2,546 research outputs found
The effect of the stochasticity of photoionization on 3D streamer simulations
Positive streamer discharges require a source of free electrons ahead of them
for their growth. In air, these electrons are typically provided by
photoionization. Here we investigate how stochastic fluctuations due to the
discreteness of ionizing photons affect positive streamers in air. We simulate
positive streamers between two planar electrodes with a 3D plasma fluid model,
using both a stochastic and a continuum method for photoionization. With
stochastic photoionization, fluctuations are visible in the streamer's
direction, maximal electric field, velocity, and electron density. The
streamers do not branch, and we find good agreement between the averaged
stochastic results and the results with continuum photoionization. The
streamers stay roughly axisymmetric, and we show that results obtained with an
axisymmetric model indeed agree well with the 3D results. However, we find that
positive streamers are sensitive to the amount of photoionization. When the
amount of photoionization is doubled, there is even better agreement between
the stochastic and continuum results, but with half the amount of
photoionization, stochastic fluctuations become more important and streamer
branching starts to occur
On the probability density function of the GNSS ambiguity residuals
Integer GNSS ambiguity resolution involves estimation and validation of the unknown integer carrier phase ambiguities. A problem then is that the classical theory of linear estimation does not apply to the integer GPS model, and hence rigorous validation is not possible when use is made of the classical results. As with the classical theory, a first step for being able to validate the integer GPS model is to make use of the residuals and their probabilistic properties. The residuals quantify the inconsistency between data and model, while their probabilistic properties can be used to measure the significance of the inconsistency. Existing validation methods are often based on incorrect assumptions with respect to the probabilistic properties of the parameters involved. In this contribution we will present and evaluate the joint probability density function (PDF) of the multivariate integer GPS carrier phase ambiguity residuals. The residuals and their properties depend on the integer estimation principle used. Since it is known that the integer least-squares estimator is the optimal choice from the class of admissible integer estimators, we will only focus on the PDF of the ambiguity residuals for this estimator. Unfortunately the PDF cannot be evaluated exactly. It will therefore be shown how to obtain a good approximation. The evaluation will be completed by some examples
Why isolated streamer discharges hardly exist above the breakdown field in atmospheric air
We investigate streamer formation in the troposphere, in electric fields
above the breakdown threshold. With fully three-dimensional particle
simulations, we study the combined effect of natural background ionization and
of photoionization on the discharge morphology. In previous investigations
based on deterministic fluid models without background ionization, so-called
double-headed streamers emerged. But in our improved model, many electron
avalanches start to grow at different locations. Eventually the avalanches
collectively screen the electric field in the interior of the discharge. This
happens after what we call the `ionization screening time', for which we give
an analytical estimate. As this time is comparable to the streamer formation
time, we conclude that isolated streamers are unlikely to exist in fields well
above breakdown in atmospheric air.Comment: Changed citation information. 6 pages, 4 figures, Geophysical
Research Letters, Vol. 40, 2417-2422, 201
Review and principles of PPP-RTK methods
PPP-RTK is integer ambiguity resolution-enabled precise point positioning. In this contribution, we present the principles of PPP-RTK, together with a review of different mechanizations that have been proposed in the literature. By application of S-system theory, the estimable parameters of the different methods are identified and compared. Their interpretation is essential for gaining a proper insight into PPP-RTK in general, and into the role of the PPP-RTK corrections in particular. We show that PPP-RTK is a relative technique for which the ‘single-receiver user’ integer ambiguities are in fact double-differenced ambiguities. We determine the transformational links between the different methods and their PPP-RTK corrections, thereby showing how different PPP-RTK methods can be mixed between network and users. We also present and discuss four different estimators of the PPP-RTK corrections. It is shown how they apply to the different PPP-RTK models, as well as why some of the proposed estimation methods cannot be accepted as PPP-RTK proper. We determine analytical expressions for the variance matrices of the ambiguity-fixed and ambiguity-float PPP-RTK corrections. This gives important insight into their precision, as well as allows us to discuss which parts of the PPP-RTK correction variance matrix are essential for the user and which are not
Testing of a new single-frequency GNSS carrier phase attitude determination method: land, ship and aircraft experiments
