scholarly journals The JPL 1986–3 Extragalactic Reference Frame

1988 ◽  
Vol 133 ◽  
pp. 461-464
Author(s):  
O.J. Sovers ◽  
C.D. Edwards ◽  
C.S. Jacobs ◽  
G.E. Lanyi ◽  
R.N. Treuhaft
Keyword(s):  
Rotation Axis ◽  
Time Variability ◽  
Radio Sources ◽  
Dual Frequency ◽  
Deep Space Network ◽  
Space Network ◽  
Celestial Sphere ◽  
Almost All ◽  
Comparable Quality

Intercontinental dual-frequency radio interferometric measurements were carried out during 1978 to 1985 between NASA's Deep Space Network stations in California, Spain, and Australia. Analysis of 6800 pairs of delay and delay rate observations made during 51 sessions produced a catalog of positions of 106 extragalactic radio sources, fairly uniformly distributed over the celestial sphere between −45° and +85° declination. Almost all of the resulting source positions have formal uncertainties between 0.5 and 3 milliarcseconds, with their distributions peaking somewhat below 1 mas. Root-mean-square uncertainties are 2.1 and 2.0 mas for RA and declination, respectively. Evidence is found for a long-term drift of the Earth's rotation axis in inertial space, relative to the 1984 IAU precession and nutation models. Tests for time variability of positions of 32 frequently observed sources place limits at the 1 mas/yr level. Comparisons with independently determined source catalogs of comparable quality show differences of positions of common sources that amount to a few mas, and may indicate the level of systematic errors in VLBI source position measurements.

scholarly journals Radio Reference Frame Stability from VLBI Data

1988 ◽  
Vol 129 ◽  
pp. 331-332
Author(s):  
O. J. Sovers ◽  
R. N. Treuhaft
Keyword(s):  
Reference Frame ◽  
Independent Sets ◽  
Radio Sources ◽  
Deep Space ◽  
Deep Space Network ◽  
Space Network ◽  
Rates Of Change ◽  
Upper Limits ◽  
Vlbi Data ◽  
Almost All

Intercontinental radio interferometric measurements between NASA's Deep Space Network stations yield a catalog of positions of 106 extragalactic radio sources, uniformly distributed between −45° and +85° declination. Almost all of the source positions have formal uncertainties between 0.5 and 3 milliarcseconds. Estimates of three independent sets of 32 source coordinates at average epochs 1980.0, 1983.2, and 1984.5 assess the level of stability of the reference frame over a five-year period. Comparisons of the three sets of source coordinates show a number of cases of > 2σ differences, particularly betwen the 1984–85 and 1981–83 periods. No sources, however, show 2σ shifts among both pairs of epochs. Upper limits of the order of 1 mas/yr can be placed on the time rates of change of the 32 source coordinates.

scholarly journals An Extragalactic Reference Frame From DSN VLBI Measurements–1989

1990 ◽  
Vol 141 ◽  
pp. 261-270 ◽  
Author(s):  
O. J. Sovers
Keyword(s):  
Reference Frame ◽  
Systematic Errors ◽  
Radio Sources ◽  
Dual Frequency ◽  
Source Position ◽  
Deep Space Network ◽  
Space Network ◽  
Data Analyses ◽  
Accuracy Level ◽  
The Impact

Assessment of the impact of recent improvements in Deep Space Network (DSN) instrumentation, as well as of joint data analyses, provide a prognosis for the accuracy level to be expected in future realizations of an inertial radio reference frame. Intercontinental dual-frequency radio interferometric measurements during 68 sessions (including two recent sessions employing Mark III instrumentation) from 1978 to 1989 using NASA's DSN stations in California, Spain, and Australia give 8900 pairs of delay and delay rate observations. Analysis yields a catalog of positions of 200 extragalactic radio sources north of —45° declination. The resulting source position formal uncertainty distributions peak below 1 milliarcsecond, with three fourths being smaller than 2 mas. Comparison with independent measurements shows some evidence for systematic errors at the milliarcsecond level.

scholarly journals Monitoring source variability with the Fast All Sky Telescope

1991 ◽  
Vol 131 ◽  
pp. 445-448
Author(s):  
Kenneth J. Johnston ◽  
Ralph L. Fiedler ◽  
Richard S. Simon
Keyword(s):  
Interstellar Medium ◽  
Radio Wave ◽  
Wave Scattering ◽  
Study Time ◽  
Time Variability ◽  
Radio Sources ◽  
Limited Sample ◽  
Extragalactic Radio Sources ◽  
Design Goal ◽  
Celestial Sphere

