has an LCOE comparable to GEO, and hence, it is concluded that MEO is a viable alternative to GEO for space solar power.

Signal-in-space range error and positioning accuracy of BDS-3

Being the first mixed-constellation global navigation system, the global BeiDou navigation system (BDS-3) designs new signals, the service performance of which has attracted extensive attention. In the present study, the Signal-in-space range error (SISRE) computation method for different types of navigation satellites was presented. The differential code bias (DCB) correction method for BDS-3 new signals was deduced. Based on these, analysis and evaluation were done by adopting the actual measured data after the official launching of BDS-3. The results showed that BDS-3 performed better than the regional navigation satellite system (BDS-2) in terms of SISRE. Specifically, the SISRE of the BDS-3 medium earth orbit (MEO) satellites reached 0.52 m, slightly inferior compared to 0.4 m from Galileo, marginally better than 0.59 m from GPS, and significantly better than 2.33 m from GLONASS. The BDS-3 inclined geostationary orbit (IGSO) satellites achieved the SISRE of 0.90 m, on par with that (0.92 m) of the QZSS IGSO satellites. However, the average SISRE of BDS-3 geostationary earth orbit (GEO) satellites was 1.15 m, which was marginally inferior to that of the QZSS GEO satellite (0.91 m). In terms of positioning accuracy, the new signals B1C and B2a are considered together with the transition signals B1I and B3I. The overall three-dimensional single-frequency standard point positioning (SPP) accuracy of BDS-3 B1C, B2a, B1I, and B3I gained an accuracy level better than 5 m. Moreover, the B1I signal exhibited the best positioning accuracy in the Asian-Pacific region, while the B1C signal set forth the best positioning accuracy in the other regions. Owing to the advantage in signal frequency, the dual-frequency SPP accuracy of B1C + B2a surpassed that of the transitional signal of B1I + B3I. Since there are more visible satellites in Asia–Pacific, the positioning accuracy of BDS-3 was moderately superior to that of GPS. The precise point positioning (PPP) accuracy of BDS-3 B1C + B2a or B1I + B3I converged to the order of centimeters, marginally inferior to that of the GPS L1 + L2. However, these three combinations had a similar convergence time of approximately 30 min.

Long-Term Orbit Prediction and Deorbit Disposal Investigation of MEO Navigation Satellites

With the increase in satellites in the medium Earth orbit (MEO) region, there should be a focus on orbit safety in the MEO region. A safe orbit disposal strategy is necessary to maintain the sustainability of the MEO region. This paper focuses on long-term evolution modeling, safety analysis of MEO objects, and different disposal techniques for end-of-life BDS-2 MEO satellites. On the one hand, a long-term numerical evolution model is established, and mean equinoctial elements are adopted to propagate a long-term orbit. Long-term evolution for the MEO region over 100 years is carried out, including the Galileo, BDS, GPS, and GLONASS constellations. The earliest orbit intersection time with other global navigation satellite system (GNSS) constellations is put forward. On the other hand, a dynamic model and an optimization model for disposal orbit are established, which minimize the eccentricity growth within 200 years and the fuel consumption for maneuvering to the disposal orbit. The bounds for the disposal region of BDS MEO satellites are also proposed, which consider the measurement and control error of BDS MEO satellites and the eccentricity bounds for end-of-life BDS MEO satellites. A genetic algorithm is adopted to optimize the orbital elements for end-of-life BDS MEO satellites. In addition, two disposal cases, namely, upraising and reducing the orbit, for end-of-life BDS MEO satellites are simulated. The long-term evolutions for the disposal of orbital elements within 200 years are implemented, and the fuel consumption is calculated. The results show that the current MEO region is relatively safe and that the eccentricity is the most important factor that influences the long-term evolution of safety analysis for BDS MEO disposal orbits. Upraising the orbit is safe for end-of-life BDS MEO satellites. This investigation provides the theoretical foundation for investigating the long-term evolutionary mechanisms of the MEO region and references disposal strategy analysis for decommissioned navigation satellites, and the spent upper stages for other GNSS constellations.

