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This paper explores the evolution of Radio Access Network (RAN) architectures and their integration into Non-Terrestrial Networks (NTN) to address escalating mobile traffic demands. Focusing on Low Earth Orbit (LEO) satellites as key components of NTN, we examine the feasibility of RAN function splits (FSs) in terms of fronthaul (FH) latency, elevation angle, and bandwidth (BW) across LEO satellites and ground stations (GS), alongside evaluating performance of Conditional Handover (CHO) procedures under diverse scenarios. By assessing performance metrics such as handover duration, disconnection time, and control traffic volume, we provide insights on several aspects such as stringent constraints for Low Layer Splits (LLSs), leading to longer delays during mobility procedures and increased control traffic across the feeder link in comparison with the case when gNodeB is onboard satellite. Despite challenges, LLSs demonstrate minimal onboard satellite computational requirements, promising reduced power consumption and payload weight. These findings underscore the architectural possibilities and challenges within the telecommunications industry, paving the way for future advancements in NTN RAN design and operation., Part of ISBN [9798350344998]QC 20240820

The deployment of satellite networks is key to providing global wireless connectivity for the Internet of Things (IoT). In this line, we consider a cluster of IoT devices served by a constellation of low-Earth-orbit (LEO) satellites, while slotted Aloha is used as a medium-access-control (MAC) technique in the uplink. To characterize the channel, we employ an ON–OFF fading channel model that estimates the quality of the links between the cluster of IoT devices and the LEO satellites within the constellation, by taking into account their relative positions. Since each relative position of the constellation with respect to the cluster of IoT devices leads to a different throughput for a given traffic load, we propose a novel traffic load distribution strategy based on successive convex approximation (SCA) to maximize the system throughput. The method adequately allocates the traffic load among the different constellation positions with respect to the IoT cluster. Finally, the results show that the proposed method outperforms other recently proposed strategies based on heuristics for traffic load allocation, while it also achieves a stable nonzero throughput even for large traffic loads.

Satellites in Low Earth Orbit (LEO) are essential for sounding the topside ionosphere. In this work, we present and validate a data set of Total Electron Content (TEC) and in situ electron density observations from the Gravity Recovery And Climate Experiment (GRACE) and GRACE-Follow-On missions as well as a TEC data set from the CHAllenging Minisatellite Payload mission. Concerning TEC, special emphasis is put to ensure optimal consistency to the already existing Swarm and Gravity field and steady-state ocean circulation explorer (GOCE) TEC data sets. The newly processed satellite missions allow covering two full solar cycles with LEO slant TEC. Furthermore, the twin satellite missions GRACE and GRACE-FO equipped with inter-satellite K-band ranging allows to derive the horizontal TEC and, due to the small inter-satellite distance of the satellite pairs, an approximation for local electron density. However, the derived value of electron density is relative and requires calibration using external information. In this work, the calibration is performed using the IRI-2016 model. Radar observations, as well as in situ electron density observations available from Swarm B Langmuir probes, are used for validation. Conjunctions between satellites are used to validate the TEC time series. The newly derived data set is shown to be highly consistent with the already existing data sets with standard deviations below 3 TECU for TEC (even 1 TECU was reached for low solar flux) and an offset below 7 × 1010 m−3 with a standard deviation near 1 × 1011 m−3 for the electron density., Astrodynamics & Space Missions

Driven by the ever increasing penetration and proliferation of data-driven applications, a new generation of wireless communication, the sixth generation (6G) mobile system enhanced by artificial intelligence, has attracted substantial research interests. Among various candidate technologies of 6G, low Earth orbit (LEO) satellites have appealing characteristics of ubiquitous wireless access. However, the costs of satellite communication (SatCom) are still high, relative to their counterparts of ground mobile networks. To support massively interconnected devices with intelligent adaptive learning and reduce expensive traffic in SatCom, we propose federated learning (FL) in LEO-based satellite communication networks. We first review the state-of-the-art LEO-based SatCom and related machine learning (ML) techniques, and then analyze four possible ways of combining ML with satellite networks. The learning performance of the proposed strategies is evaluated by simulation and results reveal that FL-based computing networks improve the performance of communication overheads and latency. Finally, we discuss future research topics along this research direction., QC 20220530

