Willis P.,IGN |
Willis P.,University Paris Diderot |
Lemoine F.G.,GSFC |
Moreaux G.,CLS |
And 8 more authors.
International Association of Geodesy Symposia | Year: 2016
The International DORIS Service (IDS) was created in 2003 under the umbrella of the International Association of Geodesy (IAG) to foster scientific research related to the French DORIS tracking system and to deliver scientific products, mostly related to the International Earth rotation and Reference systems Service (IERS). We first present some general background related to the DORIS system (current and planned satellites, current tracking network and expected evolution) and to the general IDS organization (from Data Centers, Analysis Centers and Combination Center). Then, we discuss some of the steps recently taken to prepare the IDS submission to ITRF2013 (combined weekly time series based on individual solutions from several Analysis Centers). In particular, recent results obtained from the Analysis Centers and the Combination Center show that improvements can still be made when updating physical models of some DORIS satellites, such as Envisat, Cryosat-2 or Jason-2. The DORIS contribution to ITRF2013 should also benefit from the larger number of ground observations collected by the last generation of DGXX receivers (first instrument being onboard Jason-2 satellite). In particular for polar motion, submilliarcsecond accuracy seems now to be achievable. Weekly station positioning internal consistency also seems to be improved with a larger DORIS constellation. © Springer International Publishing Switzerland 2015.
Zerbo J.L.,Bobo-Dioulasso Polytechnic University |
Zerbo J.L.,CNRS Physics Laboratory |
Amory Mazaudier C.,CNRS Physics Laboratory |
Ouattara F.,Ecole Normale Superieure Of Luniversite Of Koudougou |
Richardson J.D.,Center for Space Research
Annales Geophysicae | Year: 2012
We examined solar activity with a large series of geomagnetic data from 1868 to 2009. We have revisited the geomagnetic activity classification scheme of Legrand and Simon (1989) and improve their scheme by lowering the minimum Aa index value for shock and recurrent activity from 40 to 20 nT. This improved scheme allows us to clearly classify about 80% of the geomagnetic activity in this time period instead of only 60% for the previous Legrand and Simon classification. © Author(s) 2012.
Smith N.H.,Center for Space Research |
Bae S.,Center for Space Research |
Schutz B.E.,Center for Space Research
Journal of Spacecraft and Rockets | Year: 2010
Approximately 1% of the 10,000 stars observed by the ice, cloud, and land elevation satellite star trackers are believed to have position measurement biases caused by near-neighbor stars. The biases are in the tracker measurements, not the star catalogs. Empirical biases were derived for 49 stars. A survey was performed to detect and characterize biased stars by treating each observed star as a target, predicting the tracker measurements of the target, and then comparing the observations and predictions. The distribution of prediction accuracies for unbiased stars had a mean of 1.46 arcseconds and a standard deviation of 0.61 arcseconds. Ninety percent of the sky was covered and five million passes of 10,472 stars were processed. Stars were classified using a Mahalanobis distance parameter, which scaled position residuals by prediction uncertainties. Stars with large Mahalanobis distances were then studied individually. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.
Azimov D.M.,University of Texas at Austin |
Azimov D.M.,Center for Space Research
Journal of Guidance, Control, and Dynamics | Year: 2010
The variational problem of determining optimal trajectories of motion with constant exhaust velocity and limited mass-flow rate in a central Newtonian field is considered. The first-order necessary conditions of optimality reduce the problem to a Hamiltonian canonical system of equations for intermediate- and maximum-thrust arcs, both of which have no complete analytical solutions to date. The approach used in this work is based on the analytical integration of the canonical system by employing its first integrals and invariant expressions. Several new classes of extremal analytical solutions for planar intermediate-thrust arcs with free and fixed flight times are presented. The solutions describe families of spiral trajectories around the center of attraction. The main result of the paper is that, in their current form with known integrals, the differential equations of the variational problem for intermediate-thrust arcs are integrable in elementary functions and quadratures, and the solution of this problem with such arcs can be reduced to a system of algebraic continuity equations formed for each junction point. These solutions can be used as representative reference trajectories for guidance algorithms and to compute initial values of Lagrange multipliers for high-fidelity trajectory optimization software. As an illustrative example, the transfer maneuver to a given elliptical parking orbit using an intermediate-thrust arc is discussed. Results of simulations for three study cases containing the change of eccentricity and semiparameter of the parking orbit and specific impulses are presented. Copyright © 2010 by Dilmurat M. Azimov. Published by the American Institute of Aeronautics and Astronautics, Inc.
