Towards Space Deployment of the NDSA Concept for Tropospheric Water Vapour Measurements

A novel measurement concept specifically tuned to monitoring tropospheric water vapour's vertical distribution has been demonstrated on a theoretical basis and is currently under development for space deployment. The NDSA (Normalised Differential Spectral Attenuation) technique derives the integrated water vapour (IWV) along the radio link between a transmitter and a receiver carried by two LEO satellites, using the linear correlation between the IWV and a parameter called spectral sensitivity. This is the normalised incremental ratio of the spectral attenuation at two frequencies in the Ku and K bands, with the slope of the water vapour absorption line at 22.235 GHz. Vertical profiles of WV can be retrieved by inverting a set of IWV measurements acquired in limb geometry at different tangent altitudes. This paper provides a comprehensive insight into the NDSA approach for sounding lower tropospheric WV, from the theoretical investigations in previous ESA studies, to the first experimental developments and testing, and to the latest advancements achieved with the SATCROSS project of the Italian Space Agency. The focus is on the new results from SATCROSS activities; primarily, on the upgrading of the instrument prototype, with improved performance in terms of its power stability and the time resolution of the measurements. Special emphasis is also placed on discussing tomographic inversion methods capable of retrieving tropospheric WV content from IWV measurements, i.e., the least squares and the external reconstruction approaches, showing results with different spatial features when applied to a given atmospheric scenario. The ultimate goal of deploying the NDSA measurement technique from space is thoroughly examined and conclusions are drawn after presenting the results of an Observing System Simulation Experiment conducted to assess the impact of NDSA data assimilation on environmental model simulations

Sensing the dynamics of severe weather using 4D GPS tomography in the Australian region

The dynamics of water vapour (WV) have a strong influence on weather and climate due to the large energy transfers in the hydrological processes. This particularly correlates to WV dynamics during the formation and lifecycle of severe mesoscale convective storm and precipitation systems. Contrary to its importance, WV remains poorly understood and inadequately measured both spatially and temporally, especially in Australia and the southern hemisphere where meteorological data are sparse. Ground-based and space-borne GPS (global positioning system) meteorology are currently regarded as leading atmospheric remote sensing instruments for numerical weather prediction (NWP) and climatology due to their high spatio-temporal resolutions, multiple observing platforms and continuous operability. The GPS signals are delayed and bent due to the refractive index of the ionosphere and troposphere. This tropospheric path delay can be separated into dry and wet integral components, with the latter proportional, using a scale factor, to the integrated precipitable water vapour (PWV) in the vertical column above the GPS stations. These wet delay measurements can also be combined using a network of GPS stations to resolve the spatial distribution of WV. This method is called GPS tomography, which is a promising and developing method of reconstructing dynamically changing four dimensional (4D) wet refractivity fields. This takes advantage of the high density and homogeneity of ground-based GPS Continuously Operating Reference Station (CORS) networks to provide accurately resolved WV profiles in space and time. A distinct trend between the 4D reconstructed wet refractivity fields using GPS tomography and the formation and lifecycle of severe storm and precipitation systems was found. Sharp gradients are evident up the vertical layers The dynamics of water vapour (WV) have a strong influence on weather and climate due to the large energy transfers in the hydrological processes. This