Online Networks as Societies: User Behaviors and Contribution Incentives

Online networks like email, Facebook, LinkedIn, Wikipedia, eBay, and BitTorrent-like Peer-to-Peer (P2P) systems have become popular and powerful infrastructures for communication. They involve potentially large numbers of humans with their collective inputs and decisions, and they often rely on the cooperation and the contribution of their users. Nevertheless, users in online networks are often found to be selfish, strategic, or even malicious, rather than cooperative, and therefore they need to be incentivized for contributions. This thesis provides theoretical and practical insights into the correlation between user behaviors and contribution incentives in online networks. It contains a demonstration of user behaviors and their consequences at both the system and the individual level, an analysis of barter schemes and their limitations in incentivizing users to contribute, an evaluation of monetary schemes and their risks in causing the collapse of the entire system, and an examination of user interactions and their implications in inferring user relationships.Software and Computer TechnologyElectrical Engineering, Mathematics and Computer Scienc

Mitigating the Risks in Energy Retrofits of Residential Buildings in China

To speed up residential energy retrofitting in the Hot Summer and Cold Winter(HSCW) zone, the barriers to retrofitting projects need elimination. Energy retrofitting contributes to improving building quality and living comfort, but has not been accepted by the public. It stems from poor project performance in quality, time, costs, etc. The risk is an essential factor hindering such project objectives and project success. Residential energy retrofitting in China is exposed to various risks due to uncertainties regarding finance, organization, coordination, technology, etc. This thesis thus aims to deepen the understanding of risks in the whole process of residential energy retrofitting to smoothen its implementation and develop risk mitigation strategies for the HSCW climate zone of China. The thesis adopts Transaction Costs Theory (TCT) to identify the risks in the whole process of project implementation and assesses the importance of these risks in both objective and subjective aspects. Given the importance of homeowners-related risks and the key role of the government in retrofitting projects, this research develops s series of develop strategies for risk mitigation from the viewpoints of both homeowners and the government. The thesis contributes to the body of knowledge by conducting a systematic exploration of risks in retrofitting projects. In terms of the practical contributions, it does not only enable project managers to recognize the priority of project risks, but also help the government tackle these issues at its source for promotion of residential energy retrofitting.A+BE | Architecture and the Built Environment No. 23 (2021)Housing Quality and Process Innovatio

Thermal infrared observations of heterogeneous soil-vegetation systems

Remote SensingAerospace Engineerin

Modeling and reconstruction of time series of passive microwave data by Discrete Fourier Transform guided filtering and Harmonic Analysis

Daily time series of microwave radiometer data obtained in one-orbit direction are full of observation gaps due to satellite configuration and errors from spatial sampling. Such time series carry information about the surface signal including surface emittance and vegetation attenuation, and the atmospheric signal including atmosphere emittance and atmospheric attenuation. To extract the surface signal from this noisy time series, the Time Series Analysis Procedure (TSAP) was developed, based on the properties of the Discrete Fourier Transform (DFT). TSAP includes two stages: (1) identify the spectral features of observation gaps and errors and remove them with a modified boxcar filter; and (2) identify the spectral features of the surface signal and reconstruct it with the Harmonic Analysis of Time Series (HANTS) algorithm. Polarization Difference Brightness Temperature (PDBT) at 37 GHz data were used to illustrate the problems, to explain the implementation of TSAP and to validate this method, due to the PDBT sensitivity to the water content both at the land surface and in the atmosphere. We carried out a case study on a limited heterogeneous crop land and lake area, where the power spectrum of the PDBT time series showed that the harmonic components associated with observation gaps and errors have periods ≤8 days. After applying the modified boxcar filter with a length of 10 days, the RMSD between raw and filtered time series was above 11 K, mainly related to the power reduction in the frequency range associated with observation gaps and errors. Noise reduction is beneficial when applying PDBT observations to monitor wet areas and open water, since the PDBT range between dryland and open water is about 20 K. The spectral features of the atmospheric signal can be revealed by time series analysis of rain-gauge data, since the PDBT at 37 GHz is mainly attenuated by hydrometeors that yield precipitation. Thus, the spectral features of the surface signal were identified in the PDBT time series with the help of the rain-gauge data. HANTS reconstructed the upper envelope of the signal, i.e., correcting for atmospheric influence, while retaining the spectral features of the surface signal. To evaluate the impact of TSAP on retrieval accuracy, the fraction of Water Saturated Surface (WSS) in the region of Poyang Lake was retrieved with 37 GHz observations. The retrievals were evaluated against estimations of the lake area obtained with MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Synthetic Aperture Radar (ASAR) data. The Relative RMSE on WSS was 39.5% with unfiltered data and 23% after applying TSAP, i.e., using the estimated surface signal only.Optical and Laser Remote Sensin

Phenological response of vegetation to upstream river flow in the Heihe Rive basin by time series analysis of MODIS data

