Altitude Measurements Research Articles

Eruption reconstruction, formation of flow-lobe tumuli and eruption duration in the 5900 BP Helgafell lava field (Heimaey), south Iceland

The 5900 BP Helgafell eruption started with an intense fire-fountaining phase that deposited scoria and spatter near the vent. Spatter accumulated, agglutinated, and was subsequently emplaced as clastogenic flows. The intense initial phase was followed by effusive emplacement of tube-fed pahoehoe. This type of lava emplacement makes up the bulk of the lava field, and contains abundant inflation features such as tumuli and pressure-ridges. We measured the depths of 358 inflation clefts in tumuli and the thicknesses of the upper-vesicular crust in flow-lobes from the Helgafell lava field. Measurements of altitude between a point-source of lava tubes and the depth of inflation clefts in tumuli suggest that the magmastatic pressure affects initial tumuli formation. The overall growth of tumuli is, however, more dependent on effusion rate and collapse/clogging of individual tubes than on the magmastatic head within tubes. Three areas with abundant tumuli are recognized in the Helgafell lava field, each of which was active for at least 47 days. The average time for tumuli formation is 32 days. Using the crustal measurements of inflation features we test the hypothesis that the total eruption duration can be estimated for a pre-historic volcanic eruption. We chose the Helgafell lava field to test this hypothesis because it has (1) abundant inflation features that are easily measured, and (2) the majority of lava flows emplaced during the eruption are well exposed. Our result shows that the duration of the Helgafell eruption can be estimated to 11–12 months, yielding an average volumetric effusion rate for the Helgafell eruption of 0.05–0.06 km 3/month. This result is very similar to the effusion rates for two known eruptions in the same volcanic system (i.e. the 1963–1967 Surtsey and the 1973 Eldfell eruptions, with effusion rates of 0.02 and 0.04 km 3/month, respectively). Given that all three eruptions occurred within the same volcanic system and that they have similar volumetric effusion rates, we suggest that our estimated eruption duration for Helgafell is reasonable. Our results, building on the suggestion by Hon et al. (1994) [Hon, K., Kauhikaua, J., Denlinger, R., MacKay, K., 1994. Emplacement and inflation of pahoehoe sheet flows: observations and measurements of active lava flows on Kilauea Volcano, Hawaii. Geol. Soc. Amer. Bull. 106, 351-370] that the duration of inflation can be estimated for individual flows by measuring inflation features, show that by combining abundant and measureable inflation features with a well-established stratigraphy the duration of an entire pre-historic eruption can be estimated.

3D modeling of shock-induced trapping of solar energetic particles in the Earth's magnetosphere

The prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere around L = 2 –2.5, including protons and heavier ions, has been observed at both the Cycle 22 and 23 solar maxima, in association with high-speed interplanetary shocks and storm sudden commencements (SSCs). Recent observations include the Bastille Day 2000 CME-driven storm as well as two in November 2001, which produced a long-lived new proton belt, as well as trapping of heavy ions up to Fe in all three cases. A survey of such events around the most recent solar maximum, including high altitude measurements from Polar and HEO satellites along with low altitude measurements from `the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)', indicates similarities to the well-studied March 24, 1991 SSC event. In this event, electrons and protons in drift resonance with a magnetosonic impulse were transported radially inward. A requirement for such shock-induced acceleration is a high-speed CME-shock at 1 AU, which launches a perturbation with comparable velocity inside the magnetosphere. Secondly, there must be a source population which is drift-resonant with the impulse. The CME-shock itself is a source of solar energetic particles, both protons and heavy ions, with higher fluxes and harder spectra associated with faster moving CMEs. Arrival of the interplanetary shock compresses and changes the magnetosphere topology, leading to a reduction of the geomagnetic cutoff, initially around L = 4 for SEP protons. This effect is modeled using a 3D Lorentz integration of SEP trajectories in electric and magnetic fields taken from the Lyon–Fedder–Mobarry (LFM) global MHD code, with solar wind input parameters taken from spacecraft measurements upstream from the bow shock, carried out for the November 24, 2001 SEP event. The results indicate that an enhancement in solar wind dynamic pressure for this event plays a role in the observed injection of ions to low L-values, to form a new proton belt which has lasted for more than 2 years.

