84,621 research outputs found
Observing Exoplanets with High Dispersion Coronagraphy. I. The scientific potential of current and next-generation large ground and space telescopes
Direct imaging of exoplanets presents a formidable technical challenge owing
to the small angular separation and high contrast between exoplanets and their
host stars. High Dispersion Coronagraphy (HDC) is a pathway to achieve
unprecedented sensitivity to Earth-like planets in the habitable zone. Here, we
present a framework to simulate HDC observations and data analyses. The goal of
these simulations is to perform a detailed analysis of the trade-off between
raw star light suppression and spectral resolution for various instrument
configurations, target types, and science cases. We predict the performance of
an HDC instrument at Keck observatory for characterizing directly imaged
gas-giant planets in near infrared bands. We also simulate HDC observations of
an Earth-like planet using next-generation ground-based (TMT) and spaced-base
telescopes (HabEx and LUVOIR). We conclude that ground-based ELTs are more
suitable for HDC observations of an Earth-like planet than future space-based
missions owing to the considerable difference in collecting area. For
ground-based telescopes, HDC observations can detect an Earth-like planet in
the habitable zone around an M dwarf star at 10 starlight suppression
level. Compared to the 10 planet/star contrast, HDC relaxes the
starlight suppression requirement by a factor of 10. For space-based
telescopes, detector noise will be a major limitation at spectral resolutions
higher than 10. Considering detector noise and speckle chromatic noise,
R=400 (1600) is the optimal spectral resolutions for HabEx(LUVOIR). The
corresponding starlight suppression requirement to detect a planet with
planet/star contrast= is relaxed by a factor of 10 (100) for
HabEx (LUVOIR).Comment: 28 pages, 21 figures, 8 tables, accepted by A
The Impact of Acoustic Imaging Geometry on the Fidelity of Seabed Bathymetric Models
Attributes derived from digital bathymetric models (DBM) are a powerful means of analyzing seabed characteristics. Those models however are inherently constrained by the method of seabed sampling. Most bathymetric models are derived by collating a number of discrete corridors of multibeam sonar data. Within each corridor the data are collected over a wide range of distances, azimuths and elevation angles and thus the quality varies significantly. That variability therefore becomes imprinted into the DBM. Subsequent users of the DBM, unfamiliar with the original acquisition geometry, may potentially misinterpret such variability as attributes of the seabed. This paper examines the impact on accuracy and resolution of the resultant derived model as a function of the imaging geometry. This can be broken down into the range, angle, azimuth, density and overlap attributes. These attributes in turn are impacted by the sonar configuration including beam widths, beam spacing, bottom detection algorithms, stabilization strategies, platform speed and stability. Superimposed over the imaging geometry are residual effects due to imperfect integration of ancillary sensors. As the platform (normally a surface vessel), is moving with characteristic motions resulting from the ocean wave spectrum, periodic residuals in the seafloor can become imprinted that may again be misinterpreted as geomorphological information
An Atomic Gravitational Wave Interferometric Sensor in Low Earth Orbit (AGIS-LEO)
We propose an atom interferometer gravitational wave detector in low Earth
orbit (AGIS-LEO). Gravitational waves can be observed by comparing a pair of
atom interferometers separated over a ~30 km baseline. In the proposed
configuration, one or three of these interferometer pairs are simultaneously
operated through the use of two or three satellites in formation flight. The
three satellite configuration allows for the increased suppression of multiple
noise sources and for the detection of stochastic gravitational wave signals.
The mission will offer a strain sensitivity of < 10^(-18) / Hz^(1/2) in the 50
mHz - 10 Hz frequency range, providing access to a rich scientific region with
substantial discovery potential. This band is not currently addressed with the
LIGO or LISA instruments. We analyze systematic backgrounds that are relevant
to the mission and discuss how they can be mitigated at the required levels.
Some of these effects do not appear to have been considered previously in the
context of atom interferometry, and we therefore expect that our analysis will
be broadly relevant to atom interferometric precision measurements. Finally, we
present a brief conceptual overview of shorter-baseline (< 100 m) atom
interferometer configurations that could be deployed as proof-of-principle
instruments on the International Space Station (AGIS-ISS) or an independent
satellite.Comment: 37 pages, 21 figure
Surface-wave-enabled darkfield aperture for background suppression during weak signal detection
Sensitive optical signal detection can often be confounded by the presence of a significant background, and, as such, predetection background suppression is substantively important for weak signal detection. In this paper, we present a novel optical structure design, termed surface-wave-enabled darkfield aperture (SWEDA), which can be directly incorporated onto optical sensors to accomplish predetection background suppression. This SWEDA structure consists of a central hole and a set of groove pattern that channels incident light to the central hole via surface plasmon wave and surface-scattered wave coupling. We show that the surface wave component can mutually cancel the direct transmission component, resulting in near-zero net transmission under uniform normal incidence illumination. Here, we report the implementation of two SWEDA structures. The first structure, circular-groove-based SWEDA, is able to provide polarization-independent suppression of uniform illumination with a suppression factor of 1230. The second structure, linear-groove-based SWEDA, is able to provide a suppression factor of 5080 for transverse-magnetic wave and can serve as a highly compact (5.5 micrometer length) polarization sensor (the measured transmission ratio of two orthogonal polarizations is 6100). Because the exact destructive interference balance is highly delicate and can be easily disrupted by the nonuniformity of the localized light field or light field deviation from normal incidence, the SWEDA can therefore be used to suppress a bright background and allow for sensitive darkfield sensing and imaging (observed image contrast enhancement of 27 dB for the first SWEDA)
Passive detection of moving aerial target based on multiple collaborative GPS satellites
Passive localization is an important part of intelligent surveillance in security and emergency applications. Nowadays, Global Navigation Satellite Systems (GNSSs) have been widely deployed. As a result, the satellite signal receiver may receive multiple GPS signals simultaneously, incurring echo signal detection failure. Therefore, in this paper, a passive method leveraging signals from multiple GPS satellites is proposed for moving aerial target detection. In passive detection, the first challenge is the interference caused by multiple GPS signals transmitted upon the same spectrum resources. To address this issue, successive interference cancellation (SIC) is utilized to separate and reconstruct multiple GPS signals on the reference channel. Moreover, on the monitoring channel, direct wave and multi-path interference are eliminated by extensive cancellation algorithm (ECA). After interference from multiple GPS signals is suppressed, the cycle cross ambiguity function (CCAF) of the signal on the monitoring channel is calculated and coordinate transformation method is adopted to map multiple groups of different time delay-Doppler spectrum into the distance−velocity spectrum. The detection statistics are calculated by the superposition of multiple groups of distance-velocity spectrum. Finally, the echo signal is detected based on a properly defined adaptive detection threshold. Simulation results demonstrate the effectiveness of our proposed method. They show that the detection probability of our proposed method can reach 99%, when the echo signal signal-to-noise ratio (SNR) is only −64 dB. Moreover, our proposed method can achieve 5 dB improvement over the detection method using a single GPS satellite
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