particularly correlates to WV dynamics during the formation and lifecycle of severe mesoscale convective storm and precipitation systems. Contrary to its importance, WV remains poorly understood and inadequately measured both spatially and temporally, especially in Australia and the southern hemisphere where meteorological data are sparse. Ground-based and space-borne GPS (global positioning system) meteorology are currently regarded as leading atmospheric remote sensing instruments for numerical weather prediction (NWP) and climatology due to their high spatio-temporal resolutions, multiple observing platforms and continuous operability. The GPS signals are delayed and bent due to the refractive index of the ionosphere and troposphere. This tropospheric path delay can be separated into dry and wet integral components, with the latter proportional, using a scale factor, to the integrated precipitable water vapour (PWV) in the vertical column above the GPS stations. These wet delay measurements can also be combined using a network of GPS stations to resolve the spatial distribution of WV. This method is called GPS tomography, which is a promising and developing method of reconstructing dynamically changing four dimensional (4D) wet refractivity fields. This takes advantage of the high density and homogeneity of ground-based GPS Continuously Operating Reference Station (CORS) networks to provide accurately resolved WV profiles in space and time. A distinct trend between the 4D reconstructed wet refractivity fields using GPS tomography and the formation and lifecycle of severe storm and precipitation systems was found. Sharp gradients are evident up the vertical layers providing the wet refractivity trend of convection, with high gradient falls through the vertical layers after the storm system passed. Radiosonde is used as a reference to validate the GPS tomographic model with final accuracies of the March 2010 and January 2011 case studies presenting 8.58 and 9.36 ppm RMS errors, respectively. A wet refractivity index adopted for the GPS tomographic wet refractivity profiles showed an excessive increase above the planetary boundary layer as a response to the formation of a supercell thunderstorm. Finally, horizontal and vertical 2D cross sections, investigating the evolution of the March 2010 severe weather event concludes a high correlation between the highly dynamic spatial and temporal changes of wet refractivity, modelled using 4D GPS tomography with precipitation intensities measured using weather radars images. These gradient solutions from GPS tomography are able to identify the spatial and temporal structure of the mesoscale convective and stratiform processes during severe weather. Final investigations analyse the influence of additional observational methods introduced into the observation model of the GPS tomographic processing. This analysis is conducted during the formation and lifecycle of severe weather of the January 2011 case study. A statistical analysis compares additional observational methods including: radiosonde, synoptic weather station networks and GPS radio occultation and then the influence of all observation methods combined. The results are compared against radiosonde-derived wet refractivity estimates as the reference data to conclude RMS errors of 9.36, 8.03, 8.14, 8.56 and 7.57 ppm, respectively. These results have shown that the introduction of accurate additional information into the tomographic solution lead to a significant increase in accuracy and more robust results than the original method containing no additional data. These improvements are in the order of 14.29%, 13.09%, 8.57% and 19.15%, respectively. The major objectives of this research are satisfied by developing ground-based GPS meteorological platforms in the Australian region including the introduction of 4D tomographic reconstruction methods to the GPSnet. These developments are in view of assimilation methods for nowcasting and NWP to provide a more robust platform for early detection and prediction of severe weather and precipitation extremes

