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At least 865 records · Page 48

Carrier synchronization and detection of polyphase signals.

Digital communication networks used for the distribution of high-speed digital information are currently the subject of design studies for many civil and military applications. This paper presents results that are useful in such studies as well as in network planning. In particular, the paper is concerned with the problems of carrier synchronization and noisy reference detection of polyphase signals. Reconstruction of coherent references for the detection of polyphase signals is considered and analyzed for three carrier reconstruction loops, namely, Nth power (multiply-and-divide) loops, generalized Costas (I-Q) loops, and extensions of data-aided (modulation wipeoff) loops. General expressions for the error probability are developed when the reconstructed reference signals are noisy.

Lindsey, W. C.↗

Digital Controller For Emergency Beacon

Prototype digital controller intended for use in 406-MHz emergency beacon. Undergoing development according to international specifications, 406-MHz emergency beacon system includes satellites providing worldwide monitoring of beacons, with Doppler tracking to locate each beacon within 5 km. Controller turns beacon on and off and generates binary codes identifying source (e.g., ship, aircraft, person, or vehicle on land). Codes transmitted by phase modulation. Knowing code, monitor attempts to communicate with user, monitor uses code information to dispatch rescue team appropriate to type and locations of carrier.

Ivancic, William D.↗

A NASA Strategy for Leveraging Emerging Launch Vehicles for Routine, Small Payload Missions

Orbital flight opportunities for small payloads have always been few and far between, and then on February 1, 2002, the situation got worse. In the wake of the loss of the Columbia during STS- 107, changing NASA missions and priorities led to the termination of the Shuttle Small Payloads Projects, including Get-Away Special, Hitcbker, and Space Experiment Module. In spite of the limited opportunities, long queue, and restrictions associated with flying experiments on a man-rated transportation system; the carriers provided a sustained, high quality experiment services for education, science, and technology payloads, and was one of the few games in town. Attempts to establish routine opportunities aboard existing ELVs have been unsuccessful, as the cost-per-pound on small ELVs and conflicts with primary spacecraft on larger vehicles have proven prohibitive. Ths has led to a backlog of existing NASA-sponsored payloads and no prospects or plans for fbture opportunities within the NASA community. The prospects for breaking out of this paradigm appear promising as a result of NASA s partnership with DARPA in pursuit of low-cost, responsive small ELVs under the Falcon Program. Through this partnership several new small ELVs, providing 1000 lbs. to LEO will be demonstrated in less than two years that promise costs that are reasonable enough that NASA, DoD, and other sponsors can once again invest in small payload opportunities. Within NASA, planning has already begun. NASA will be populating one or more of the Falcon demonstration flights with small payloads that are already under development. To accommodate these experiments, Goddard s Wallops Flight Facility has been tasked to develop a multi-payload ejector (MPE) to accommodate the needs of these payloads. The MPE capabilities and design is described in detail in a separately submitted abstract. Beyond use of the demonstration flights however, Goddard has already begun developing strategies to leverage these new ELVs as elements of a larger system designed to provide routine, low-cost end-to-end services for small science, Exploration, and education payloads. The plan leverages the management approaches of the successful Sounding Rocket Program and Shuttle Small Payloads Projects. The strategy consists of using a systems implementation approach of elements, including 1) Falcon ELVs, 2) advanced launch site technologies and processes, 3) suite of experiment carriers accommodating different mission requirements, 4) streamlined integration and test operations, 5 ) experiment brokering and management, and 6) standardized, distributed payload operations. The envisioned suite of carriers includes the MPE, a standard interface experiment carrier, and potentially a reentry fieeflyer experiment carrier. Key to the success of this strategy is standard experiment interfaces within the carriers to limit mission- unique tasks, establishmg and managing a program of scheduled reoccurring flights rather than discrete missions, and streamlined, centralized implementation of the elements. These individual elements are each under development and Goddard will demonstrate the overall system strategy low-cost small payload missions on the initial Falcon demonstration launches from Wallops. goal is to show that this model should be converted to a sustained NASA program supporting science, technology, and education, with annual flight opportunities. The paper will define in detail the various elements of the overall program, as well as provide status, philosophy, and strategy for the program that will hopefully once-and-for-all provide low-cost, routine access to space for the small payloads community.

