132 research outputs found
Dynamical Processes in the Planet-Forming Environment
The transfer of circumstellar disk mass and momentum onto the protostar and out into the environment occurs via a variety of mechanisms including magnetospheric accretion, jets, outflows, and disk winds. The interplay of these processes determine both the conditions under which planet formation occurs and the lifetime of the disk. Metallic emission lines, along with the Balmer series of hydrogen, probe the kinematics of gas within the planet-forming and central regions of circumstellar disks. High-spectral resolution study of these emission lines provides critical information on mass and momentum loss, turbulence, and disk wind origins
A new approach for optimising GNSS positioning performance in harsh observation environments
Maintaining good positioning performance has always been a challenging task for Global Navigation Satellite Systems (GNSS) applications in partially obstructed environments. A method that can optimise positioning performance in harsh environments is proposed. Using a carrier double-difference (DD) model, the influence of the satellite-pair geometry on the correlation among different equations has been researched. This addresses the critical relationship between DD equations and its ill-posedness. From analysing the collected multi-constellation observations, a strong correlation between the condition number and the positioning standard deviation is detected as the correlation coefficient is larger than 0·92. Based on this finding, a new method for determining the reference satellites by using the minimum condition number rather than the maximum elevation is proposed. This reduces the ill-posedness of the co-factor matrix, which improves the single-epoch positioning solution with a fixed DD ambiguity. Finally, evaluation trials are carried out by masking some satellites to simulate common satellite obstruction scenarios including azimuth shielding, elevation shielding and strip shielding. Results indicate the proposed approach improves the positioning stability with multi-constellation satellites notably in harsh environments
Tuning a Kalman filter carrier tracking algorithm in the presence of ionospheric scintillation
© 2017, The Author(s). Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems (GNSS). This phenomenon, known as ionospheric scintillation, is capable of deteriorating the tracking performance of a GNSS receiver, leading to increased phase and Doppler errors, cycle slips and also to complete losses of signal lock. In order to mitigate scintillation effects at receiver level, the robustness of the carrier tracking loop, the receiver weakest link under scintillation, must be enhanced. Kalman filter (KF)-based tracking algorithms are particularly suitable to cope with the variable working conditions imposed by scintillation. However, the effectiveness of this tracking approach strongly depends on the accuracy of the assumed dynamic model, which can quickly become inaccurate under randomly variable situations. This study first shows how inaccurate dynamic models can lead to a KF suboptimum solution or divergence, when both strong phase and amplitude scintillation are present. Then, to overcome this issue, it proposes two self-tuning KF-based carrier tracking algorithms, which self-tune their dynamic models by exploiting the knowledge about scintillation that can be achieved through scintillation monitoring. The algorithms have been assessed with live equatorial data affected by strong scintillation. Results show that the algorithms are able to maintain the signal lock and provide reliable scintillation indices when classical architectures and commercial ionospheric scintillation monitoring receivers fail
A Filtering Method Developed to Improve GNSS Receiver Data Quality in the CALIBRA Project
To study ionospheric scintillation on L-band radio signals, it is nowadays typical to acquire
data with GNSS (Global Navigation Satellite System) receivers working at high frequency
sampling rate (50-100 Hz). When dealing with such data, it is common to consider the
contribution coming solely from observations at elevation angles, calculated from the receiver
to the selected satellite, above an arbitrary threshold, typically 15-30°. Filtering out measurements
made at low elevation angles helps keeping a high SNR (Signal to Noise Ratio) and
eliminating non-ionospheric related effects, such as multipath.
The downside of that well consolidated method is a reduction of the field of view spanned by
the GNSS receiver antenna, and, if it is the case, of the whole network. This is not crucial for
dense networks or well covered areas, but it can be in the case of not well covered regions, for
logistics (e.g. forests, deserts, etc.) and/or environmental reasons (e.g. oceans). The loss of
information in many applications could be meaningful.
