9 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
RadioAstron as a target and as an instrument: Enhancing the Space VLBI mission's scientific output
The accuracy of orbit determination has a strong impact on the scientific
output of the Space VLBI mission RadioAstron. The aim of this work is to
improve the RadioAstron orbit reconstruction by means of sophisticated
dynamical modelling of its motion in combination with multi-station Doppler
tracking of the RadioAstron spacecraft. The improved orbital solution is
demonstrated using Doppler measurements of the RadioAstron downlink signal and
by correlating VLBI observations made by RadioAstron with ground-based
telescopes using the enhanced orbit determination data. Orbit determination
accuracy has been significantly improved from ~ 600 m in 3D position and ~ 2
cm/s in 3D velocity to several tens of metres and mm/s, respectively.Comment: 8 pages, 8 figures. Astronomy and Astrophysics, accepte
Overview of the coordinated ground-based observations of Titan during the Huygens mission
Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Satumian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental imnortance for the interpretatinn of results from the Huygens mission
RadioAstron gravitational redshift experiment: Status update
A test of a cornerstone of general relativity, the gravitational redshift effect, is currently being conducted with the RadioAstron spacecraft, which is on a highly eccentric orbit around Earth. Using ground radio telescopes to record the spacecraft signal, synchronized to its ultra-stable on-board H-maser, we can probe the varying flow of time on board with unprecedented accuracy. The observations performed so far, currently being analyzed, have already allowed us to measure the effect with a relative accuracy of 4
7 10−4. We expect to reach 2.5
7 10−5 with additional observations in 2016, an improvement of almost a magnitude over the 40-year old result of the GP-A mission
Water masers in the Kronian system
The presence of water has been considered for a long time as a key condition for life in planetary environments. The Cassini mission discovered water vapour in the Kronian system by detecting absorption of UV emission from a background star (Hansen et al. 2006). Prompted by this discovery, we started an observational campaign for search of another manifestation of the water vapour in the Kronian system, its maser emission at the frequency of 22 GHz (1.35 cm wavelength). Observations with the 32 m Medicina radio telescope (INAF-IRA, Italy) started in 2006 using Mk5A data recording and the JIVE-Huygens software correlator. Later on, an on-line spectrometer was used at Medicina. The 14 m Metsähovi radio telescope (TKK-MRO, Finland) joined the observational campaign in 2008 using a locally developed data capture unit and software spectrometer. More than 300 hours of observations were collected in 2006-2008 campaign with the two radio telescopes. The data were analysed at JIVE using the Doppler tracking technique to compensate the observed spectra for the radial Doppler shift for various bodies in the Kronian system (Pogrebenko et al. 2009). Here we report the observational results for Hyperion, Titan, Enceladus and Atlas, and their physical interpretation. Encouraged by these results we started a campaign of follow up observations including other radio telescopes
Overview of the coordinated ground-based observations of Titan during the Huygens mission
[1] Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan’s atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan’s atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, ar