Interplanetary Trajectory Optimization with Powerlimited Propulsion Systems

A trajectory-optimization process is described in which the optimum thrust equations are derived using the calculus of variations. The magnitude of the thrust is constrained within an upper and a lower bound, but the thrust direction is arbitrary. This formulation allows both the constant-thrust program and the variable-thrust program to be considered. For the constant-thrust program, certain propulsion-system parameters are optimized for maximum final vehicle mass. This theory has been used to study interplanetary missions to Venus and Mars using a power-limited propulsion system. Both one-way and round trip rendezvous trajectories are considered. The analysis employs a two-body inverse-square force-field model of three dimensions. An iterative routine used to solve the two-point boundary-value problem is described in the Appendix

JPL development ephemeris no. 69

Improved lunar and planetary ephemeri

Moment release rate of Cascadia tremor constrained by GPS

A comparison of GPS and seismic analyses of 23 distinct episodic tremor and slip events, located throughout the Cascadia subduction zone over an 11-year period, yields a highly linear relationship between moment release, as estimated from GPS, and total duration of nonvolcanic tremor, as summed from regional seismic arrays. The events last 1–5 weeks, typically produce ~5 mm of static forearc deformation, and show cumulative totals of tremor that range from 40 to 280 h. Moment released by each event is estimated by inverting GPS-measured deformation, which is sensitive to all rates of tremor-synchronous faulting, including aseismic creep, for total slip along the North American-Juan de Fuca plate interface. Tremor, which is shown to be largely invariant in amplitude and frequency content both between events and with respect to its duration, is quantified using several different parameterizations that agree to within 10%. All known Cascadia events detected since 1997, which collectively span the Cascadia arc from northern California to Vancouver Island, Canada, release moment during tremor at a rate of 5.2 ± 0.4 X 1016 N m per hour of recorded tremor. This relationship enables estimation of moment dissipation, via seismic monitoring of tremor, along the deeper Cascadia subduction zone that poses the greatest threat to its major metropolitan centers

Data intensive scientific analysis with grid computing

At the end of September 2009, a new Italian GPS receiver for radio occultation was launched from the Satish Dhawan Space Center (Sriharikota, India) on the Indian Remote Sensing OCEANSAT-2 satellite. The Italian Space Agency has established a set of Italian universities and research centers to implement the overall processing radio occultation chain. After a brief description of the adopted algorithms, which can be used to characterize the temperature, pressure and humidity, the contribution will focus on a method for automatic processing these data, based on the use of a distributed architecture. This paper aims at being a possible application of grid computing for scientific research

Real-time Monitoring of Tectonic Displacements in the Pacific Northwest through an Array of GPS Receivers

The Pacific Northwest Geodesic Array at Central Washington University collects telemetered streaming data from 450 GPS stations. These real-time data are used to monitor and mitigate natural hazards arising from earthquakes, volcanic eruptions, landslides, and coastal sea-level hazards in the Pacific Northwest. The displacement measurements are performed at millimeter-scale, and require stringent analysis and parameter estimation techniques. Recent improvements in both accuracy of positioning measurements and latency of terrestrial data communication have led to the ability to collect data with higher sampling rates, of up to 1 Hz. For seismic monitoring applications, this means 1350 separate position streams from stations located across 1200 km along the West Coast of North America must be able to be both visually observed and analyzed automatically. We aim to make the real-time information from GPS sensors easily available, including public access via interfaces for all intelligent devices with a connection to the Internet. Our contribution is a dashboard application that monitors the real-time status of the network of GPS sensors. We are able to visualize individual and multiple sensors using similar time series scales. We are also able to visualize groups of sensors based on time-dependent statistical similarity, such as sensors with the the highest variance, in real-time. In addition to raw positioning data, users can also display derived quantities, such as the Allan variance or the second derivative of a data stream

A moving target : matching graduate education with available careers for ocean scientists

Author Posting. © The Oceanography Society, 2016. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 29, no. 1 (2016): 22–30, doi:10.5670/oceanog.2016.05.The objective of this paper is to look at past assessments and available data to examine the match (or mismatch) between university curricula and programs available to graduate students in the ocean sciences and the career possibilities available to those students. We conclude there is a need for fundamental change in how we educate graduate students in the ocean sciences. The change should accommodate the interests of students as well as the needs of a changing society; the change should not be constrained by the traditions or resource challenges of the graduate institutions themselves. The limited data we have been able to obtain from schools and employers are consistent with this view: desirable careers for ocean scientists are moving rapidly toward interdisciplinary, collaborative, societally relevant activities, away from traditional academic-research/professorial jobs, but the training available to the students is not keeping pace. We offer some suggestions to mitigate the mismatch. Most importantly, although anecdotes and “gut feelings” abound, the quantitative data backing our conclusions and suggestions are very sparse and barely compelling; we urge better data collection to support curricular revision, perhaps with the involvement of professional societies

Noise Characteristics of Operational Real‐Time High‐Rate GNSS Positions in a Large Aperture Network

