2,171 research outputs found
PHARAO Laser Source Flight Model: Design and Performances
In this paper, we describe the design and the main performances of the PHARAO
laser source flight model. PHARAO is a laser cooled cesium clock specially
designed for operation in space and the laser source is one of the main
sub-systems. The flight model presented in this work is the first
remote-controlled laser system designed for spaceborne cold atom manipulation.
The main challenges arise from mechanical compatibility with space constraints,
which impose a high level of compactness, a low electric power consumption, a
wide range of operating temperature and a vacuum environment. We describe the
main functions of the laser source and give an overview of the main
technologies developed for this instrument. We present some results of the
qualification process. The characteristics of the laser source flight model,
and their impact on the clock performances, have been verified in operational
conditions.Comment: Accepted for publication in Review of Scientific Instrument
Study of high voltage solar array configurations with integrated power control electronics
Solar array electrical configurations for voltage regulatio
Planning assistance for the NASA 30/20 GHz program. Network control architecture study.
Network Control Architecture for a 30/20 GHz flight experiment system operating in the Time Division Multiple Access (TDMA) was studied. Architecture development, identification of processing functions, and performance requirements for the Master Control Station (MCS), diversity trunking stations, and Customer Premises Service (CPS) stations are covered. Preliminary hardware and software processing requirements as well as budgetary cost estimates for the network control system are given. For the trunking system control, areas covered include on board SS-TDMA switch organization, frame structure, acquisition and synchronization, channel assignment, fade detection and adaptive power control, on board oscillator control, and terrestrial network timing. For the CPS control, they include on board processing and adaptive forward error correction control
Progress in Atomic Fountains at LNE-SYRTE
We give an overview of the work done with the Laboratoire National de
M\'etrologie et d'Essais-Syst\`emes de R\'ef\'erence Temps-Espace (LNE-SYRTE)
fountain ensemble during the last five years. After a description of the clock
ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest
developments, we review recent studies of several systematic frequency shifts.
This includes the distributed cavity phase shift, which we evaluate for the FO1
and FOM fountains, applying the techniques of our recent work on FO2. We also
report calculations of the microwave lensing frequency shift for the three
fountains, review the status of the blackbody radiation shift, and summarize
recent experimental work to control microwave leakage and spurious phase
perturbations. We give current accuracy budgets. We also describe several
applications in time and frequency metrology: fountain comparisons,
calibrations of the international atomic time, secondary representation of the
SI second based on the 87Rb hyperfine frequency, absolute measurements of
optical frequencies, tests of the T2L2 satellite laser link, and review
fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally,
we give a summary of the tests of the PHARAO cold atom space clock performed
using the FOM transportable fountain.Comment: 19 pages, 12 figures, 5 tables, 126 reference
Viking '75 spacecraft design and test summary. Volume 1: Lander design
The Viking Mars program is summarized. The design of the Viking lander spacecraft is described
Navigation/traffic control satellite mission study. Volume 3 - System concepts
Satellite network for air traffic control, solar flare warning, and collision avoidanc
Investigating Performance and Reliability of Process Bus Networks for Digital Protective Relaying
To reduce the cost of complex and long copper wiring, as well as to achieve flexibility in signal communications, IEC 61850 part 9-2 proposes a process bus communication network between process level switchyard equipments, and bay level protection and control (P&C) Intelligent Electronic Devices (IEDs). After successful implementation of Ethernet networks for IEC 61850 standard part 8-1 (station bus) at several substations worldwide, major manufacturers are currently working on the development of interoperable products for the IEC 61850-9-2 based process bus. The major technical challenges for applying Ethernet networks at process level include: 1) the performance of time critical messages for protection applications; 2) impacts of process bus Ethernet networks on the reliability of substation protection systems.
This work starts with the performance analysis in terms of time critical Sampled Value (SV) messages loss and/or delay over the IEC 61850-9-2 process bus networks of a typical substation. Unlike GOOSE, the SV message is not repeated several times, and therefore, there is no assurance that each SV message will be received from the process bus network at protection IEDs. Therefore, the detailed modeling of IEC 61850 based substation protection devices, communication protocols, and packet format is carried out using an industry-trusted simulation tool OPNET, to study and quantify number of SV loss and delay over the process bus.
