GPS tropospheric modelling: new developments and insights

GPS is widely used to monitor temporal and spatial variations of Earth’s crust, oceans and atmosphere. Of particular interest to this research is the use of GPS for studying variations in the Earth’s lower atmosphere. While there have been significant advances in the techniques and models used in GPS analyses over the past two decades, there is still room for improvement. In particular, observations at very low elevation angles still suffer greatly from modelling errors. These low-elevation observations provide useful information about the moisture content of the atmosphere and its variability around a GPS station, and are thus valuable data for meteorological studies if properly modelled. The main focus of this thesis is on optimization of the techniques and models used in GPS analysis for more accurate estimates of the tropospheric delays. Particular attention is paid to modelling low-elevation observations and challenging weather conditions. Throughout the thesis, we investigate several different aspects of modelling techniques and how each of them affect the tropospheric estimates. By applying a previously developed empirical model [Moore, 2015], the site-specific errors are shown to have large impacts on the tropospheric delay estimates: empirical mitigation of site-specific errors leads to improved repeatabilities of heights and tropospheric zenith delays for the majority of the stations in our analysis. The empirical site-specific model also significantly reduces the sensitivity of tropospheric zenith delay estimates to the choice of elevation cut-off. Another important potential source of error, the GPS estimates of tropospheric horizontal gradients are shown to be more accurate than the model values currently available. However, the conventional two-axis planar model of gradients does not accurately represent the actual gradients of the refractivity under weather conditions with asymmetric horizontal changes of refractivity. Such abnormal conditions may occur due to topography-driven gravity waves in the troposphere, and the mismodelled tropospheric horizontal gradients induce errors in the parameter estimates, sometimes leading to skewed position time series and inaccurate tropospheric zenith delays. A new parametrization of tropospheric gradients whereby an arbitrary number of gradients are estimated as discrete directional wedges is shown via both simulations and real case studies to largely improve the accuracy of recovered tropospheric zenith delays in asymmetric gradient scenarios. The new directional model significantly improves the repeatabilities of the station height time series in asymmetric gradient situations while causing slightly degraded repeatabilities for the stations in normal symmetric gradient conditions. The constraints on the temporal variations of the tropospheric delays are also investigated. It is shown via simulations and real experiments that it is generally preferable to avoid constraints on both tropospheric zenith delays and horizontal gradients. However, since the conventional model of horizontal gradients oversimplifies the horizontal variations of the refractivity in asymmetric gradient conditions, it is important to use a more complete model of gradients like the directional gradient model introduced in this thesis in conjunction with the relaxed constraints to avoid errors caused by the simplifying assumption of symmetric gradients by the conventional model

Selected Papers from the 2018 IEEE International Workshop on Metrology for the Sea

This Special Issue is devoted to recent developments in instrumentation and measurement techniques applied to the marine field. ¶The sea is the medium that has allowed people to travel from one continent to another using vessels, even today despite the use of aircraft. It has also been acting as a great reservoir and source of food for all living beings. However, for many generations, it served as a landfill for depositing conventional and nuclear wastes, especially in its deep seabeds, and we are assisting in a race to exploit minerals and resources, different from foods, encompassed in it. Its health is a great challenge for the survival of all humanity since it is one of the most important environmental components targeted by global warming. ¶ As everyone may know, measuring is a step that generates substantial knowledge about a phenomenon or an asset, which is the basis for proposing correct solutions and making proper decisions. However, measurements in the sea environment pose unique difficulties and opportunities, which is made clear from the research results presented in this Special Issue

Identifying Ionospheric Scintillation in the South Atlantic Magnetic Anomaly using motion-affected GPS data from a ship-based receiver

