Altitude | EPFL Graph Search

L'altitude (du altitudo) est historiquement une notion géographique qui désigne la hauteur géométrique verticale entre un point et un référent altimétrique, le plus souvent le niveau de la mer. En géodésie, elle exprime également l'éloignement d'un point par rapport au géoïde. Il existe plusieurs manières de calculer l'altitude : altitude orthométrique, , altitude géopotentielle et . En aéronautique, l'altitude est la distance entre un point et le sol. Elle mesurée en pieds, sauf dans les pays de l'ex-Union soviétique et en Chine, où elle est exprimée en mètres. L'altitude est souvent confondue avec l' et la hauteur. L'élévation est la distance entre le niveau de la mer et le niveau des . La hauteur est la distance géométrique entre un point et son projeté sur l'ellipsoïde. Les différents lieux sont associés à une altitude à l'aide d'une surface de niveau, calculée par divers moyens indirects (géodésie, triangulation). L'altitude est également une donnée utile pour le calcul num

Current and future spatial distribution of ticks (Ixodes ricinus) and of hosted Chlamydia in Switzerland

Morgan Bruhin

This project aims to study the spatial distribution of tick, I. ricinus, and emergent Chlamydiales families hosted by them. We used presence data associated with environmental variables to construct habitat suitability indexes by using species distribution modeling from the Maxent software. Most of the variables are climatic coming from the Worldclim website, also available for future climatic predictions. Additionally, the possible evolution of those habitats was estimated considering future climate conditions in 2070. We applied the previous species distribution modeling method with predictive variables to characterize their evolution. Finally, an estimation of the population exposed to this possible emergent threat to the human health under the form of a synthetic index. This index regroups the relative population density with the combination of the possible habitat of the tick and the hosted Chlamydiae. The results showed an expected general presence over the low altitude areas for the tick, but the predictive model failed to clearly identify areas of possible important modification of their suitable habitat. The Chlamydiae also admitted a possible habitat over most of the low altitude, mostly the Plateau. Their future evolution seemed to point out a homogenization of the distribution for those families over the Plateau.

2016

Doppler-based positioning using LEO augmented satellite constellation - Simulations of perspectives

Alice Andreetti

The Global Navigation Satellite System (GNSS) refers to a constellation of satellites emitting signals from space, used to provide Position, Navigation and Timing (PNT) services to the receivers on Earth. Nowadays, the GNSS is exploited in a wide range of applications among which ground mapping, transportation, machine control, timing, surveying, defense, or aerial photogrammetry [1]. However, as we become more dependent on GNSS technology, we also become more vulnerable to its limitations [2]. The GNSS operates with satellites orbiting in Medium Earth Orbits (MEO, approximately 20200 [km] in altitude). On one hand, this great distance allows to have a large footprint of the satellite signal on Earth, but on the other hand, it requires the signal to cross a wide path in the atmosphere before reaching the receiver. This results in a decrease of signal strength, which is not an issue in open-sky condition, but becomes a limitation in deep attenuation environments, with little or no service in urban canyon, in dense cities or indoors [3]. In this contribution, a possible strategy to cope with this issue is proposed and evaluated through a simulation process. It implies the use of Low Earth Orbits (LEO) satellites as a support for the GNSS PNT service. Since nowadays no broadband big LEO constellation broadcasting navigation messages exist, in this project various LEO constellation setups have been simulated and tested. Because LEOs are much closer to Earth (200 to 1500 [km]), they move faster in the sky, passing overhead in minutes instead of hours like MEOs. This gives rise to the possibility to exploit the Doppler effect to accurately locate the user. Therefore, three different PNT algorithms, employing the Kalman filter, are implemented and the evaluation of the localization quality is made at four distinct locations on the globe. The results show that, for most of the cases, compared to the standard GPS pseudorange only PNT, the addition of LEO pseudoranges already improve the localization quality. The performance of the PNT algorithm increases even more with the exploitation of both pseudoranges and pseudorangerates (Doppler) measurements. Nevertheless, the choice of constellation parameters such as satellites altitude, total number of satellites, orbit inclination or the frequency of the emitted signal, has a great impact on the magnitude of the improvement in the localization quality. The factor that has the most significant influence seems to be the signal frequency and for that reason it must be carefully chosen: the higher the better. Moreover, it has been demonstrated that at high elevation cut-off angles (denied environments), the addition of LEO constellation is a valid option and allows us to obtain an accurate position even when the GNSS only can not provide a solution. Finally, the LEO only system seems to be a promising alternative to cope with GNSS jamming or spoofing. However, the optimal LEO parameters providing an accurate solution for all the location on the globe has not yet been identified. In conclusion, the result of this effort is the awareness that, once the optimal parameters for the LEO system have been found, it is a valid option to augment the GNSS, or even as a standalone backup, especially when the Doppler-based positioning is applied. Nevertheless, we are still far from that end and much more research needs to be done.