Wang F.,University of Texas at Austin |
Wang F.,Center for Space Research |
Bettadpur S.,University of Texas at Austin |
Bettadpur S.,Center for Space Research |
And 4 more authors.
Journal of Spacecraft and Rockets | Year: 2010
The Gravity Recovery and Climate Experiment (GRACE) mission, launched on 17 March 2002, uses radiometric tracking between win, coorbiting satellites in a polar 500-km-alt orbit, in order to make detailed measurements of Earth's gravity field. These measurements have led to significant, new insights into climate-driven mass transport in the Earth system. A key element of the GRACE scientific measurement suite is the high-precision accelerometer required to measure the nongravitational accelerations acting on the GRACE satellites. To avoid contamination of nongravitational acceleration measurements, the GRACE mission requires the proof mass of the accelerometer to be positioned within 100 |im (0.1 mm) of the center-of-mass of satellite. This is accomplished using a dedicated center-of-mass calibration maneuver every few months. This paper describes the GRACE center-of-mass calibration maneuver design and implementation details and presents the data analysis used to routinely measure the center-of-mass offset. Using external validation and internal consistency checks, we show that the GRACE satellite center-of-mass offset is being measured routinely to approximately 25 to 40 μm precision along the three satellite axes.
Smith N.H.,University of Texas at Austin |
Smith N.H.,Center for Space Research |
Bae S.,University of Texas at Austin |
Bae S.,Center for Space Research |
And 4 more authors.
Journal of Spacecraft and Rockets | Year: 2014
The Laser Reference Sensor is the central instrument in the Ice, Cloud, and land Elevation Satellite laser pointing knowledge system, simultaneously observing the altimetry laser, stars, and a reference signal in a single instrument coordinate frame. The reference signal is intended to provide direct observations of the alignment between the Laser Reference Sensor and the Instrument Star Tracker. The reference signal failed early in the mission and a method was developed to partially replace it by comparing two attitude time series: an attitude filter time series for the Instrument Star Tracker and a pure-gyro time series for the Laser Reference Sensor. Only the Instrument Star Tracker and gyros are used in the replacement method, with the gyros tracking the Laser Reference Sensor attitude in order to make the relative motion of the Instrument Star Tracker observable. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
News Article | April 8, 2016
FILE - In this July 26, 2011 file photo, drops of water fall from a melting iceberg near Nuuk, Greenland. Global warming is shifting the way the Earth wobbles on its polar axis, a new NASA study finds. Melting ice sheets, especially in Greenland, are changing the distribution of weight on Earth. And that has caused both the North Pole and the wobble, which is called polar motion, to change course, according to a study published Friday, April 8, 2016, in the journal Science Advances. (AP Photo/Brennan Linsley, File) WASHINGTON (AP) — Global warming is shifting the way the Earth wobbles on its polar axis, a new NASA study finds. Melting ice sheets — especially in Greenland — are changing the distribution of weight on Earth. And that has caused both the North Pole and the wobble, which is called polar motion, to change course, according to a study published Friday in the journal Science Advances. Scientists and navigators have been accurately measuring the true pole and polar motion since 1899 and for almost the entire 20th century they migrated a bit toward Canada. But that has changed with this century and now it's moving toward England, said study lead author Surendra Adhikari at NASA's Jet Propulsion Lab. "The recent shift from the 20th-century direction is very dramatic," Adhikari said. While scientists say the shift is harmless, it is meaningful. Jonathan Overpeck, professor of geosciences at the University of Arizona who wasn't part of the study, said "this highlights how real and profoundly large an impact humans are having on the planet." Since 2003, Greenland has lost on average more than 600 trillion pounds of ice a year and that affects the way the Earth wobbles in a manner similar to a figure skater lifting one leg while spinning, said NASA scientist Eirk Ivins, the study's co-author. Ivins said he likes to think of it as a billion trucks each year dumping ice out of Greenland. On top of that, West Antarctica loses 275 trillion pounds of ice and East Antarctica gains about 165 trillion pounds of ice yearly, helping tilt the wobble further, Ivins said. They all combine to pull polar motion toward the east, Adhikari said. Jianli Chen, a senior research scientist at the University of Texas' Center for Space Research, first attributed the pole shift to climate change in 2013 and he said this new study takes his work a step further. "There is nothing to worry about," said Chen, who wasn't part of the NASA study. "It is just another interesting effect of climate change." Follow Seth Borenstein at http://twitter.com/borenbears and his work can be found at http://bigstory.ap.org/content/seth-borenstein