his dissertation serves to report on the novel use of a geodetic-grade, dual-frequency Global Positioning System (GPS) Ionospheric Scintillation and Total Electron Content Monitor (GISTM), in an attempt to identify instances of ionospheric scintillation over the South Atlantic Magnetic Anomaly (SAMA) while located aboard the moving polar research vessel SA Agulhas II. The SAMA is a region in the South Atlantic Ocean where the Earth’s magnetic field is weakest in relation to other regions at comparable latitudes, resulting in the precipitation of high-energy particles into the ionosphere during geomagnetic storms. Ionospheric scintillations are rapid fluctuations in the phase and amplitude of trans-ionospheric radio signals resulting from electron density variations along the ray path. As a result, spacebased navigation systems can encounter increased errors in position accuracy or complete loss of lock. These are risk factors for modern aircraft and ocean vessels which rely on access to accurate Position, Navigation and Timing (PNT) services to operate safely. In this research, only the radio signals from GPS satellites are specifically used to measure these fluctuations. Traditional scintillation measurements are done using dedicated dual-frequency GPS receivers at fixed terrestrial locations. Most of the SAMA lies beyond the reach of the land-based sensors. The South African National Space Agency (SANSA) operates several GISTM stations in Southern Africa, at Marion Island, Gough Island, and the SANAE-IV base in Antarctica. The NovAtel GSV4004B GPS Ionospheric Scintillation and Total Electron Content Monitor (GISTM) installed on board the SA Agulhas II in 2012 has enabled for the first time the terrestrial measurement of scintillation from within the SAMA region. In this project, the amplitude scintillation (S4) and phase scintillation (σφ) indices from 50 Hz L1 GPS signals recorded during the 2014 and 2015 voyages of the SA Agulhas II were analysed for the first time. The scintillation effects are characterised in terms of position and motion data, carrierto-noise-density ratio, number of satellites, and satellite lock time. The goal is to develop an understanding of the effect of motion on the quality of data recorded by the receiver. The roll angle thresholds for the SA Agulhas II are calculated and it is shown that multipath errors are unlikely to be experienced. Significant data challenges were identified stemming from the incorrect setup of the SA Agulhas II GISTM. Data from elevations below 10° were missing because of hard-coded limitations within the GISTM on-board software. The data underwent significant reprocessing before being used. Comparisons were done in-harbour and out at sea with data from the nearest stationary GISTM receivers. It was shown that the movement of the receiver induces significant noise in the data. The noise levels are proportional to the velocity of the ship. An attempt to filter out the noise was unsuccessful. The motion-induced noise in the ship data masked the presence of any potential scintillations. With the ability to detect scintillation compromised, it was decided that a comparison with a land-based receiver within the SAMA would be necessary. Only one identical GISTM receiver met these requirements, located on Gough Island, at 40°20’ 58.90" S, 9°52’ 49.35" W. Data was isolated from both the SA Agulhas II GISTM and Gough Island GISTM for a period where the separation between the two receiver locations was less than 100 km. The Symmetric-Horizontal disturbance index (SYM-H) was used to identify geomagnetic storm conditions. GPS visibility maps were used to identify any potential signal obstructions. No correlation could be seen between position error and the number of satellites locked due to the high number of GPS satellites available at all times. It was discovered that the high noise levels had no effect on the position accuracy of the moving receiver, but that rapid changes in the instantaneous velocity coincided with peaks in the position error. No scintillation events were identified using the SA Agulhas II GISTM as a result of masking by the noise, however, the Gough Island GISTM data showed that no scintillation events occurred during the period in question anyway. Wind was identified as a potential contributing factor to the motion noise effect. This study provided justification for the purchase and installation of a newly developed motion-compensated GISTM receiver on board the SA Agulhas II, running off the same antenna and thus the same received signals. These data sets can be used for a direct receiver comparison in future work

