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 Cassini spacecraft orbited Saturn for over 13 years and collected stellar occultations using an Ultraviolet Imaging Spectrograph (UVIS). Chord occultations were analyzed using autocorrelations at minimum ring plane radius to visualize the structure and correlation in the azimuthal direction. These particle tracking occultations cut a chord across the rings in the path of the star. By taking the autocorrelation of these chord occultations, 8 out of the 66 showed clumping within the first 3.0 km in azimuth, representing signs of a structure. Six of those occultations could be moonlets or propellers as their minimum ring plane radii are in the Propeller Belt region. The Fast Fourier Transform Power Spectrum of the autocorrelation was also taken, and 6 of the 8 had high peak power outputs at certain wavelengths. It is also observed that five of the occultations may contain self-gravity wakes.

An ultraviolet instrument compatible with a CubeSat form factor is currently being developed at Virginia Tech for the purpose of measuring nitric oxide in the polar night through the stellar occultation technique. This instrument will allow the investigation of how the Sun and Earth systems are related via energetic particle precipitation in the auroral regions. The work performed in this thesis supports the instrument design and requirements development by modelling the stellar occultation geometry to identify orbit parameters and target stars that could yield nitric oxide measurements during the polar winter at consistent latitudes, to best observe the build-up and fall-off of nitric oxide. The orbit study was accomplished through the development of an open-source tool in MATLAB, the Stellar Occultation Mission Planner. The results of this analysis were used to model the instrument performance and identify the required narrowband filter parameters to meet science requirements. Additional studies were performed to explore system performance for a future flight opportunity. / Master of Science / A small, light weight instrument is being designed at Virginia Tech to allow for nitric oxide in the atmosphere to be measured during the long polar nights that occur during winter in the Arctic and Antarctic regions. This instrument will allow scientists to explore how the Sun and Earth interact through space weather at high latitudes. This will be accomplished by using star light to probe the atmosphere while the instrument is onboard a small spacecraft called a CubeSat. The work performed in this thesis simulated the spacecraft orbit to determine which stars yielded the best measurements over the course of the polar night. Using these results, the instrument performance was simulated to inform the design of a filter for the instrument. Additional studies were performed to support the design of a future mission to fly the instrument in space.

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Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

L'observation d'occultations du Soleil et d'étoiles par Titan permet d'étudier l'atmosphère épaisse de ce satellite de Saturne du point de vue de sa composition en gaz et en aérosols. Le principe de ces observations, réalisées par le spectro-imageur visible/infrarouge VIMS de la sonde Cassini, est de mesurer la transmission du flux solaire ou stellaire à travers l'atmosphère de Titan. Les données sont constituées de courbes de lumière à différentes longueurs d'onde et de spectres infrarouges pour différentes altitudes de visée. L'étude des courbes de lumière montre qu'il s'agit d'occultations par absorption et non par réfraction différentielle comme c'est le cas pour les occultations observées depuis la Terre. La baisse de signal observée est donc due à l'absorption du flux lumineux par le gaz et les aérosols de l'atmosphère.<br>Les spectres en transmission présentent des bandes d'absorption du méthane à 1,2, 1,4, 1,7, 2,3 et 3,3 µm et du monoxyde de carbone à 4,7 µm. Un code de transfert radiatif en géométrie sphérique et utilisant la méthode de calcul raie par raie a été développé afin de modéliser les bandes observées. L'étude du méthane est centrée principalement sur la bande à 2,3 µm. Au-dessus de 200 km, nos données sont compatibles avec une abondance uniforme de 1,4 - 1,6% telle que mesurée par d'autres instruments. En dessous de 200 km, un effet systématique mal compris empêche une mesure fiable. La molécule de CO est détectée en dessous de 180 km. Une abondance de 33±10 ppm est mesurée entre 70 et 130 km d'altitude. En dessous de 500 km environ, une absorption supplémentaire, centrée sur 3,4 µm se superpose à la bande du méthane à 3,3 µm. Cette bande caractérise la vibration des liaisons C - H au sein de longues chaînes aliphatiques rattachées à de larges molécules organiques qui composent les aérosols. <br>L'absorption des aérosols fixe le niveau de continu des spectres étudiés. Celle-ci est plus forte aux courtes longueurs d'onde et augmente lorsque l'altitude décroît. Un code d'inversion du continu a été développé afin de déterminer les profils de densité des aérosols et de modéliser leur transmission. L'hypothèse de départ est que les aérosols sont des agrégats fractals composés de sphères de 0,05 µm de rayon dont les propriétés optiques sont celles des tholins de Khare et al. (1984). Les modèles de transmission obtenus révèlent que seuls les agrégats comportant plus de 1 000 sphères sont compatibles avec les observations. De plus, l'absence des deux absorptions caractéristiques à 3 et 4,6 µm dans nos données soulignent les différences significatives entre les tholins et les aérosols réels. Les profils de densité des aérosols indiquent une augmentation exponentielle en dessous de 450 km, caractérisée par une échelle de hauteur de l'ordre de 60 km pour les données de l'occultation solaire (71°S) et de l'ordre de 50 km pour celle de l'occultation de Gamma Crucis (24°N). L'écart constaté est peut-être attribuable à la différence de latitude entre ces deux observations. Enfin, les données de l'occultation rasante d'Antarès comportent de<br>nombreuses variations de flux rapides et intenses (« spikes ») : elles sont attribuées à des ondes de gravité se propageant dans l'atmosphère de Titan.