Temporal Characteristics of Boreal Forest Radar Measurements

Radar observations of forests are sensitive to seasonal changes, meteorological variables and variations in soil and tree water content. These phenomena cause temporal variations in radar measurements, limiting the accuracy of tree height and biomass estimates using radar data. The temporal characteristics of radar measurements of forests, especially boreal forests, are not well understood. To fill this knowledge gap, a tower-based radar experiment was established for studying temporal variations in radar measurements of a boreal forest site in southern Sweden. The work in this thesis involves the design and implementation of the experiment and the analysis of data acquired. The instrument allowed radar signatures from the forest to be monitored over timescales ranging from less than a second to years. A purpose-built, 50 m high tower was equipped with 30 antennas for tomographic imaging at microwave frequencies of P-band (420-450 MHz), L-band (1240-1375 MHz) and C-band (5250-5570 MHz) for multiple polarisation combinations. Parallel measurements using a 20-port vector network analyser resulted in significantly shorter measurement times and better tomographic image quality than previous tower-based radars. A new method was developed for suppressing mutual antenna coupling without affecting the range resolution. Algorithms were developed for compensating for phase errors using an array radar and for correcting for pixel-variant impulse responses in tomographic images. Time series results showed large freeze/thaw backscatter variations due to freezing moisture in trees. P-band canopy backscatter variations of up to 10 dB occurred near instantaneously as the air temperature crossed 0⁰C, with ground backscatter responding over longer timescales. During nonfrozen conditions, the canopy backscatter was very stable with time. Evidence of backscatter variations due to tree water content were observed during hot summer periods only. A high vapour pressure deficit and strong winds increased the rate of transpiration fast enough to reduce the tree water content, which was visible as 0.5-2 dB backscatter drops during the day. Ground backscatter for cross-polarised observations increased during strong winds due to bending tree stems. Significant temporal decorrelation was only seen at P-band during freezing, thawing and strong winds. Suitable conditions for repeat-pass L-band interferometry were only seen during the summer. C-band temporal coherence was high over timescales of seconds and occasionally for several hours for night-time observations during the summer. Decorrelation coinciding with high transpiration rates was observed at L- and C-band, suggesting sensitivity to tree water dynamics.The observations from this experiment are important for understanding, modelling and mitigating temporal variations in radar observables in forest parameter estimation algorithms. The results also are also useful in the design of spaceborne synthetic aperture radar missions with interferometric and tomographic capabilities. The results motivate the implementation of single-pass interferometric synthetic aperture radars for forest applications at P-, L- and C-band

FIREX mission requirements document for nonrenewable resources

The proposed mission requirements and a proposed experimental program for satellite synthetic aperture radar (SAR) system named FIREX (Free-Flying Imaging Radar Experiment) for nonrenewable resources is described. The recommended spacecraft minimum SAR system is a C-band imager operating in four modes: (1) low look angle HH-polarized; (2) intermediate look angle, HH-polarized; (3) intermediate look angle, IIV-polarized; and (4) high look angle HH-polarized. This SAR system is complementary to other future spaceborne imagers such as the Thematic Mapper on LANDSAT-D. A near term aircraft SAR based research program is outlined which addresses specific mission design issues such as preferred incidence angles or polarizations for geologic targets of interest

The End of the Rainbow: What Can We Say About the Extragalactic Sub-Megahertz Radio Sky?

