When a radio signal traverses the atmosphere it will be delayed by not only thedistance between transmitter and receiver, but also the atmosphere. Given knowl-edge of the characteristics of the sent signal the effect of the atmosphere can beobtained from the received signal. This concept is called radio occultation. Radiooccultation can provide high accuracy profiles of temperature, pressure and watervapour troughout the atmosphere.This report aims to present the work and results from a thesis performed atRUAG Space in Göteborg. The purpose of the thesis was to implement a simulatorwhich with high accuracy could generate a signal as it would have been receivedhad it propagated through the atmosphere.We will show that the generated signal passes the requirements that have beenset. / När en radiovåg passerar genom atmosfären kommer den att fördröjas, inte bara avavståndet mellan sändare och mottagare utan också av atmosfären. Givet kunskapom karaktäristiken hos den sända signalen kan atmosfärens effekt erhållas från denmottagna signalen. Detta koncept kallas för radio-ockultation. Radio-ockultationkan med hög noggrannhet ge profiler för temperatur, tryck och vattenånga genomatmosfären.Denna rapport ämnar presentera det jobb och de resultat som uppnåtts genomett examensarbete genomfört på RUAG Space i Göteborg. Examensarbetets syftevar att implementera en simulator som med hög noggrannhet kan generera ensignal så som den hade sett ut då den propagerat genom atmosfären.Vi kommer att visa att den genererade signalen uppnår de krav som ställts.

This thesis is executed in cooperation with RUAG Space AB, which specializes in highly reliable on-board satellite equipment. The thesis focuses on the effect, which disturbs the amplitude and phase of a Global Positioning System (GPS) signal, called scintillation effect. It has a substantial impact on a GPS signal, during Radio Occultation (RO). RO is a method of analysis of a refracted signal which passes through the atmosphere. RO can be used for measuring climate change and for weather forecasting. By retrieving the bending angle of a GPS signal, three basic parameters of the Earth’s atmosphere can be obtained at different heights: temperature, pressure and humidity. As the scintillation effect causes prominent errors in the bending angle calculations, it is crucial to provide possibly the most precise mathematical model, which allows to conceive proper ionospheric corrections. In this thesis, the model using Rytov approach is implemented and optimised with different optimisation functions. It is shown that the scintillation model can be optimized, which may contribute to a more accurate retrieval of the atmospheric profiles.

Several measurement campaigns took place at the ALOMAR observatory at Andøya, Northern Norway during July-November 2001 to validate ionospheric electron density and dry temperature profiles in the troposphere and lower stratosphere derived from radio occultation measurements of the low earth orbiter satellite CHAMP. For temperature sounding, three balloons are released around GPS satellite occultation events that occurred inbetween a distance of 200 km around Andøya. At altitudes of 7–20 km the CHAMP profile shows a positive mean deviation increasing with height by about 1.5-2 Kelvin/ 10 km overlayed with variations of ±1 K when compared to the radiosonde. Taking into account the previous and following radiosonde ascents the mean deviation seems to be of systematic nature due to the occultation principle or the retrieval algorithm and the variations are related to geographical variations of temperature and to the horizontal averaging by the radio occultation technique. During the period from mid July to mid August, four occultations for ionospheric soundings occurred. The values of the F2 layer calculated from the CHAMP derived electron density profiles are compared to the readings of the Alomar and Tromsø ionosondes for these times. Comparison shows that using the radio occultation technique electron densities of the maximum value layer are calculated inbetween the same order of magnitude as the ionosondes measurements, however, they overestimate it in the cases discussed here. / Zur Validierung von Elekronendichte- und Temperaturprofilen, abgeleitet aus Radiookkultationsmessungen von CHAMP wurden im Juli-November 2001 mehrere Messkampagnen am ALOMAR Institut auf Andøya, Nordnorwegen durchgeführt. Zur Temperatursondierung wurden drei aufeinanderfolgende Radiosonden um den Zeitpunkt von Okkultationsereignissen im Umkreis von weniger als 200 km gestartet. Das hier diskutierte Temperaturprofil von CHAMP zeigt im Höhenbereich 7-20 km eine mit der Höhe zunehmende positive Abweichung von ca. 1,5-2 K/10 km mit Variationen um ±1 K verglichen mit dem Temperaturprofil der zum Okkultationszeitpunkt fliegenden Radiosonde. Der Vergleich mit den vorhergehenden und nachfolgenden Sondierungen lässt darauf schließen, dass die mittlere Abweichung durch systematische Fehler des Okkultationsverfahrens oder den Retrieval-Algorithmus bedingt sind, die Variationen jedoch durch die örtliche Abweichung und die horizontale Mittelung des Messverfahrens. Während des Zeitraumes von Mitte Juli bis Mitte August ereigneten sich vier Okkultationen zur Sondierung der Ionosphäre. Von den abgeleiteten Elektronendichteprofilen werden jeweils die Werte der F2-Schicht mit den zur gleichen Zeit gemessenen Elektronendichten der Ionosonden auf Andøya und bei Tromsø verglichen. Der Vergleich zeigt, dass mit Hilfe der Radiookkultaktionstechnik die Elektronendichtewerte der F2-Schicht in der gleichen Größenordnung berechnet, in diesen konkreten Fällen jedoch überschätzt werden.

