scholarly journals Determining the accuracy of VLBI radio source catalogs

2018 ◽  
Vol 620 ◽  
pp. A160 ◽  
Author(s):  
N. Liu ◽  
S. B. Lambert ◽  
Z. Zhu
Keyword(s):  
Radio Source ◽  
Major Axis ◽  
Scale Factor ◽  
Noise Floor ◽  
Long Baseline ◽  
Formal Error ◽  
Standard Solution ◽  
The Difference ◽  
Improved Accuracy ◽  
Right Ascension

Aims. We propose to estimate the accuracy of current very long baseline interferometry (VLBI) catalogs. Methods. The difference of source position estimated from two decimation solutions was analyzed to estimate the scale factor and noise floor for the formal error of radio source positions by two different methods. In one method, we investigated the weighted root-square-mean (wrms) scatter of source positional differences versus the number of observed sessions; for the other one, we compared the wrms difference versus the formal error. Based on the estimated noise floor and scale factor, we determined the realistic error of radio source positions in the standard solution and compared it with that of Gaia DR2 and ICRF2 catalogs. Results. The estimated scale factors from two methods are rather consistent, which is of ∼1.3 in both coordinates. As for the noise floor, it is estimated to be 20–25 μas for sources observed in at least ten sessions, and it could reduce down to ∼10 μas for sources which have been observed more than 1000 times. The inflated median formal error of our solution is of the same order as the Gaia DR2 catalog in declination and the direction of major axis of the error ellipse, but smaller by a factor of two in right ascension. With respect to the ICRF2 catalog, our solution yields an improved accuracy by a factor of about three. Conclusions. Currently, the VLBI radio source catalog still provides source positions with the best accuracy which is about 20–25 μas. Moreover, the noise floor of VLBI catalogs could potentially reach 10 μas with more observations in the future.

scholarly journals Measurement of the solar system acceleration using the Earth scale factor

2018 ◽  
Vol 610 ◽  
pp. A36 ◽  
Author(s):  
O. Titov ◽  
H. Krásná
Keyword(s):  
Solar System ◽  
Radio Source ◽  
Scale Factor ◽  
Intrinsic Structure ◽  
Structure Variation ◽  
Long Baseline ◽  
The Galaxy ◽  
Vlbi Data ◽  
The Individual ◽  
Right Ascension

Aim. We propose an alternative method to detect the secular aberration drift induced by the solar system acceleration due to the attraction to the Galaxy centre. This method is free of the individual radio source proper motion caused by intrinsic structure variation. Methods. We developed a procedure to estimate the scale factor directly from very long baseline interferometry (VLBI) data analysis in a source-wise mode within a global solution. The scale factor is estimated for each reference radio source individually as a function of astrometric coordinates (right ascension and declination). This approach splits the systematic dipole effect and uncorrelated motions on the level of observational parameters. Results. We processed VLBI observations from 1979.7 to 2016.5 to obtain the scale factor estimates for more than 4000 reference radio sources. We show that the estimates highlight a dipole systematics aligned with the direction to the centre of the Galaxy. With this method we obtained a Galactocentric acceleration vector with an amplitude of 5.2 ± 0.2 μas/yr and direction αG = 281∘± 3∘ and δG = −35∘± 3∘.

scholarly journals Systematics and accuracy of VLBI astrometry: A comparison with Gaia Data Release 2

2020 ◽  
Vol 634 ◽  
pp. A28
Author(s):  
N. Liu ◽  
S. B. Lambert ◽  
Z. Zhu ◽  
J.-C. Liu
Keyword(s):  
Radio Source ◽  
Positional Error ◽  
Third Generation ◽  
Noise Floor ◽  
Vector Spherical Harmonics ◽  
The Third ◽  
Data Release ◽  
Improved Accuracy ◽  
External Check

Context. The third generation of the ICRF – ICRF3 – was published in 2018. This new fundamental catalog provides radio source positions measured independently at three bands: S/X, K, and X/Ka, representing three independent radio celestial frames which altogether constitute a multi-frequency ICRF. Aims. We aim to investigate the overall properties of the ICRF3 with the help of the Gaia Data Release 2 (Gaia DR2). This could serve as an external check of the quality of the ICRF3. Methods. The radio source positions of the ICRF3 catalog were compared with the Gaia DR2 positions of their optical counterparts at G <  18.7. Their properties were analyzed in terms of the dependency of the quoted error on the number of observations, on the declination, and the global difference, the latter revealed by means of expansions in the vector spherical harmonics. Results. The ICRF3 S/X-band catalog shows a more smooth dependency on the number of observations than the ICRF1 and ICRF2, while the K and X/Ka-band yield a dependency discrepancy at the number of observations of ∼50. The rotation of all ICRF catalogs show consistent results, except for the X-component of the X/Ka-band which arises from the positional error in the non-defining sources. No significant glides were found between the ICRF3 S/X-band component and Gaia DR2. However, the K- and X/Ka-band frames show a dipolar deformation in Y-component of +50 μas and several quadrupolar terms of 50 μas in an absolute sense. A significant glide along Z-axis exceeding 200 μas in the X/Ka-band was also reported. These systematics in the ICRF catalog are shown to be less dependent on the limiting magnitude of the Gaia sample when the number of common sources is sufficient (>100). Conclusions. The ICRF3 S/X-band catalog shows improved accuracy and systematics at the level of noise floor. But the zonal errors in the X/Ka-band should be noted, especially in the context of comparisons of multi-frequency positions for individual sources.

