Forward and Inverse Modeling of GPS Multipath for Snow Monitoring

Snowpacks provide reservoirs of freshwater, storing solid precipitation and delaying runoff to be released later in the spring and summer when it is most needed. The goal of this dissertation is to develop the technique of GPS multipath reflectometry (GPS-MR) for ground-based measurement of snow depth. The phenomenon of multipath in GPS constitutes the reception of reflected signals in conjunction with the direct signal from a satellite. As these coherent direct and reflected signals go in and out of phase, signal-to-noise ratio (SNR) exhibits peaks and troughs that can be related to land surface characteristics. In contrast to other GPS reflectometry modes, in GPS-MR the poorly separated composite signal is collected utilizing a single antenna and correlated against a single replica. SNR observations derived from the newer L2-frequency civilian GPS signal (L2C) are used, as recorded by commercial off-the-shelf receivers and geodetic-quality antennas in existing GPS sites. I developed a forward/inverse approach for modeling GPS multipath present in SNR observations. The model here is unique in that it capitalizes on known information about the antenna response and the physics of surface scattering to aid in retrieving the unknown snow conditions in the antenna surroundings. This physically-based forward model is utilized to simulate the surface and antenna coupling. The statistically-rigorous inverse model is considered in two parts. Part I (theory) explains how the snow characteristics are parameterized; the observation/parameter sensitivity; inversion errors; and parameter uncertainty, which serves to indicate the sensing footprint where the reflection originates. Part II (practice) applies the multipath model to SNR observations and validates the resulting GPS retrievals against independent in situ measurements during a 1-3 year period in three different environments - grasslands, alpine, and forested. The assessment yields a correlation of 0.98 and an RMS error of 6-8 cm, with the GPS under-estimating in situ snow depth by approximately 15%. GPS daily site averages were found effective in mitigating random noise without unduly smoothing the sharp transitions as captured in new snow events. This work corroborates the readiness of quality-controlled GPS-MR for snow depth monitoring, reinforcing its maturity for operational usage

An open‑source low‑cost sensor for SNR‑based GNSS reflectometry : design and long‑term validation towards sea‑level altimetry

Monitoring sea level is critical due to climate change observed over the years. Global Navigation Satellite System Reflectometry (GNSS-R) has been widely demonstrated for coastal sea-level monitoring. The use of signal-to-noise ratio (SNR) observations from ground-based stations has been especially productive for altimetry applications. SNR records an interference pattern whose oscillation frequency allows retrieving the unknown reflector height. Here we report the development and validation of a complete hardware and software system for SNR-based GNSS-R. We make it available as open source based on the Arduino platform. It costs about US$200 (including solar power supply) and requires minimal assembly of commercial off-the-shelf components. As an initial validation towards applications in coastal regions, we have evaluated the system over approximately 1 year by the Guaíba Lake in Brazil. We have compared water-level altimetry retrievals with independent measurements from a co-located radar tide gauge (within 10 m). The GNSS-R device ran practically uninterruptedly, while the reference radar gauge suffered two malfunctioning periods, resulting in gaps lasting for 44 and 38 days. The stability of GNSS-R altimetry results enabled the detection of miscalibration steps (10 cm and 15 cm) inadvertently introduced in the radar gauge after it underwent maintenance. Excluding the radar gaps and its malfunctioning periods (reducing the time series duration from 317 to 147 days), we have found a correlation of 0.989 and RMSE of 2.9 cm in daily means. To foster open science and lower the barriers for entry in SNR-based GNSS-R research and applications, we make a complete bill of materials and build tutorials freely available on the Internet so that interested researchers can replicate the system

Effects of Parent and Peer Behaviors on Adolescent Sexual Behavior: Are Positive and Negative Peer Behaviors Moderators?