AbstractThe proposed Fast All Sky Telescope (FAST) is an interferometer which is intended to monitor the northern four-fifths of the celestial sphere every two days at 8.1 GHz and daily at 2.7 GHz. The design goal is to have a rms sensitivity of 10 mJy/beam at both frequencies. The array is planned to comprise 20 3-meter diameter antennas with a maximum baseline of 0.7 km. FAST will provide a valuable database that may be used to study time variability in a sensitivity limited sample of radio sources. This will significantly impact on the understanding of active Galactic and extragalactic radio sources, as well as on the understanding of radio wave scattering in the interstellar medium.

scholarly journals The JPL/DSN J2000 Radio Reference Frame

1986 ◽  
Vol 109 ◽  
pp. 163-167 ◽  
Author(s):  
A. E. Niell ◽  
J. L. Fanselow ◽  
K. M. Liewer ◽  
O. J. Sovers ◽  
J. B. Thomas ◽  
...  
Keyword(s):  
Reference Frame ◽  
High Accuracy ◽  
Radio Sources ◽  
Deep Space ◽  
Deep Space Network ◽  
Extragalactic Radio Sources ◽  
Space Network ◽  
Spacecraft Navigation ◽  
Planetary Missions ◽  
Very High

Development of a radio reference frame of very high accuracy has become necessary in order to obtain the required accuracy in spacecraft navigation for current and future planetary missions. The antennas of the NASA Deep Space Network are being utilized on a regular basis to obtain the positions of over 100 compact extragalactic radio sources distributed over the sky down to a declination of −45 degrees with uncertainties of less than 0.005 arcseconds.

Long Coherence Intercontinental Interferometry

1972 ◽  
Vol 2 (2) ◽  
pp. 114-115 ◽  
Author(s):  
J. S. Gubbay
Keyword(s):  
Fine Structure ◽  
Flux Density ◽  
Total Flux ◽  
Radio Sources ◽  
Deep Space ◽  
Deep Space Network ◽  
Space Network ◽  
Order Of Magnitude

Compact radio sources have been observed over several years at 2.3 GHz by interferometers of the NASA-JPL Deep Space Network with trans- or inter-continental baselines to study the secular behaviour of fine structure appearing in these sources. A comparison of the variation in flux density of components of diameter < 0.001 arcsec with the corresponding variation in total flux density of the source at the same frequency indicated that variations could be wholly ascribed to components of this order of magnitude.

scholarly journals Long-term Variability of Extragalactic Sources at 843 MHz

1996 ◽  
Vol 175 ◽  
pp. 103-104
Author(s):  
R. W. Hunstead ◽  
B. M. Gaensler
Keyword(s):  
Electron Density ◽  
High Frequency ◽  
Low Frequency ◽  
Time Variability ◽  
Radio Sources ◽  
Interstellar Scintillation ◽  
Low Frequency Variability ◽  
Signal Path ◽  
Frequency Variations

Time variability is commonly observed in the most compact extragalactic radio sources. Low-frequency variability (LFV)—at frequencies <1 GHz—is thought to arise through two different mechanisms, intrinsic and extrinsic. The former is just an extension of the often rapid high-frequency variations, delayed and reduced in amplitude. The latter is usually attributed to refractive interstellar scintillation (RISS; Rickett et al. 1984), whereby the variations in intensity are the result of wavefront distortions caused by transverse gradients in electron density. If RISS arises predominantly along the signal path through our Galaxy, we might expect to find evidence for a dependence on Galactic coordinates.

scholarly journals Accurate Positions of 120 Radio Sources with Declinations Above −45°

1984 ◽  
Vol 110 ◽  
pp. 363-364
Author(s):  
A. E. Niell ◽  
J. L. Fanselow ◽  
O. J. Sovers ◽  
J. B. Thomas ◽  
K. M. Liewer ◽  
...  
Keyword(s):  
Reference Frame ◽  
Radio Sources ◽  
Deep Space ◽  
Entry Angle ◽  
Positional Accuracy ◽  
Deep Space Network ◽  
Space Network ◽  
Vlbi Observations