scholarly journals Realization of a multifrequency celestial reference frame through a combination of normal equation systems

2019 ◽  
Vol 630 ◽  
pp. A101 ◽  
Author(s):  
M. Karbon ◽  
A. Nothnagel
Keyword(s):  
Radio Source ◽  
Reference Frame ◽  
Reference Frames ◽  
Very Long Baseline Interferometry ◽  
Normal Equation ◽  
Long Baseline ◽  
Combination Approach ◽  
Celestial Reference Frame ◽  
Right Ascension ◽  
24 Ghz

Context. We present a celestial reference frame (CRF) based on the combination of independent, multifrequency radio source position catalogs using nearly 40 years of very long baseline interferometry observations at the standard geodetic frequencies at SX band and about 15 years of observations at higher frequencies (K and XKa). The final catalog contains 4617 sources. Aims. We produce a multifrequency catalog of radio source positions with full variance–covariance information across all radio source positions of all input catalogs. Methods. We combined three catalogs, one observed at 8 GHz (X band), one at 24 GHz (K band) and one at 32 GHz (Ka band). Rather than only using the radio source positions, we developed a new, rigorous combination approach by carrying over the full covariance information through the process of adding normal equation systems. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined catalog. Results. The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at 8 GHz, 824 sources also observed at 24 GHz, and 674 at 32 GHz. The frame is aligned with ICRF3 within ±3 μas and shows an average positional uncertainty of 0.1 mas in right ascension and declination. No significant deformations can be identified. Comparisons with Gaia-CRF remain inconclusive, nonetheless significant differences between all frames can be attested.

scholarly journals Absorption by Neutral Hydrogen in the Irregular Galaxy M82

1972 ◽  
Vol 44 ◽  
pp. 74-74
Author(s):  
M. Guélin ◽  
L. Weliachew
Keyword(s):  
Radio Source ◽  
Velocity Gradient ◽  
Neutral Hydrogen ◽  
Major Axis ◽  
Velocity Spread ◽  
Absorption Profile ◽  
Irregular Galaxy ◽  
Negative Temperatures ◽  
The Galaxy ◽  
Right Ascension

A neutral hydrogen survey of the irregular galaxy M82 has been carried out with the transit radio telescope at Nançay, France. The resolving power was 4′ in right ascension and 34′ in declination. The velocity resolution was 59 km s−1.Drift scans covering 58′ in right ascension were taken across the center of the galaxy where the radio source is located; fourteen scans were averaged.Line profiles were derived every 2′ in right ascension. The profiles at ±4′ from the radio source are similar within the measurement errors. In particular, they do not show any rotation effect within ±15 km s−1. They were averaged in order to provide an estimate of the expected emission profile at the radio source position.Subtraction of this average from the line profile measured in front of the radio source yielded significant negative temperatures at all velocities from 180 to 360 km s−1 and no positive temperatures at other velocities.These negative temperatures were assigned to absorption of the radiation from the radio source by neutral hydrogen in M82.Absorption is running from 3 to 6% in depth. The average velocity of the absorption profile is lying between the central emission velocity and the optically-determined velocity which are known to show a large disagreement.The width of the absorption profile shows a velocity gradient of 200 km s−1 across the 35′ × 20′ radio source. Such a large velocity spread across this angular extent is only shown by the excited gas showing emission lines at optical wavelengths (Burbidge et al., 1964).Since then, the absorption in M82 at the neutral hydrogen wavelength has been confirmed by measurements done with the Owens Valley Radio interferometer at 2 spacings where any emission is completely resolved (interfringes were 3′ and 1′.5).In addition to the conclusions of the single dish observations done at Nançay, the interferometer data have shown that the steeper velocity gradient occurs along the major axis of the galaxy which coincides with the major axis of the radio source.