In the height range 10–30 km atmospheric gravity waves lead to periodic perturbations of the background temperature field in the order of 2-3 K, that are resolved in temperature profiles derived from radio occultation measurements. Due to the spherical symmetry assumption in the retrieval algorithm and the low horizontal resolution of the measurement damping in the amplitude and phase shift of the waves occurs leading to remarkable errors in the retrieved temperatures. The influence of the geometric wave parameters and the measurement geometry on plane gravity waves in the range 100-1000 km horizontal and 1-10 km vertical wavelength is investigated with a 2D model ranging ±1000 km around the tangent point and 10-50 km in height. The investigation shows, that with radio occultation measurements more than 90 % of the simulated waves can be resolved and more than 50% with amplitudes above 90%. But the geometrical parameters cannot be identified, since one signal can be attributed to different combinations of wave parameters and view angle. Even short waves with horizontal wavelengths below 200 km can be derived correctly in amplitude and phase if the vertical tilt is small or the view angle of the receiver satellite is in direction of the wave crests. / Atmosphärische Schwerewellen führen im Höhenbereich 10-30 km zu periodischen Störungendes Hintergrundtemperaturfeldes in der Größenordnung von 2-3 K, die in Temperaturprofilen aus Radiookkultationsmessungen aufgelöst werden. Aufgrund der sphärischen Symmetrieannahme im Retrievalverfahren und durch die niedrige horizontale Auflösung des Messverfahrens werden Phasenverschiebungen und Dämpfung der Amplitude verursacht, die zu beachtlichen Fehlern bei den abgeleiteten Temperaturen führen. Der Einfluss der geometrischen Wellenparameter und der Messgeometrie auf ebene Schwerewellen im Bereich 100-1000 km horizontale und 1-10 km vertikale Wellenlänge wird untersucht mit einem 2D-Modell, dass sich auf ein Gebiet von ±1000 km um den Tangentenpunkt und von 10-50 km in der Höhe erstreckt. Die Untersuchung zeigt, dass mit Radiookkultationsmessungen mehr als 90% der simulierten Wellen aufgelöst werden und mehr als 50% mit Amplituden oberhalb von 90% der ursprünglichen. Die geometrischen Parameter können jedoch nicht aus Einzelmessungen abgeleitet werden, da ein Signal zu verschiedenen Kombinationen von Wellenparametern und Sichtwinkel zugeordnet werden kann. Auch relativ kurze Wellen mit horizontalen Wellenlängen unterhalb von 200 km können korrekt in der Amplitude und Phase aufgelöst werden, falls die Neigung des Wellenvektors gegen die vertikale gering ist oder der Sichtwinkel des Empfängersatelliten in Richtung der Wellenberge ist.

