Effect of atmospheric anisoplanatism on earth-to-satellite time transfer over laser communication links

© 2017 [2017 Optical Society of America.]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.The need for an accurate time reference on orbiting platforms motivates study of time transfer via free-space optical communication links. The impact of atmospheric turbulence on earth-to-satellite optical time transfer has not been fully characterized, however. We analyze limits to two-way laser time transfer accuracy posed by anisoplanatic non-reciprocity between uplink and downlink. We show that despite limited reciprocity, two- way time transfer can still achieve sub-picosecond accuracy in realistic propagation scenarios over a single satellite visibility period.Peer ReviewedPostprint (published version

Impact and implementation of higher-order ionospheric effects on precise GNSS applications

High precision Global Navigation Satellite Systems (GNSS) positioning and time transfer require correcting signal delays, in particular higher-order ionospheric (I2+) terms. We present a consolidated model to correct second- and third-order terms, geometric bending and differential STEC bending effects in GNSS data. The model has been implemented in an online service correcting observations from submitted RINEX files for I2+ effects. We performed GNSS data processing with and without including I2+ corrections, in order to investigate the impact of I2+ corrections on GNSS products. We selected three time periods representing different ionospheric conditions. We used GPS and GLONASS observations from a global network and two regional networks in Poland and Brazil. We estimated satellite orbits, satellite clock corrections, Earth rotation parameters, troposphere delays, horizontal gradients, and receiver positions using global GNSS solution, Real-Time Kinematic (RTK), and Precise Point Positioning (PPP) techniques. The satellite-related products captured most of the impact of I2+ corrections, with the magnitude up to 2 cm for clock corrections, 1 cm for the along- and cross-track orbit components, and below 5 mm for the radial component. The impact of I2+ on troposphere products turned out to be insignificant in general. I2+ corrections had limited influence on the performance of ambiguity resolution and the reliability of RTK positioning. Finally, we found that I2+ corrections caused a systematic shift in the coordinate domain that was time- and region-dependent and reached up to -11 mm for the north component of the Brazilian stations during the most active ionospheric conditions.Peer ReviewedPostprint (published version

Ultrastructural analysis of changes in neurons of the mouse internal anal sphincter during ageing

Gastrointestinal disorders, including chronic constipation, faecal impaction and incontinence, are a major cause of morbidity in the elderly

Age-related changes in blood-brain barrier integrity in C57BL/6J mice

The blood-brain barrier (BBB) is formed by the endothelial cells of the brain microvasculature, which control the molecular traffic between the blood and brain to maintain the neural microenvironment

Age-related changes to lumbosacral spinal cord motoneurons that modulate bladder and bowel functions in male C57BL/6 mice

Incontinence and sexual dysfunction are often increased in the aged human population. In rats and mice the pattern of micturition and faecal clearance also changes with ageing and is suggestive of bladder and bowel dysfunction

Exploring space situational awareness using neuromorphic event-based cameras

The orbits around earth are a limited natural resource and one that hosts a vast range of vital space-based systems that support international systems use by both commercial industries, civil organisations, and national defence. The availability of this space resource is rapidly depleting due to the ever-growing presence of space debris and rampant overcrowding, especially in the limited and highly desirable slots in geosynchronous orbit. The field of Space Situational Awareness encompasses tasks aimed at mitigating these hazards to on-orbit systems through the monitoring of satellite traffic. Essential to this task is the collection of accurate and timely observation data. This thesis explores the use of a novel sensor paradigm to optically collect and process sensor data to enhance and improve space situational awareness tasks. Solving this issue is critical to ensure that we can continue to utilise the space environment in a sustainable way. However, these tasks pose significant engineering challenges that involve the detection and characterisation of faint, highly distant, and high-speed targets. Recent advances in neuromorphic engineering have led to the availability of high-quality neuromorphic event-based cameras that provide a promising alternative to the conventional cameras used in space imaging. These cameras offer the potential to improve the capabilities of existing space tracking systems and have been shown to detect and track satellites or ‘Resident Space Objects’ at low data rates, high temporal resolutions, and in conditions typically unsuitable for conventional optical cameras. This thesis presents a thorough exploration of neuromorphic event-based cameras for space situational awareness tasks and establishes a rigorous foundation for event-based space imaging. The work conducted in this project demonstrates how to enable event-based space imaging systems that serve the goals of space situational awareness by providing accurate and timely information on the space domain. By developing and implementing event-based processing techniques, the asynchronous operation, high temporal resolution, and dynamic range of these novel sensors are leveraged to provide low latency target acquisition and rapid reaction to challenging satellite tracking scenarios. The algorithms and experiments developed in this thesis successfully study the properties and trade-offs of event-based space imaging and provide comparisons with traditional observing methods and conventional frame-based sensors. The outcomes of this thesis demonstrate the viability of event-based cameras for use in tracking and space imaging tasks and therefore contribute to the growing efforts of the international space situational awareness community and the development of the event-based technology in astronomy and space science applications