Effect of J3 perturbation on satellite position in LEO

2018 ◽  
Vol 90 (1) ◽  
pp. 74-86
Author(s):  
Nai-ming Qi ◽  
Qilong Sun ◽  
Yong Yang
Keyword(s):  
Satellite Orbit ◽  
Major Axis ◽  
Variation Method ◽  
Semi Major Axis ◽  
Content Type ◽  
The Difference ◽  
The Right ◽  
Perturbation Effect ◽  
Earth’S Oblateness ◽  
Right Ascension

Purpose The purpose of this paper is to study the effect of J3 perturbation of the Earth’s oblateness on satellite orbit compared with J2 perturbation. Design/methodology/approach Based on the parametric variation method in the time domain, considering more accurate Earth potential function by considering J3-perturbation effect, the perturbation equations about satellite’s six orbital elements (including semi-major axis, orbit inclination, right ascension of the ascending node, true anomaly, eccentricity and argument of perigee) has been deduced theoretically. The disturbance effects of J2 and J3 perturbations on the satellite orbit with different orbit inclinations have been studied numerically. Findings With the inclination increasing, the maximum of the semi-major axis increases weakly. The difference of inclination disturbed by the J2 and J3 perturbation is relative to orbit inclinations. J3 perturbation has weak effect on the right ascension and argument of perigee. The critical angle of the right ascension and argument of perigee which decides the precession direction is 90° and 63.43°, respectively. The disturbance effects of J2 and J3 perturbations on the argument of perigee, right ascension and eccentricity are weakened when the eccentricity increases, simultaneously, the difference of J2 and J3 perturbations on argument of perigee, right ascension and argument of perigee decreases with eccentricity increasing, respectively. Practical implications In the future, satellites need to orbit the Earth much more precisely for a long period. The J3 perturbation effect and the weight compared to J2 perturbation in LEO can provide a theoretical reference for researchers who want to improve the control accuracy of satellite. On the other hand, the theoretical analysis and simulation results can help people to design the satellite orbit to avoid or diminish the disturbance effect of the Earth’s oblateness. Originality/value The J3 perturbation equations of satellite orbit elements are deduced theoretically by using parametric variation method in this paper. Additionally, the comparison studies of J2 perturbation and J3 perturbation of the Earth’s oblateness on the satellite orbit with different initial conditions are presented.

scholarly journals Improved accuracy fullerene polarizability measurements in a long-baseline matter-wave interferometer

2019 ◽  
Vol 1 (3) ◽  
Author(s):  
Yaakov Y. Fein ◽  
Philipp Geyer ◽  
Filip Kiałka ◽  
Stefan Gerlich ◽  
Markus Arndt
Keyword(s):  
Matter Wave ◽  
Long Baseline ◽  
Improved Accuracy

scholarly journals VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz

2018 ◽  
Vol 616 ◽  
pp. A128 ◽  
Author(s):  
N. Herrera Ruiz ◽  
E. Middelberg ◽  
A. Deller ◽  
V. Smolčić ◽  
R. P. Norris ◽  
...  
Keyword(s):  
Radio Source ◽  
Flux Density ◽  
Lower Limit ◽  
High Sensitivity ◽  
Host Galaxy ◽  
Radio Sources ◽  
Source Population ◽  
Star Forming ◽  
Brightness Temperatures ◽  
Long Baseline

We present very long baseline interferometry (VLBI) observations of 179 radio sources in the COSMOS field with extremely high sensitivity using the Green Bank Telescope (GBT) together with the Very Long Baseline Array (VLBA) (VLBA+GBT) at 1.4 GHz, to explore the faint radio population in the flux density regime of tens of μJy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, meaning that it is independent of X-ray or infrared properties. The milli-arcsecond resolution provided by the VLBI technique implies that the detected sources must be compact and have large brightness temperatures, and therefore they are most likely AGN (when the host galaxy is located at z ≥ 0.1). On the other hand, this technique only allows us to positively identify when a radio-active AGN is present, in other words, we cannot affirm that there is no AGN when the source is not detected. For this reason, the number of identified AGN using VLBI should be always treated as a lower limit. We present a catalogue containing the 35 radio sources detected with the VLBA+GBT, ten of which were not previously detected using only the VLBA. We have constructed the radio source counts at 1.4 GHz using the samples of the VLBA and VLBA+GBT detected sources of the COSMOS field to determine a lower limit for the AGN contribution to the faint radio source population. We found an AGN contribution of >40−75% at flux density levels between 150 μJy and 1 mJy. This flux density range is characterised by the upturn of the Euclidean-normalised radio source counts, which implies a contribution of a new population. This result supports the idea that the sub-mJy radio population is composed of a significant fraction of radio-emitting AGN, rather than solely by star-forming galaxies, in agreement with previous studies.

