Absolute Calibration of an Ashtech Z12-T GPS Receiver

Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated. We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate uncertainty of order 1 ns in the absolute calibration seems to be attainable. ? 2001 John Wiley & Sons, Inc.     

Enclosure experiments on survival and growth of black drum eggs and larvae in lower Chesapeake Bay

Experiments in 2.2 m³, in situ mesocosm enclosures indicate that black drum, Pogonias cromis, eggs and larvae potentially can survive in the lower Chesapeake Bay at ambient microzooplankton prey levels (≈200 prey 1^?1) in the absence of predators. In growth experiments, larva mean growth rates to 10 d posthatch were similar (0.17 mm d^?1 and 0.18 mm d^?1) when fed at prey levels of 50 prey 1^?1 and 200 prey 1^?1. Individual growth rates, however, were more variable at 50 prey 1^?1. Mortality rates also were comparable in 50 (27% d^?1) and 200 (23% d^?1) prey 1^?1 enclosures. In a second experiment, the predation potentials of the hydromedusa Nemopsis bachei and the lobate ctenophore Mnemiopsis leidyi were estimated in relation to initial black drum egg prey density, presence of alternative <1 mm zooplankton prey, and estimated daily abundance of the jellyfish on the black drum spawning grounds. Mortality rates per medusa and ctenophore were similar (0.02–0.03 d^?1), were not affected by presence of alternative prey, and were directly related to initial egg density. Results suggest that the gelatinous predators, especially the hydromedusa, could have cleared a high (≈38%) but variable fraction of the water column daily of fish eggs and yolk-sac larvae during the black drum spawning season. We hypothesize that the poor or episodic recruitment success of black drum in Chesapeake Bay results from a short spawning season that often coincides with abundance peaks of gelatinous predators and that predation on eggs and yolk-sac larvae may control recruitment.     

Laboratory Demonstration of Hydrocarbon Migration in the Vadose Zone: Effectiveness of the U-Tube Design for Underground Storage Tank Leak Detection Monitoring

The experimental results derived from a laboratory model of gasoline migration from a simulated leaking underground storage tank (UST) demonstrate that the direction of migration through the simulated fill material is essentially straight down. Consequently, the U-tube design for leak detection monitoring of USTs has limited utility because that design relies on the assumption that the path of migration of the leaked fluids will follow the surface of the tank to the bottom and be intercepted by the U-tube below.     

Sheared shale response to deep TBM excavation

Phien-wej, N. and Cording, E.J., 1991. Sheared shale response to deep TBM excavation. Eng. Geol., 30: 371–391.

Ravelling and squeezing of sheared shale of Stillwater Tunnel caused severe problems in tunneling with a tunnel boring machine (TBM) that led to termination of the contract. The tunnel was finally holed through with two TBM's specially designed for squeezing ground. Although the shale mass in all geological conditions exhibited time-dependent response, significant squeezing was confined to sheared shale with large amounts of clay gouge infill, wherein creep of the clay gouge was the prime mechanism controlling the ground response. However, when the tunnel face was advanced at a slow rate, the observed ground squeezing in the early period was largely induced by the effect of stress change from face advance, not the creep. Ground ravelling was very significant in sheared shale due to the high degree of fissuring and fracturing of this thinly bedded shale. Failure of the first TBM resulted mainly from the incompatibility of the shield design with the sheared shale. The shield was too long and stiff and had variable diameters. Extensive observation and instrumentation programs of the project provided valuable information on rapid mechanized tunneling in heavy ground.     

