Jointly Optimizing Placement and Inference for Beacon-based Localization

The ability of robots to estimate their location is crucial for a wide variety of autonomous operations. In settings where GPS is unavailable, measurements of transmissions from fixed beacons provide an effective means of estimating a robot's location as it navigates. The accuracy of such a beacon-based localization system depends both on how beacons are distributed in the environment, and how the robot's location is inferred based on noisy and potentially ambiguous measurements. We propose an approach for making these design decisions automatically and without expert supervision, by explicitly searching for the placement and inference strategies that, together, are optimal for a given environment. Since this search is computationally expensive, our approach encodes beacon placement as a differential neural layer that interfaces with a neural network for inference. This formulation allows us to employ standard techniques for training neural networks to carry out the joint optimization. We evaluate this approach on a variety of environments and settings, and find that it is able to discover designs that enable high localization accuracy.Comment: Appeared at 2017 International Conference on Intelligent Robots and Systems (IROS

Large-scale relocation of two decades of Northern California seismicity using cross-correlation and double-difference methods

We simultaneously reanalyzed two decades (1984–2003) of the digital seismic archive of Northern California using waveform cross-correlation (CC) and double-difference (DD) methods to improve the resolution in hypocenter locations in the existing earthquake catalog generated at the Northern California Seismic Network (NCSN) by up to three orders of magnitude. We used a combination of ∼3 billion CC differential times measured from all correlated pairs of events that are separated by less than 5 km and ∼7 million P wave arrival-time picks listed in the NCSN bulletin. Data were inverted for precise relative locations of 311,273 events using the DD method. The relocated catalog is able to image the fine-scale structure of seismicity associated with active faults and revealed characteristic spatiotemporal structures such as streaks and repeating earthquakes. We found that 90% of the earthquakes have correlated P wave and S wave trains at common stations and that 12% are colocated repeating events. An analysis of the repeating events indicates that uncertainties at the 95% confidence level in the existing network locations are on average 0.7 km laterally and 2 km vertically. Correlation characteristics and relative location improvement are remarkably similar across most of Northern California, implying the general applicability of these techniques to image high-resolution seismicity caused by a variety of plate tectonic and anthropogenic processes. We show that consistent long-term seismic monitoring and data archiving practices are key to increase resolution in existing hypocenter catalogs and to estimate the precise location of future events on a routine basis

Jointly Learning to Construct and Control Agents using Deep Reinforcement Learning

The physical design of a robot and the policy that controls its motion are inherently coupled, and should be determined according to the task and environment. In an increasing number of applications, data-driven and learning-based approaches, such as deep reinforcement learning, have proven effective at designing control policies. For most tasks, the only way to evaluate a physical design with respect to such control policies is empirical--i.e., by picking a design and training a control policy for it. Since training these policies is time-consuming, it is computationally infeasible to train separate policies for all possible designs as a means to identify the best one. In this work, we address this limitation by introducing a method that performs simultaneous joint optimization of the physical design and control network. Our approach maintains a distribution over designs and uses reinforcement learning to optimize a control policy to maximize expected reward over the design distribution. We give the controller access to design parameters to allow it to tailor its policy to each design in the distribution. Throughout training, we shift the distribution towards higher-performing designs, eventually converging to a design and control policy that are jointly optimal. We evaluate our approach in the context of legged locomotion, and demonstrate that it discovers novel designs and walking gaits, outperforming baselines in both performance and efficiency

Post seismic response of repeating aftershocks

The recurrence intervals of repeating earthquakes on the San Andreas Fault in the Loma Prieta aftershock zone follow the characteristic 1/t decay of Omori's law. A model in which these earthquakes occur on isolated patches of the fault that fail in stick-slip with creep around them can explain this observation. In this model the recurrence interval is inversely proportional to the loading rate due to creep. Logarithmic velocity strengthening friction predicts 1/t decay in creep rate following the mainshock. The time dependence of recurrence is inconsistent with a simple viscous constitutive relationship, which predicts an exponential decay of loading rate. Thus, our observations imply postseismic slip at seismogenic depth under a power law rheology. The time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths

High resolution 3D laser scanner measurements of a strike-slip fault quantify its morphological anisotropy at all scales

The surface roughness of a recently exhumed strikeslip fault plane has been measured by three independent 3D portable laser scanners. Digital elevation models of several fault surface areas, from 1 m2 to 600 m2, have been measured at a resolution ranging from 5 mm to 80 mm. Out of plane height fluctuations are described by non-Gaussian distribution with exponential long range tails. Statistical scaling analyses show that the striated fault surface exhibits self-affine scaling invariance with a small but significant directional morphological anisotropy that can be described by two scaling roughness exponents, H1 = 0.7 in the direction of slip and H2 = 0.8 perpendicular to the direction of slip

Regional and teleseismic double-difference earthquake relocation using waveform cross-correlation and global bulletin data