Large earthquakes are difficult to model in real‐time with traditional inertial seismic measurements. Several algorithms that leverage high‐rate real‐time Global Navigation Satellite Systems (HR‐GNSS) positions have been proposed, and it has been shown that they can supplement the earthquake monitoring effort. However, analyses of the long‐term noise behavior of high‐rate real‐time GNSS positions, which are important to understand how the data can be used operationally by monitoring agencies, have been limited to just a few sites and to short time spans. Here, we show results from an analysis of the noise characteristics of 1 year of positions at 213 GNSS sites spanning a large geographic region from Southern California to Alaska. We characterize the behavior of noise and propose several references noise models which can be used as baselines to compare against as technological improvements allow for higher precision solutions. We also show how to use the reference noise models to generate realistic synthetic noise that can be used in simulations of HR‐GNSS waveforms. We discuss spatiotemporal variations in the noise and their potential sources and significance. We also detail how noise analysis can be used in a dynamic quality control to determine which sites should or should not contribute positions to an earthquake modeling algorithm at a particular moment in time. We posit that while there remain important improvements yet to be made, such as reducing the number of outliers in the time series, the present quality of real‐time HR‐GNSS waveforms is more than sufficient for monitoring large earthquakes

Unstable Attractors: Existence and Robustness in Networks of Oscillators With Delayed Pulse Coupling

We consider unstable attractors; Milnor attractors $A$ such that, for some neighbourhood $U$ of $A$, almost all initial conditions leave $U$. Previous research strongly suggests that unstable attractors exist and even occur robustly (i.e. for open sets of parameter values) in a system modelling biological phenomena, namely in globally coupled oscillators with delayed pulse interactions. In the first part of this paper we give a rigorous definition of unstable attractors for general dynamical systems. We classify unstable attractors into two types, depending on whether or not there is a neighbourhood of the attractor that intersects the basin in a set of positive measure. We give examples of both types of unstable attractor; these examples have non-invertible dynamics that collapse certain open sets onto stable manifolds of saddle orbits. In the second part we give the first rigorous demonstration of existence and robust occurrence of unstable attractors in a network of oscillators with delayed pulse coupling. Although such systems are technically hybrid systems of delay differential equations with discontinuous `firing' events, we show that their dynamics reduces to a finite dimensional hybrid system system after a finite time and hence we can discuss Milnor attractors for this reduced finite dimensional system. We prove that for an open set of phase resetting functions there are saddle periodic orbits that are unstable attractors.Comment: 29 pages, 8 figures,submitted to Nonlinearit

IMECE2002-32160 SURFACE ROUGHNESS EFFECTS ON CIRCULAR CYLINDERS AT HIGH REYNOLDS NUMBERS

ABSTRACT The problem of flow-induced vibration has been studied extensively. However, much of this research has focused on the smooth cylinder to gain an understanding of the mechanisms that cause vortex-induced vibration. In this paper results of an investigation of the effect of surface roughness on the crosswind forces are presented. Measurements of the sectional RMS fluctuating lift forces and the axial correlation of the pressures for Reynolds numbers from 1 x 10 5 to 1.4 x 10 6 are given. It was found that surface roughness significantly increased the axial correlation of the pressures to similar values found at high subcritical Reynolds numbers. There was little effect of the surface roughness on the sectional lift forces. The improved correlation of the vortex shedding means rough cylinders will be subject to larger cross-wind forces and an increased possibility of vortex-induced vibration compared to smooth cylinders. INTRODUCTION The problem of vortex-induced vibration confronts us in many everyday situations; from the vibration of a domestic television aerial to flow over heat exchanger tube banks to large towers, and in some situations left untreated can lead to structure failures. The importance of finding causes and solutions to vortex induced vibration is reflected in the quantity of research. Many of the circular cylinder applications mentioned in the previous paragraph are not highly polished, instead they have a degree of surface roughness caused by manufacturing tolerances or environmental conditions. Research on smooth cylinders in low turbulence flow has shown the fluid-structure mechanisms that result in vibration over a wide range of Reynolds numbers. However, the quantity of research data available showing the effect of surface roughness an

Classification and stability of simple homoclinic cycles in R^5

The paper presents a complete study of simple homoclinic cycles in R^5. We find all symmetry groups Gamma such that a Gamma-equivariant dynamical system in R^5 can possess a simple homoclinic cycle. We introduce a classification of simple homoclinic cycles in R^n based on the action of the system symmetry group. For systems in R^5, we list all classes of simple homoclinic cycles. For each class, we derive necessary and sufficient conditions for asymptotic stability and fragmentary asymptotic stability in terms of eigenvalues of linearisation near the steady state involved in the cycle. For any action of the groups Gamma which can give rise to a simple homoclinic cycle, we list classes to which the respective homoclinic cycles belong, thus determining conditions for asymptotic stability of these cycles.Comment: 34 pp., 4 tables, 30 references. Submitted to Nonlinearit