The Galactic disc is opaque to radio waves from extragalactic sources with frequencies nu less than ~3 MHz. However, radio waves with kHz, Hz, and even lower frequencies may propagate through the intergalactic medium (IGM). I argue that the presence of these waves can be inferred by using the Universe as our detector. I discuss possible sub-MHz sources and set new non-trivial upper limits on the energy density of sub-MHz radio waves in galaxy clusters and the average cosmic background. Limits based on five effects are considered: (1) changes in the expansion of the Universe from the radiation energy density (2) heating of the IGM by free-free absorption; (3) radiation pressure squeezing of IGM clouds by external radio waves; (4) synchrotron heating of electrons in clusters; and (5) Inverse Compton upscattering of sub-MHz radio photons. Any sub-MHz background must have an energy density much smaller than the CMB at frequencies below 1 MHz. The free-free absorption bounds from the Lyman-alpha forest are potentially the strongest, but are highly dependent on the properties of sub-MHz radio scattering in the IGM. I estimate an upper limit of 6 * 10^4 L_sun Mpc^-3 for the emissivity within Lyman-alpha forest clouds in the frequency range 5 - 200 Hz. The sub-MHz energy density in the Coma cluster is constrained to be less than ~10^-15 erg cm^-3. At present, none of the limits is strong enough to rule out a maximal T_b = 10^12 K sub-MHz synchrotron background, but other sources may be constrained with a better knowledge of sub-MHz radio propagation in the IGM.Comment: Accepted by MNRAS, 4 figures, 20 pages

Forest cover estimation in Ireland using radar remote sensing: a comparative analysis of forest cover assessment methodologies

Quantification of spatial and temporal changes in forest cover is an essential component of forest monitoring programs. Due to its cloud free capability, Synthetic Aperture Radar (SAR) is an ideal source of information on forest dynamics in countries with near-constant cloud-cover. However, few studies have investigated the use of SAR for forest cover estimation in landscapes with highly sparse and fragmented forest cover. In this study, the potential use of L-band SAR for forest cover estimation in two regions (Longford and Sligo) in Ireland is investigated and compared to forest cover estimates derived from three national (Forestry2010, Prime2, National Forest Inventory), one pan-European (Forest Map 2006) and one global forest cover (Global Forest Change) product. Two machine-learning approaches (Random Forests and Extremely Randomised Trees) are evaluated. Both Random Forests and Extremely Randomised Trees classification accuracies were high (98.1–98.5%), with differences between the two classifiers being minimal (<0.5%). Increasing levels of post classification filtering led to a decrease in estimated forest area and an increase in overall accuracy of SAR-derived forest cover maps. All forest cover products were evaluated using an independent validation dataset. For the Longford region, the highest overall accuracy was recorded with the Forestry2010 dataset (97.42%) whereas in Sligo, highest overall accuracy was obtained for the Prime2 dataset (97.43%), although accuracies of SAR-derived forest maps were comparable. Our findings indicate that spaceborne radar could aid inventories in regions with low levels of forest cover in fragmented landscapes. The reduced accuracies observed for the global and pan-continental forest cover maps in comparison to national and SAR-derived forest maps indicate that caution should be exercised when applying these datasets for national reporting

BIOSAR 2010 - A SAR campaign in support to the BIOMASS mission

The ESA funded campaign BioSAR 2010 was carried out at the forestry test site Remningstorp in southern Sweden, in support to the BIOMASS satellite mission under study. Fully polarimetric SAR data were successfully acquired at L- and P-band using ONERA's multi-frequency system SETHI. In addition with other data types gathered, e.g. LiDAR and in-situ measurements, the compiled data set will be used for analyses and comparisons with biomass estimation results obtained at the same test site in the campaign BioSAR 2007, in which DLR's E-SAR made the SAR imaging. Detection of forest changes, robustness of biomass retrieval algorithms and long-term P-band coherence will be in focus as well as cross-validations between the two SAR sensors

Calibrating CHIME, A New Radio Interferometer to Probe Dark Energy

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a transit interferometer currently being built at the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, BC, Canada. We will use CHIME to map neutral hydrogen in the frequency range 400 -- 800\,MHz over half of the sky, producing a measurement of baryon acoustic oscillations (BAO) at redshifts between 0.8 -- 2.5 to probe dark energy. We have deployed a pathfinder version of CHIME that will yield constraints on the BAO power spectrum and provide a test-bed for our calibration scheme. I will discuss the CHIME calibration requirements and describe instrumentation we are developing to meet these requirements

Detection of particles, bacteria and viruses using consumer optoelectronic components

