The effect of errors in the initial data on the reference variance factor and accuracy estimates

The issue of the effect of errors in initial data on the reference variance factor and the accuracy of the estimated values in a least squares adjustment are discussed. General formulas for the reference variance factor and covariance of estimated values are derived. The accuracy of a densified third-order triangulation controlled by a second-order network is evaluated using the suggested formulas, and results utilizing actual data are given.     

An Overview of Multi-Reference Station Methods for cm-Level Positioning

Over the past few years, a significant amount of research has been conducted on the formulation of carrier phase corrections in order to enhance ambiguity resolution and to increase the distances over which precise positioning can be achieved. Recently the use of a network of multiple GPS reference stations for generating carrier phase-based corrections has emerged with great promise for use in real-time environments. However, little research has been conducted on the distribution of these corrections to potential GPS users located within, and surrounding, the network coverage area. This is an integral part of real-time kinematic DGPS, and it must be adequately addressed before a practical realization of the multireference station concept is implemented. The focus of this paper is to present a comprehensive summary of some of the multiple reference station methods, with specific attention directed toward the correction generation and dissemination processes. More specifically, the various multi-reference station methodologies have been categorized according to their underlying correction generation framework, but will be discussed in terms of the correction dissemination options presented by the various authors. The for main categories of methods investigated in this paper are: (a) partial derivative algorithms, (b) linear interpolation algorithms, (c) condition adjustment algorithms, and (d) virtual reference station methodologies. ? 2001 John Wiley & Sons, Inc.     

A consistency test of airborne GPS using multiple monitor stations

In October 1990, several airborne GPS tests were conducted in the Ottawa region by the Canada Centre for Surveying (CCS) and the Canada Centre for Remote Sensing (CCRS). Ashtech XII receivers were located at up to three monitor stations with baseline lengths to the aircraft ranging from 1–200 km. Approximately two hours of airborne data, collected at a 2 Hz rate, were available for each of the three test days. Post-processing of the differential data was done using the University of Calgary's SEMIKIN package which utilizes a Kalman filter algorithm to estimate both the remote receiver's position and velocity. Comparisons were made between the aircraft position and velocity determined from each of the monitor stations to assess the consistency of differential GPS when different reference stations are used. Results show that the degree of consistency is dependent upon the distance to the monitor stations. Agreement at the decimetre-level is achieved in position when the baseline lengths are within 100 km. Agreement in velocity is usually better than 1 cm s^–1 (RMS).     

Microstructure of pulsar radio pulses measured with a time resolution of 62.5 ns at 1650 MHz

We present an analysis of pulsar observations carried out on two frequency channels at 1634 MHz and 1650 MHz with a time resolution of 62.5 ns on the 70-m radio telescope of the NASA Deep Space Network in Tidbinbilla. The data were recorded using the S2 system, intended primarily for VLBI observations. Microstructure with characteristic timescales of 270, 80, and 150 µs was detected in pulsars B0833-45, B1749-28, and B1933 + 16, respectively. The distribution of microstructure timescales for the Vela pulsar (B0833-45) is characterized by a gradual growth with decreasing timescale to 200 µs; the distribution has a maximum at 20–200 µs and falls off sharply for timescales below 20 µs. The statistical relation between the microstructure modulation index m and the corresponding timescale τ_µ can be approximated by the power law dependence R∝τ _⊙ ^0.5 ; i.e., the intensity is higher for micropulses with longer durations. This contradicts the predictions of nonlinear models for the formation of micropulses by supercompact soliton wave packets. In all the pulsars studied, the time delays of the micropulses between the two frequency channels deviate from the expected dispersion laws for the interstellar plasma. In particular, the micropulses in the low-frequency channel arrive earlier than predicted by the dispersion measures derived previously from the mean pulse profiles. The deviation from the dispersion delay is determined most accurately for B0833-45, and is 4.9±0.2 µs. Such anomalous delays are probably associated with the effects of propagation of the radio emission within the pulsar magnetosphere.     