Adolescents and young adults account for a significantly high proportion of unintended pregnancy and sexually transmitted infection cases in the United States. According to Jessor\u27s Problem Behavior Theory, combined protective factors, such as exposure to positive parenting and peer behaviors, create an environment that is supportive of conventional behaviors and discouraging of problem behaviors. There is an extensive amount of literature on parent and peer influences on adolescent sexual behavior but few studies address the interactive influence of both parent and peer behaviors on adolescent sexual risk-taking. The purpose of this study was to examine the relationship between maternal supportiveness and strictness on adolescent sexual risk-taking, as well as the moderating influence of peer involvement in positive or negative activities. A sample of 14-16 year old adolescents was drawn from the National Longitudinal Survey of Youth-1997 (NLSY-97; N = 4,008, 50.5% male, 59.4% White, 26.5% Black, and 13.3% other). Higher levels of maternal supportiveness, maternal strictness, and positive peer behaviors were each associated with lower levels of sexual risk-taking two years later. High levels of negative peer behaviors were related to high sexual-risk taking two years later. No interaction terms were significant. Important implications for positive peer relationships were also found. Future research should focus on the comparison of parental warmth and control variables as moderators for the relationship between peer influence and adolescent sexual risk-taking

Analyzing Anomalous Artefacts in TDS-1 Delay Doppler Maps

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Global Navigation Satellite System Reflectometry (GNSS-R) uses the GNSS reflected signals to study parameters of the Earth's surface such as ocean surface height, wind speed, soil moisture, sea surface target detection. In this paper fourteen DDMs (Delay Doppler Maps) of TechDemoSat-1 (TDS-1) containing anomalous artefacts are presented and analyzed. Anomalous artefacts are not caused by the reflection from Earth surface targets, occultation, nor the leakages of direct signals, but likely - according to their delays- from reflection of targets above the Earth's surface (either airborne or spaceborne).Postprint (author's final draft

Precise Point Positioning Inertial Navigation Integration for Kinematic Airborne Applications

UAVs have the potential for autonomous airborne remote sensing applications that require rapid response to natural hazards (e.g. volcano eruptions, earthquakes). As these applications require very accurate positioning, tightly coupled Global Positioning System (GPS) Precise Point Positioning (PPP) Inertial Navigation Systems (INS) are an attractive method to perform real-time aircraft positioning. In particular, PPP can achieve a level of positioning accuracy that is similar to Real-Time Kinematic (RTK) GPS, without the need of a relatively close GPS reference station. However, the PPP method is known to converge to accurate positioning estimate more slowly when compared to RTK, a drawback of PPP that is amplified whenever the receiver platform is faced with GPS challenged environments, such as poor satellite visibility and frequent phase breaks.;This thesis presents the use of a simulation environment that characterizes the position estimation performance sensitivity of PPP/INS through a Monte Carlo analysis that is considered under various conditions: such as, the intensity of multipath errors, the number of phase breaks, the satellite geometry, the atmospheric conditions, the noise characteristics of the inertial sensor, and the accuracy of GPS orbit products. After the PPP/INS formulation was verified in a simulation environment, the INS formulation was incorporated into NASA JPL\u27s Real-Time GIPSY-x. This software was then verified using eight recorded flight data sets provided by the National Geodetic Survey (NGS), National Oceanic and Atmospheric Administration (NOAA) program called Gravity for the Redefinition of the American Vertical Datum (GRAV-D)