The focus of this thesis is on the design, development and validation of two novel photonic sensors for the detection and characterisation of industrial and biological samples. The first one is a PSA in a collimated beam configuration using an innovative angular spatial filter, and a consumer electronic camera similar to that used in a smartphone. The small form factor angular spatial filter allows for the collection of diffused light from particles up to predefined discrete angles. By using angularly resolved scattering images acquired by the camera, a machine learning (ML) algorithm predicts the volume median diameter of the particles. Our system has achieved a mean absolute percentage error of only 0.72% for spherical particles in solution with sizes greater than 10 µm and at concentrations up to 40 mg mL-1. Compared to traditional laser diffraction systems, the proposed PSA is an order of magnitude smaller in size, weight and cost, and offers a promising approach to online industrial process monitoring. As light scattering is influenced by factors other than particle size, including shape, refractive index contrast and suspension concentration, the PSA can also be employed in biological applications. To this end, the second part of the thesis aims to optimise the PSA for the measurement of small (< 10 µm) particles such as microorganisms. The results demonstrate that the modified PSA in combination with ML is able to accurately classify different types of bacteria (Escherichia coli and Enterococcus sp.) and distinguish them from silica beads of comparable sizes, with an accuracy of 89%. Moreover, it can detect the concentration of bacteria in water with a limit of detection (LOD) of approximately 105 cells mL-1. The final part of the thesis is dedicated to the development of a low-cost, portable optical biosensor for the specific detection of particles smaller than bacteria, such as viruses (< 1 µm). The proposed system, which we have called flow virometry reader (FVR), is a modification of a flow cytometer and relies on measuring light emissions from fluorescent antibodies that bind to specific viral particles. An LOD of 3,834 copies mL-1 for SARS-CoV-2 in saliva can be achieved with the device. The FVR is clinically validated using 54 saliva samples in a blind test, with high sensitivity and specificity of 91.2% and 90%, respectively. These findings suggest that the FVR has the potential to be a highly viable alternative to current diagnostic methods for pandemic events, as it is faster (< 30 min) and less expensive than PCR tests, while being more sensitive than today’s COVID-19 rapid antigen tests. The photonic sensing technologies developed in the thesis show significant potential for use in a wide range of applications, including: • particulate air pollution, causing cardiovascular and respiratory problems • particulate water pollution, which affects the ecosystems of rivers, lakes and oceans • total bacterial count in environmental or bathing water • viral pandemics The technologies are particularly appealing in countries with limited resources due to their simple design, portability, short time-to-result and affordability, as well as the fact that they do not require a specialised laboratory or trained personnel to operate them.El objetivo de esta tesis es el diseño, desarrollo y validación de dos nuevos sensores fotónicos para la detección y caracterización de muestras industriales y biológicas. El primero es un PSA en configuración de haz colimado que usa un innovador filtro espacial angular y una cámara electrónica similar a la usada en móviles. El pequeño factor de tamaño del filtro angular espacial permite la detección de la luz difusa de las partículas hasta ángulos discretos predefinidos. A partir del uso de imágenes difusas angularmente resueltas obtenidas por la cámara, un algoritmo de aprendizaje automático, machine learning (ML) en inglés, puede predecir la mediana del diámetro del volumen de las partículas. Nuestro sistema ha conseguido un error absoluto medio porcentual de solamente un 0.72% para partículas esféricas en disoluciones con tamaños superiores a 10 µm y concentraciones de hasta 40 mg mL-1. En comparación a sistemas tradicionales de difracción láser, el propuesto PSA es un orden de magnitud más pequeño en tamaño, peso y coste, y ofrece un enfoque prometedor para la supervisión online de procesos industriales. Dado que la difusión de luz depende de más factores aparte del tamaño de la partícula, incluyendo la forma, el contraste del índice de refracción y la suspensión de la concentración, el PSA también puede ser empleado en aplicaciones biológicas. Con este objetivo, la segunda parte de la tesis busca optimizar el PSA para la medida de partículas pequeñas (< 10 µm) como microorganismos. Los resultados demuestran que el PSA modificado en combinación con ML es capaz de clasificar con exactitud diferentes tipos de bacterias (Escherichia coli y Enterococcus sp.) y diferéncialas de partículas de silicio con tamaños similares, con una precisión del 89%. Además, puede detectar una concentración de bacterias en agua con un límite de detección (LOD en inglés) de aproximadamente 105 células mL-1. La parte final de tesis está dedicada al desarrollo de un biosensor óptico de bajo coste y portátil para la detección especifica de partículas más pequeñas que bacterias, como virus (< 1 µm). El sistema propuesto, el cual hemos llamado flow virometry reader (FVR), es una modificación de un citómetro de flujo y se basa en la medida de emisiones de luz provenientes de anticuerpos fluorescentes que son unidos a partículas virales específicas. Con este dispositivo se puede conseguir un LOD de 3,834 copias mL-1 para el SARS-CoV-2 en saliva. El FVR ha sido validado clínicamente usando 54 muestras de saliva en un test a ciegas, con una sensibilidad y especificidad del 91.2% y 90%, respectivamente. Estos hallazgos sugieren que el FVR tiene el potencial de ser una alternativa viable a los métodos de diagnóstico actuales en escenarios de pandemias, pues es rápido (< 30 min) y menos costoso que los test por PCR, mientras que es más sensible que los actuales test de antígenos para COVID-19. Las tecnologías de detección fotónicas desarrolladas en esta tesis muestran un potencial significativo para su uso en un amplio rango de aplicaciones, incluyendo: -contaminación de aire por partículas, causantes de problemas cardiovasculares y respiratorios -contaminación de agua por partículas, el cual afecta a ecosistemas como ríos, lagos y océanos -recuento total de bacterias en aguas de baño o ambientales -pandemias víricas. Estas tecnologías son particularmente atractivas en países con recursos limitados, dado sus simples diseños, portabilidad, el poco tiempo de espera para obtener resultados y asequibilidad, así como el hecho de que estos no requieren un laboratorio especializado o un personal cualificado para operar con ellas.Postprint (published version