Contributions to land, sea, and sea ice remote sensing using GNSS-reflectometry

This PhD thesis researches the use of passive remote sensing techniques using signals transmitted from the navigation satellites (GNSS) in order to retrieve different geophysical parameters. The thesis consists of two different parts without taking into account the introduction, the state of the art and the conclusions. The first part analyzes the Interference Pattern Technique (IPT), which was previously used in another PhD thesis, and proposes some key improvements. First, the addition of horizontal polarization to the existing vertical polarization is proposed. Then, the retrieval of soil moisture is studied using the horizontal polarization only and combining both polarizations to correct for the surface roughness effects. It is also demonstrated that the phase difference between the two interference patterns is directly related to soil moisture content. A field campaign was conducted in Australia to test empirically all the theoretical developments and algorithms. Secondly, the possibility of measuring Significant Wave Height (SWH) and Mean Sea Surface Level (MSSL) using the IPT is studied. A three month field campaign over coastal sea is devoted to that study. The SWH retrieval is a new estimation algorithm based on measuring the point where the interference pattern loses coherence. The MSSL retrieval is based on the estimation of the IPT oscillation frequency, testing different spectral estimators to improve the accuracy. Since the IPT is limited in coverage due to its static requirements, the research conducted in this thesis migrated to scatterometric GNSS-R techniques. The main goal that migration was to increase coverage of the different GNSS-R instruments. Therefore, the second part of this thesis analyzes the applicability of a scatterometric technique from different platforms: ground-based (mobile and fixed), airborne, and spaceborne. The ground-based still platforms have allowed to develop a soil moisture retrieval algorithm. The ground-based moving platforms have extended the validity of that algorithm. Airborne platforms have been used to study the reflected electric field statistics when the surface reflecting surface is varying (smooth or rough land, and sea). They have also been used to develop different algorithms to measure the coherent and incoherent scattered components depending on the data structure (real-data or complex data). Coherent reflectivity measured from airborne platforms has been compared to other techniques such microwave radiometry, which is highly used in the soil moisture retrieval from spaceborne sensors, and other sensors using optical, multispectral and thermal frequency bands. These relationships between microwave radiometry and GNSS-R measurements suggests the potential synergy of both techniques. A sea ice detection algorithm is also developed using scatterometric GNSS-R data from the UK TDS-1 mission. This algorithm is based on measuring the degree of coherence of the reflected waveform. Finally, a field campaign was conducted to study the effect of vegetation on the GNSS signals that pass through it in order to take into account and correct the effect of vegetation in the GNSS-R data and in the soil moisture retrieval algorithms.Aquesta tesi doctoral aprofundeix en el coneixement de les tècniques de teledetecció passives utilitzant senyals emesos pels satèl·lits de navegació (GNSS) amb l'objectiu de recuperar diferents paràmetres geofísics del terreny. La tesi conté dues parts ben diferenciades a banda de la introducció, estat de l'art i conclusions. La primera part analitza la tècnica coneguda com a patró d'interferències, utilitzada prèviament en una altra tesi doctoral, i proposa certes millores per la seva aplicabilitat. En primer lloc es decideix afegir polarització horitzontal a la ja existent polarització vertical, i s'estudia la recuperació d'humitat del sòl utilitzant només polarització horitzontal i combinant les dues polaritzacions per corregir els efectes de la rugositat del terreny. A continuació es demostra que la mesura de desfasament entre els dos patrons d'interferència està directament relacionada amb la humitat del terreny. Es va realitzar una campanya de mesures a Austràlia per provar empíricament tots els desenvolupaments teòrics i algorismes proposats. En segon lloc s'analitza l'aplicabilitat del patró d'interferències en la mesura de l'altura de les onades (SWH) i del nivell del mar (MSSL), tots dos de forma precisa. L'estimació de l'alçada de les onades és un procés totalment nou basat en mesurar el punt on el patró d'interferències perd la coherència. L'estimació del nivell del mar es basa en l'anàlisi espectral del patró d'interferències provant diferents estimadors espectrals. Atès que la tècnica del patró d'interferència està limitada en cobertura per les seves característiques estàtiques, la investigació duta a terme en aquesta tesi doctoral va migrar cap a tècniques GNSS-R escateromètriques. El principal objectiu a assolir va ser el d'augmentar la cobertura dels diferents instruments GNSS-R de mesura. En conseqüència, la segona part d'aquesta tesi analitza l'aplicabilitat d'aquestes tècniques des de diferents plataformes terrestres (mòbils i fixes), aerotransportades i satèl·lit. Les plataformes terrestres fixes han permès derivar algoritmes de recuperació d'humitat i les mòbils estendre la validació d'aquests. Les plataformes aerotransportades s'han utilitzat per mirar l'estadística del camp elèctric reflectit quan la superfície on es reflecteixen els senyals GNSS va variant (terra plana o terra rugosa, i mar). També han servit per desenvolupar diferents algorismes amb l'objectiu de determinar les components coherent i incoherent del senyal reflectit. De la mateixa manera, dades de reflectivitat coherent mesurades des d'aquestes plataformes han estat comparades amb altres tècniques de teledetecció passiva com la radiometria de microones, altament utilitzada en la mesura d'humitat de terreny, i altres sensors òptics, multi-espectrals, i tèrmics. Aquests resultats han permès suggerir la possible sinergia de dades d'ambdues tecnologies. Un algorisme per detectar la presència de gel sobre el mar també ha estat desenvolupat mitjançant l'ús de dades GNSS-R escateromètriques satel·litals de la missió UK TDS-1. Aquest algorisme es basa en mesurar el grau de coherència de la forma d'ona reflectida. Finalment, s'ha realitzat un estudi de l'efecte de la vegetació en els senyals GNSS que la travessen, per tal de poder corregir aquest efecte en els algoritmes de recuperació d'humitat del terreny