Bayesian approach to ionospheric imaging with Gaussian Markov random field priors

Ionosfääri on noin 60–1000 kilometrin korkeudella sijaitseva ilmakehän kerros, jossa kaasuatomien ja -molekyylien elektroneja on päässyt irtoamaan auringon säteilyn ja auringosta peräisin olevien nopeiden hiukkasten vaikutuksesta. Näin syntyneillä ioneilla ja vapailla elektroneilla on sähkö- ja magneettikenttien kanssa vuorovaikuttava sähkövaraus. Ionosfäärillä on siksi merkittävä rooli radioliikenteessä. Se voi mahdollistaa horisontin yli tapahtuvat pitkät radiolähetykset heijastamalla lähetetyn sähkömagneettisen signaalin takaisin maata kohti. Toisaalta ionosfääri vaikuttaa myös sen läpäiseviin korkeampitaajuuksisiin signaaleihin. Esimerkiksi satelliittipaikannuksessa ionosfäärin vaikutus on parhaassakin tapauksessa otettava huomioon, mutta huonoimmassa se voi estää paikannuksen täysin. Näkyvin ja tunnetuin ionosfääriin liittyvä ilmiö lienee revontulet. Yksi keskeisistä suureista ionosfäärin tutkimuksessa on vapaiden elektronien määrä kuutiometrin tilavuudessa. Käytännössä elektronitiheyden mittaaminen on mahdollista mm. tutkilla, kuten Norjan, Suomen ja Ruotsin alueilla sijaitsevalla EISCAT-tutkajärjestelmällä, sekä raketti- tai satelliittimittauksilla. Mittaukset voivat olla hyvinkin tarkkoja, mutta tietoa saadaan ainoastaan tutkakeilan suunnassa tai mittalaitteen läheisyydestä. Näillä menetelmillä ionosfäärin tutkiminen laajemmalla alueella on siten vaikeaa ja kallista. Olemassa olevat paikannussatelliitit ja vastaanotinverkot mahdollistavat ionosfäärin elektronitiheyden mittaamisen alueellisessa, ja jopa globaalissa mittakaavassa, ensisijaisen käyttötarkoituksensa sivutuotteena. Satelliittimittausten ajallinen ja paikallinen kattavuus on hyvä, ja kaiken aikaa kasvava, mutta esimerkiksi tarkkoihin tutkamittauksiin verrattuna yksittäisten mittausten tuottama informaatio on huomattavasti vähäisempää. Tässä väitöstyössä kehitettiin tietokoneohjelmisto ionosfäärin elektronitiheyden kolmiulotteiseen kuvantamiseen. Menetelmä perustuu matemaattisten käänteisongelmien teoriaan ja muistuttaa lääketieteessä käytettyjä viipalekuvausmenetelmiä. Satelliittimittausten puutteellisesta informaatiosta johtuen työssä on keskitytty etenkin siihen, miten ratkaisun löytymistä voidaan auttaa tilastollisesti esitetyllä fysikaalisella ennakkotiedolla. Erityisesti työssä sovellettiin gaussisiin Markovin satunnaiskenttiin perustuvaa uutta korrelaatiopriori-menetelmää. Menetelmä vähentää merkittävästi tietokonelaskennassa käytettävän muistin tarvetta, mikä lyhentää laskenta-aikaa ja mahdollistaa korkeamman kuvantamisresoluution.Ionosphere is the partly ionised layer of Earth's atmosphere caused by solar radiation and particle precipitation. The ionisation can start from 60 km and extend up to 1000 km altitude. Often the interest in ionosphere is in the quantity and distribution of the free electrons. The electron density is related to the ionospheric refractive index and thus sufficiently high densities affect the electromagnetic waves propagating in the ionised medium. This is the reason for HF radio signals being able to reflect from the ionosphere allowing broadcast over the horizon, but also an error source in satellite positioning systems. The ionospheric electron density can be studied e.g. with specific radars and satellite in situ measurements. These instruments can provide very precise observations, however, typically only in the vicinity of the instrument. To make observations in regional and global scales, due to the volume of the domain and price of the aforementioned instruments, indirect satellite measurements and imaging methods are required. Mathematically ionospheric imaging suffers from two main complications. First, due to very sparse and limited measurement geometry between satellites and receivers, it is an ill-posed inverse problem. The measurements do not have enough information to reconstruct the electron density and thus additional information is required in some form. Second, to obtain sufficient resolution, the resulting numerical model can become computationally infeasible. In this thesis, the Bayesian statistical background for the ionospheric imaging is presented. The Bayesian approach provides a natural way to account for different sources of information with corresponding uncertainties and to update the estimated ionospheric state as new information becomes available. Most importantly, the Gaussian Markov Random Field (GMRF) priors are introduced for the application of ionospheric imaging. The GMRF approach makes the Bayesian approach computationally feasible by sparse prior precision matrices. The Bayesian method is indeed practicable and many of the widely used methods in ionospheric imaging revert back to the Bayesian approach. Unfortunately, the approach cannot escape the inherent lack of information provided by the measurement set-up, and similarly to other approaches, it is highly dependent on the additional subjective information required to solve the problem. It is here shown that the use of GMRF provides a genuine improvement for the task as this subjective information can be understood and described probabilistically in a meaningful and physically interpretative way while keeping the computational costs low

Geosynchronous synthetic aperture radar for Earth continuous observation missions