News Article | April 9, 2016
Global warming is changing the way the Earth wobbles on its polar axis, a new Nasa study has found. Melting ice sheets, especially in Greenland, are changing the distribution of weight on Earth. And that has caused both the North Pole and the wobble, which is called polar motion, to change course, according to a study published on Friday in the journal Science Advances. Scientists and navigators have been accurately measuring the true pole and polar motion since 1899, and for almost the entire 20th century they migrated a bit toward Canada. But that has changed with this century, and now it’s moving toward England, according to study lead author Surendra Adhikari at Nasa’s Jet Propulsion Lab. “The recent shift from the 20th-century direction is very dramatic,” Adhikari said. While scientists say the shift is harmless, it is meaningful. Jonathan Overpeck, professor of geosciences at the University of Arizona, who wasn’t part of the study, said that “this highlights how real and profoundly large an impact humans are having on the planet.” Since 2003, Greenland has lost on average more than 272 trillion kilograms of ice a year, and that affects the way the Earth wobbles in a manner similar to a figure skater lifting one leg while spinning, said Nasa scientist Eirk Ivins, the study’s co-author. On top of that, West Antarctica loses 124 trillion kgs of ice and East Antarctica gains about 74 trillion kgs of ice yearly, helping tilt the wobble further, Ivins said. They all combine to pull polar motion toward the east, Adhikari said. Jianli Chen, a senior research scientist at the University of Texas’ Center for Space Research, first attributed the pole shift to climate change in 2013, and he said this new study takes his work a step further. “There is nothing to worry about,” said Chen, who wasn’t part of the Nasa study. “It is just another interesting effect of climate change.”
News Article | January 20, 2016
The Texas Advanced Computing Center (TACC) at The University of Texas at Austin (UT Austin) announced that the Lonestar 5 supercomputer is in full production and is ready to contribute to advancing science across the state of Texas. Managed by TACC, the center's second petaflop system is primed to be a leading computing resource for the engineering and science research community. The supercomputer is sponsored by UT System in partnership with UT Austin, Texas Tech University, Texas A&M University, and the Institute for Computational Engineering and Sciences (ICES) and the Center for Space Research at The University of Texas at Austin. The technology partners are Cray, Intel and DataDirect Networks. Lonestar 5 is designed for academic researchers, serving as the primary high performance computing resource in the UT Research Cyberinfrastructure (UTRC) initiative. Sponsored by The University of Texas System (UT System), UTRC provides a combination of advanced computational systems, a large data storage opportunity, and high bandwidth data access. UTRC enables researchers within all 14 UT System institutions to collaborate with each other and compete at the forefront of science and discovery. The new Lonestar 5 Cray XC40 supercomputer, which contains more than 30,000 Intel Xeon processing cores from the E5-2600 v3 product family, provides a peak performance of 1.25 petaflops. With 24 processing cores per compute node, Lonestar 5 follows the trend of more cores per node that the industry sees in every generation of microprocessors. The system is the fifth in a long line of systems available for Texas researchers, dating back over 15 years to the original Lonestar 1 system (also a Cray). The system will continue to serve its mainstay user communities with an emphasis on addressing a wide variety of research areas in engineering, medicine and the sciences. A number of researchers have been using Lonestar 5 in an "early user" mode over the last few months. Researchers from UT System institutions and contributing partners wishing to request access to Lonestar 5 should do so via the TACC User Portal.