The impact of SV loss/delay on digital substation protection systems is evident, and recognized by several manufacturers. Therefore, a sample value estimation algorithm is developed in order to enhance the performance of digital substation protection functions by estimating the lost and delayed sampled values. The error of estimation is evaluated in detail considering several scenarios of power system relaying. The work is further carried out to investigate the possible impact of SV loss/delay on protection functions, and test the proposed SV estimation algorithm using the hardware setup. Therefore, a state-of-the-art process bus laboratory with the protection IEDs and merging unit playback simulator using industrial computers on the QNX hard-real-time platform, is developed for a typical IEC 61850-9-2 based process bus network. Moreover, the proposed SV estimation algorithm is implemented as a part of bus differential and transmission line distance protection IEDs, and it is tested using the developed experimental setup for various SV loss/delay scenarios and power system fault conditions.
In addition to the performance analysis, this work also focuses on the reliability aspects of protection systems with process bus communication network. To study the impact of process bus communication on reliability indices of a substation protection function, the detailed reliability modeling and analysis is carried out for a typical substation layout. First of all, reliability analysis is done using Reliability Block Diagrams (RBD) considering various practical process bus architectures, as well as, time synchronization techniques. After obtaining important failure rates from the RBD, an extended Markov model is proposed to analyze the reliability indices of protection systems, such as, protection unavailability, abnormal unavailability, and loss of security. It is shown with the proposed Markov model that the implementation of sampled value estimation improves the reliability indices of a protection system
Space-based Aperture Array For Ultra-Long Wavelength Radio Astronomy
The past decade has seen the rise of various radio astronomy arrays,
particularly for low-frequency observations below 100MHz. These developments
have been primarily driven by interesting and fundamental scientific questions,
such as studying the dark ages and epoch of re-ionization, by detecting the
highly red-shifted 21cm line emission. However, Earth-based radio astronomy
below frequencies of 30MHz is severely restricted due to man-made interference,
ionospheric distortion and almost complete non-transparency of the ionosphere
below 10MHz. Therefore, this narrow spectral band remains possibly the last
unexplored frequency range in radio astronomy. A straightforward solution to
study the universe at these frequencies is to deploy a space-based antenna
array far away from Earths' ionosphere. Various studies in the past were
principally limited by technology and computing resources, however current
processing and communication trends indicate otherwise. We briefly present the
achievable science cases, and discuss the system design for selected scenarios,
such as extra-galactic surveys. An extensive discussion is presented on various
sub-systems of the potential satellite array, such as radio astronomical
antenna design, the on-board signal processing, communication architectures and
joint space-time estimation of the satellite network. In light of a scalable
array and to avert single point of failure, we propose both centralized and
distributed solutions for the ULW space-based array. We highlight the benefits
of various deployment locations and summarize the technological challenges for
future space-based radio arrays.Comment: Submitte
High Energy Astronomy Observatory, Mission C, Phase A. Volume 2: Preliminary analyses and conceptual design
An analysis and conceptual design of a baseline mission and spacecraft are presented. Aspects of the HEAO-C discussed include: baseline experiments with X-ray observations of space, analysis of mission requirements, observatory design, structural analysis, thermal control, attitude sensing and control system, communication and data handling, and space shuttle launch and retrieval of HEAO-C
Quantum Physics Exploring Gravity in the Outer Solar System: The Sagas Project
We summarise the scientific and technological aspects of the SAGAS (Search
for Anomalous Gravitation using Atomic Sensors) project, submitted to ESA in
June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The
proposed mission aims at flying highly sensitive atomic sensors (optical clock,
cold atom accelerometer, optical link) on a Solar System escape trajectory in
the 2020 to 2030 time-frame. SAGAS has numerous science objectives in
fundamental physics and Solar System science, for example numerous tests of
general relativity and the exploration of the Kuiper belt. The combination of
highly sensitive atomic sensors and of the laser link well adapted for large
distances will allow measurements with unprecedented accuracy and on scales
never reached before. We present the proposed mission in some detail, with
particular emphasis on the science goals and associated measurements.Comment: 39 pages. Submitted in abridged version to Experimental Astronom
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