gAGE  grup d'Astronomia i GEomàtica
http://hdl.handle.net/2117/1128
Sun, 30 Aug 2015 06:17:32 GMT20150830T06:17:32ZConditions of consistency for multicomponent stellar systems. II. Is a pointaxial symmetric model suitable for the Galaxy?
http://hdl.handle.net/2117/28482
Conditions of consistency for multicomponent stellar systems. II. Is a pointaxial symmetric model suitable for the Galaxy?
Cubarsí Morera, Rafael
Under a common potential, a finite mixture of ellipsoidal velocity distributions satisfying the Boltzmann collisionless
equation provides a set of integrability conditions that may constrain the population kinematics. They are referred to as
conditions of consistency and were discussed in a previous paper on mixtures of axisymmetric populations. As a corollary,
these conditions are now extended to pointaxial symmetry, that is, point symmetry around the rotation axis or bisymmetry,
by determining which potentials are connected with a more flexible superposition of stellar populations. Under pointaxial
symmetry, the potential is still axisymmetric, but the velocity and mass distributions are not necessarily. A pointaxial stellar
system is, in a natural way, consistent with a flat velocity distribution of a disc population. Therefore, no additional integrability
conditions are required to solve the Boltzmann collisionless equation for such a population. For other populations, if the
potential is additively separable in cylindrical coordinates, the populations are not kinematically constrained, although under
pointaxial symmetry, the potential is reduced to the harmonic function, which, for the Galaxy, is proven to be nonrealistic.
In contrast, a nonseparable potential provides additional conditions of consistency. When mean velocities for the populations
are unconstrained, the potential becomes quasistationary, being a particular case of the axisymmetric model. Then, the radial
and vertical mean velocities of the populations can differ and produce an apparent vertex deviation of the whole velocity
distribution. However, single population velocity ellipsoids still have no vertex deviation in the Galactic plane and no tilt in
their intersection with a meridional Galactic plane. If the thick disc and halo ellipsoids actually have nonvanishing tilt, as the
surveys of the solar neighbourhood that include RAdial Velocity Experiment (RAVE) data seem to show, the pointaxial model
is unable to fit the local velocity distribution. Conversely, the axisymmetric model is capable of making a better approach. If,
in the end, more accurate data confirm a negligible tilt of the populations, then the pointaxisymmetric model will be able to
describe nonaxisymmetric mass and velocity distributions, although in the Galactic plane the velocity distribution will still be
axisymmetric.
Tue, 27 May 2014 00:00:00 GMThttp://hdl.handle.net/2117/2848220140527T00:00:00ZConditions of consistency for multicomponent axisymmetric stellar systems. Is an axisymmetric model suitable yet?
http://hdl.handle.net/2117/28464
Conditions of consistency for multicomponent axisymmetric stellar systems. Is an axisymmetric model suitable yet?
Cubarsí Morera, Rafael
Solving the Boltzmann collisionless equatio n under the axisymmetric hypot hesis introduces serious lim itations on describing the kinematics of a single stellar system according to the local Galactic observables. Instead of relaxing the hypothesis of axisymmetry, one alternative is to assume a mixture model. For a finite mixture of ellipsoidal velocity distributions, the coexistence of several stellar populations sharing a co mmon potential introduces a set of conditions of consistency that m ay also constrain the population kinematics. For only a few potentials, the populations may have independent mean veloc ities and unconstrained velocity ellipsoids. In this paper, we determine which axisymmetric potentials are connect ed with a more flexible superposition of the stellar populations. The conditions of consistency are checked against recent results derived from kinematic surveys of the solar neighbourhood that include RAdial Velocity Experiment (RAVE) data. Several key observables are used to determine whether the axisymmetric mixture model is able to account for the main features of the local velocity distribution, such as the vertex deviation associated with the second central moment µ ¿ , the population radial mean velocities, the radial gradient of the moment µ z , the tilt of the velocity ellipsoids, and the existence of stars with no net rotation. In addition, the mixture moments for an arbitrary number of populations are derived in terms of the onetoone mean velocity di ff erences in order to study whether a more populated mixture could add any new features to the velocity distribution that remain unnoticed in a twocomponent mixture. According to this analysis, the quasistationary potential is the only potential allowing arbitrary directions of the population mean velocities. Then, the apparent vertex deviation of the total velocity distribution is due to the di ff erence of the mean velocities of the populations whose velocity ellipsoids hav e no vertex deviation. For a nonseparable pot ential, the population v elocity ellipsoids have the same orientation and point towards the Galactic centre. For a potential separable in addition in cylindrical coordinates, the population velocity ellipsoids may ha ve arbitrary tilt.