Low earth orbit (LEO) satellite-assisted communications have been considered as one of the key elements in beyond 5G systems to provide wide coverage and cost-efficient data services. Such dynamic space-terrestrial topologies impose an exponential increase in the degrees of freedom in network management. In this paper, we address two practical issues for an over-loaded LEO-terrestrial system. The first challenge is how to efficiently schedule resources to serve a massive number of connected users, such that more data and users can be delivered/served. The second challenge is how to make the algorithmic solution more resilient in adapting to dynamic wireless environments. We first propose an iterative suboptimal algorithm to provide an offline benchmark. To adapt to unforeseen variations, we propose an enhanced meta-critic learning algorithm (EMCL), where a hybrid neural network for parameterization and the Wolpertinger policy for action mapping are designed in EMCL. The results demonstrate EMCL’s effectiveness and fast-response capabilities in over-loaded systems and in adapting to dynamic environments compare to previous actor-critic and meta-learning methods.

Towards the next generation networks, low earth orbit (LEO) satellites have been considered as a promising component for beyond 5G networks. In this paper, we study downlink LEO-5G communication systems in a practical scenario, where the integrated LEO-terrestrial system is over-loaded by serving a number of terminals with high-volume traffic requests. Our goal is to optimize resource scheduling such that the amount of undelivered data and the number of unserved terminals can be minimized. Due to the inherent hardness of the formulated quadratic integer programming problem, the optimal algorithm requires unaffordable complexity. To solve the problem, we propose a near-optimal algorithm based on alternating direction method of multipliers (ADMM-HEU), which saves computational time by taking advantage of the distributed ADMM structure, and a low-complexity heuristic algorithm (LC-HEU), which is based on estimation and greedy methods. The results demonstrate the near-optimality of ADMM-HEU and the computational efficiency of LC-HEU compared to the benchmarks.

Low earth orbit (LEO) satellite-assisted communications have been considered as one of the key elements in beyond 5G systems to provide wide coverage and cost-efficient data services. Such dynamic space-terrestrial topologies impose an exponential increase in the degrees of freedom in network management. In this paper, we address two practical issues for an over-loaded LEO-terrestrial system. The first challenge is how to efficiently schedule resources to serve a massive number of connected users, such that more data and users can be delivered/served. The second challenge is how to make the algorithmic solution more resilient in adapting to dynamic wireless environments. We first propose an iterative suboptimal algorithm to provide an offline benchmark. To adapt to unforeseen variations, we propose an enhanced meta-critic learning algorithm (EMCL), where a hybrid neural network for parameterization and the Wolpertinger policy for action mapping are designed in EMCL. The results demonstrate EMCL’s effectiveness and fast-response capabilities in over-loaded systems and in adapting to dynamic environments compare to previous actor-critic and meta-learning methods.

Towards the next generation networks, low earth orbit (LEO) satellites have been considered as a promising component for beyond 5G networks. In this paper, we study downlink LEO-5G communication systems in a practical scenario, where the integrated LEO-terrestrial system is over-loaded by serving a number of terminals with high-volume traffic requests. Our goal is to optimize resource scheduling such that the amount of undelivered data and the number of unserved terminals can be minimized. Due to the inherent hardness of the formulated quadratic integer programming problem, the optimal algorithm requires unaffordable complexity. To solve the problem, we propose a near-optimal algorithm based on alternating direction method of multipliers (ADMM-HEU), which saves computational time by taking advantage of the distributed ADMM structure, and a low-complexity heuristic algorithm (LC-HEU), which is based on estimation and greedy methods. The results demonstrate the near-optimality of ADMM-HEU and the computational efficiency of LC-HEU compared to the benchmarks.