Underwood, Bruce E.↗

A Stream lined Approach for the Payload Customer in Identifying Payload Design Requirements

NASA payload developers from across various disciplines were asked to identify areas where process changes would simplify their task of developing and flying flight hardware. Responses to this query included a central location for consistent hardware design requirements for middeck payloads. The multidisciplinary team assigned to review the numerous payload interface design documents is assessing the Space Shuttle middeck, the SPACEHAB Inc. locker, as well as the MultiPurpose Logistics Module (MPLM) and EXpedite the PRocessing of Experiments to Space Station (EXPRESS) rack design requirements for the payloads. They are comparing the multiple carriers and platform requirements and developing a matrix which illustrates the individual requirements, and where possible, the envelope that encompasses all of the possibilities. The matrix will be expanded to form an overall envelope that the payload developers will have the option to utilize when designing their payload's hardware. This will optimize the flexibility for payload hardware and ancillary items to be manifested on multiple carriers and platforms with minimal impact to the payload developer.

Miller, Ladonna J.↗

Concept of Operations for a Prospective "Proving Ground" in the Lunar Vicinity

NASA is studying conceptual architectures for a "Proving Ground" near the Moon or in high lunar orbit to conduct human space exploration missions that bridge the gap between today's operations with the International Space Station (ISS) and future human exploration of Mars beginning in the 2030s. This paper describes the framework of a concept of operations ("Conops") for candidate activities in the Proving Ground. The Conops discusses broad goals that the Proving Ground might address, such as participation from commercial entities, support for human landings on the Moon, use of mature technologies, and growth of capability through a steady cadence of increasingly ambitious piloted missions. Additional Proving Ground objectives are outlined in a companion paper. Key elements in the Conops include the Orion spacecraft (with mission kits for docking and other specialized operations) and the Space Launch System (SLS) heavy-lift rocket. Potential additions include a new space suit, commercial launch vehicles and logistics carriers, Solar Electric Propulsion (SEP) stages to move elements between different orbits and eventually take them on excursions to deep space, a core module with multiple docking ports, a habitation block, and robotic and piloted lunar landers. The landers might include reusable ascent modules which could remain docked to in-space elements between lunar sorties. A module providing advanced regenerative life support functions could launch to the ISS, and later move to the Proving Ground. The architecture will include infrastructure for launch preparation, communication, mission control, and range safety. The Conops describes notional missions chosen to guide the design of the architecture and its elements. One such mission might be the delivery of a approximately 10-t Transit Habitat element, comanifested with Orion on a Block 1B SLS launcher, to the Proving Ground. In another mission, the architecture might participate in direct human exploration of an asteroidal boulder brought to high lunar orbit by the Asteroid Redirect Mission. The Proving Ground stack could serve as a staging point and tele-operation center for robotic and piloted Moon landings. With the addition of a SEP stage, the architecture could support months-long excursions within and beyond the Earth's sphere of influence, possibly culminating in a year-long mission to land humans on a near-Earth asteroid. In the last case, after returning to near-lunar space, two of the asteroid explorers could join two crewmembers freshly arrived from Earth for a Moon landing, helping to quantify the risk of landing deconditioned crews on Mars. In a conceptual mission particularly stressing to system design, Proving Ground elements could transit to Mars orbit. Other possible design-driving operations include relocation of the stack with no crew on board, the unpiloted journey of the advanced life support module from ISS to the lunar vicinity, excursions to other destinations in near-Earth space, and additional support for Mars exploration in conjunction with the Evolvable Mars Campaign. The Proving Ground Conops concludes with a discussion of aborts and contingency operations

Love, Stanley G.↗

Electrochemical mechanisms of leakage‐current in photovoltaic modules

Abstract This paper analyzes the mechanisms and pathways for leakage current flow observed in Si photovoltaic modules subjected to high temperature and humidity and a large voltage bias with respect to ground. The current inside of the frame is in the form of electron motion, but in the glass and polymer, it is in at least a large part attributable to the movement of ions. When the mode of current flow changes from electronic to ionic conduction, electrochemical reactions will take place at the interface. This can include reactions that produce volatile chemical species like H 2 , CO x , and O 2 , along with ionic species such as OH − and H 3 O + . Here, we see evidence of the importance of different charge carriers with different diffusion rates and the influence of electrochemical processes involved. The application of negative voltage to the cell circuit affects the resistivity of glass producing surfaces with poor conductivity but with some increases in the electrochemical potential producing complicated interactions that are important when the voltage is changed. In the polymer, there is the development of a space charge region and a chemical gradient providing two oppositional forces to current flow, which when released create a complicated discharge process. Here, we give a basic understanding of the charge/discharge of PV cells highlighting how the specific mechanisms are important in understanding some of the degradation processes in PV modules. We find that there is evidence of multiple significant charge carrier species with different diffusion time scales. The glass/polymer interface forms a depleted region of higher resistance after prolonged exposure to current. Charge also builds up at the polymer to cell antireflective coating interface and mostly flows to the gridlines to experience electrochemical reactions. These complexities result in non‐linear behavior where the apparent resistivities of the different layers change during charge/discharging processes, making the modeling of the current flow extremely difficult.