GNSS Radio Occultation (GNSS-RO) is an opportunistic Earth sensing technique where GNSS signals passing through the atmosphere are received in low Earth orbit and processed to extract meteorological parameters. As signals are received along an orbit, the measured Doppler shift is transformed to a bending angle profile (commonly referred to as bending angle retrieval), which, in turn, is inverted to a refractivity profile. Thanks to its high vertical resolution and SI traceability, GNSS-RO is an important complement to other Earth sensing endeavors. In the lower troposphere, GNSS-RO measurements often get degraded and biased due to sharp refractive gradients and other complex structures. The main objective of this thesis is to explore contemporary retrieval methods such as phase matching and full spectrum inversion to improve their performance in these conditions. To avoid the bias caused by the standard inversion, we attempt to derive additional information from the amplitude output of the examined retrieval operators. While simulations indicate that such information could be found, it is not immediately straightforward how to achieve this with real measurements. The approach chosen is to examine reflected signal components and their effect on the amplitude output.

While dual-frequency GPS receivers have been used in space for more than two decades, the size, power, and cost of this technology is an important driver for future space missions. The growing availability of launch opportunities for very small satellites known as nanosatellites and CubeSats raises the possibility of more affordable access to space measurements if the observation quality is sufficient to support the user's needs. This thesis presents the initial development and testing of the Fast, Orbital, TEC, Observables, and Navigation (FOTON) receiver: a small, reconfigurable, dual-frequency, space-based GPS receiver. Originally developed as a science-grade software receiver for monitoring ionospheric scintillation and total electron content (TEC), this receiver was designed to provide high-quality GPS signal observations. The original receiver hardware was miniaturized and the software has been adapted for low earth orbit (LEO) operations. FOTON now fits within a 0.5U CubeSat form factor (8.3 x 9.6 x 3.8 cm), weighs 326 g, and consumes 4.5 W of instantaneous power, which can be reduced to <1 W orbit average power with on-off duty cycling. The receiver has been designed with commercial parts to keep manufacturing costs low. Significant testing of FOTON has been performed with live signals and with signals generated by a Spirent GPS signal simulator. Initial terrestrial tests demonstrate behavioral consistency with the original heritage high-performance receiver. Several LEO simulations are presented, demonstrating FOTON's single-frequency and dual-frequency-enhanced positioning down to 0.5 m and 1.5 m, respectively, which can be improved using Kalman filter based POD. FOTON's potential for GPS radio occultation observation is also demonstrated. In addition, its acquisition and reacquisition performance is presented; on average, FOTON's time to first fix is approximately 45 seconds. Finally, navigation in geostationary orbit (GEO), a challenging application for space-based GPS receivers, is demonstrated. Extensive testing demonstrates that FOTON is a robust, versatile, high-precision dual-frequency space receiver. Its low cost, size, weight, and power requirements are key enablers for future small-satellite missions. / text

Proper and precise interpretation of radio occultation soundings of planetary atmospheres requires understanding the signal amplitude and phase variations caused by random perturbations in the complex index of refraction caused by atmospheric turbulence. This research focuses on understanding the turbulence and its impact on these soundings.From aircraft temperature, pressure and humidity measurements we obtained a parametric model for estimating the strength of the atmospheric turbulence in the troposphere. We used high-resolution balloon measurements to understand the spatial spectrum of turbulence in the vertical dimension.We also review and extend electromagnetic scintillation theory to include a complex index of refraction of the propagating medium. In contrast to when the fluctuations in only the real component of the index of refraction are considered, this work quantifies how atmospheric turbulent eddies contribute to the signal amplitude and phase fluctuations and the amplitude frequency correlation function when the index of refraction is complex. The generalized expressions developed for determining the signal's amplitude and phase fluctuations can be solved for planar, spherical or beam electromagnetic wave propagation.We then apply our mathematical model to the case of a plane wave propagating through a homogenous turbulence medium and estimate the amplitude variance for signals at various frequencies near the 22 GHz and 183 GHz water vapor absorption features. The theoretical results predict the impact of random fluctuations in the absorption coefficient along the signal propagation path on the signal's amplitude fluctuations. These results indicate that amplitude fluctuations arising from perturbations of the absorption field can be comparable to those when the medium has a purely real index of refraction. This clearly indicates that the differential optical depth approach devised by Kursinski et al. (2002) to ratio out the effects of turbulence on signals passing through a medium of a purely real index of refraction must be modified to include the effects of turbulent variations in the imaginary part of the refractivity.