GNSS Receiver Architectures For Remote Sensing Applications

Global Navigation Satellite System (GNSS) signals designed to support position, navigation and time, can also be used as signals of opportunity for remote sensing applications. As GNSS evolve, the receiver architectures need to evolve as well. This work explores how to improve the GNSS receiver for remote sensing. Three areas are developed throughout the thesis: utilization of modernized signals, signal cancellation for reduction of cross-correlation noise, and development of a vector delay frequency lock loop with support for Low Earth Orbit (LEO). The improved structures of modernized signals offer considerable advantages for GNSS Reflectometry. One of those signals is GPS L5, which features increased signal power and a higher chipping rate. A comparison study of the transmit power between GPS L1 and L5 showed that the transmit power difference for L5 is close to the specified 3.6 dB. Despite this, due to performance differences of the receive antennas, the benefit might not be obtained. The higher chipping rate of the L5 signal provide advantages in terms of increased waveform resolution for GNSS-R. A study of altimetry performance with WAAS L5 was undertaken. After optimizing the coherent integration time, the altitude standard deviation of the L5 signal, compared to the L1 signal was found to be 2.2 times smaller. The improvements were primarily seen when the aircraft changed altitude. Reducing cross-correlation noise by means of signal cancellation is an effective tool to improve weak signal tracking. Factors influencing performance of signal cancellation: filtering, analog quantization and processing resolution were studied. It was found that canceling a signal collected with a narrow filter bandwidth introduced a residual signal that contributed additional noise. A cancellation efficiency metric was developed in order to analyze how well a signal could be canceled. It was found that high cancellation efficiency could be obtained with 4 bit analog quantization and 9 bit processing resolution. GPS Radio Occultation measurements originate from the excess phase and amplitude of the occulted signal. The measurements can be obtained through open loop tracking, which requires precise orbit determination. The use of a vector delay frequency lock loop (VDFLL) with a low Earth orbit dynamic model is proposed to satisfy both of those requirements. The VDFLL takes advantage of inter-channel aiding for precision orbit determination as well as provides a natural extension to open loop processing. The algorithm was tested on simulated and live data, and was found to successfully track a low elevation signal from a spacecraft with geometric open loop processing through VDFLL.</p

Estimativa da umidade do solo por refletometria GNSS : uma revisão conceitual

Soil moisture monitoring enables efficient management and use of water resources, having great importance for several purposes, such as: monitoring of risk areas; delimitation of areas susceptible to flooding; geotechnical activities; and in agriculture development. GNSS Reflectometry (GNSS-R) is a scientific and technological development that allows one to perform proximal or remote sensing, depending on the antenna height concerning the surface, by means of navigation satellites. This method exploits GNSS signals indirectly reaching a receiver antenna after they are reflected on the surrounding surfaces. In this method, direct and indirect GNSS signals that reach the receiving antenna are exploited, after reflection on the surfaces existing around the antenna. The combination of these two signals causes the multipath effect, which affects GNSS observable and deteriorates positioning. On the other hand, when interacting with these reflecting surfaces one can estimate their properties. One of the main advantages of GNSS-R, when compared with the conventional methods, is the intermediate coverage area, as well as, the use of the well-defined structure of GNSS systems that guarantee appropriate temporal resolution. The scope of this paper is to present a conceptual review of GNSS-R applied to soil moisture monitoring.O monitoramento da umidade do solo possibilita o manejo e uso eficiente de recursos hídricos, sendo uma atividade importante em diversas áreas, tais como: no monitoramento de áreas de risco; delimitação de áreas suscetíveis a enchentes; atividades da geotecnia; e na agricultura. A Refletometria GNSS (GNSS-R) é um desenvolvimento científico e tecnológico que permite realizar sensoriamento remoto ou proximal, a depender da altura da antena em relação à superfície, com satélites de navegação. Neste método, explora-se os sinais GNSS que chegam à antena receptora de maneira direta e indireta, após reflexão nas superfícies existentes no entorno da antena. A combinação destes dois sinais ocasiona o efeito de multicaminho, que afeta as observáveis GNSS e deteriora o posicionamento. Por outro lado, ao interagir com estas superfícies, o sinal indireto permite estimar atributos acerca destas superfícies, como por exemplo a umidade do solo. Uma das principais vantagens em relação aos métodos convencionais reside no fato do GNSS-R proporcionar uma área de abrangência intermediária e o uso da estrutura bem estabelecida dos satélites GNSS, que garantem resolução temporal apropriada. O escopo deste trabalho é apresentar uma revisão conceitual acerca do GNSS-R aplicado no monitoramento da umidade do solo