Multipath Error Modeling Methodology for GNSS Integrity Monitoring Using a Global Optimization Strategy

Valid multipath error model is the prerequisite for high-performance GNSS integrity monitoring. It is indispensable to civil aviation and other Safety-of-Life (SoL) users. The model must perfectly bound multipath error while preventing the constructed model from being too conservative. Nevertheless, no sound methodologies to meet both the requirements have been introduced in previous literatures, and subsequently, practices always require iterative manual trade-offs. To improve the efficiency of multipath modeling, we propose a new automatic multipath error modeling methodology. It quantifies the above requirements in the objective function of multiobjective genetic algorithm (GA) so that multipath modeling can be managed automatically. Moreover, through introducing a new model that is based on two inflation factors, conservatism of modeling results can be significantly reduced. Experiments based on a 4-month dataset of BDS-3 Medium Earth Orbit (MEO) satellites show that constructed multipath models effectively bound actual error in each elevation bin. In addition, the new model form with two inflation factors brings average CDF difference reduction of 67.4% at B1I and 50.6% at B3I, which means significantly mitigation in terms of conservatism.

Advancing the Solar Radiation Pressure Model for BeiDou-3 IGSO Satellites

In the absence of detailed surface information, empirical solar radiation pressure (SRP) models, such as the five-parameter Empirical CODE Orbit Model (ECOM1) and its extended version-ECOM2, are widely used for modeling SRP forces acting on GNSS satellites. This study shows that the orbits of BeiDou-3 Inclined Geosynchronous Orbit satellites (IGSOs) determined with the ECOM1 model suffer from systematic once-per-revolution radial orbit errors, which can be partly reduced by the ECOM2 model. To eliminate such orbit errors, the BeiDou-3 IGSO optical coefficients are solved by using an adjustable box-wing (ABW) model and then introduced into an a priori box-wing SRP model to enhance the ECOM1 model (ECOM1 + BW). In the ABW solution, in addition to satellite body and solar panels, the contributions of the communication payloads installed on BeiDou-3 IGSO ±X panels on the SRP are also considered, which markedly improves the stability of the optical coefficient estimates. The efficiency of the developed a priori box-wing model is demonstrated through eliminated once-per-revolution radial orbit errors and decreased day boundary discontinuities. However, the orbit solutions still show significant degradations during eclipse seasons. The results of the first yaw-attitude analysis for eclipsing BeiDou-3 IGSOs show that their yaw behaviors are the same as those of BeiDou-3 CAST (China Academy of Space Technology) MEOs (Medium Earth Orbit satellites), and have been well considered in the study. This rules out the possibility that attitude errors are the potential reason for the orbit deterioration. By introducing a once-per-revolution sine term in the Sun direction (Ds term) and keeping Ds active during the Earth’s shadow transitions to the ECOM1 + BW model, the orbit performance inside the eclipse seasons is significantly improved and can be comparable to that outside the eclipse seasons.

Analysis of BDS-3 Onboard Clocks Based on GFZ Precise Clock Products

The characteristics and performance of satellite clocks are important to the positioning, navigation, and timing (PNT) services of Global Navigation Satellite System (GNSS) users. Although China’s BeiDou-3 Navigation Satellite System (BDS-3) has been fully operational for more than one year, there is still a lack of comprehensive research on the onboard clocks of the entire BDS-3 constellation. In this study, the precise clock products of GeoForschungsZentrum (GFZ) from day-of-year (DOY) 1, 2021 to DOY 300, 2021 were used to analyze the characteristics and performance of BDS-3 onboard clocks from the following aspects: clock bias, frequency, drift rate, fitting residuals, periodicity, and frequency stability. Compared with BDS-2, the clock quality of BDS-3 satellites has been greatly improved, but there are still jumps in the clock offsets and frequency series of BDS-3 clocks. The drift rate of BDS-3 clocks varies within the range between −2×10−18 and 2×10−18 s/s2. The daily model fitting residuals of passive hydrogen masers (PHM) on BDS-3 medium Earth orbit (MEO), inclined geosynchronous orbit (IGSO), and geostationary (GEO) satellites are 0.15, 0.28, and 0.46 ns, respectively. The overlapping Allan deviation (OADEV) of BDS-3 MEO clocks is 4.0 × 10−14 s/s at a time interval of 1000 s. The PHMs on BDS-3 MEO satellites exhibit fewer periodic signals than those of Rb clocks. In addition, the precise clock offsets of the BDS-3 PHMs carried on the MEO, IGSO, and GEO satellites show different periodicities, which are similar to those of the corresponding types of BDS-2 satellites.