[ES] El objetivo de este trabajo Fin de Grado es el estudio del efecto de la atmósfera sobre la trayectoria de los satélites en órbita LEO (Low Earth Orbit) atendiendo a las características del satélite, al coeficiente de Drag asociado y al modelo de atmósfera elegido. Inicialmente se implementará un propagador orbital para la integración de las ecuaciones de movimiento del problema de dos cuerpos, al cual se le añadirán diversas perturbaciones como las debidas al achatamiento terrestre, dada la relevancia de éstas en satélites que orbitan a distancias cercanas a la Tierra. Además, teniendo en cuenta el objetivo principal de este trabajo, se incorporará también la perturbación debida a la atmósfera terrestre utilizando un modelo exponencial de atmósfera. Se validará la eficacia del propagador generado comparando con los resultados calculados por la herramienta JPL¿s HORIZONS de la NASA. Una vez validado el integrador, se procederá a realizar un estudio de las variaciones orbitales a las que da lugar la consideración de diferentes valores del coeficiente de Drag dentro de un rango admisible teniendo en cuenta las dimensiones y forma del satélite., [EN] The main goal of this work is to study the effect of the atmosphere on the trajectory of satellites in LEO (Low Earth Orbit) orbit, taking into account the characteristics of the satellite, the associated Drag coefficient and the chosen atmosphere model. Initially, an orbital propagator will be implemented to integrate the equations of motion of the two-body problem. Next, different perturbations will be added, as those due to the Earth oblateness since they are very relevant for LEO satellites. In addition, taking into account the main objective of this work, the perturbation due to the Earth's atmosphere will also be incorporated using an exponential model of the atmosphere. The goodness of the generated propagator will be validated by comparison with the results given by NASA's JPL's HORIZONS tool. Finally, a study of the orbital variations due to different values of the Drag coefficient will be carried out with our validated propagator. The Drag coefficients will vary in an admissible range for the the dimensions and shape of the satellite., [CA] En aquest treball es dissenya un propagador utilitzant el mètode de Cowell i incloent les pertorbacions degudes a laplanament terrestre i la resistència aerodinàmica per a òrbites baixes (també conegudes com òrbites LEO). La validesa i precisió de lintegrador dissenyat sobté mitjançant la comparació dels resultats amb el propagador de la interfície Horizons de la NASA. Donat que linterès es centra en els satèl·lits LEO, aquesta comparativa es realitza propagant les seues òrbites en un interval de temps de una setmana, ja que aquests pateixen correccions en la seua trajectòria amb freqüència.

[ES] El objetivo de este trabajo Fin de Grado es el estudio del efecto de la atmósfera sobre la trayectoria de los satélites en órbita LEO (Low Earth Orbit) atendiendo a las características del satélite, al coeficiente de Drag asociado y al modelo de atmósfera elegido. Inicialmente se implementará un propagador orbital para la integración de las ecuaciones de movimiento del problema de dos cuerpos, al cual se le añadirán diversas perturbaciones como las debidas al achatamiento terrestre, dada la relevancia de éstas en satélites que orbitan a distancias cercanas a la Tierra. Además, teniendo en cuenta el objetivo principal de este trabajo, se incorporará también la perturbación debida a la atmósfera terrestre utilizando un modelo exponencial de atmósfera. Se validará la eficacia del propagador generado comparando con los resultados calculados por la herramienta JPL¿s HORIZONS de la NASA. Una vez validado el integrador, se procederá a realizar un estudio de las variaciones orbitales a las que da lugar la consideración de diferentes valores del coeficiente de Drag dentro de un rango admisible teniendo en cuenta las dimensiones y forma del satélite., [EN] The main goal of this work is to study the effect of the atmosphere on the trajectory of satellites in LEO (Low Earth Orbit) orbit, taking into account the characteristics of the satellite, the associated Drag coefficient and the chosen atmosphere model. Initially, an orbital propagator will be implemented to integrate the equations of motion of the two-body problem. Next, different perturbations will be added, as those due to the Earth oblateness since they are very relevant for LEO satellites. In addition, taking into account the main objective of this work, the perturbation due to the Earth's atmosphere will also be incorporated using an exponential model of the atmosphere. The goodness of the generated propagator will be validated by comparison with the results given by NASA's JPL's HORIZONS tool. Finally, a study of the orbital variations due to different values of the Drag coefficient will be carried out with our validated propagator. The Drag coefficients will vary in an admissible range for the the dimensions and shape of the satellite., [CA] En aquest treball es dissenya un propagador utilitzant el mètode de Cowell i incloent les pertorbacions degudes a laplanament terrestre i la resistència aerodinàmica per a òrbites baixes (també conegudes com òrbites LEO). La validesa i precisió de lintegrador dissenyat sobté mitjançant la comparació dels resultats amb el propagador de la interfície Horizons de la NASA. Donat que linterès es centra en els satèl·lits LEO, aquesta comparativa es realitza propagant les seues òrbites en un interval de temps de una setmana, ja que aquests pateixen correccions en la seua trajectòria amb freqüència.