ACTIVE AND PASSIVE REMOTE SENSING FOR ATMOSPHERIC APPLICATIONS

The atmosphere surrounding our planet is vital for the existence of many living organisms, including humans. Although this layer is quite thin, there are numerous components interacting with each other with processes taking place across widely different spatial and temporal scales. No single instrument is able to cover all of these scales, and therefore, in order to advance our knowledge of atmospheric processes and composition, different instruments, methods and synergy of instruments have to be applied. Remote sensing techniques offer a variety of possibilities for atmospheric research. Satellite remote sensing is exploited to get a regional or global view on the problems, to verify climate models, as well as to reach locations which are not accessible for measurements otherwise. Ground-based remote sensing allows a continuous monitoring of the vertical structure of the atmosphere and, due to exploiting various wavelengths, the observation of atmospheric compounds of various sizes from gases to aerosol particles to snowflakes. In this thesis, several remote sensing techniques have been utilized to find new methods of utilizing existing observations as well as the application of known methods to new geographical locations. A novel method is proposed for retrieving convective boundary layer height during spring and summer months using insect echoes in radar returns. Observations from several different radar frequencies were analysed and the proposed method was proven applicable at all frequencies given some limitations. Moreover, this method can serve as a platform for future research in different geographical locations where insects might behave differently. The synergy of ground-based lidar and airborne in situ measurements were used to study elevated aerosol layers in Southern Finland. Based on two cases, a clear-sky and partly cloudy case, the temporal and spatial variability of aerosol particle number concentration in the boundary layer and several elevated layers were investigated. Nucleation mode particles (the smallest aerosol sizes) were also detected in one of the elevated layers, which was probably not mixing with the boundary layer during a new particle formation event. In addition to aerosol particles, some lidars have the capability to measure water vapor profiles. Several calibration methods for this type of lidar were analysed in order to find an alternative to the usual method of using a radiosonde launched close by, since radiosondes may not always be available at every site. Output from a weather forecast model, or a radiosonde profile which was 100 km away, were both found to be reliable, whereas the use of satellite products required more caution in the absence of other methods. The seasonal variability of water vapour profiles was also studied. Satellite remote sensing observations were probed to obtain proxies of nucleation mode aerosol particles, which are otherwise not seen from space. So far, the results were not very successful, however, some bottlenecks were identified with a potential to improve the proxies in the future.Atmosfären som omger vår planet är avgörande för existensen av olika levande organismer, inklusive människor. Trots att detta lager är ganska tunt, finns det massor av komponenter som interagerar med varandra och processer som äger rum i olika rumsliga och tidsmässiga skalor. Inget enskilt instrument kan täcka alla dessa skalor och därför måste olika instrument, metoder och synergier av instrument användas för att föra fram vår kunskap om atmosfäriska processer och sammansättningar. Fjärranalysmetoder erbjuder en mängd olika