Global navigation satellite system (GNSS) ambiguity resolution is the process of resolving the unknown cycle ambiguities of the carrier phase data as integers. The sole purpose of ambiguity resolution is to use the integer ambiguity constraints as a means of improving significantly on the precision of the remaining GNSS model parameters. In this contribution, we consider the problem of ambiguity resolution for GNSS attitude determination. We analyse the performance of a new ambiguity resolution method for GNSS attitude determination. As it will be shown, this method provides a numerically efficient, highly reliable and robust solution of the nonlinearly constrained integer least-squares GNSS compass estimators. The analyses have been done by means of a unique set of extensive experimental tests, using simulated as well as actual GNSS data and using receivers of different manufacturers and type as well as different platforms. The executed field tests cover two static land experiments, one in the Netherlands and one in Australia, and two dynamic experiments, a low-dynamics vessel experiment and high-dynamics aircraft experiment. In our analyses, we focus on stand-alone, unaided, single-frequency, single epoch attitude determination, as this is the most challenging case of GNSS compass processing
DIA-datasnooping and identifiability
In this contribution, we present and analyze datasnooping in the context of the DIA method. As the DIA method for the detection, identification and adaptation of mismodelling errors is concerned with estimation and testing, it is the combination of both that needs to be considered. This combination is rigorously captured by the DIA estimator. We discuss and analyze the DIA-datasnooping decision probabilities and the construction of the corresponding partitioning of misclosure space. We also investigate the circumstances under which two or more hypotheses are nonseparable in the identification step. By means of a theorem on the equivalence between the nonseparability of hypotheses and the inestimability of parameters, we demonstrate that one can forget about adapting the parameter vector for hypotheses that are nonseparable. However, as this concerns the complete vector and not necessarily functions of it, we also show that parameter functions may exist for which adaptation is still possible. It is shown how this adaptation looks like and how it changes the structure of the DIA estimator. To demonstrate the performance of the various elements of DIA-datasnooping, we apply the theory to some selected examples. We analyze how geometry changes in the measurement setup affect the testing procedure, by studying their partitioning of misclosure space, the decision probabilities and the minimal detectable and identifiable biases. The difference between these two minimal biases is highlighted by showing the difference between their corresponding contributing factors. We also show that if two alternative hypotheses, say (Formula presented.) and (Formula presented.), are nonseparable, the testing procedure may have different levels of sensitivity to (Formula presented.)-biases compared to the same (Formula presented.)-biases
GLONASS CDMA L3 ambiguity resolution and positioning
A first assessment of GLONASS CDMA L3 ambiguity resolution and positioning performance is provided. Our analyses are based on GLONASS L3 data from the satellite pair SVNs 755-801, received by two JAVAD receivers at Curtin University, Perth, Australia. In our analyses, four different versions of the two-satellite model are applied: the geometry-free model, the geometry-based model , the height-constrained geometry-based model, and the geometry-fixed model. We study the noise characteristics (carrier-to-noise density, measurement precision), the integer ambiguity resolution performance (success rates and distribution of the ambiguity residuals), and the positioning performance (ambiguity float and ambiguity fixed). The results show that our empirical outcomes are consistent with their formal counterparts and that the GLONASS data have a lower noise level than that of GPS, particularly in case of the code data. This difference is not only seen in the noise levels but also in their onward propagation to the ambiguity time series and ambiguity residuals distribution
Optical Position and Time Resolved Measurement of Magnetic Field Distribution in High Speed Metal Forming
In the area of the position and time resolved measurement of the magnetic field distribution in small gaps between workpieces and coils in high-speed sheet metal forming optical sensors are predestined to be integrated into the very small geometries of experimental setups. Optical sensors for current measurement based on the magneto-optic Faraday effect are well known for a long time. This effect can also be used for the direct measurement of magnetic fields. For the use in electromagnetic high-speed metal forming applications, only very small field probes are probable. The measurement of axial symmetric fields can be achieved with two connected fibres with different Verdet constants. They solve the problem with the not given measurement value, which occurs by the use of only one fibre because of its closed integral domain. A continuous time signal of the magnetic field can be calculated for discrete regions. Likewise, it is possible to employ miniature fibre-optic magnetic field point sensors for the field determination in the gap of an electromagnetic high-speed forming device. It is necessary to examine the influence on the polarisation state and the intensity of the light in a fibre. There are two different sensors shown in this paper. One is based on a piece of flint glass fibre spliced between two polarising fibres, and the other sensor arrangement consists of two glued SiO_2 blocks. A workbench for assembling of fibre-optic sensors using the splice technology has been constructed and will be presented. First trial measurements of the magnetic field, compared to the causing current, show the functionality of these kinds of optical sensors and are discussed under the aspect of optimisation
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