14 SOLAR ENERGY↗

Negative polarity of phenyl-C{sub 61} butyric acid methyl ester adjacent to donor macromolecule domains

Interfacial fields within organic photovoltaics influence the movement of free charge carriers, including exciton dissociation and recombination. Open circuit voltage (V{sub oc}) can also be dependent on the interfacial fields, in the event that they modulate the energy gap between donor HOMO and acceptor LUMO. A rise in the vacuum level of the acceptor will increase the gap and the V{sub oc}, which can be beneficial for device efficiency. Here, we measure the interfacial potential differences at donor-acceptor junctions using Scanning Kelvin Probe Microscopy, and quantify how much of the potential difference originates from physical contact between the donor and acceptor. We see a statistically significant and pervasive negative polarity on the phenyl-C{sub 61} butyric acid methyl ester (PCBM) side of PCBM/donor junctions, which should also be present at the complex interfaces in bulk heterojunctions. This potential difference may originate from molecular dipoles, interfacial interactions with donor materials, and/or equilibrium charge transfer due to the higher work function and electron affinity of PCBM. We show that the contact between PCBM and poly(3-hexylthiophene) doubles the interfacial potential difference, a statistically significant difference. Control experiments determined that this potential difference was not due to charges trapped in the underlying substrate. The direction of the observed potential difference would lead to increased V{sub oc}, but would also pose a barrier to electrons being injected into the PCBM and make recombination more favorable. Our method may allow unique information to be obtained in new donor-acceptor junctions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions

We demonstrate a substantial modulation of the optical properties of multilayer graphene (~100 layers) using a simple device consisting of a multilayer graphene/polymer electrolyte membrane/gold film stack. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI] - [ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]] resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm -1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 $μ$m (2000–6000 cm -1 ), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. Furthermore, all of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI - ion and the associated change in the free carrier density (Δ$n$ = 10 20 cm -3 ). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of Δ$E$ F = 1.1 eV.

42 ENGINEERING↗

Spin-dependent transport across Co/LaAlO{sub 3}/SrTiO{sub 3} heterojunctions

The conducting interface formed between LaAlO{sub 3} and SrTiO{sub 3} exhibits high electron mobility, tunable carrier densities, and is theoretically predicted to have long spin lifetimes. Thus, the LaAlO{sub 3}/SrTiO{sub 3} heterostructure is a promising system for the transportation of spin information. We have investigated spin-dependent transport across Co/LaAlO{sub 3}/SrTiO{sub 3} heterostructures using an all electrical three terminal geometry and observed Lorentzian magnetoresistance typically associated with Hanle spin dephasing. However, such a picture fails to explain all the experimentally observed behavior. Further, experiments with spin-unpolarized Au/LaAlO{sub 3}/SrTiO{sub 3} heterojunctions support an alternative scenario involving current modulation by spin-dependent transport through defect states in the LaAlO{sub 3} barrier.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Low-PAPR interleaved single-carrier FDM scheme for optical wireless communications

As a potential complementary access technology for the 5G wireless systems, optical wireless communication (OWC) gains extensive attention for decades owing to its numerous advantages of broad license-free spectrum, immunity to electromagnetic interference and high-level privacy. However, two rigorous obstacles to achieve high data-rate OWC transmission are the severe nonlinear clipping of transmitter and high-frequency attenuation of OWC system. In this paper, we propose the first interleaved single-carrier frequency-division multiplexing (I-SC-FDM) scheme for OWC systems. Compared with orthogonal frequency-division multiplexing (OFDM), I-SC-FDM has a lower peak-to-average power ratio, which makes it more immune to the nonlinearity clipping in OWC systems. Meanwhile, for the bandwidth-limited OWC systems, I-SC-FDM has better performance on resistance to the serious high-frequency distortion. The simulation results indicate that, under the transmitter nonlinearity and optical-wireless diffuse fading channel, the maximum Q{sup 2} of I-SC-FDM is about 3.44 dB and 3.14 dB higher than that of OFDM when 4-ary quadrature amplitude modulation (4-QAM) and 16-QAM are modulated, respectively. The results show the feasibility and advantages of I-SC-FDM for cost-sensitive OWC systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Phase-amplitude coupling characteristics in directly modulated quantum dot lasers

