7 research outputs found
Coastal California's Fog as a Unique Habitable Niche: Design for Autonomous Sampling and Preliminary Aerobiological Characterization
Just as on the land or in the ocean, atmospheric regions may be more or less hospitable to life. The aerobiosphere, or collection of living things in Earth's atmosphere, is poorly understood due to the small number and ad hoc nature of samples studied. However, we know viable airborne microbes play important roles, such as providing cloud condensation nuclei. Knowing the distribution of such microorganisms and how their activity can alter water, carbon, and other geochemical cycles is key to developing criteria for planetary habitability, particularly for potential habitats with wet atmospheres but little stable surface water. Coastal California has regular, dense fog known to play a major transport role in the local ecosystem. In addition to the significant local (1 km) geographical variation in typical fog, previous studies have found that changes in height above surface of as little as a few meters can yield significant differences in typical concentrations, populations and residence times. No single current sampling platform (ground-based impactors, towers, balloons, aircraft) is capable of accessing all of these regions of interest.A novel passive fog and cloud water sampler, consisting of a lightweight passive impactor suspended from autonomous aerial vehicles (UAVs), is being developed to allow 4D point sampling within a single fog bank, allowing closer study of small-scale (100 m) system dynamics. Fog and cloud droplet water samples from low-altitude aircraft flights in nearby coastal waters were collected and assayed to estimate the required sample volumes, flight times, and sensitivity thresholds of the system under design.125 cloud water samples were collected from 16 flights of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) instrumented Twin Otter, equipped with a sampling tube collector, occurring between 18 July and 12 August 2016 below 1 km altitude off the central coast. The collector was flushed first with 70 ethanol, then with sterile DI water, between sampling regions. Collected volumes ranged from 100 L to 12 mL. All samples were diluted serially and plated on two different types of agar, nutrient-dense (PCA) and sparse (R-2A). Plates were incubated at room temperature and counted when colonies first appeared and again at 2 weeks.Preliminary results from seven flights are consistent with generally reported colony-forming unit (CFU) values for terrestrial fog water (e.g., [4]). The PCA assay ranged from 400 to 125,000 CFU/mL, R-2A from 700 to 130,000 CFU/mL. PCA and R-2A counts were not significantly different from each other at I^ plus or minus +/- 0.05, although observationally, the R2A plates had more pigmented colonies. CFU counts from the majority of flights were not different from each other in mean at the same level of significance, but about half differed in median, indicating differences in underlying distribution. These results validate the presence of viable microorganisms in coastal California fog at levels that should be easily detectable by our sampling system. The indicated distribution differences underscore the need for small-scale, long-term sampling surveys. Future planned work includes ion chromatography for limiting nutrients, ATP quantification, and qPCR for several microbial classes of interest
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Hybrid Clock Synchronization in Networked Control Systems
Clock synchronization over networks is a nontrivial problem that has long been an important topic in the fields of computer science and engineering as it pertains to digital networks and distributed systems. Recently, clock synchronization has received much attention in the study of networked control theory due to the importance of consensus on time in distributed control and estimation settings.This dissertation addresses the need for new clock synchronization schemes with the presentation of several hybrid based approach to clock synchronization problem. To motivate this work, the problem of a hybrid observer, with a clock synchronization scheme, that receives information sporadically over a network is presented. Through an attractivity result on the convergence properties of the observer system, sufficient conditions on the convergence properties of the accompanying clock synchronization scheme demonstrate the need for clock synchronization algorithms with performance guarantees. In one of the solutions to the problem, a distributed hybrid algorithm that synchronizes the time and rate of a set of clocks connected over a network is presented. Clock measurements of the nodes are given at aperiodic time instants and the controller at each node uses these measurements to achieve synchronization. Due to the continuous and impulsive nature of the clocks and the network, a hybrid system model to effectively capture the dynamics of the system and proposed hybrid algorithm is introduced. Moreover, the hybrid algorithm allows each agent to estimate the skew of its internal clock in order to allow for synchronization to a common timer rate. Sufficient conditions guaranteeing synchronization of the timers, exponentially fast are provided. Numerical results illustrate the synchronization property induced by the proposed algorithm as well as robustness to communication noise.Next, an innovative hybrid systems approach to the sender-receiver synchronization of timers is presented. Via the hybrid systems framework, the traditional sender-receiver algorithm for clock synchronization is united with an online, adaptive strategy to achieve synchronization of the clock rates to exponentially synchronize a pair of clocks connected over a network. Following the conventions of the algorithm, clock measurements of the nodes are given at periodic time instants, and each node uses these measurements to achieve synchronization. For this purpose, a hybrid system model of a network with continuous and impulsive dynamics that captures the sender-receiver algorithm as a state-feedback controller to synchronize the network clocks is introduced. Moreover, sufficient design conditions that ensure attractivity of the synchronization set are provided with numerical examples to validate the theoretical results.Finally, a general approach and framework to modeling clock synchronization protocols using hybrid systems is presented. Using the general framework, several existing algorithms from the literature are modeled. The models are then simulated numerically to demonstrate the feasibility of the proposed modeling framework
State Estimation of a Linear System on a Network subject to Sporadic Measurements and Time-Delays
This paper proposes a hybrid observer for state estimation over a network. The network provides delayed measurements of the output of the plant at time instants that are not necessarily periodic and are accompanied by timestamps provided by a clock that synchronizes with the clock of the observer in finite time. The proposed observer, along with the plant and communication network, are modeled by a hybrid dynamical system that has two timers, a logic variable, and two memory states to capture the mechanisms involved in the events associated with sampling and arrival of information, as well as the logic in the estimation algorithm. The hybrid model also includes a generic clock synchronization scheme to cope with a mismatch between the clocks at the plant and the observer. Convergence properties of the estimation error of the system are shown analytically and supported by numerical examples
HyNTP: A Distributed Hybrid Algorithm for Time Synchronization
This paper presents a distributed hybrid algorithm that synchronizes the time
and rate of a set of clocks connected over a network. Clock measurements of the
nodes are given at aperiodic time instants and the controller at each node uses
these measurements to achieve synchronization. Due to the continuous and
impulsive nature of the clocks and the network, we introduce a hybrid system
model to effectively capture the dynamics of the system and the proposed hybrid
algorithm. Moreover, the hybrid algorithm allows each agent to estimate the
skew of its internal clock in order to allow for synchronization to a common
timer rate. We provide sufficient conditions guaranteeing synchronization of
the timers, exponentially fast. Numerical results illustrate the
synchronization property induced by the proposed algorithm as well as its
performance against comparable algorithms from the literature