Surveying at the limits of local RTK networks: Test results from the perspective of high accuracy users

Precise GNSS-based differential positioning in real time is usually known as the real-time kinematics (RTK) technique. RTK reduces the effects of orbit errors and ionospheric and tropospheric refraction by forming differences between the observables (e.g. double-differences). These effects, however, grow with increasing baseline length, although the use of corrections generated in real-time from an active GNSS network allows these distance-dependent errors to be reduced. This technology increases the reliability of the system and thereby the accuracy of real-time positioning. In this study, the test results of RTK positioning at different test points located in the border area between the Autonomous Communities of the Region of Murcia and the Community of Valencia, in SE Spain, are presented. The analysis is based on three GNSS local active networks present in this border area, namely MERISTEMUM, REGAM and ERVA networks. Test measurements with RTK rover were performed in this region in order to analyze the real-time services offered by these three networks. Moreover, precise coordinates for each test point were determined using Bernese 5.0. The results confirm that it is possible to achieve centimetre-scale accuracy with RTK positioning based on the networks studied, even in border regions.     

Pluto's Atmosphere And A Targeted-Occultation Search For Other Bound Kbo Atmospheres

Processes relevant to Pluto's atmosphere are discussed,and our current knowledge is summarized, including results of two stellar occultationsby Pluto that were observed in 2002. The question of whether other Kuiper belt objects (KBOs)may have bound atmospheres is considered, and observational indicators for KBO atmospheresare described. The definitive detection of a KBO atmosphere could be established withtargeted stellar-occultation observations. These data can also establish accurate diametersfor these objects and be used to detect possible nearby companions. Strategies for acquiringoccultation data with portable, airborne, and fixed telescopes are evaluated in terms of thenumber of KBO occultations per year that should be observable. For the sample of 29 currentlyknown KBOs with H ≤ 5.2, (radius ≤ 300 km for a geometricalbedo of 0.04), we expect about 4 events per year would yield good results for a (stationary) 6.5-m telescope. A network of portable0.36-m telescopes should be able to observe 6 events per year, and a 2.5-m airborne telescopewould have about 200 annual opportunities to observe KBO occultations.     

The Deep Impact oblique impact cratering experiment

The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼5mp (projectile masses) of water ice or 6mp of CO₂. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130-220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10-30 m) within the diffuse plume above the crater and shallower (5-10 m) materials within the ejecta curtain.     

Pedo‐Geochemical Signatures of Archeological Sites in the Tapirapé‐Aquiri National Forest in Marabá, Amazonia,Brazil

The present study aims to interpret the occupation of terra firme (nonflooded uplands) archeological sites located at Tapirapé‐Aquiri National Forest in the Brazilian state of Pará, through an integrated analysis of pedological, archeological, and geochemical data. We focus on seven archaeological sites, selected among 22 identified in the region. Radiocarbon and thermoluminescence dating indicate distinct periods of occupation over the past ∼6000 years, and the pedo‐geochemical data identify intra‐ and inter‐site differences in soil. Archaeological, chronometric, and pedo‐geochemical data provide a basis for the functional classification of archeological sites found in the region and help to identify specific human activity areas. The results lead us to infer that many of the archeological sites were the result of multiple occupations that left a persistent pedological signature on the landscape.     

PEGASE, an infrared interferometer to study stellar environments and low mass companions around nearby stars

PEGASE is a mission dedicated to the exploration of the environment (including habitable zone) of young and solar-type stars (particularly those in the DARWIN catalogue) and the observation of low mass companions around nearby stars. It is a space interferometer project composed of three free flying spacecraft, respectively featuring two 40 cm siderostats and a beam combiner working in the visible and near infrared. It has been proposed to ESA as an answer to the first “Cosmic Vision” call for proposals, as an M mission. The concept also enables full-scale demonstration of space nulling interferometry operation for DARWIN.

Machine Learning Based Predictive Modeling of Debris Flow Probability Following Wildfire in the Intermountain Western United States

It has been recognized that wildfire, followed by large precipitation events, triggers both flooding and debris flows in mountainous regions. The ability to predict and mitigate these hazards is crucial in protecting public safety and infrastructure. A need for advanced modeling techniques was highlighted by re-evaluating existing prediction models from the literature. Data from 15 individual burn basins in the intermountain western United States, which contained 388 instances and 26 variables, were obtained from the United States Geological Survey (USGS). After randomly selecting a subset of the data to serve as a validation set, advanced predictive modeling techniques, using machine learning, were implemented using the remaining training data. Tenfold cross-validation was applied to the training data to ensure nearly unbiased error estimation and also to avoid model over-fitting. Linear, nonlinear, and rule-based predictive models including naïve Bayes, mixture discriminant analysis, classification trees, and logistic regression models were developed and tested on the validation dataset. Results for the new non-linear approaches were nearly twice as successful as those for the linear models, previously published in debris flow prediction literature. The new prediction models advance the current state-of-the-art of debris flow prediction and improve the ability to accurately predict debris flow events in wildfire-prone intermountain western United States.     

Reactive transport benchmark of MoMaS

We present here the definition of the reactive transport benchmark of Groupement Mathematical Modeling and Numerical Simulation for Nuclear Waste Management Problems. The aim of this benchmark is to propose a challenging test for numerical methods used for reactive transport modeling in porous media. In order to focus on numerical methods, the problem presented here is of quite a small size, both from a hydrodynamical and from a geochemical point of view. Though the chemical coefficients used in this benchmark are not taken from a real chemical system, they are realistic, and the test case is quite challenging.     