Aerospace Engineerin

Estimating user interaction strength in distributed online networks

User interactions are indispensable for any online network to thrive, especially for BitTorrent‐like and Web real‐time communication‐based distributed online networks that rely on users' collective contributions instead of the help of central servers. User interactions provide fine‐grained information for many applications, such as security enhancement and cooperation promotion. To date, several schemes for estimating user interaction strength in centralized online networks have been proposed. In contrast, we present design, deployment, and analysis of UISE for user interaction strength estimation in distributed online networks. Among the strong points of UISE is that it captures both direct and indirect user interactions, and that it scales with only partial information dissemination. We apply UISE to devise the first distributed scheme for online time estimation and we implement it into Tribler, a distributed online network for media and social applications like file sharing, streaming, and voting. We demonstrate the accuracy and the scalability of UISE with different information dissemination protocols and user behaviors using simulations, emulations, and a real‐world deployment.Distributed System

Estimation of Aerodynamic Roughness Length over Oasis in the Heihe River Basin by Utilizing Remote Sensing and Ground Data

Most land surface models require information on aerodynamic roughness length and its temporal and spatial variability. This research presents a practical approach for determining the aerodynamic roughness length at fine temporal and spatial resolution over the landscape by combining remote sensing and ground measurements. The basic framework of Raupach, with the bulk surface parameters redefined by Jasinski et al., has been applied to optical remote sensing data collected by the HJ-1A/1B satellites. In addition, a method for estimating vegetation height was introduced to derive the aerodynamic roughness length, which is preferred by users over the height-normalized form. Finally, mapping different vegetation classes was validated taking advantage of the data-dense field experiments conducted in the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) project. Overall, the roughness model performed well against the measurements collected at most HiWATER flux tower sites. However, deviations still occurred at some sites, which have been further analyzed.Geoscience & Remote SensingCivil Engineering and Geoscience

Improved Surface Reflectance from Remote Sensing Data with Sub-Pixel Topographic Information

Several methods currently exist to efficiently correct topographic effects on the radiance measured by satellites. Most of those methods use topographic information and satellite data at the same spatial resolution. In this study, the 30 m spatial resolution data of the Digital Elevation Model (DEM) from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) are used to account for those topographic effects when retrieving land surface reflectance from satellite data at lower spatial resolution (e.g., 1 km). The methodology integrates the effects of sub-pixel topography on the estimation of the total irradiance received at the surface considering direct, diffuse and terrain irradiance. The corrected total irradiance is then used to compute the topographically corrected surface reflectance. The proposed method has been developed to be applied on various kilometric pixel size satellite data. In this study, it was tested and validated with synthetic Landsat data aggregated at 1 km. The results obtained after a sub-pixel topographic correction are compared with the ones obtained after a pixel level topographic correction and show that in rough terrain, the sub-pixel topography correction method provides better results even if it tends to slightly overestimate the retrieved land surface reflectance in some cases.Geoscience & Remote SensingCivil Engineering and Geoscience

Consistent nighttime light time series in 1992–2020 in Northern Africa by combining DMSP-OLS and NPP-VIIRS data

Human activities modulate the impact of environmental forcing in general and of climate in particular. Information on the spatial and temporal patterns of human activities is in high demand, but scarce in sparsely populated and data-poor regions such as Northern Africa. The intensity and spatial distribution of nighttime lights provide useful information on human activities and can be observed by space-borne imaging radiometers. Our study helps to bridge the gap between the DMSP–OLS data available until 2013 and the NPP–VIIRS data available since 2013. The approach to calibrate the OLS data includes three steps: a) inter-calibrate the OLS DN data acquired by different sensors in 1992–2013; b) calibrate the OLS DN data using VIIRS data in 2013; c) generate synthetic OLS radiance data by degrading the VIIRS data in 2013–2020. We generated a) a time series of calibrated OLS nighttime light radiance data (1992–2013); b) mean annual VIIRS radiance on stable lights at the OLS spatial resolution for 2013–2020; c) synthetic OLS radiance data generated using VIIRS radiance data degraded to match the radiometric specifications of OLS for 2013–2020. The evaluation of these data products in 2013 documented their accuracy and consistency.Geoscience and Remote SensingOptical and Laser Remote Sensin

Stakeholders’ Risk Perception: A Perspective for Proactive Risk Management in Residential Building Energy Retrofits in China

The implementation of energy retrofit of residential buildings faces many risks around the world, especially in China, leading to low retrofit progress. Stakeholders’ proactive risk management is the key to the smooth implementation of retrofit projects but is normally affected by risk perception. Perceived risks instead of real risks are the motivators of their proactive behaviours. This paper aims to understand and address the present risk perception of stakeholders in order to drive effective proactive risk mitigation practices. Based on a risk list identified through a literature review and interviews, a questionnaire survey was then made to analyse and compare different stakeholders’ perceptions of each risk by measuring the levels of their concern about risks. It is validated that all the stakeholder groups tend to mitigate risks perceived highly proactively. Proactive risk management of risk-source-related stakeholders deserves more attention and responsibility-sharing with transaction costs (TCs) considerations contribute to the enhancement of risk perception. More responsibilities of construction quality and maintenance is taken by the government and contractors should be clarified, and the government should also be responsible for assisting design work. Effective information is beneficial to the decrease in homeowners’ risk perception that can motivate their initiative of cooperation.Housing Quality and Process InnovationBouwkunde OT