Strategies to Recover from Satellite Communication Failures

In natural and manmade disasters, inadequate strategies to recover from satellite communication (SATCOM) failures can affect the ability of humanitarian organizations to provide timely assistance to the affected populations. This single case study explored strategies used by network administrators (NAs) to recover from SATCOM failures in humanitarian operations. The study population were NAs in Asia, the Middle East, Central Africa, East Africa, and West Africa. Data were collected from semistructured interviews with 9 NAs and an analysis of network statistics for their locations. The resource-based view was used as the conceptual framework for the study. Using inductive analysis, 3 themes emerged from coding and triangulation: redundancy of equipment, knowledge transfer, and the use of spare parts to service the SATCOM infrastructure. The findings showed that the organization\u27s use of knowledge, and collaboration among NAs and nontechnical staff improved the organization\u27s ability to recover from SATCOM failures. The implication of this study for social change was the reduced cost of satellite services due to the efficient use of the bandwidth. These savings can be channeled into the purchase of vaccines, shelter, and the improvement in the quality of water and sanitation for displaced persons in humanitarian disasters, which improve the organization\u27s delivery of humanitarian services to the affected populations in the disaster

Stacked Modulation Formats Enabling Highest-Sensitivity Optical Free-Space Communications

Die vorliegende Arbeit befasst sich mit hochempfindlichen optischen Kommunikationssystemen, wie sie z.B. bei Intersatellitenlinks verwendet werden. Theoretische Überlegungen zur Steigerung der Empfängerempfindlichkeit werden mit Simulations- und Messergebnissen ergänzt und verifiziert. Auf Grund der steigenden Nachfrage nach optischen Links zwischen Satelliten stellt sich die Frage, was sind geeignete Eckparameter, um ein solches System zu beschreiben. Die gigantischen Datenmengen, die von diversen Messgeräten, wie z.B. hochauflösende Kameras auf einem Satelliten generiert werden, bringen die Kapazitäten klassischer HF-Datenlinks an ihre Grenzen. Hier können optische Kommunikationssysteme auf Grund ihrer hohen Trägerfrequenz im Infrarotbereich sehr hohe Datenraten im Terabit/s Bereich ermöglichen. Systeme mit Radiowellen im GHz Bereich als Trägerfrequenz sind hier deutlich limitierter. [7] Linkdistanz, verfügbare Leistung, Pointinggenauigkeit und verfügbare Antennengröße sind einige Parameter, die einen wichtigen Einfluss auf die Leistungsfähigkeit des Systems haben. Je größer die Distanz und desto kleiner die verfügbare Antennengröße sowohl am Sender als auch am Empfänger sind, desto weniger Signalleistung wird den Detektor erreichen. Nimmt man dann noch ungenaues Pointing hinzu, d.h. Sender und Empfänger sind nicht exakt aufeinander ausgerichtet, treten zusätzliche Verluste auf. [7] Ziel dieser Arbeit ist es, ein vereinfachtes System zu implementieren und zu testen, das mit möglichst wenigen Photonen pro Bit bei einer gegebenen Bitfehlerwahrscheinlichkeit bei einer möglichst hohen Datenrate arbeiten kann. Hierfür werden alle Freiheitsgrade einer optischen Welle zur Modulation verwendet, um mit sog. „Stapeln“ von Modulationsformaten eine Empfindlichkeitssteigerung zu erreichen. Die Amplitude des Signals wird durch Pulspositionsmodulation (PPM) moduliert, wobei das zeitlich variable Vorhandensein eines Pulses innerhalb des Symbols die Information enthält. Dieses Modulationsformat weist bis dato die höchste Empfindlichkeit in Literatur und Experimenten auf [4]. Je mehr Möglichkeiten es gibt, einen Puls in einem Symbol zu platzieren, desto höher ist die zu erwartende Empfindlichkeit des Systems. Mit anderen Worten: Steigert man die zeitliche Dauer eines PPM-Symbols, so wächst ebenfalls die Empfängerempfindlichkeit. Da bei diesem Ansatz die Datenrate sinkt, wird in dieser Arbeit eine andere Methode vorgestellt, die Empfindlichkeit eines Übertragungssystems zu steigern, ohne die Symbollänge unnötig in die Länge zu ziehen. Diese Arbeit befasst sich mit dem Stapeln (sog. „Stacking“) von Modulationsformaten, in dem neben der Amplitudenmodulation weitere Freiheitsgrade, wie die Frequenz, Phase und Polarisation geschickt genutzt werden. Bei der Frequenzumtastung (FSK) wird die optische Frequenz je nach Symbol um ein gewisses Maß verschoben. Bei der polarisations-geschalteten Quadratur-Phasenumtastung (PS-QPSK) werden sowohl die Phase, als auch die Polarisation der optischen Welle moduliert [12]. Als Endergebnis erhält man PPM-FSK-PS-QPSK als Modulationsformat mit hoher Empfindlichkeit. Gegenüber dem reinen PPM wird eine theoretische Empfindlichkeitssteigerung von mehr als 1 dB erreicht. Sowohl Simulations- als auch Messergebnisse bestätigen den Empfindlichkeitsgewinn