Radio Occultation (RO) is a remote sensing technique which uses Global Navigation Satellite System (GNSS) signals tracked by a Low-Earth Orbit (LEO) satellite to sound the earth's atmosphere both in low (troposphere, stratosphere) and high (ionosphere) altitudes. GNSS-RO provides global coverage and SI traceable measurements of atmospheric data with high-vertical resolution. Refractivity, dry temperature, pressure and water vapour profiles retrieved from RO measurements have a relevant contribution in Numerical Weather Prediction (NWP) systems and in climate-monitoring. Due to the partial propagation through the ionosphere, a systematic bias is added to the lower atmospheric data product. Most of this contribution is removed by a linear combination of data for two frequencies. In climatology studies, one can apply a second-order correction - so called κ-correction - which relies on a priori information on the conditions in the ionosphere. However, both approaches do not remove high-order terms in the error due to horizontal gradient and earth's geomagnetic fields. The remaining residual ionospheric error (RIE) and its systematic bias in RO atmospheric data is a well-known issue and its mitigation is an open research topic. In this licentiate dissertation, the residual ionospheric error after the standard correction is evaluated with computational simulations using a wave optics propagator (WOP). Multiple Phase Screen (MPS) method is used to simulate occultation events in different ionospheric scenarios, e.g. quiet and disturbed conditions. Electron density profiles (EDP) assumed in simulations are either defined by analytical equations or measurements. The disturbed cases are modelled as small-scale irregularities within F-region in two different ways: as sinusoidal fluctuations; and by using a more complex approach, where the irregularities follow a single-slope power-law that yields moderate to strong scintillation in the signal amplitude. Possible errors in MPS simulations assuming long segment of orbit and ionosphere are also evaluated. The results obtained with the sinusoidal disturbances show minor influence in the RIE after the standard correction, with the major part of the error due to the F-region peak. The implementation of the single-slope power-law is validated and the fluctuations obtained in simulation show good agreement to the ones observed in RO measurements. Finally, an alternative to overcome limitations in MPS simulations considering occultations with long segment of orbit and ionosphere is introduced and validated. The small-scale irregularities modelled in F-region with the power-law can be added in simulations of a large dataset subjected to κ-correction, in order to evaluate the RIE bending angle and the consequences in atmospheric parameters, e.g. temperature. / NRPF-3, Rymdstyrelsen, 241/15

The marine boundary layer (MBL) is the region where energy, momentum and masses are exchanged between the ocean surface and the free troposphere. The lack of observations with high vertical resolution over the ocean significantly restricts the understanding of the complex physical processes that occur inside the MBL. The relatively short vertical extent of the MBL (average about 1~2 km) and the frequent cloudiness at its top make probing the MBL extremely difficult from the space. Several features of the Global Positioning System (GPS) radio occultation (RO) technique suggest that it has a great potential for sensing the MBL. These features include global coverage, high vertical resolution, and the ability of GPS signals to penetrate clouds.Over moist marine areas, a large negative moisture gradient often exists across the thermal inversion capping the MBL, which can cause super-refraction (SR) or ducting. A large number of high-resolution soundings have shown that SR occurs about 90% of the time in a year over the subtropical and tropical oceans and even 50% at high-latitudes during the summer. In the presence of SR, the reconstruction of refractivity from RO data becomes an ill-posed inverse problem, i.e., a given RO bending angle profile is consistent with a continuum (an infinite number) of refractivity profiles. The standard Abel retrieval gives the minimum refractivity solution of the continuum and thus produces the largest negative bias, consistent with a negative bias that is often present in the retrieved refractivity profiles in the moist lower troposphere. Simulation studies indicate a large variation of the negative refractivity biases (could be over -15%). The impact of diffraction effects and the open-loop receiver tracking on the bending angle and refractivity retrievals are assessed. A novel approach is developed and tested to reconstruct the vertical refractivity structure within and below the SR layer, which yields a much-improved retrieval, especially below the SR layer (less than 0.5% error). Such a reconstruction method should greatly enhance our ability to measure the MBL globally using the GPS RO technique as well as to improve the MBL parameterizations used in weather and climate models.