Spaceborne Synthetic Aperture Radar Imaging Algorithms: An overview

Spaceborne synthetic aperture radar (SAR) has been widely used in the national defense, geologic mapping, disaster estimation, and other fields. This technique has realized the technological leap forward from low-Earth-orbit (LEO) to medium-Earth-orbit (MEO)/geosynchronous-Earth-orbit (GEO) SAR imaging, single- to multichannel processing, the resolution level of the meter to submeter, and single to multiple imaging modes in recent years.

Survey on Optimization Methods for LEO-Satellite-Based Networks with Applications in Future Autonomous Transportation.

Future autonomous transportation is one of the most demanding application areas in terms of connectivity, as it has to simultaneously meet stringent criteria that do not typically go hand in hand, such as high throughput, low latency, high coverage/availability, high positioning and sensing accuracies, high security and robustness to interferences, etc. In order to meet the future demands of challenging applications, such as applications relying on autonomous vehicles, terrestrial networks are no longer sufficient and are to be augmented in the future with satellite-based networks. Among the emerging satellite networks, Low Earth Orbit (LEO) networks are able to provide advantages over traditional Medium Earth Orbit (MEO) and Geo-Stationary Earth Orbit (GEO) networks in terms of signal latency, cost, and performance. Nevertheless, several challenges exist in LEO system design, which have not been fully addressed in the existing literature. In particular, the problem of LEO-system optimization of design parameters is a multi-dimensional problem with many aspects to be considered. This paper offers a comprehensive survey of the LEO-system design parameters, of the challenges in LEO system design process, and of the optimization methods for satellite communication, positioning, and sensing applications, as well as a summarizing discussion on the design considerations for LEO-based networks to support future autonomous transportation.

Characterizing doubly-averaged dynamical models in medium earth orbit

Understanding long term stability in the GNSS region is crucial to maintaining this orbital regime. Averaged dynamics pose a useful tool for running long term analysis on these orbits for low computational power and quick run times. Because this region is subject to luni-solar resonances, averaged dynamics do not necessarily closely match the actual dynamics. This paper seeks to understand how to utilize averaged dynamics to characterize regions of strongly dynamic secular eccentricity growth despite these disagreements. By analyzing the distribution of a set of initial conditions, we are able to describe the general dynamics of the region with far less computational effort. This paper also builds upon existing models for doubly-averaged solar radiation pressure to examine how high area-to-mass ratio objects can be studied in this region.

Analysis and Design of an X-Band Reflectarray Antenna for Remote Sensing Satellite System.

This paper presents the analysis and design of an X-band reflectarray. The proposed antenna can be used for a medium Earth orbit (MEO) remote sensing satellite system in the 8.5 GHz band. To obtain a nearly constant response along the coverage area of this satellite system, the proposed antenna was designed with a flat-top radiation pattern with a beam width of around 29° for the required MEO system. In addition, broadside pencil beam and tilted pencil beam reflectarrays were also investigated. The feeding element of the proposed reflectarray antennas is a Yagi–Uda array. The amplitude and phase distribution of the fields due to the feeding element on the aperture of the reflectarray antenna are obtained directly by numerical simulation without introducing any approximation. The required phase distribution along the aperture of the reflectarray to obtain the required flat-top radiation pattern is obtained using the genetic algorithm (GA) optimization method. The reflecting elements of the reflectarray are composed of stacked circular patches. This stacked configuration was found to be appropriate for obtaining a wide range of reflection phase shift, which is required to implement the required phase distribution on the reflectarray aperture. The antenna was fabricated and measured for verification.

Improving the Orbits of the BDS-2 IGSO and MEO Satellites with Compensating Thermal Radiation Pressure Parameters

The orbit accuracy of the navigation satellites relies on the accurate knowledge of the forces on the spacecraft, in particular the non-conservative perturbations. This study focuses on the Inclined Geosynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites of the regional Chinese BeiDou Navigation Satellite System (BDS-2), for which apparent deficiencies of non-conservative models are identified and evidenced in the Satellite Laser Ranging (SLR) residuals. The orbit errors derived from the empirical 5-parameter Extended CODE Orbit Model (ECOM) as well as a semi-analytical adjustable box-wing model show prominent dependency on the Sun elongation angle, even in the yaw-steering attitude mode. Hence, a periodic acceleration in the normal direction of the +X surface, presumably generated by the mismodeled thermal radiation pressure, is introduced. The SLR validations reveal that the Sun elongation angle-dependent systematic errors were significantly reduced, and the orbit accuracy was improved by 10–30% to approximately 4.5 cm and 3.0 cm for the BDS-2 IGSO and MEO satellites, respectively.