We present a semi-analytical model for studying the phase-amplitude coupling (α-factor) in quantum dot (QD) semiconductor lasers, which takes into account the influence of carrier populations in the excited state and in the two-dimensional carrier reservoir on the refractive index change. Calculations of the α-factor based on the amplified spontaneous emission method and on the “FM/AM” technique are both investigated. It is shown that the α-factor of a QD laser strongly depends on the energy separation between the ground state and the off-resonant states. Through band structure engineering, the α-factor can be reduced by enlarging this energy separation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Gas adsorption and light interaction mechanism in phosphorene-based field-effect transistors

Phosphorene-based field effect transistor (FET) structures were fabricated to study the gas- and photo-detection properties of phosphorene. The interplay between device performance and environmental conditions was probed and analyzed using in situ transport measurements. The device structures were exposed to different chemical and light environments to understand how they perform under different external stimuli. For the gas/molecule detection studies, inert (Ar), as well as, oxidizing (N 2 O), and reducing (H 2 and also N 2 H 4 ) agents were selected. The FET structure was exposed to these different gases, and the effect of each gas on the device resistance was measured. The study showed varying response towards different molecules. Specifically, no significant resistance change was observed upon exposure to Ar, while H 2 and N 2 H 4 were found to decrease the resistance and N 2 O had the opposite effect resulting in an increase in resistance. This work is the first demonstration for the detection of N 2 H 2 and N 2 O using a phosphorene-based system. These phosphorene-based FET structures were also found to be sensitive to light exposure. When such structure was irradiated with light, the current modulation was lost. The observed resistance changes can be explained as a result of the modulation of the Schottky barrier at the phosphorene-electrical contact interface due to the adsorbed molecules and charge transfer, and/or photo-induced carrier generation. Furthermore, the results were consistent with the transfer characteristics of V ds v s . V g .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Spatially resolved charge-transfer kinetics at the quantum dot–microbe interface using fluorescence lifetime imaging microscopy

Integrating the optoelectronic properties of quantum dots (QDs) with biological enzymatic systems to form microbe-semiconductor biohybrids offers promising prospects for both solar-to-chemical conversion and light-modulated biochemical processes. Developing these nano–bio hybrid systems necessitates a deep understanding of charge-transfer dynamics at the nano–bio interface. Photoexcited carrier transfer from QDs to microbes is driven by complex interactions, with emerging insights into the relevant thermodynamic and kinetic factors. The heterogeneities of both microbes and QD ensembles pose significant challenges in mechanistic understanding, which is critical for designing advanced nano–bio hybrids. We used fluorescence lifetime imaging microscopy to analyze charge transfer between a CdSe QD film andShewanella oneidensismicrobes. We correlated the spatiotemporal fluorescence data with an analytical model. Our analysis revealed two distinct distributions of QD de-excitation pathways. The characteristics of these distributions: 1) a faster transfer rate ( k ¯ E T 1 = 1.5 10 9 s - 1 ), with a lower acceptor number ( N ¯ a 1 = 0.03 ) and 2) a slower transfer rate ( k ¯ E T 2 = 4.1 10 8 s - 1 ) with a higher acceptor number ( N ¯ a 2 = 0.18 ). We assign these distributions to the indirect and direct electron transfer mechanisms, respectively. Our findings demonstrate how spectroscopic imaging can uncover fundamental electron transfer mechanisms at complex interfaces, offering valuable design principles for future nano–bio hybrids.

Science & Technology - Other Topics↗

Junction range finder

Electronic system locates interferences in radar reception. System utilizes well known frequency-modulated continuous-wave technique to locate objects with nonlinear impedances. FM transmitter generates signal through bandpass filter which eliminates higher order harmonics around carrier frequency.