möjligheter för atmosfärisk forskning. Satellitfjärranalys utnyttjas för att få en regional eller global syn på problemen, för att verifiera klimatmodeller, samt för att nå platser som annars inte är tillgängliga för mätningar. Markbaserad fjärranalys möjliggör en kontinuerlig övervakning av atmosfärens vertikala struktur och, genom att utnyttja olika våglängder, observation av atmosfäriska sammansättningar av olika storlekar från aerosolpartiklar till snöflingor. I denna avhandling har flera fjärranalysmetoder använts för att hitta nya sätt att utnyttja befintliga observationer samt för att tillämpa kända metoder på nya geografiska platser. En ny metod föreslås för att skapa en konvektiv gränsskiktshöjd under vår- och sommarmånaderna med hjälp av insektsekon i radarsignalen. Observationer från flera olika radarfrekvenser analyserades och den föreslagna metoden visade sig vara tillämpbar på alla frekvenser med vissa begränsningar. Dessutom kan denna metod fungera som en plattform för framtida forskning på olika geografiska platser där insekter kan bete sig annorlunda. Synergin mellan markbaserad lidar och luftburna in situ-mätningar användes för att studera förhöjda aerosollager i södra Finland. Den temporala och rumsliga variationen av aerosolpartikelkoncentrationen i gränsskiktet och förhöjda lager undersöktes baserat på två fall, ett med klar himmel och ett delvis molnigt. Nukleationsmodpartiklar (de minsta aerosolstorlekarna) detekterades också i ett av de förhöjda skikten, som troligtvis inte blandades med gränsskiktet under en ny partikelbildningshändelse. Förutom aerosolpartiklar har vissa lidarer förmågan att mäta vattenångsprofiler. Flera kalibreringsmetoder för denna typ av lidar analyserades för att hitta ett alternativ till den vanliga metoden att använda en radiosonde som lanseras i närheten, eftersom radiosonder inte alltid är tillgängliga på alla platser. Utdata från en väderprognosmodell eller en radiosondeprofil på 100 km avstånd, visade sig båda vara tillförlitliga, medan användningen av satellitprodukter krävde mer försiktighet i avsaknad av andra metoder. Den säsongsmässiga variationen av vattenångprofiler studerades också. Satellitfjärranalysobservationer undersöktes för att erhålla proxies för aerosolpartiklar i kärnbildningsläge, vilka annars inte kan ses från rymden. Hittills har resultaten dock inte varit särskilt framgångsrika, men vissa flaskhalsar har identifierats med potential att förbättra fullmakterna i framtiden

Spaceborne synthetic aperture radar: Current status and future directions. A report to the Committee on Earth Sciences, Space Studies Board, National Research Council

This report provides a context in which questions put forth by NASA's Office of Mission to Planet Earth (OMPTE) regarding the next steps in spaceborne synthetic aperture radar (SAR) science and technology can be addressed. It summarizes the state-of-the-art in theory, experimental design, technology, data analysis, and utilization of SAR data for studies of the Earth, and describes potential new applications. The report is divided into five science chapters and a technology assessment. The chapters summarize the value of existing SAR data and currently planned SAR systems, and identify gaps in observational capabilities needing to be filled to address the scientific questions. Cases where SAR provides complementary data to other (non-SAR) measurement techniques are also described. The chapter on technology assessment outlines SAR technology development which is critical not only to NASA's providing societally relevant geophysical parameters but to maintaining competitiveness in SAR technology, and promoting economic development