In this paper, we present a method to filter out spurious data based on an “outliers analysis”
able to efficiently remove multipath affected measurements, reducing the data loss from
35-45% to 10-20%. It is based upon the Ground Based Scintillation Climatology (GBSC) and the station characterization based upon GBSC [5] is applied to the CIGALA1/
CALIBRA2 network in Brazil. The research shown herein was carried out in the context of the CALIBRA (http://www.calibra-ionosphere.net) project and exploits the CIGALA/
CALIBRA network in Brazil, to which the method was applied, enlarging the field of view
and, then, improving the capability of inferring the dynamics of the low latitude ionosphere
Surgical intervention for complications caused by femoral artery catheterization in pediatric patients
AbstractPurpose: This study evaluated the risk factors and surgical management of complications caused by femoral artery catheterization in pediatric patients. Methods: From January 1986 to March 2001, the hospital records of all children who underwent operative repairs for complications caused by femoral artery catheterization were reviewed. A prospective cardiac data bank containing 1674 catheterization procedures during the study period was used as a means of determining risk factors associated with iatrogenic femoral artery injury. Results: Thirty-six operations were performed in 34 patients (age range, 1 week-17.4 years) in whom iatrogenic complications developed after either diagnostic or therapeutic femoral artery catheterizations during the study period. Non-ischemic complications included femoral artery pseudoaneurysms (n = 4), arteriovenous fistulae (n = 5), uncontrollable bleeding, and expanding hematoma (n = 4). Operative repairs were performed successfully in all patients with non-ischemic iatrogenic femoral artery injuries. In contrast, ischemic complications occurred in 21 patients. Among them, 14 patients had acute femoral ischemia and underwent surgical interventions including femoral artery thrombectomy with primary closure (n = 6), saphenous vein patch angioplasty (n = 6), and resection with primary anastomosis (n = 2). Chronic femoral artery occlusion (> 30 days) occurred in seven patients, with symptoms including either severe claudication (n = 4) or gait disturbance or limb growth impairment (n = 3). Operative treatments in these patients included ileofemoral bypass grafting (n = 5), femorofemoral bypass grafting (n = 1), and femoral artery patch angioplasty (n = 1). During a mean follow-up period of 38 months, no instances of limb loss occurred, and 84% of children with ischemic complications eventually gained normal circulation. Factors that correlated with an increased risk of iatrogenic groin complications that necessitated surgical intervention included age younger than 3 years, therapeutic intervention, number of catheterizations (≥ 3), and use of 6F or larger guiding catheter. Conclusion: Although excellent operative results can be achieved in cases of non-ischemic complications, acute femoral occlusion in children younger than 2 years often leads to less satisfactory outcomes. Operative intervention can provide successful outcome in children with claudication caused by chronic limb ischemia. Variables that correlated with significant iatrogenic groin complications included a young age, therapeutic intervention, earlier catheterization, and the use of a large guiding catheter. (J Vasc Surg 2001;33:1071-8.
Erratic Jet Wobbling in the BL Lacertae Object OJ287 Revealed by Sixteen Years of 7mm VLBA Observations
We present the results from an ultra-high-resolution 7mm Very Long Baseline
Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from
1995 to 2011 containing 136 total intensity images. Analysis of the image
sequence reveals a sharp jet-position-angle swing by >100 deg. during
[2004,2006], as viewed in the plane of the sky, that we interpret as the
crossing of the jet from one side of the line of sight to the other during a
softer and longer term swing of the inner jet. Modulating such long term swing,
our images also show for the first time a prominent erratic wobbling behavior
of the innermost ~0.4mas of the jet with fluctuations in position angle of up
to ~40 deg. over time scales ~2yr. This is accompanied by highly superluminal
motions along non-radial trajectories, which reflect the remarkable
non-ballistic nature of the jet plasma on these scales. The erratic nature and
short time scales of the observed behavior rules out scenarios such as binary
black hole systems, accretion disk precession, and interaction with the ambient
medium as possible origins of the phenomenon on the scales probed by our
observations, although such processes may cause longer-term modulation of the
jet direction. We propose that variable asymmetric injection of the jet flow;
perhaps related to turbulence in the accretion disk; coupled with hydrodynamic
instabilities, leads to the non-ballistic dynamics that cause the observed
non-periodic changes in the direction of the inner jet.Comment: Accepted for Publication in The Astrophysical Journal. 11 pages, 6
figures, 4 tables. High resolution images on figure 1 and complete tables 1
and 2 may be provided on reques
Dynamical Processes in the Planet-Forming Environment
The transfer of circumstellar disk mass and momentum onto the protostar and out into the environment occurs via a variety of mechanisms including magnetospheric accretion, jets, outflows, and disk winds. The interplay of these processes determine both the conditions under which planet formation occurs and the lifetime of the disk. Metallic emission lines, along with the Balmer series of hydrogen, probe the kinematics of gas within the planet-forming and central regions of circumstellar disks. High-spectral resolution study of these emission lines provides critical information on mass and momentum loss, turbulence, and disk wind origins
On the source of Faraday rotation in the jet of the radio galaxy 3C120
The source of Faraday rotation in the jet of the radio galaxy 3C120 is
analyzed through Very Long Baseline Array observations carried out between 1999
and 2007 at 86, 43, 22, 15, 12, 8, 5, 2, and 1.7 GHz. Comparison of
observations from 1999 to 2001 reveals uncorrelated changes in the linear
polarization of the underlying jet emission and the Faraday rotation screen:
while the rotation measure (RM) remains constant between approximately 2 and 5
mas from the core, the RM-corrected electric vector position angles (EVPAs) of
two superluminal components are rotated by almost 90 degrees when compared to
other components moving through similar jet locations. On the other hand, the
innermost 2 mas experiences a significant change in RM -- including a sign
reversal -- but without variations in the RM-corrected EVPAs. Similarly,
observations in 2007 reveal a double sign reversal in RM along the jet, while
the RM-corrected EVPAs remain perpendicular to the jet axis. Although the
observed coherent structure and gradient of the RM along the jet supports the
idea that the Faraday rotation is produced by a sheath of thermal electrons
that surrounds the emitting jet, the uncorrelated changes in the RM and
RM-corrected EVPAs indicate that the emitting jet and the source of Faraday
rotation are not closely connected physically and have different configurations
for the magnetic field and/or kinematical properties. Furthermore, the
existence of a region of enhanced RM whose properties remain constant over
three years requires a localized source of Faraday rotation, favoring a model
in which a significant fraction of the RM originates in foreground clouds.Comment: 12 pages, 11 figures; Accepted for publication in Ap
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