Morissette, S.↗

Microwave Photonics for Distributed Sensing

In the past few years, microwave-photonics technologies have been investigated for optical fiber sensing. By introducing microwave modulation into the optical system, the optical detection is synchronized with the microwave modulation frequency. As a result, the system has a high SNR and thus an improved detection limit. In addition, the phase of the microwave-modulated light can be obtained and Fourier transformed to find the time-of-arrival information for distributed sensing. Recently, an incoherent optical-carrier-based microwave interferometry (OCMI) technique has been demonstrated for fully distributed sensing with high spatial resolution and large measurement range. Since the modal interference has little influence on the OCMI signal, the OCMI is insensitive to the types of optical waveguide. Motivated by the eeds of distributed measurement in the harsh environment, in the first part of this paper, several OCMI-based sensing systems were built by using special multimode waveguides to perform sensing for heavy duty applications. Driven by an interest on the high-resolution sensing, in the second part of the paper, I propose a coherence-gated microwave photonics interferometry (CMPI) technique, which uses a coherent light source to obtain the optical interference signal from cascaded weak reflectors. The coherence length of the light source is carefully chosen or controlled to gate the signal so that distributed sensing can be achieved. The experimental results indicate that the strain resolution can be better than 0.6 μ using a Fabry-Perot interferometer (FPI) iii with a cavity length of 1.5 cm. Further improvement of the strain resolution to the 1 n level is achievable by increasing the cavity length of the FPI to over 1m. The CMPI has also been utilized for distributed dynamic measurement of vibration by using a new signal processing method. The fast time-varying optical interference intensity change induced by the sub-scan rate vibration is recorded in the frequency domain. After Fourier transform, distinctive features are shown at the vibration location in the time domain signal, where the vibration frequency and intensity can be retrieved. The signal processing method supports vibration measurement of multiple points with the measurable frequency of up to 20 kHz.

Hua, Liwei↗

High field-effect mobility at the (Sr,Ba)SnO{sub 3}/BaSnO{sub 3} interface

A perovskite oxide, BaSnO{sub 3}, has been classified as one of transparent conducting materials with high electron mobility, and its application for field-effect transistors has been the focus of recent research. Here we report transistor operation in BaSnO{sub 3}-based heterostructures with atomically smooth surfaces, fabricated on SrTiO{sub 3} substrates by the (Sr,Ba)SnO{sub 3} buffer technique. Indeed, modulation of band profiles at the channel interfaces with the insertion of wide bandgap (Sr,Ba)SnO{sub 3} as a barrier layer results in a significant improvement of field-effect mobility, implying effective carrier doping at the regulated heterointerface. These results provide an important step towards realization of high-performance BaSnO{sub 3}-based field-effect transistors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Insulin promotes cell migration by regulating PSA-NCAM

Cellular interactions with the extracellular environment are modulated by cell surface polysialic acid (PSA) carried by the neural cell adhesion molecule (NCAM). PSA-NCAM is involved in cellular processes such as differentiation, plasticity, and migration, and is elevated in Alzheimer's disease as well as in metastatic tumour cells. Our previous work demonstrated that insulin enhances the abundance of cell surface PSA by inhibiting PSA-NCAM endocytosis. In the present study we have identified a mechanism for insulin-dependent inhibition of PSA-NCAM turnover affecting cell migration. Insulin enhanced the phosphorylation of the focal adhesion kinase leading to dissociation of αv-integrin/PSA-NCAM clusters, and promoted cell migration. Our results show that αv-integrin plays a key role in the PSA-NCAM turnover process. αv-integrin knockdown stopped PSA-NCAM from being endocytosed, and αv-integrin/PSA-NCAM clusters co-labelled intracellularly with Rab5, altogether indicating a role for αv-integrin as a carrier for PSA-NCAM during internalisation. Furthermore, inhibition of p-FAK caused dissociation of αv-integrin/PSA-NCAM clusters and counteracted the insulin-induced accumulation of PSA at the cell surface and cell migration was impaired. Our data reveal a functional association between the insulin/p-FAK-dependent regulation of PSA-NCAM turnover and cell migration through the extracellular matrix. Most importantly, they identify a novel mechanism for insulin-stimulated cell migration. - Highlights: • Insulin modulates PSA-NCAM turnover through upregulation of p-FAK. • P-FAK modulates αv-integrin/PSA-NCAM clustering. • αv-integrin acts as a carrier for PSA-NCAM endocytosis. • Cell migration is promoted by cell surface PSA. • Insulin promotes PSA-dependent migration in vitro.

60 APPLIED LIFE SCIENCES↗