Cette thèse est basée sur cinq publications étudiant les ionosphères terrestre et martienne en s'appuyant sur la combinaison d'observations provenant d'instruments variés ainsi que sur des techniques de modélisation. L'ionosphère terrestre est un système complexe fortement couplé à la magnétosphère et est par conséquent affectée par les perturbations provenant du vent solaire. De nombreux types d'instruments peuvent être utilisés pour étudier la variabilité de l'ionosphère, qu'il s'agisse de systèmes d'observation au sol ou d'instruments à bord de satellites. Deux des articles se focalisent sur les réponses de l'ionosphère terrestre aurorale et subaurorale aux courants de vent solaire rapide émanant des trous coronaux à la surface du soleil. Ces deux études sont basées sur la méthode des époques superposées, qui permet d'obtenir un comportement statistique des paramètres considérés. Pour la première étude, qui s'intéresse à la concentration électronique du pic de la région F de l'ionosphère à l'aide de l'ionosonde de Sodankylä (Finlande, L = 5.2), la méthode des époques superposées a été modifiée en ajoutant un verrouillage de phase permettant de distinguer les réponses de la région F dans différents secteurs de temps magnétique local. La deuxième étude s'intéresse aux précipitations d'électrons énergétiques (>30 keV) durant les courants de vent solaire rapide, en s'appuyant sur des mesures d'absorption du bruit cosmique par des riomètres situés entre L = 3.8 et L = 5.7. Une troisième étude met en évidence pour la première fois des signatures de pulsations dans les données riométriques durant une aurore pulsante. Cela révèle que le flux de précipitation d'électrons est modulé simultanément sur une grande plage d'énergies - de quelques kiloélectronvolts à plusieurs dizaines de kiloélectronvolts - durant une aurore pulsante. Les quatrième et cinquième articles traitent de l'ionosphère martienne. Ils présentent une nouvelle méthode d'analyse des données d'occultation radio fournies par la sonde Mars Express, qui s'appuie non pas sur une inversion des mesures tel qu'effectué classiquement, mais sur une modélisation directe de l'environnement martien - atmosphère neutre et ionosphère - et de la propagation des ondes radio entre la station sol sur Terre et la sonde Mars Express. L'ajustement des paramètres dont dépendent l'atmosphère et l'ionosphère martiennes permet d'obtenir des données d'occultation radio simulées s'approchant le plus possible des données mesurées. L'ajustement optimal donne alors les profils de température et de concentration des neutres ainsi que les profils de concentrations ioniques et électronique dans l'ionosphère martienne au voisinage du point d'occultation. / This thesis is based on five publications studying the terrestrial and Martian ionospheres by making use of versatile instruments and of modelling techniques. The terrestrial ionosphere is a complex system strongly coupled to the magnetosphere and hence very sensitive to solar wind driving. Various kinds of instruments may be used to study the ionosphere, from ground-based instruments to satellite-borne systems. Two papers study the response of the auroral and subauroral ionosphere to solar wind high-speed streams, which originate from coronal holes at the surface of the Sun. These two studies make use of the superposed epoch analysis method, which enables to derive the statistical behaviour of the studied parameters. For the first study, which focuses on the F-region peak electron density measured by the Sodankylä ionosonde (at L = 5.2), the superposed epoch method has been modified so that a study of the effects of high-speed streams in the F region in different magnetic local time sectors becomes possible. The modified method is called phase-locked superposed epoch analysis. The second paper focuses on energetic (>30 keV) electron precipitation during high-speed streams by making use of cosmic noise absorption measurements from a chain of riometers located between L = 3.8 and L = 5.7. A third study reveals for the first time pulsation signatures in cosmic noise absorption data during a pulsating aurora event. This indicates that the electron precipitation flux is modulated simultaneously over a broad range of energies (from a few keV to several tens of keV) in relation to pulsating aurora. The fourth and fifth articles study the Martian ionosphere. They present a novel analysis method for Mars Express radio-occultation data. Contrary to the classical inversion approach, this new method is based on a direct simulation of the radio wave propagation between the ground-based station at Earth and the Mars Express spacecraft, in a modelled Martian environment. The parameters determining the properties of the neutral atmosphere and the ionosphere of Mars are adjusted in order for the simulated radio-occultation data to fit the measured data. The optimal set of parameters provides the retrieved neutral temperature and density profiles in the atmosphere, and the ion and electron density profiles in the ionosphere near the occultation point.