2020

Development of the Jungfraujoch UV DIAL Lidar to Observe the Vertical Ozone Distribution in the Context of Stratosphere Troposphere Exchange and Long Range Transport

Marcel Bartlome

Tropospheric ozone is a climate relevant greenhouse gas, as well as an atmospheric pollutant. Its abundance in the troposphere is mainly governed by the influx of stratospheric air masses and the photochemical production due to anthropogenic and biogenic ozone precursors. Precise model simulations and measurements are needed to assess the exact impact of these sources onto the total tropospheric ozone content. In the latter case, vertical tropospheric ozone profiles are commonly obtained from in-situ balloon-borne soundings. These routine observations are performed with a maximum frequency of several measurements per week. Therefore, they are not suited to resolve the rather fast changes in the tropospheric ozone concentration. However, accurate vertical ozone profiles with temporal and spatial resolutions suitable for studying those changes have been produced for decades using remote sensing by means of the ozone UV differential absorption lidar (DIAL) technique. The main goals of this thesis aimed to address some of the unresolved problems of the ozone origin and evolution in the upper troposphere and lower stratosphere and were: to develop an UV DIAL for the observation of the vertical ozone distribution in the free troposphere and at the tropopause region. to perform experimental measurements of vertical ozone profiles in the free troposphere and the tropopause in order to assess the ability of the ozone UV DIAL to observe fast variations in vertical ozone distribution caused by stratosphere-troposphere exchange (STE), long range transport, and advection from the boundary layer. to assess the feasibility of systematic ozone profile retrieval by the ozone UV DIAL technique from the High Altitude Research Station Jungfraujoch (HARSJ, 3580 m ASL, 7°59'2''E, 46°32'53''N). The HARSJ was chosen for the experiment because of its location, which allows ozone UV DIAL measurements of the tropopause region without perturbation of the lower troposphere. Furthermore, previous atmospheric studies have shown that because of the station's location, the upper tropospheric data taken at the HARSJ can be regarded as representative for central and western Europe. During the first two and a half years of the thesis project the ozone UV DIAL was designed and built. The ozone UV DIAL transmitter is based on a commercial, fourth harmonic Nd:YAG laser. The on- (284 nm) and off- (304 nm) ozone UV DIAL wavelengths are produced by stimulated Raman scattering in high-pressure nitrogen. The receiver uses the existing astronomical Cassegrain telescope (76 cm in diameter) at the HARSJ. Spectral separation of the backscattered ozone UV DIAL wavelengths is carried out by a polychromator based on an concave imaging diffraction grating. The entire ozone UV DIAL detection unit is integrated into the existing long-range multi-wavelength polychromator box optically coupled at the Cassegrain telescope's rear end. With the current design, the ozone UV DIAL system provides hourly averaged ozone profiles reaching from 6 km to 12 km ASL with a vertical resolution better than 400 m at 6 km ASL and 1000 m at 12 km ASL. The relative statistical error of the profiles is 10% at 12 km ASL. The experimental measurements started in the spring of 2008 but were interrupted several times because of serious laser failures. As a first step of the experiment, the ozone UV DIAL profiles were compared to balloon borne ozone profiles taken by electrochemical concentration cell (ECC) sondes. The sondes were launched by the Swiss Meteorological Institute – Payerne (SMI, 46°49'N, 6°56'E, 491 m ASL) situated 80 km in northwestern direction of the HARSJ. The ozone UV DIAL and the sonde measurements were taken quasi simultaneously. The relative differences between the ozone UV DIAL and the sonde profiles were found to be below ±25% in a horizontally homogeneous atmosphere. The intercomparison has shown that the ozone UV DIAL is capable to accurately reproduce the vertical ozone distribution. Intercomparison with vertical ozone profiles taken in the vicinity of the HARSJ by an airplane-borne UV-photometer confirmed the performance of the ozone UV DIAL. Time series with duration of up to 21 hours were taken in order to study the time evolution of tropospheric events characterized by elevated ozone concentrations. As a result of these measurements three events, demonstrating different processes have been identified. In the first case an air layer with elevated ozone concentration was observed in the altitude range between 6 and 7 km ASL. The lower than 10% relative humidity and the time evolution of this layer allowed to determine its stratospheric origin. The observations are in a good agreement with the EURopean Air Pollution Dispersion (EURAD) model. The model predictions also agree with the elevated ozone concentration measured by the UV-photometer of the National Air Pollution Monitoring Network (NABEL) operated at the HARSJ. The evolution of an ozone-enriched layer in the tropopause region was followed for 21 hours during the second case study. The time evolution, the ECC data, and the EURAD model predictions suggest the possibility of a short filamentation process developing at the tropopause region. The measurements of the third event were made according to a prediction of STE forecasted by a dynamical model developed at the ETHZ. A short living enhancement of the ozone concentration has been observed at an altitude of about 9.5 km ASL. There are two possible explanations for this event. The first one could be a short living reversible STE event. A second explanation is a possible long-range transport of ozone and ozone precursor rich air from the forest fires in the Athens region suggested by the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) backward trajectories. However, the available information does not allow to make a definite conclusion. The ozone UV DIAL has been built successfully and has shown its ability to observe fast variation in vertical ozone distribution with high accuracy and precision. In combination with additional water vapor and temperature lidar observations and supported by model forecasting, this system could become a powerful tool for the improving of the understanding of upper troposphere and lower stratosphere ozone related processes. However, to allow future systematic observations the ozone UV DIAL will need an essential upgrade to resolve the problems related to the long preparation time, and the limited by technical reasons time of observation.

EPFL2010