[ES] El objetivo de este trabajo Fin de Grado es el estudio del efecto de la atmósfera sobre la trayectoria de los satélites en órbita LEO (Low Earth Orbit) atendiendo a las características del satélite, al coeficiente de Drag asociado y al modelo de atmósfera elegido. Inicialmente se implementará un propagador orbital para la integración de las ecuaciones de movimiento del problema de dos cuerpos, al cual se le añadirán diversas perturbaciones como las debidas al achatamiento terrestre, dada la relevancia de éstas en satélites que orbitan a distancias cercanas a la Tierra. Además, teniendo en cuenta el objetivo principal de este trabajo, se incorporará también la perturbación debida a la atmósfera terrestre utilizando un modelo exponencial de atmósfera. Se validará la eficacia del propagador generado comparando con los resultados calculados por la herramienta JPL¿s HORIZONS de la NASA. Una vez validado el integrador, se procederá a realizar un estudio de las variaciones orbitales a las que da lugar la consideración de diferentes valores del coeficiente de Drag dentro de un rango admisible teniendo en cuenta las dimensiones y forma del satélite., [EN] The main goal of this work is to study the effect of the atmosphere on the trajectory of satellites in LEO (Low Earth Orbit) orbit, taking into account the characteristics of the satellite, the associated Drag coefficient and the chosen atmosphere model. Initially, an orbital propagator will be implemented to integrate the equations of motion of the two-body problem. Next, different perturbations will be added, as those due to the Earth oblateness since they are very relevant for LEO satellites. In addition, taking into account the main objective of this work, the perturbation due to the Earth's atmosphere will also be incorporated using an exponential model of the atmosphere. The goodness of the generated propagator will be validated by comparison with the results given by NASA's JPL's HORIZONS tool. Finally, a study of the orbital variations due to different values of the Drag coefficient will be carried out with our validated propagator. The Drag coefficients will vary in an admissible range for the the dimensions and shape of the satellite., [CA] En aquest treball es dissenya un propagador utilitzant el mètode de Cowell i incloent les pertorbacions degudes a laplanament terrestre i la resistència aerodinàmica per a òrbites baixes (també conegudes com òrbites LEO). La validesa i precisió de lintegrador dissenyat sobté mitjançant la comparació dels resultats amb el propagador de la interfície Horizons de la NASA. Donat que linterès es centra en els satèl·lits LEO, aquesta comparativa es realitza propagant les seues òrbites en un interval de temps de una setmana, ja que aquests pateixen correccions en la seua trajectòria amb freqüència.

[ES] El objetivo de este trabajo Fin de Grado es el estudio del efecto de la atmósfera sobre la trayectoria de los satélites en órbita LEO (Low Earth Orbit) atendiendo a las características del satélite, al coeficiente de Drag asociado y al modelo de atmósfera elegido. Inicialmente se implementará un propagador orbital para la integración de las ecuaciones de movimiento del problema de dos cuerpos, al cual se le añadirán diversas perturbaciones como las debidas al achatamiento terrestre, dada la relevancia de éstas en satélites que orbitan a distancias cercanas a la Tierra. Además, teniendo en cuenta el objetivo principal de este trabajo, se incorporará también la perturbación debida a la atmósfera terrestre utilizando un modelo exponencial de atmósfera. Se validará la eficacia del propagador generado comparando con los resultados calculados por la herramienta JPL¿s HORIZONS de la NASA. Una vez validado el integrador, se procederá a realizar un estudio de las variaciones orbitales a las que da lugar la consideración de diferentes valores del coeficiente de Drag dentro de un rango admisible teniendo en cuenta las dimensiones y forma del satélite., [EN] The main goal of this work is to study the effect of the atmosphere on the trajectory of satellites in LEO (Low Earth Orbit) orbit, taking into account the characteristics of the satellite, the associated Drag coefficient and the chosen atmosphere model. Initially, an orbital propagator will be implemented to integrate the equations of motion of the two-body problem. Next, different perturbations will be added, as those due to the Earth oblateness since they are very relevant for LEO satellites. In addition, taking into account the main objective of this work, the perturbation due to the Earth's atmosphere will also be incorporated using an exponential model of the atmosphere. The goodness of the generated propagator will be validated by comparison with the results given by NASA's JPL's HORIZONS tool. Finally, a study of the orbital variations due to different values of the Drag coefficient will be carried out with our validated propagator. The Drag coefficients will vary in an admissible range for the the dimensions and shape of the satellite., [CA] En aquest treball es dissenya un propagador utilitzant el mètode de Cowell i incloent les pertorbacions degudes a laplanament terrestre i la resistència aerodinàmica per a òrbites baixes (també conegudes com òrbites LEO). La validesa i precisió de lintegrador dissenyat sobté mitjançant la comparació dels resultats amb el propagador de la interfície Horizons de la NASA. Donat que linterès es centra en els satèl·lits LEO, aquesta comparativa es realitza propagant les seues òrbites en un interval de temps de una setmana, ja que aquests pateixen correccions en la seua trajectòria amb freqüència.