Working borders and shifting identities in the Russian Far North

State-imposed borders inform socio-spatial identities, often encouraging divergent identities for those living of different sides of the border. However, these identities may be discursively appropriated by the groups affected by the borders, in order to manage their relations with the state. We describe how one group of aboriginal people in the Russian Far North forged a common identity based on evasion of state institutions in the 1930–1950s. This group, once articulated with state institutions and divided by the enforcement of a provincial/republican border, developed two socio-spatial identities––and employed these identities as counterhegemonic tactics to state pressures over their lifeways. As types of pressures change, the affordances that the borders provide also change, as evident in the shifting discourses of difference and similitude.     

Seasonal changes in vocalization during behavior of the Atlantic bottlenose dolphin

Four vocalizations (whistles, buzzes, quacks, and pops) were quantified during three behavioral categories (socializing, traveling, and feeding) of the bottlenose dolphin in the Newport River Estuary, North Carolina. This study tested the hypothesis that specific vocalizations of dolphins in this area are associated with specific behavioral categories during the summer and fall. Approximately twice the total number of vocalizations per unit time occurred during the fall as compared to the summer. Each type of vocalization had a distinctive and consistent acoustical structure. The relative frequency of the different vocalization types was significantly dependent on the behavior of the dolphins. Pops were consistently associated with feeding, quacks with socializing, and buzzes with traveling. Whistles, the most common vocalization, were associated with traveling and feeding during the summer and socializing in the fall. Dolphins spent more time involved in undirected activity (socialization) during the summer but spent more time traveling in search of food and feeding during the fall in preparation for migration out of the estuary and movement south.     

Mechanical models for correlation of ring-fracture eruptions at Pantelleria,Strait of Sicily,with glacial sea-level drawdown

Recent K-Ar dating of eruptions at Pantelleria, a peralkaline volcanic island in the Strait of Sicily, shows a correlation between eruption of pantellerite lavas from caldera ring fractures and low stands of sea level as determined from ¹⁸O stratigraphy. Post-caldera pantellerite lavas associated with an 114-ky-old caldera erupted along the ring-fracture zone during a major low stand of sea level at about 67 Ka. The most recent episode of lava-flow emplacement began about 20 ky ago during the last glacial maximum. Magma vented along the ring fault of a 45-ky-old caldera, from fractures radial to the caldera, and along faults formed by intracaldera trapdoor uplift. Two mechanical models based on elasticity theory are presented to explain the correlation of post-caldera ring-fracture eruptions at Pantelleria with lowering of sea level. A simple analysis of a bending circular plate of thickness,T _r, and radius,R, representing the magma-chamber roof block, shows that tensile stress is concentrated by a factor of 0.75R ²/T _r ² at the lower perimeter of the plate when sea level drops. Stress changes may be even greater ifT _r is effectively less than the stratigraphic thickness due to layering of rocks in the roof block. Calculated stress changes due to a 100-m drawdown of sea level are similar in magnitude to stresses associated with dike propagation. More realistic model geometries, including different chamber shapes, a conical volcanic edifice, and sea-level drawdown beyond the surface projection of the magma chamber, were tested using the boundary-element method. Lowering sea level generates a horizontal tensile stress above the chamber, even when sea water is removed outboard of the magma chamber. For some chamber geometries the magnitude of the tensile stress maximum is greater than the 1 MPa pressure of the 100 m of removed water and is of the right order of magnitude for dike propagation. Dikes initiated by the change of the stress field may originate and propagate along fractures inboard of the chamber margin. The magnitudes of tensile maxima along the top of the chamber decrease as original sea level is moved outboard of the chamber margin and as the chamber thickness decreases. When the depth to the top of the magma chamber reaches a critical value, dependent on chamber geometry, the propagation of dikes to the surface is inhibited.     

Sedimentation processes in an impoundment,Union lake,New Jersey

A small (3 75 km²) impoundment. Union Lake, on the Maurice River in southern New Jersey contains fine-grained sediment and organic material being transported through the surface water system Sedimentation is slow and rates calculated from¹³⁷Cs-dated cores indicate a decrease through time from 2 6 mm/yr to 0 57 mm/yr Sediment from shallow areas (<3 m) consists of winnowed preimpoundment fluvial sand, whereas sediment from deeper areas (>3 m) is dy composed of diatoms, organic matter, and silt- and clay-sized mineral grains Sediment distribution is controlled by water moving from the river entrance southward toward the outlet and by wind-generated water motion from shallow areas fringing the shore toward deeper portions of the impoundment