Tue, 17 Dec 2013 00:00:00 GMThttp://hdl.handle.net/2117/2846420131217T00:00:00ZA methodology to assess ionospheric models for GNSS
http://hdl.handle.net/2117/27981
A methodology to assess ionospheric models for GNSS
Rovira Garcia, Adrià; Juan Zornoza, José Miguel; Sanz Subirana, Jaume; González Casado, Guillermo; Ibáñez Segura, Marcos  Deimos
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/2117/2798120150101T00:00:00ZNovel ionospheric activity indicator specifically tailored for GNSS users
http://hdl.handle.net/2117/27928
Novel ionospheric activity indicator specifically tailored for GNSS users
Sanz Subirana, Jaume; Juan Zornoza, José Miguel; González Casado, Guillermo; Prieto Cerdeira, Roberto; Schlüter, S.; Orús Pérez, Raul
This work introduces a novel ionospheric activity indicator useful for identifying disturbed periods affecting performance for GNSS users, at regional level. This indicator is based in the “Along Arc TEC Rate (AATR) and can be easily computed from GNSS data. The AATR indicator has been assessed over more than one Solar Cycle (20022013) involving 140 receivers distributed worldwide. Results show that it is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity (i.e. DST, Ap), it is sensitive to regional behaviour the ionosphere and identifies specific effects on GNSS users. Moreover from a devoted analysis of EGNOS performances in different ionospheric conditions, it follows that the AATR indicator is able to predict SBAS user availability anomalies linked to the ionosphere. The AATR indicator has been chosen as the metric to characterise the ionosphere operational conditions in the frame of EGNOS activities. This indicator has been also proposed for joint analysis in the International SBASIonosphere Working Group.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/2117/2792820140101T00:00:00ZA Worldwide ionospheric model for fast precise point positioning
http://hdl.handle.net/2117/27895
A Worldwide ionospheric model for fast precise point positioning
Rovira Garcia, Adrià; Juan Zornoza, José Miguel; Sanz Subirana, Jaume; González Casado, Guillermo
Fast precise point positioning (FastPPP) is a satellitebased navigation technique using an accurate realtime ionospheric modeling to achieve high accuracy quickly. In this paper, an endtoend performance assessment of FastPPP is presented in nearmaximum Solar Cycle conditions; from the accuracy of the Central Processing Facility corrections, to the user positioning. A planetary distribution of permanent receivers including challenging conditions at equatorial latitudes, is navigated in pure kinematic mode, located from 100 to 1300 km away from the nearest reference station used to derive the ionospheric model.
It is shown that satellite orbits and clocks accurate to few centimeters
and few tenths of nanoseconds, used in conjunction with an ionosphere with an accuracy better than 1 Total Electron Content Unit (16 cm in L1) reduce the convergence time of dualfrequency Precise Point Positioning, to decimeterlevel (3D) solutions. Horizontal convergence times are shortened 40% to 90%, whereas the vertical components are reduced by 20% to 60%. A metric to evaluate the quality of any ionospheric model for Global Navigation Satellite System is also proposed. The ionospheric modeling accuracy is directly translated to massmarket singlefrequency
users. The 95th percentile of horizontal and vertical accuracies is shown to be 40 and 60 cm for singlefrequency users and 9 and 16 cm for dualfrequency users. The tradeoff between the formal and actual positioning errors has been carefully studied to set realistic confidence levels to the corrections.
Tue, 03 Mar 2015 00:00:00 GMThttp://hdl.handle.net/2117/2789520150303T00:00:00ZA realtime worldwide ionospheric model for single and multifrequency precise navigation
http://hdl.handle.net/2117/27516
A realtime worldwide ionospheric model for single and multifrequency precise navigation
Rovira Garcia, Adrià; Juan Zornoza, José Miguel; Sanz Subirana, Jaume
The ionosphere plays an important role in satellitebased navigation, either in standard navigation, with single frequency massmarket receivers, or in precise navigation, with dual frequency receivers.