This thesis belongs to the field of remote sensing, particularly Synthetic Aperture Radar (SAR) systems from the space. These systems acquire the signals along the orbital track of one or more satellites where the transmitter and receiver are mounted, and coherently process the echoes in order to form the synthetic aperture. So, high resolution images can be obtained without using large arrays of antennas. The study presented in this thesis is centred in a novel concept in SAR, which is known as Geosynchronous SAR or GEOSAR, where the transmitter and/or receiver are placed in a platform in a geostationary orbit. In this case, the small relative motions between the satellite and the Earth surface are taken to get the necessary motion to form the synthetic aperture and focus the image. The main advantage of these systems with respect to the current technology (where LEO satellites with lower height are considered) is the possibility of permanently acquire images from the same region thanks to the small motion of the platform. Therefore, the different possibilities in the orbital design that offer this novel technology as well as the geometric resolutions obtained in the final image have been firstly studied. However, the use of geosynchronous satellites as illuminators results in slant ranges between 35.000-38.000 Km, which are much higher than the typical values obtained in LEOSAR, under 1.000 Km. Fortunately, the slow motion of the satellite makes possible large integration of pulses during minutes or even hours, reaching Signal-to-Noise Ratio (SNR) levels in the order of LEO acquisitions without using high transmitted power or large antennas. Moreover, such large integration times, increases the length of the synthetic aperture to get the desired geometric resolutions of the image (in the order of a few meters or kilometres depending on the application). On the other hand, the use of long integration time presents some drawbacks such as the scene targets decorrelation, atmospheric artefacts due to the refraction index variations in the tropospheric layer, transmitter and receiver clock jitter, clutter decorrelation or orbital positioning errors; which will affect the correct focusing of the image. For this reason, a detailed theoretical study is presented in the thesis in order to characterize and model these artefacts. Several simulations have been performed in order to see their effects on the final images. Some techniques and algorithms to track and remove these errors from the focused image are presented and the improvement of the final focused image is analysed. Additionally, the real data from a GB-SAR (Ground-Based SAR) have been reused to simulate a long integration time acquisition and see the effects in the image focusing as well as to check the performance of compensation algorithms in the final image. Finally, a ground receiver to reuse signals of opportunity from a broadcasting satellite have been designed and manufactured. This hardware is expected to be an important tool for experimental testing in future GEOSAR analysis.Aquesta tesi s'emmarca dins de l'àmbit de la teledetecció, en particular, en els sistemes coneguts com a radar d'obertura sintètica (SAR en anglès) des de l'espai. Aquests sistemes adquireixen senyal al llarg de l'òrbita d'un o més satèl·lits on estan situats el transmissor i el receptor, i processa els ecos de forma coherent per a formar l'obertura sintètica. D'aquesta manera es poden aconseguir imatge d'alta resolució sense la necessitat d'emprar un array d'antenes molt gran. El treball realitzat en aquest estudi es centra en un nou concepte dins del món SAR que consisteix en l'ús de satèl·lits en òrbita geostacionària per a l'adquisició d'imatges, sistemes coneguts com a Geosynchronous SAR o GEOSAR. En aquest cas, els petits moviments relatius dels satèl·lits respecte de la superfície terrestre s'empren per a aconseguir el desplaçament necessari per a formar l'obertura sintètica i així obtenir la imatge. El principal avantatge d'aquests sistemes respecte a la tecnologia actual (on s'utilitzen satèl·lits en orbites més baixes LEO) és la possibilitat d'adquirir imatges d'una mateixa zona de forma permanent gràcies als petits desplaçaments del satèl·lit. Així doncs, en aquesta tesi s'estudien les diferents possibilitats en el disseny orbital que ofereixen aquests sistemes així com les resolucions d'imatge que s'obtindrien. Tot i així, l'ús de satèl·lits en òrbita geoestacionària, resulta en una distància entre el transmissor/receptor i l'escena entre 35000-38000 Km, molt més gran que les distàncies típiques en els sistemes LEO per sota dels 1000 Km. Tot i així, el moviment lent de les plataformes geostacionàries fa possible la integració de polsos durant minuts o hores, arribant a nivells acceptables de relació senyal a soroll (SNR) sense necessitat d'utilitzar potències transmeses i antenes massa grans. A més a més, aquesta llarga integració també permet assolir unes longituds d'obertura sintètica adients per a arribar a resolucions d'imatge desitjades (de l'ordre de pocs metres o kilòmetres segons l'aplicació). Malgrat això, l'ús de temps d'integració llargs té una sèrie d'inconvenients com poden ser la decorrelació dels blancs de l'escena, l'aparició d'artefactes atmosfèrics deguts als canvis d'índex de refracció en la troposfera, derives dels rellotges del transmissor i receptor, decorrelació del clutter o errors en el posicionament orbital, que poden afectar la correcta focalització de la imatge. Així doncs, en la tesi s'ha fet un detallat estudi teòric d'aquests problemes per tal de modelitzar-los i posteriorment s'han realitzat diverses simulacions per veure els seus efectes en una imatge. Diverses tècniques per a compensar aquests errors i millorar la qualitat de la imatge també s'han estudiat al llarg de la tesi. Per altra banda, dades reals d'un GB-SAR (SAR en una base terrestre) s'han reutilitzat per adaptar-les a una possible adquisició de llarga durada i veure així de forma experimental com afecta la llarga integració en les imatges i com millora l'enfocament després d'aplicar els algoritmes de compensació. Per últim, en la tesi es presenta un sistema receptor terrestre per a poder realitzar un anàlisi experimental del cas GEOSAR utilitzant un il·luminador d'oportunitat. Els primers passos en el disseny i la fabricació del hardware també es presenten en aquesta tes