The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015. The work is a wider review of available ground and satellite data and techniques available to detect seismically induced traveling ionospheric disturbances (TID) and irregularities of smaller scale. The data used in the experiment includes several types of ground and satellite observations from low-Earth orbit (LEO) satellites. The number of techniques applied here is also extended and includes spectral analysis of LEO along-track data and composed analysis of ground GNSS data. The timeframe of the analyses is focused on September 16 and 17, 2015 but also extended to several adjacent days, where an enhanced seismic activity has been recorded. Several examples of seismically triggered TIDs are shown, as detected by combined observations from more than one source and applying different methods, including spectral analysis. These disturbances occur before the mainshock, just after, or in time following this large EQ, and can be found in close neighborhood of Chile-Illapel or far away from the epicenter. The objective of the work was to demonstrate an increasing number of available data and techniques, which can be limited when applied alone, but their combination can provide many advantages in the analysis of seismically disturbed ionosphere. The combination of LEO satellite data reaching all regions of the globe with local but dense ground-based GNSS data and ionospheric HF sounders looks promising, especially in view of the nearby availability of CubeSat constellations equipped with instruments for ionosphere sounding. An important conclusion coming from the study is a need for spectral analysis techniques in the processing of LEO along-track data and the requirement of the validation of LEO observations with separate LEO

The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015. The work is a wider review of available ground and satellite data and techniques available to detect seismically induced traveling ionospheric disturbances (TID) and irregularities of smaller scale. The data used in the experiment includes several types of ground and satellite observations from low-Earth orbit (LEO) satellites. The number of techniques applied here is also extended and includes spectral analysis of LEO along-track data and composed analysis of ground GNSS data. The timeframe of the analyses is focused on September 16 and 17, 2015 but also extended to several adjacent days, where an enhanced seismic activity has been recorded. Several examples of seismically triggered TIDs are shown, as detected by combined observations from more than one source and applying different methods, including spectral analysis. These disturbances occur before the mainshock, just after, or in time following this large EQ, and can be found in close neighborhood of Chile-Illapel or far away from the epicenter. The objective of the work was to demonstrate an increasing number of available data and techniques, which can be limited when applied alone, but their combination can provide many advantages in the analysis of seismically disturbed ionosphere. The combination of LEO satellite data reaching all regions of the globe with local but dense ground-based GNSS data and ionospheric HF sounders looks promising, especially in view of the nearby availability of CubeSat constellations equipped with instruments for ionosphere sounding. An important conclusion coming from the study is a need for spectral analysis techniques in the processing of LEO along-track data and the requirement of the validation of LEO observations with separate LEO, Peer Reviewed, Postprint (published version)

Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a highly valuable remote sensing technique for probing the Earth’s atmosphere, due to its global coverage, high accuracy, long-term stability, and essentially all-weather capability. In order to ensure the highest quality of essential climate variables (ECVs), derived from GNSS signal tracking by RO satellites in low Earth orbit (LEO), the orbit positions and velocities of the GNSS transmitter and LEO receiver satellites need to be determined with high and proven accuracy and reliability. Wegener Center’s new Reference Occultation Processing System (rOPS) hence aims to integrate uncertainty estimation at all stages of the processing. Here we present a novel setup for precise orbit determination (POD) within the rOPS, which routinely and in parallel performs the LEO POD with the two independent software packages Bernese GNSS software (v5.2) and NAPEOS (v3.3.1), employing two different GNSS orbit data products. This POD setup enables mutual consistency checks of the calculated orbit solutions and is used for position and velocity uncertainty estimation, including estimated systematic and random uncertainties. For LEOs enabling laser tracking we involve position uncertainty estimates from satellite laser ranging. Furthermore, we intercompare the LEO orbit solutions with solutions from other leading orbit processing centers for cross-validation. We carefully analyze multi-month, multi-satellite POD result statistics and find a strong overall consistency of estimates within LEO orbit uncertainty target specifications of 5 cm in position and 0.05 mm/s in velocity for the CHAMP, GRACE-A, and Metop-A/B missions. In 92% of the days investigated over two representative 3-month periods (July to September in 2008 and 2013) these POD uncertainty targets, which enable highly accurate climate-quality RO processing, are satisfied. The moderately higher uncertainty estimates found for the remaining 8% of days ( ∼ 5–, Fonds zur Förderung der Wissenschaftlichen Forschung W1256-G15, Österreichische Forschungsförderungsgesellschaft OPSCLIMPROP-840070 OPSCLIMTRACE-844395 EOPCLIMTRACK-859773, Version of record

