68,653 research outputs found

    Goldstone intracomplex connected element interferometry

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    Interferometric observations of the radio source pair 3C 84 and OE 400 were made on the 21 km baseline between Deep Space Station (DSS) 13 and DSS 15 to explore the angular navigation potential of intracomplex connected element interferometry (CEI). The differential phase-delay observable formed from pairs of 3 minute scans exhibited a precision of 1 psec, while the actual scatter of the phase-delay residuals for eleven scans over the 90 minute observing session was about 10 psec, consistent with the expected few millimeter fluctuations in the wet tropospheric path delay. Fitting for the position of OE 400 relative to 3C 84 yielded an error ellipse with a semi-minor axis of 60 nrad. Given the short data arc in this experiment, the orthogonal direction in the plane of the sky is not well determined; however, a second baseline or a data arc spanning a larger fraction of the source mutual visibility window could provide simultaneous determination of both right ascension and declination. Examination of the phase-delay residuals supports the accuracy of the cycle ambiguity resolution. However, reliable phase ambiguity resolution will pose the most significant challenge to routine use of CEI for spacecraft tracking, particularly when the a priori spacecraft source position is not well known. Several approaches for ambiguity resolution are briefly outlined

    The effect of spatial and temporal wet-troposphere fluctuations on connected element interferometry

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    Numerical integrations of the structure function of tropospheric inhomogeneities have been performed to assess the impact of water vapor fluctuations on connected element interferometry (CEI). The expectation value of the RMS troposphere error for a differential spacecraft-quasar observation is derived by integrating the spatial refractivity structure function along raypaths to both the spacecraft and quasar from two spatially separated sites. Correlations between the tropospheric conditions at the two sites, which can become significant for short baseline observations, are fully accounted for in this calculation. Temporal effects are treated by assuming a frozen-flow model in which a fixed spatial distribution blows over both sites. Two nominal observation scenarios are considered, along with variations to study the dependence of the resultant differential troposphere errors on baseline length, observation time, source separation angle, and elevation. Consecutive differential observations are found to be almost completely uncorrelated, implying that averaging many repeated differential observations can quickly reduce the troposphere error

    Short baseline phase delay interferometry

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    The high precision of the phase delay data type allows angular navigation accuracy on relatively short baselines to compete with the angular accuracy achieved with long baseline group delay measurements. Differential phase delay observations of close quasar pairs on both a 5.9-km baseline (DSS 12-DSS 13) and a 253-km baseline (DSS 13-Owens Valley Radio Observatory) have been performed to study the potential navigational precision and accuracy of the short baseline interferometry. As a first step toward demonstration of a connected element system at Goldstone, the DSS 12-DSS 13 baseline was operated coherently, distributing a common frequency reference via a recently installed fiber optic cable. The observed phase delay residuals of about 10 psec or less on both baselines appear to be dominated by short term troposphere fluctuations, and correspond to navigational accuracies of well below 50 nrad for the 253-km baseline. Additional experiments will be required to probe the full range of systematic errors

    The Role of Transportation in Campus Emergency Planning, MTI Report 08-06

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    In 2005, Hurricane Katrina created the greatest natural disaster in American history. The states of Louisiana, Mississippi and Alabama sustained significant damage, including 31 colleges and universities. Other institutions of higher education, most notably Louisiana State University (LSU), became resources to the disaster area. This is just one of the many examples of disaster impacts on institutions of higher education. The Federal Department of Homeland Security, under Homeland Security Presidential Directive–5, requires all public agencies that want to receive federal preparedness assistance to comply with the National Incident Management System (NIMS), which includes the creation of an Emergency Operations Plan (EOP). Universities, which may be victims or resources during disasters, must write NIMS–compliant emergency plans. While most university emergency plans address public safety and logistics management, few adequately address the transportation aspects of disaster response and recovery. This MTI report describes the value of integrating transportation infrastructure into the campus emergency plan, including planning for helicopter operations. It offers a list of materials that can be used to educate and inform campus leadership on campus emergency impacts, including books about the Katrina response by LSU and Tulane Hospital, contained in the report´s bibliography. It provides a complete set of Emergency Operations Plan checklists and organization charts updated to acknowledge lessons learned from Katrina, 9/11 and other wide–scale emergencies. Campus emergency planners can quickly update their existing emergency management documents by integrating selected annexes and elements, or create new NIMS–compliant plans by adapting the complete set of annexes to their university´s structures
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