In this work, the requirements of a realtime ionospheric model suitable for GNSS applications are explored, in terms of accuracy and confidence bounds. Key factors for an ionospheric determination better than 1 Total Electron Content Unit (TECU) (16 centimeters in L1) are shown to be whether the model has been derived using an ambiguityfixing strategy and the number of layers used to reproduce the ionospheric delay. Different models are assessed both in midlatitudes and equatorial regions, near the Solar Cycle maximum.
It will be shown how dualfrequency users take benefit from a precise modelling of the ionosphere. If accurate enough, the convergence of the navigation filter is reduced to achieve high accuracy positioning quickly, (i.e., the Fast Precise Point Positioning technique). Satellite orbits and clocks computed for FastPPP will be shown to be accurate to few centimeters and few tenths of nanoseconds, respectively.
Singlefrequency users correct its measurements with the predictions provided by any ionospheric model. Thence, the accuracy of the FastPPP ionospheric corrections is directly translated to the measurements modelling and, consequently, to the user solution.
Horizontal and vertical 95% accuracies are shown to be better than 36 and 63 centimeters for singlefrequency users and 11 and 15 centimeters for dualfrequency users. The assessment is done for several locations, including the equatorial region, for a month of data close to the last Solar Maximum. The tradeoff between the formal and actual positioning errors has been carefully studied by means of the Stanford plots to set realistic confidence bounds to the corrections.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/2117/2751620140101T00:00:00ZBiomedical applications of bacterial inclusion bodies
http://hdl.handle.net/2117/27044
Biomedical applications of bacterial inclusion bodies
Ratera, Imma; Peternel, Espela; Seras Franzoso, Joaquín; Cano Garrido, Olivia; García Fruitós, Elena; Cubarsí Morera, Rafael; Vazquez, Esther; Corchero Nieto, Jose Luis; RodriguezCarmona, Escar; Veciana Miró, Jaume; Villaverde, A.
Fri, 01 Aug 2014 00:00:00 GMThttp://hdl.handle.net/2117/2704420140801T00:00:00ZConsolidated characterisation of an ionospheric indicator for the definition of EGNOS Ionospheric Conditions
http://hdl.handle.net/2117/23040
Consolidated characterisation of an ionospheric indicator for the definition of EGNOS Ionospheric Conditions
Juan Zornoza, José Miguel; Sanz Subirana, Jaume
Tue, 29 Apr 2014 00:00:00 GMThttp://hdl.handle.net/2117/2304020140429T00:00:00ZFeasibility analysis of a methodology to estimate hourly DCBs for Feared Events Characterization
http://hdl.handle.net/2117/23039
Feasibility analysis of a methodology to estimate hourly DCBs for Feared Events Characterization
Juan Zornoza, José Miguel; Sanz Subirana, Jaume
Analysis of Hardware Biases Feared Events
Mon, 19 May 2014 00:00:00 GMThttp://hdl.handle.net/2117/2303920140519T00:00:00ZExperiments on the Ionospheric Models in GNSS
http://hdl.handle.net/2117/22728
Experiments on the Ionospheric Models in GNSS
Vinh, La The; Quang, Phuong Xuan; García Rigo, Alberto; Rovira Garcia, Adrià; Ibáñez Segura, Marcos  Deimos
In GNSS, one of the main error sources of the Standard Positioning Service (SPS) is introduced by the ionosphere.
Although
this error can be cancelled by combining two signals at different
frequencies, most of the single

frequency
mass

market receivers do not benefit from this cancel
l
ation. For that reason, a set of parameter
s
is included in the navigation
message in order to compute the ionospheric delay of any observation by the
Klobuchar model. The Klobuchar model is a very
simple model that is able to remove more than the 50% of the ionospheric delay.
Recently, more accurate ionospheric models
have been introduced such as Global Ionospheric Map (GIM) or the
F
ast
P
recise
P
oint
P
ositioning
(
F
PPP
) ionospheric model.
In previous works, with data gathered in Europe, it was shown the advantage of the
F
PPP’s ionospheric model. In this work,
we conduct experiments
to compare the performance of different ionospheric
modelling
methods including: Klobuchar, GIM
s
and
F
PPP. Our preliminary results show how
F
PPP and GIM
s
lead to better positioning precisions compared to the Klobuchar
model. However, since data is not wide enough to cover different ionospheric cond
itions, more experiments will be
carried out
in our future work to validate the current result
s
.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/2117/2272820130101T00:00:00Z