Three dilemmas in the integrated assessment of climate change

Conclusions These three dilemmas embody the hardest, most important, and most enduring problems of doing assessment well. None admits simple, obvious solutions. Each can be managed better or worse for any particular assessment endeavor, but doing better requires clear understanding of the purpose of the endeavor. What ways of combining different pieces of disciplinary knowledge, of making projections, and of pursuing policy relevance are more or less appropriate will differ, depending on whether a project seeks to characterize uncertainties and gaps in knowledge; to advise a particular policy choice; to support dialog among policy actors; or to facilitate inquiry into relevant values or goals. Evaluation of the relative emphasis, the methods, and the process of an assessment can only be done relative to some such purpose.Of course, some pitfalls may be so serious as to thwart any purpose, as Risbey et al.'s discussion of the global modeling movement reminds us. The global models' most obvious pitfalls — inadequate treatment of uncertainty, neglect of economic adjustment, excessive confidence in predictions — have largely been seen and avoided by the current assessment community (though there may be more to be learned even here). But on the subtler questions of how assessment or modeling can contribute most usefully to policy, little progress has been made since the 1970s. Consequently, though assessment has advanced in many ways since then, IA remains at risk of suffering the same fate as the global models: a cycle of early enthusiasm, followed by a reaction of frustration and excessive, undeserved rejection.Current endeavors in IA have made substantial contributions to identifying and prioritizing knowledge needs, less to informing specific policy choice. Further progress cannot be guided by a single canonical view of what assessment should be and do, but will proceed incrementally down multiple paths. Several paths currently appear promising: analytic approaches to better represent multiple actors, diverse preferences, and multiple valued outcomes; better representation and application of uncertainty, including diverse expert opinion; novel methods to link assessment with policy communities; and broader participation in assessment teams and explicit focus on negotiating and elaborating pragmatic, viable critical standards. Risbey et al.'s call to develop institutions for critical reflection, mutual learning, and self-improvement will be crucial in developing and evaluating the progress made down these paths.Morgan and Dowlatabadi's checklist for desiderata of IA is a good starting point for a conversation about assessment standards, to which I would propose a few extensions and elaborations. First, there should be not just multiple assessments, but multiple assessment projects using diverse collections of methods and approaches. Second, assessment projects should explore novel methods for connecting their work with the policy community. Third, the approach should be iterative not just within each project, but across assessment projects and between them and the policy community. Fourth, assessors should not be embarrassed by, or seek to disguise, results that are merely illustrative, non-authoritative, and suggestive; these should be acknowledged as such, and the vigorous questioning and critique that will come, including partisan critique, accepted. Do not seek to avoid criticism by mumbling. An important limit to this checklist approach is suggested, though, by the way various writers have groped to define assessment standards by analogy to other domains, revealing how limited is our understanding of how to evaluate assessment. Risbey et al. refer to connoisseurship, as if assessment is like fine wine; Clark and Majone (1985) refer to artistic criticism, as if assessment is like opera singing. If these analogies are appropriate, then pursuing a single set of critical standards for assessment is at least premature, possibly erroneous. Rather, there should be a diversity of approaches, perhaps so broad that no single set of criteria for excellence could be defined. The pragmatic middle way between the too-limiting application of a single set of standards, and an anarchic refusal to evaluate, will have to be negotiated, defined, and improved incrementally.     

Do stony meteorites come from comets?

The place of origin of stony meteorites can be determined from their trapped solar-wind gases. “Gas-rich” meteorites have only 10^?3?10^?4 the solar noble gas content and ?10^?2?10^?4 the surface exposure age of lunar soils. These differences suggest that the gas implantation took place between 1 and 8 AU from the Sun, in a region where the cratering rate was 10²?10³ times higher than at 1 AU. Both characteristics point to the asteroid belt. The predicted Ne²⁰ content a gas-rich meteorite formed at 2.5 AU is 1.2 × 10^?5 cc STP g^?1, compared to an observed mean for H-chondrites of 0.5 × 10^?5 cc STP g^?1. The observed prevalence of gas-rich meteorites (40–100% among carbonaceous chondrites, 2–33% among other classes) requires that the parent body remained long enough in the asteroid belt to develop a substantial regolith. This condition can be met by asteroids (～ 10% of mass converted to regolith.in 4.5 × 10⁹ yr), but not by short period comets (～0.04% converted in 10⁷ yr). It appears that a cometary origin can be ruled out for all stony meteorite clases that have gas-rich members. This includes carbonaceous chondrites.