Transient signal loss of the global positioning system (GPS) has been frequently observed by receivers on board the European Space Agency's Swarm mission when the satellites encounter ionospheric plasma irregularities. In this study we provided the first comparison of the GPS signal amplitude degradations from receivers on board low Earth orbiting satellites at different altitudes. Intense carrier phase variations but almost no amplitude fades (less than 2 dB Hz) are observed when the spaceborne receiver lies right inside the ionospheric plasma irregularities, like the case for the Swarm and CHAMP satellites flying at about 400–500 km. This indicates that the strong phase variation, but not the amplitude fades, causes the receivers to stop tracking the GPS signals. When the receiver is located 100–200 km below the slab of plasma irregularities, like the case for the GOCE satellite flying at about 250 km, signal amplitude fades exceeding 10 dB Hz are observed, in addition to strong phase variation. Our results suggest that a considerable distance of the receiver to the plasma irregularity slab is needed to affect the Fresnel diffractive process and further causes GPS signal amplitude fades., Astrodynamics & Space Missions

With the advent of an increasing number of high-throughput telecommunication satellite constellations in Low Earth Orbit (LEO), tracking performance requirements for User Terminals (UTs) have become more stringent. A way to efficiently perform the Acquisition (ACQ) and Tracking (TRK) of a fast-moving LEO satellite along its trajectory is needed. Phased Array Antennas (PAA) are particularly suited for this type of application since their radiation patterns can be rapidly changed and their main lobe quickly steered without any need of mechanical components. In particular, Active PAAs, possessing an individual transceiver module per each array element, can extract the information of all individual array elements. This information can later be processed by algorithms to get the Directions of Arrival (DOAs) of the satellite signals arriving at the array. Namely, subspace-based DOA estimation methods are commonly implemented in systems employing Active PAAs, since they make full use of the detailed array information available. The aim of this work is to determine which subspace-based DOA estimation algorithms for UTs based on Active PAA technology work best for ACQ and TRK scenarios involving LEO satellite constellations. An extensive bibliographic study of array data modelling, subspace-based DOA estimation methods, and subspace tracking algorithms was carried out. The most promising methods were implemented in MATLAB. Furthermore, a full ACQ plus TRK simulation system was coded and used to test application scenarios with real satellite constellation data to conclude which algorithms yielded the best real-time performance. From the results of several worst-case scenario simulations, it was concluded that satellite DOA estimations were the most accurate when using the Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT) method for both ACQ and TRK alongside the Fast Data Projection Method (FDPM) algorithm for updating the spatial covariance matrix

Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a highly valuable remote sensing technique for probing the Earth’s atmosphere, due to its global coverage, high accuracy, long-term stability, and essentially all-weather capability. In order to ensure the highest quality of essential climate variables (ECVs), derived from GNSS signal tracking by RO satellites in low Earth orbit (LEO), the orbit positions and velocities of the GNSS transmitter and LEO receiver satellites need to be determined with high and proven accuracy and reliability. Wegener Center’s new Reference Occultation Processing System (rOPS) hence aims to integrate uncertainty estimation at all stages of the processing. Here we present a novel setup for precise orbit determination (POD) within the rOPS, which routinely and in parallel performs the LEO POD with the two independent software packages Bernese GNSS software (v5.2) and NAPEOS (v3.3.1), employing two different GNSS orbit data products. This POD setup enables mutual consistency checks of the calculated orbit solutions and is used for position and velocity uncertainty estimation, including estimated systematic and random uncertainties. For LEOs enabling laser tracking we involve position uncertainty estimates from satellite laser ranging. Furthermore, we intercompare the LEO orbit solutions with solutions from other leading orbit processing centers for cross-validation. We carefully analyze multi-month, multi-satellite POD result statistics and find a strong overall consistency of estimates within LEO orbit uncertainty target specifications of 5 cm in position and 0.05 mm/s in velocity for the CHAMP, GRACE-A, and Metop-A/B missions. In 92% of the days investigated over two representative 3-month periods (July to September in 2008 and 2013) these POD uncertainty targets, which enable highly accurate climate-quality RO processing, are satisfied. The moderately higher uncertainty estimates found for the remaining 8% of days ( ∼ 5–, Fonds zur Förderung der Wissenschaftlichen Forschung W1256-G15, Österreichische Forschungsförderungsgesellschaft OPSCLIMPROP-840070 OPSCLIMTRACE-844395 EOPCLIMTRACK-859773, Version of record