Geodetic Sciences

Space geodetic techniques, e.g., global navigation satellite systems (GNSS), Very Long Baseline Interferometry (VLBI), satellite gravimetry and altimetry, and GNSS Reflectometry & Radio Occultation, are capable of measuring small changes of the Earth�s shape, rotation, and gravity field, as well as mass changes in the Earth system with an unprecedented accuracy. This book is devoted to presenting recent results and development in space geodetic techniques and sciences, including GNSS, VLBI, gravimetry, geoid, geodetic atmosphere, geodetic geophysics and geodetic mass transport associated with the ocean, hydrology, cryosphere and solid-Earth. This book provides a good reference for geodetic techniques, engineers, scientists as well as user community

Development of a Small Satellite Remote Sensing Payload for Passive Limb Sounding of the Atmospheric Oxygen Emission

The Mesosphere/ Lower Thermosphere (MLT) is the region of the atmosphere in the altitude range from 60 km to 110 km. This region becomes more and more important for climate predictions and weather forecasts with the extension of simulation models to higher altitudes. The global dynamics of the MLT is driven by gravity waves. Gravity waves are generated in the lower atmosphere and transport momentum to the MLT, where these waves break and dissipate. The resulting gravity wave drag influences the wind fields and, thus, the global circulation in this region. However, gravity waves are not yet sufficiently well represented in global circulation models, because their scales are often below the grid size of the simulation models, requiring that gravity waves are parameterized. The parameterization is one of the major uncertainties in current simulation models. Thus, observational data are required to better understand the underlying processes and to constrain gravity waves in the global circulation models. However, current gravity wave observing satellites for the MLT exceeded their operational lifetimes and succeeding missions are sparse. The observational gap in the near future is already conceivable. The goal of this work is to propose a novel satellite mission with the corresponding remote sensing instrument that can reduce the data gap through a low-cost, agile, and scalable satellite. The satellite is based on the 3U CubeSat form factor that limits the mass to 4 kg and the launch volume to 34cm x 10 cm x 10 cm. CubeSats are nano satellites that can be launched on many different rockets through a standardized interface that eases the access to space. The here proposed AtmoCube-1 mission is described on a conceptual level. The focus of this work lies on the development of the remote sensing instrument that enables the characterization of gravity waves through temperature soundings in the MLT with a limb viewing geometry. The instrument measures the oxygen atmospheric band emission around 762 nm with a high spectral resolution in a small bandwidth to derive the kinetic temperature in the MLT from the temperature dependence of individual rotational fine structure lines. Thereby, the instrument uses a monolithic and temperature stabilized Fourier-transform spectrometer of the type Spatial Heterodyne Spectrometer that is characterized by a high resolving power and a high etendué at a small form factor. Thus, this instrument can be miniaturized to fit into the volume of a CubeSat. The development of the instrument and of the satellite mission started with this work. Accordingly, the specification of the satellite instrument is a major part of this work, followed by the actual development of the instrument within the mission AtmoHIT. The Atmospheric Heterodyne Interferometer Test (AtmoHIT) is an experiment on-board the sounding rocket REXUS 22 that was launched in Kiruna, Sweden, in March 2017, within the Rocket/Ballon Experiments for University Students program. AtmoHIT had the goal to verify the satellite instrument under near-space conditions by measuring the oxygen atmospheric band. The temperature stabilized design of the spectrometer has been verified in a thermal vacuum chamber test before the flight, where also the operations in the temperature range from -20 degC to 46 degC have been confirmed. Vibration tests indicated that the instrument can sustain the loads during the flight, which was demonstrated with the successful rocket flight campaign. The campaign showed also that the instrument operates under near-space conditions. The oxygen atmospheric band was measured, demonstrating the functionality of the instrument. An anomaly occurred during the separation of the payload module and the rocket motor that resulted in a strongly tumbling payload. Thus, the goal of temperature sounding in the MLT could not be fulfilled. Nevertheless, the sounding rocket campaign was deemed successful, because it showed that the instrument performed as expected. This work concludes by a discussion of the major results from the instrument development and possible enhancements to the instrument. The here developed methods and design tools are already employed in the related projects AtmoSHINE and AtmoWINDS that eventually lead to the launch of the AtmoCube-1 satellite.</p

Remote Sensing

This dual conception of remote sensing brought us to the idea of preparing two different books; in addition to the first book which displays recent advances in remote sensing applications, this book is devoted to new techniques for data processing, sensors and platforms. We do not intend this book to cover all aspects of remote sensing techniques and platforms, since it would be an impossible task for a single volume. Instead, we have collected a number of high-quality, original and representative contributions in those areas

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Abstracts on Radio Direction Finding (1899 - 1995)

The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography). Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM. The contents of these files are: 1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format]; 2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format]; 3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion