74 research outputs found
Spacecraft VLBI and Doppler tracking: algorithms and implementation
We present the results of several multi-station Very Long Baseline
Interferometry (VLBI) experiments conducted with the ESA spacecraft Venus
Express as a target. To determine the true capabilities of VLBI tracking for
future planetary missions in the solar system, it is necessary to demonstrate
the accuracy of the method for existing operational spacecraft. We describe the
software pipeline for the processing of phase referencing near-field VLBI
observations and present results of the ESA Venus Express spacecraft observing
campaign conducted in 2010-2011. We show that a highly accurate determination
of spacecraft state-vectors is achievable with our method. The consistency of
the positions indicates that an internal rms accuracy of 0.1 mas has been
achieved. However, systematic effects produce offsets up to 1 mas, but can be
reduced by better modelling of the troposphere and ionosphere and closer
target-calibrator configurations.Comment: 10 pages, 10 figures. Astronomy and Astrophysics, accepte
Planning of an Experiment for VLBI Tracking of GNSS Satellites
As a preparation for future possible orbit determination of global navigation satellite system (GNSS) satellites by VLBI observations an initial three-station experiment was planned and performed in January 2009. The goal was to get first experience and to verify the feasibility of using the method for accurate satellite tracking. GNSS orbits related to a satellite constellation can be expressed in the Terrestrial Reference Frame. A comparison with orbit results that might be obtained by VLBI can give valuable information on how the GNSS reference frame and the VLBI reference frame are linked. We present GNSS transmitter specifications and experimental results of the observations of some GLONASS satellites together with evaluations for the expected signal strengths at telescopes. The satellite flux densities detected on the Earth s surface are very high. The narrow bandwidth of the GNSS signal partly compensates for potential problems at the receiving stations, and signal attenuation is necessary. Attempts to correlate recorded data have been performed with different software
The use of the Long Baseline Array in Australia for precise geodesy and absolute astrometry
We report the results of a successful 12 hour 22 GHz VLBI experiment using a
heterogeneous network that includes radio telescopes of the Long Baseline Array
(LBA) in Australia and several VLBI stations that regularly observe in geodetic
VLBI campaigns. We have determined positions of three VLBI stations, ATCA-104,
CEDUNA and MOPRA, with an accuracy of 4-30mm using a novel technique of data
analysis. These stations have never before participated in geodetic
experiments. We observed 105 radio sources, and amongst them 5 objects which
have not previously been observed with VLBI. We have determined positions of
these new sources with the accuracy of 2-5 mas. We make conclusion that the LBA
network is capable of conducting absolute astrometry VLBI surveys with accuracy
better than 5 mas.Comment: Published in Publications of the Astronomical Society of Australia.
10 pages, 3 figures, 5 tables. Table 5 in a machine readable form is
accessible in the source of this submissio
Planetary Radio Interferometry and Doppler Experiment (PRIDE) Technique: a Test Case of the Mars Express Phobos Fly-by. 2. Doppler tracking: Formulation of observed and computed values, and noise budget
Context. Closed-loop Doppler data obtained by deep space tracking networks
(e.g., NASA's DSN and ESA's Estrack) are routinely used for navigation and
science applications. By "shadow tracking" the spacecraft signal, Earth-based
radio telescopes involved in Planetary Radio Interferometry and Doppler
Experiment (PRIDE) can provide open-loop Doppler tracking data when the
dedicated deep space tracking facilities are operating in closed-loop mode
only. Aims. We explain in detail the data processing pipeline, discuss the
capabilities of the technique and its potential applications in planetary
science. Methods. We provide the formulation of the observed and computed
values of the Doppler data in PRIDE tracking of spacecraft, and demonstrate the
quality of the results using as a test case an experiment with ESA's Mars
Express spacecraft. Results. We find that the Doppler residuals and the
corresponding noise budget of the open-loop Doppler detections obtained with
the PRIDE stations are comparable to the closed-loop Doppler detections
obtained with the dedicated deep space tracking facilities
First Results of Venus Express Spacecraft Observations with Wettzell
The ESA Venus Express spacecraft was observed at X-band with the Wettzell radio telescope in October-December 2009 in the framework of an assessment study of the possible contribution of the European VLBI Network to the upcoming ESA deep space missions. A major goal of these observations was to develop and test the scheduling, data capture, transfer, processing, and analysis pipeline. Recorded data were transferred from Wettzell to Metsahovi for processing, and the processed data were sent from Mets ahovi to JIVE for analysis. A turnover time of 24 hours from observations to analysis results was achieved. The high dynamic range of the detections allowed us to achieve a milliHz level of spectral resolution accuracy and to extract the phase of the spacecraft signal carrier line. Several physical parameters can be determined from these observational results with more observational data collected. Among other important results, the measured phase fluctuations of the carrier line at different time scales can be used to determine the influence of the solar wind plasma density fluctuations on the accuracy of the astrometric VLBI observations
A search for water masers toward extrasolar planets
Water is the most common triatomic molecule in the universe and the basis of
life on Earth. Astrophysical masers have been widely studied in recent years
and have been shown to be invaluable probes of the details of the environment
in which they are found. Water masers, for instance, are often detected toward
low-mass star-forming regions. Doppler radial-velocity surveys have detected
about 160 exoplanets. Observations of water masers from exoplanetary systems
would give us a new detailed window through which to explore them. We present a
search for water masers toward eighteen extrasolar planets using the newly
upgraded Australia Telescope Compact Array at 12 mm. A sensitivity of about 25
mJy/beam and an angular resolution of about 10'' were achieved at 22.235 GHz.
No maser lines are clearly observed.Comment: accepted for publication in A&
The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation
In this paper a description is given of the SFXC software correlator,
developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The
software is designed to run on generic Linux-based computing clusters. The
correlation algorithm is explained in detail, as are some of the novel modes
that software correlation has enabled, such as wide-field VLBI imaging through
the use of multiple phase centres and pulsar gating and binning. This is
followed by an overview of the software architecture. Finally, the performance
of the correlator as a function of number of CPU cores, telescopes and spectral
channels is shown.Comment: Accepted by Experimental Astronom
Radiative transfer effects in primordial hydrogen recombination
The calculation of a highly accurate cosmological recombination history has
been the object of particular attention recently, as it constitutes the major
theoretical uncertainty when predicting the angular power spectrum of Cosmic
Microwave Background anisotropies. Lyman transitions, in particular the
Lyman-alpha line, have long been recognized as one of the bottlenecks of
recombination, due to their very low escape probabilities. The Sobolev
approximation does not describe radiative transfer in the vicinity of Lyman
lines to a sufficient degree of accuracy, and several corrections have already
been computed in other works. In this paper, the impact of some previously
ignored radiative transfer effects is calculated. First, the effect of Thomson
scattering in the vicinity of the Lyman-alpha line is evaluated, using a full
redistribution kernel incorporated into a radiative transfer code. The effect
of feedback of distortions generated by the optically thick deuterium
Lyman-alpha line blueward of the hydrogen line is investigated with an analytic
approximation. It is shown that both effects are negligible during cosmological
hydrogen recombination. Secondly, the importance of high-lying, non overlapping
Lyman transitions is assessed. It is shown that escape from lines above
Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected
without loss of accuracy. Thirdly, a formalism generalizing the Sobolev
approximation is developed to account for the overlap of the high-lying Lyman
lines, which is shown to lead to negligible changes to the recombination
history. Finally, the possibility of a cosmological hydrogen recombination
maser is investigated. It is shown that there is no such maser in the purely
radiative treatment presented here.Comment: 23 pages, 4 figures, to be submitted to PR
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