Survival of endospores of Bacillus subtilis on spacecraft surfaces under simulated martian environments: implications for the forward contamination of Mars

Experiments were conducted in a Mars simulation chamber (MSC) to characterize the survival of endospores of Bacillus subtilis under high UV irradiation and simulated martian conditions. The MSC was used to create Mars surface environments in which pressure (8.5 mb), temperature (-80, -40, -10, or +23 degrees C), gas composition (Earth-normal N2/O2 mix, pure N2, pure CO2, or a Mars gas mix), and UV-VIS-NIR fluence rates (200-1200 nm) were maintained within tight limits. The Mars gas mix was composed of CO2 (95.3%), N2 (2.7%), Ar (1.7%), O2 (0.2%), and water vapor (0.03%). Experiments were conducted to measure the effects of pressure, gas composition, and temperature alone or in combination with Mars-normal UV-VIS-NIR light environments. Endospores of B. subtilis, were deposited on aluminum coupons as monolayers in which the average density applied to coupons was 2.47 x 10(6) bacteria per sample. Populations of B. subtilis placed on aluminum coupons and subjected to an Earth-normal temperature (23 degrees C), pressure (1013 mb), and gas mix (normal N2/O2 ratio) but illuminated with a Mars-normal UV-VIS-NIR spectrum were reduced by over 99.9% after 30 sec exposure to Mars-normal UV fluence rates. However, it required at least 15 min of Mars-normal UV exposure to reduce bacterial populations on aluminum coupons to non-recoverable levels. These results were duplicated when bacteria were exposed to Mars-normal environments of temperature (-10 degrees C), pressure (8.5 mb), gas composition (pure CO2), and UV fluence rates. In other experiments, results indicated that the gas composition of the atmosphere and the temperature of the bacterial monolayers at the time of Mars UV exposure had no effects on the survival of bacterial endospores. But Mars-normal pressures (8.5 mb) were found to reduce survival by approximately 20-35% compared to Earth-normal pressures (1013 mb). The primary implications of these results are (a) that greater than 99.9% of bacterial populations on sun-exposed surfaces of spacecraft are likely to be inactivated within a few tens of seconds to a few minutes on the surface of Mars, and (b) that within a single Mars day under clear-sky conditions bacterial populations on sun-exposed surfaces of spacecraft will be sterilized. Furthermore, these results suggest that the high UV fluence rates on the martian surface can be an important resource in minimizing the forward contamination of Mars.     

Numerical Studies on the Harmonic Downward Continuation of Band-Limited Airborne Gravity

In this paper, the numerical stability and efficiency of methods of harmonic downward continuation from flying altitudes are treated for sampled gravity field data. The problem is first formulated in its continuous form, i.e. as the inverse solution of the spherical Dirichlet problem, and is then approximated by Gaussian quadrature to yield a finite system of linear equations. The numerical stability of this system is investigated for both error-free gravity data and for the noisy and band-limited gravity measurements usually obtained from airborne gravity surveys. It can be shown that the system becomes ill-conditioned, once the ratio between flying altitude and data sampling rate exceeds a certain limit. It can also be shown that noisy measurements tend to generate a solution that is practically useless, long before the system becomes ill-conditioned. Therefore, instead of treating the general solution of the discrete downward continuation problem, the more modest question is studied, for which range of flying altitudes and sampling rates, the numerical solution of the discrete linear system can be considered as practically useful. Practically useful will be defined heuristically as of sufficient accuracy and stability to satisfy the requirements of the user. The question will be investigated for the specific application of geoid computation from gravity data sampled at flying altitudes. In this case, a stable solution with a standard deviation of a few centimeters is required. Typical flight parameters are heights of 2–6 km, a minimum half-wavelength resolution of 2 km, and data noise between 0.5 and 1.5 mGal. Different methods of geoid determination, different solution techniques for the resulting systems of linear equations, and different minimization principles will be compared. As a result operational parameters will be defined which, for a given noise level, will result in a geoid accuracy of a few centimeters for the estimated band-limited gravity field spectrum.     

Calculation of ground water ages--a comparative analysis

Ground water age is a fundamental, yet complex, concept in ground water hydrology. Discrepancies between results obtained through different modeling approaches for ground water age calculation have been reported, in particular, between ground water ages modeled by advection and direct simulation of ground water ages (e.g., age-mass approach), which includes effects of advection and dispersion. Here, through a series of two-dimensional (2D) simulations, the impact of water mixing through advection and dispersion on modeled 14C and directly simulated ground water ages is assessed. Impact of dispersion on modeled ages is systematically stronger in areas where water velocities are smaller and far more pronounced on 14C ages. This effect is also observed in one-dimensional models. 2D simulations show that longitudinal dispersion generally acts as a "source" of 14C, while vertical dispersion acts as a "sink," leading to apparent younger or older modeled 14C ages as compared to advective and directly simulated ground water ages. The presence of permeable and impermeable faults provides an equally important source for discrepancies, leading to major differences in modeled ages among the three methods considered. Overall, our results show that a 14C modeling approach using a solute transport model for calculating ground water age appears to be more reliable in ground water systems without faults and where water velocities are relatively high than in systems that are relatively more heterogeneous and those where faults are present. Among the three modeling approaches considered here, direct simulation of ground water age seems to yield the most consistent results in complex, heterogeneous ground water flow systems, giving a vertical age structure consistent with ages expected from consideration of the flow system.