67,256 research outputs found
Characterization of Exoplanet Atmospheres with the Optical Coronagraph on WFIRST
WFIRST-CGI is a NASA technology demonstration mission that is charged with
demonstrating key technologies for future exo-Earth imaging missions in space.
In the process, it will obtain images and low-resolution spectra of a handful
to a dozen extrasolar planets and possibly protoplanetary disks. Its
unprecedented contrast levels in the optical will provide astronomers with
their first direct look at mature, Jupiter sized planets at moderate
separations. This paper addresses the question: what science can be done with
such data? An analytic noise model, which is informed by the ongoing
engineering developments, is used to compute maximum achievable signal-to-noise
ratios and scientifically viable integration times for hypothetical star planet
systems, as well as to investigate the constraining power of various
combinations of WFIRST-CGI photometric and spectral observations. This work
introduces two simple models for planetary geometric albedos, which are
inspired largely by the solar system's gas giants. The first planet model is a
hybrid Jupiter-Neptune model, which separately treats the short and long
wavelengths where chromophores and methane dominate absorption, respectively.
The second planet model fixes cloud and haze properties in CoolTLusty to match
Jupiter's albedo spectrum, it then perturbs only the metallicity. MCMC
retrievals performed on simulated observations are used to assess the precision
with which planet model parameters can be measured subject to different
exposure times and observing cases. Fit results for both models'
parameterizations of geometric albedo spectra demonstrate that a rough
indication of the metallicity or methane content should be possible for some
WFIRST-CGI targets. We conclude that real observations will likely be able to
differentiate between extreme cases using these models, but will lack the
precision necessary to uncover subtle trends.Comment: 29 pages, 25 figures, 2 table
Statistical Power Supply Dynamic Noise Prediction in Hierarchical Power Grid and Package Networks
One of the most crucial high performance systems-on-chip design challenge is to front their power supply noise sufferance due to high frequencies, huge number of functional blocks and technology scaling down. Marking a difference from traditional post physical-design static voltage drop analysis, /a priori dynamic voltage drop/evaluation is the focus of this work. It takes into account transient currents and on-chip and package /RLC/ parasitics while exploring the power grid design solution space: Design countermeasures can be thus early defined and long post physical-design verification cycles can be shortened. As shown by an extensive set of results, a carefully extracted and modular grid library assures realistic evaluation of parasitics impact on noise and facilitates the power network construction; furthermore statistical analysis guarantees a correct current envelope evaluation and Spice simulations endorse reliable result
PMU-Based ROCOF Measurements: Uncertainty Limits and Metrological Significance in Power System Applications
In modern power systems, the Rate-of-Change-of-Frequency (ROCOF) may be
largely employed in Wide Area Monitoring, Protection and Control (WAMPAC)
applications. However, a standard approach towards ROCOF measurements is still
missing. In this paper, we investigate the feasibility of Phasor Measurement
Units (PMUs) deployment in ROCOF-based applications, with a specific focus on
Under-Frequency Load-Shedding (UFLS). For this analysis, we select three
state-of-the-art window-based synchrophasor estimation algorithms and compare
different signal models, ROCOF estimation techniques and window lengths in
datasets inspired by real-world acquisitions. In this sense, we are able to
carry out a sensitivity analysis of the behavior of a PMU-based UFLS control
scheme. Based on the proposed results, PMUs prove to be accurate ROCOF meters,
as long as the harmonic and inter-harmonic distortion within the measurement
pass-bandwidth is scarce. In the presence of transient events, the
synchrophasor model looses its appropriateness as the signal energy spreads
over the entire spectrum and cannot be approximated as a sequence of
narrow-band components. Finally, we validate the actual feasibility of
PMU-based UFLS in a real-time simulated scenario where we compare two different
ROCOF estimation techniques with a frequency-based control scheme and we show
their impact on the successful grid restoration.Comment: Manuscript IM-18-20133R. Accepted for publication on IEEE
Transactions on Instrumentation and Measurement (acceptance date: 9 March
2019
Global Distribution of Water Vapor and Cloud Cover--Sites for High Performance THz Applications
Absorption of terahertz radiation by atmospheric water vapor is a serious
impediment for radio astronomy and for long-distance communications.
Transmission in the THz regime is dependent almost exclusively on atmospheric
precipitable water vapor (PWV). Though much of the Earth has PWV that is too
high for good transmission above 200 GHz, there are a number of dry sites with
very low attenuation. We performed a global analysis of PWV with
high-resolution measurements from the Moderate Resolution Imaging Spectrometer
(MODIS) on two NASA Earth Observing System (EOS) satellites over the year of
2011. We determined PWV and cloud cover distributions and then developed a
model to find transmission and atmospheric radiance as well as necessary
integration times in the various windows. We produced global maps over the
common THz windows for astronomical and satellite communications scenarios.
Notably, we show that up through 1 THz, systems could be built in excellent
sites of Chile, Greenland and the Tibetan Plateau, while Antarctic performance
is good to 1.6 THz. For a ground-to-space communication link up through 847
GHz, we found several sites in the Continental United States where mean
atmospheric attenuation is less than 40 dB; not an insurmountable challenge for
a link.Comment: 15 pages, 23 figure
- …