Transient signal loss of the global positioning system (GPS) has been frequently observed by receivers on board the European Space Agency's Swarm mission when the satellites encounter ionospheric plasma irregularities. In this study we provided the first comparison of the GPS signal amplitude degradations from receivers on board low Earth orbiting satellites at different altitudes. Intense carrier phase variations but almost no amplitude fades (less than 2 dB Hz) are observed when the spaceborne receiver lies right inside the ionospheric plasma irregularities, like the case for the Swarm and CHAMP satellites flying at about 400–500 km. This indicates that the strong phase variation, but not the amplitude fades, causes the receivers to stop tracking the GPS signals. When the receiver is located 100–200 km below the slab of plasma irregularities, like the case for the GOCE satellite flying at about 250 km, signal amplitude fades exceeding 10 dB Hz are observed, in addition to strong phase variation. Our results suggest that a considerable distance of the receiver to the plasma irregularity slab is needed to affect the Fresnel diffractive process and further causes GPS signal amplitude fades., Astrodynamics & Space Missions

The intent of this project was to test and evaluate a new data collection concept; that of utilizing a "store and forward" message system in a low earth orbiting satellite, and to determine its suitabilty for oceanographic data telemetry. This new generation of satellites, dubbed "MicroSats" because of their small size (a 9 in. cube), was developed by the Amateur Radio Satellite Corporation (AMSAT) to complement the existing HF and VHG terrestral Packet data switching networks., Funding was provided by the Office of Naval Research under Contract No. N00014-90-0013.

The intent of this project was to test and evaluate a new data collection concept; that of utilizing a "store and forward" message system in a low earth orbiting satellite, and to determine its suitabilty for oceanographic data telemetry. This new generation of satellites, dubbed "MicroSats" because of their small size (a 9 in. cube), was developed by the Amateur Radio Satellite Corporation (AMSAT) to complement the existing HF and VHG terrestral Packet data switching networks., Funding was provided by the Office of Naval Research under Contract No. N00014-90-0013.

The intent of this project was to test and evaluate a new data collection concept; that of utilizing a "store and forward" message system in a low earth orbiting satellite, and to determine its suitabilty for oceanographic data telemetry. This new generation of satellites, dubbed "MicroSats" because of their small size (a 9 in. cube), was developed by the Amateur Radio Satellite Corporation (AMSAT) to complement the existing HF and VHG terrestral Packet data switching networks., Funding was provided by the Office of Naval Research under Contract No. N00014-90-0013.

In this thesis, two LEO satellite systems with different multiple access schemes are analyzed. The first system, GLOBALSTAR, uses CDMA. Equations are developed to calculate the maximum capacity of one satellite, of one satellite's user beam, and of the entire GLOBALSTAR system over CONUS. A detailed description of GLOBALSTAR's outage probability, the probability that a call will be dropped from the system or blocked from connection with the system, is given and graphed against varying average call time and varying call arrival rate. The second system, IRIDIUM, uses TDMA/FDMA. Equations are similarly developed to calculate the maximum capacity of one satellite, of one satellite's user beam, and of the entire IRIDIUM system over CONUS. The probability of success or failure of an IRIDIUM subscriber obtaining a system channel is given by the Erlang Loss Formula and graphed against varying average call time and varying call arrival rate. Results show GLOBALSTAR provides five times the service capacity of IRIDIUM over CONUS and provides a better probability of obtaining a system channel for a call than will IRIDIUM.