Development of a catalogue of approximately 100 radio sources having positional accuracy of ~0.005 arcseconds is one goal of the DSN radio reference frame program. This objective is set by the navigation requirements of the Galileo Project for reconstruction of the probe entry angle into the atmosphere of Jupiter. These radio positions are determined by VLBI observations using the antennas of the NASA Deep Space Network on two intercontinental baselines – California-Spain and California-Australia. Since 1978, measurements have been made simultaneously at 2.3 and 8.4 Ghz. Sixteen to twenty-four hours of data on each of the two baselines are usually obtained within a few days of each other, and since mid-1982 these pairs of observations have been made at approximately six week intervals.

Is 3 years adequate for tracking completely occluded coiled aneurysms?

2020 ◽  
Vol 133 (3) ◽  
pp. 758-764
Author(s):  
Eung Koo Yeon ◽  
Young Dae Cho ◽  
Dong Hyun Yoo ◽  
Su Hwan Lee ◽  
Hyun-Seung Kang ◽  
...  
Keyword(s):  
De Novo ◽  
Careful Monitoring ◽  
Surveillance Period ◽  
Radiological Data ◽  
De Novo Aneurysm ◽  
Low Probability ◽  
Almost All ◽  
Annual Risk

OBJECTIVEThe authors conducted a study to ascertain the long-term durability of coiled aneurysms completely occluded at 36 months’ follow-up given the potential for delayed recanalization.METHODSIn this retrospective review, the authors examined 299 patients with 339 aneurysms, all shown to be completely occluded at 36 months on follow-up images obtained between 2011 and 2013. Medical records and radiological data acquired during the extended monitoring period (mean 74.3 ± 22.5 months) were retrieved, and the authors analyzed the incidence of (including mean annual risk) and risk factors for delayed recanalization.RESULTSA total of 5 coiled aneurysms (1.5%) occluded completely at 36 months showed recanalization (0.46% per aneurysm-year) during the long-term surveillance period (1081.9 aneurysm-years), 2 surfacing within 60 months and 3 developing thereafter. Four showed minor recanalization, with only one instance of major recanalization. The latter involved the posterior communicating artery as an apparent de novo lesion, arising at the neck of a firmly coiled sac, and was unrelated to coil compaction or growth. Additional embolization was undertaken. In a multivariate analysis, a second embolization for a recurrent aneurysm (HR = 22.088, p = 0.003) independently correlated with delayed recanalization.CONCLUSIONSAlmost all coiled aneurysms (98.5%) showing complete occlusion at 36 months postembolization proved to be stable during extended observation. However, recurrent aneurysms were predisposed to delayed recanalization. Given the low probability yet seriousness of delayed recanalization and the possibility of de novo aneurysm formation, careful monitoring may be still considered in this setting but at less frequent intervals beyond 36 months.

The geography of climate migration

2021 ◽  
pp. 1-37
Author(s):  
Michał Burzyński ◽  
Frédéric Docquier ◽  
Hendrik Scheewel
Keyword(s):  
Climate Change ◽  
International Migration ◽  
First Century ◽  
Long Term Effects ◽  
Mobility Data ◽  
Rural And Urban ◽  
Working Age ◽  
Internal Mobility ◽  
Almost All

Abstract In this paper, we investigate the long-term effects of climate change on the mobility of working-age people. We use a world economy model that covers almost all the countries around the world, and distinguishes between rural and urban regions as well as between flooded and unflooded areas. The model is calibrated to match international and internal mobility data by education level for the last 30 years, and is then simulated under climate change variants. We endogenize the size, dyadic, and skill structure of climate migration. When considering moderate climate scenarios, we predict mobility responses in the range of 70–108 million workers over the course of the twenty-first century. Most of these movements are local or inter-regional. South–South international migration responses are smaller, while the South–North migration response is of the “brain drain” type and induces a permanent increase in the number of foreigners in OECD countries in the range of 6–9% only. Changes in the sea level mainly translate into forced local movements. By contrast, inter-regional and international movements are sensitive to temperature-related changes in productivity. Lastly, we show that relaxing international migration restrictions may exacerbate the poverty effect of climate change at origin if policymakers are unable to select/screen individuals in extreme poverty.

Sign in / Sign up

Export Citation Format

Share Document