We have developed a double-difference algorithm to relocate earthquakes recorded at global seismic networks, using differential arrival times for first and later arriving regional and global phases to invert for the vectors connecting the hypocenters. Differential times are formed from global seismic bulletins and are accurately measured on similar seismograms by time domain waveform cross correlation. We evaluate the performance of this spherical, multiphase double-difference algorithm using three-dimensional regional-scale synthetic data and two sets of earthquake data in different tectonic settings. The first includes 3783 intermediate depth earthquakes that occurred between 1964 and 2000 in the subducting Nazca plate beneath northern Chile, where the relocated seismicity confirms a narrowly spaced double seismic zone previously imaged with temporary local seismic data. Residual statistics and comparison with accurately known locations indicate mean relative location errors at the 90% confidence level of 2.4 km laterally and 1.8 km vertically. Later events typically constrained by cross-correlation data have errors of 1.6 km laterally and 1.4 km vertically. The second data set includes 75 crustal earthquakes in the 1999 Izmit and Düzce, Turkey, aftershock sequences, where the double-difference solutions image orientation and dip of individual fault segments that are consistent with focal mechanisms and near-surface information. Fault complexity likely causes a low level of waveform similarity in this aftershock sequence and thus generates fewer correlated events compared to the Chile earthquakes. Differences between the double-difference locations and corresponding locations in global seismicity catalogs (Earthquake Data Report, EDR; International Seismological Centre, ISC; Engdahl-Hilst-Buland, EHB) are typically greater than 10 km. We evaluate the potential of cross-correlation and double-difference methods to improve hypocenter locations on a global scale

Gravity-Assist Mechanical Simulator for Outreach

There is no convenient way to demonstrate mechanically, as an outreach (or inreach) topic, the angular momentum trade-offs and the conservation of angular momentum associated with gravityassist interplanetary trajectories. The mechanical concepts that underlie gravity assist are often misunderstood or confused, possibly because there is no mechanical analog to it in everyday experience. The Gravity Assist Mech - anical Simulator is a hands-on solution to this longstanding technical communications challenge. Users intuitively grasp the concepts, meeting specific educational objectives. A manually spun wheel with high angular mass and low-friction bearings supplies momentum to an attached spherical neodymium magnet that represents a planet orbiting the Sun. A steel bearing ball following a trajectory across a glass plate above the wheel and magnet undergoes an elastic collision with the revolving magnet, illustrating the gravitational elastic collision between spacecraft and planet on a gravity-assist interplanetary trajectory. Manually supplying the angular momentum for the elastic collision, rather than observing an animation, intuitively conveys the concepts, meeting nine specific educational objectives. Many NASA and JPL interplanetary missions are enabled by the gravity-assist technique

Valence band offset of InN/AlN heterojunctions measured by X-ray photoelectron spectroscopy

The valence band offset of wurtzite-InN/AlN (0001) heterojunctions is determined by x-ray photoelectron spectroscopy to be 1.52±0.17 eV. Together with the resulting conduction band offset of 4.0±0.2 eV, a type-I heterojunction forms between InN and AlN in the straddling arrangement

Surface electronic properties of undoped InAlN alloys

The variation in surface electronic properties of undoped c-plane InxAl1−xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed

Sounding the Alarm Round 2: Protecting Democracy in Ukraine

In 2010 Freedom House released its first special report on Ukraine, "Sounding the Alarm: Protecting Democracy in Ukraine". That report, as the title suggested, warned that Ukraine was heading in the wrong direction on a number of fronts: consolidation of power in the executive branch at the expense of democratic development, a more restrictive environment for the media, selective prosecution of opposition figures, worrisome instances of intrusiveness by the Security Service of Ukraine (SBU), widely criticized local elections in October 2010, a pliant Verkhovna Rada (Ukraine's parliament), an erosion of basic freedoms of assembly and speech, and widening corruption. "Ukraine under President Yanukovych," last year's report warned, "has become less democratic and, if current trends are left unchecked, may head down a path toward autocracy and kleptocracy."A year later, most of those key concerns remain, and in some cases the problems have grown considerably worse, especially in the area of selective prosecution of opposition figures and corruption. The mayoral election in Obukhiv in March was widely criticized for its alleged rigging and fraud and bodes badly for the upcoming Verkhovna Rada elections. The term "familyization" was commonly used by interlocutors, implying that President Yanukovych's family has not only benefitted personally from his presidency (see the section below on corruption) but is increasingly at the center of power and governance. Freedom House's ranking of Ukraine in its Freedom in the World 2012 report remained in the Partly Free category with a negative trend; the same assessment can be found in Freedom House's just-released "Nations in Transit." Against this backdrop, Freedom House, with support from the Open Society Foundations' Ukrainian arm, the International Renaissance Foundation, undertook this follow-up special report on Ukraine