Stem eccentricity in coastal western hemlock

1981 ◽  
Vol 11 (3) ◽  
pp. 715-718 ◽  
Author(s):  
Robert M. Kellogg ◽  
Francis J. Barber
Keyword(s):  
Major Axis ◽  
Western Hemlock ◽  
Eccentricity Ratio ◽  
Angular Rotation ◽  
Minimum Diameter ◽  
Breast Height ◽  
Second Growth ◽  
The Difference ◽  
Average Eccentricity ◽  
Practical Field

Stem eccentricity in second-growth western hemlock (Tsugaheterophylla (Raf.) Sarg.) from southern coastal British Columbia appears to be the rule rather than the exception. Although the difference was small, the average eccentricity ratios (ratios of minimum diameter inside bark (DIB) to maximum DIB) for two Lower Mainland sites were significantly larger than those of two Vancouver Island sites. The average eccentricity ratio of all 87 trees studied was 0.929, with a standard deviation of 0.0231. The angular rotation of the major axis of the stem cross section varied greatly, and was positively related to the average stem eccentricity ratio within 5-m log lengths for each stem. Average eccentricity of the whole stem was relatively independent of diameter at breast height (DBH), age, stem height, and eccentricity at breast height. It does not appear that a practical field technique could be devised for the rapid estimation of average stem eccentricity in western hemlock.

scholarly journals Comparison of Buoy-Mounted and Bottom-Moored ADCP Performance at Gray’s Reef

2007 ◽  
Vol 24 (2) ◽  
pp. 270-284 ◽  
Author(s):  
Harvey E. Seim ◽  
Catherine R. Edwards
Keyword(s):  
High Frequency ◽  
Major Axis ◽  
National Data ◽  
Noise Floor ◽  
The North ◽  
Experimental Configuration ◽  
Adcp Measurements ◽  
Adcp Data ◽  
Adcp Observations ◽  
East West

Abstract Simultaneous ADCP profile measurements are compared over a 2-month period in late 2003. One set of measurements comes from a National Data Buoy Center (NDBC) buoy-mounted ADCP, the other from a bottom-mounted, upward-looking ADCP moored roughly 500 m from the buoy. The study was undertaken to evaluate the proficiency of an experimental configuration by NDBC; unfortunately, the ADCP was not optimally configured. The higher temporally and vertically resolved bottom-mounted ADCP data are interpolated in time and depth to match the buoy-mounted ADCP measurements. It is found that the two ADCP measurements are significantly different. The buoy-mounted measurements are affected by high-frequency (&lt;10 h period) noise that is vertically coherent throughout the profiles. This noise results in autospectra that are essentially white, unlike the classic red spectra formed from the bottom-mounted ADCP observations. The spectra imply a practical noise floor of 0.045 m s−1 for the buoy-mounted system. Contamination by surface waves is the likely cause of this problem. At tidal frequencies the buoy-mounted system underestimates major axis tidal current magnitude by 10%–40%; interference from the buoy chain and/or fish or plankton are considered the most likely cause of the bias. The subtidal velocity field (periods greater than 40 h) is only partially captured; the correlation coefficient for the east–west current is 0.49 and for the north–south current is 0.64.

Anisotropy Gas Reservoir Drainage Radius and Quantitative Well Interference Study

2013 ◽  
Vol 275-277 ◽  
pp. 1285-1291 ◽  
Author(s):  
Zheng Long Gao ◽  
Hong Fu Fan ◽  
Zhi Bin Gao
Keyword(s):  
Production Rate ◽  
Major Axis ◽  
Productivity Analysis ◽  
Analysis Method ◽  
Equivalent Radius ◽  
Average Value ◽  
Drainage Radius ◽  
Well Spacing ◽  
Cumulative Production ◽  
The Difference

Unstable productivity analysis method was used to obtain the equivalent radius of 77 wells and the result shows that the equivalent radius ranges from 30 to 970m with an average value of 230m in McKittrick Hills. The difference range of the radius is mainly caused by varying formation properties, gas saturation, production time, etc. Permeability anisotropy changes the drainage from round to ellipse. The major axis and the minor axis of the ellipse are determined by the ratio of major and minor permeability. Current pressure distribution was obtained and was found to be consistent with the modified drainage results, which demonstrates that the unstable productivity analysis method is applicable in the study of gas well drainage radius. An interference well and an observation well’s model was constructed to study well interference quantitatively. When the well spacing is larger than 750m, the productivity will be reduced by 20%. The production rate of interference well is more sensitive to the cumulative production of observation well, when the production rate of interference well is below 16.8×104m3/d.

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