An efficient unified algorithm for filtering and smoothing problems

The problem of optimal filtering and smoothing of the processes described by shaping filters in the framework of the state-space approach are considered. An economical, from the computational standpoint, algorithm for the problem of smoothing and filtering is suggested. The results are illustrated by a methodical example as well as for the problem when the gravity anomaly is determined from an aircraft by using precision satellite measurements of altitude.

Application of the Robust Approach to the Problem of Airborne Gravimetry

The problem of gravity anomaly (GA) estimation aboard an aircraft solved by using the data from a gravimeter and phase measurements of altitude from the differential satellite navigation system is discussed. The problem is solved by using optimal and robust filters. The optimal filter is obtained under the assumption that the spectral density for GA is known. Only two of the GA parameters, - the variance of GA and the variance of its first derivative are assumed to be known in the synthesis of the robust filter. These two filters are compared from the viewpoint of accuracy and sensitivity.

Predicting the L-position of the storm-injected relativistic electron belt

In the paper we confirm earlier work on the dependence of the L-position (Lmax) of the peak intensity of storm-injected relativistic electrons upon the magnetic storm amplitude | Dst|max. Early results for | Dst|max=30–140 nT were reported by Williams et al. (1968). Later studies gave the following formula: | Dst| max= 2.75·10 4/ L max 4 for | Dst| up to 400 nT (Tverskaya, 1986). This sort of relationship also can be applied to the lowest position of the westward electrojet center during the storm, and the boundaries of both the trapped radiation zone and discrete auroral forms. Data from high-altitude (Polar and 1997-068) satellites, and a low-altitude satellite (SAMPEX) were analyzed and compared to the predictions of the above formula. The congruence between formula and observations is excellent: the discrepancy is not larger than 0.2 Re. There are several reasons why the discrepancies might be expected including: fast radial transport due to sudden impulses; the overlapping in time of multiple storms; and the additional injections onto L>Lmax in the storm recovery phase. There could be difficulty relating high and low altitude measurements due to problems with magnetic field models, but we believe this is not a major factor in our analysis.

Real-time digital processing of GPS measurements for transmission engineering

The Global Positioning System (GPS) is a state-of-the-art timing and positioning system based on 24 or more satellites launched and maintained by the US government. Power engineering applications based on GPS include phasor measurement, positioning applications, such as surveying and mapping, and potentially in deriving real-time data on transmission lines that will allow them to be loaded to a dynamic (thermal or security) limit. Inherent errors in GPS technologies are discussed, and the differential GPS method is described for accuracy enhancement. Further digital-processing needs are necessary for meeting the accuracy requirements of certain specific applications. The focus of this paper is on the digital signal processing (DSP) of differential-GPS (DGPS) measurements. The paper describes a methodology for further improving DGPS altitude measurements for the purpose of accurate determination of high-voltage overhead conductor sag. The Haar wavelet transforms (HWT) and least-squares parameter estimation (LSPE) techniques are considered.

Atmospheric multiple scattering effects on GLAS altimetry. II. Analysis of expected errors in Antarctic altitude measurements

Atmospheric multiple scattering effects on GLAS altimetry. II. Analysis of expected errors in Antarctic altitude measurements

The canopy effect in airborne EM

Airborne electromagnetic (AEM) surveys may be used to provide environmental subsurface assessments. Trial fixed‐wing AEM surveys were conducted in the central English Midlands for such purposes. This paper investigates a specific issue, that of the canopy effect, which needs to be addressed when high‐resolution AEM data are to be interpreted accurately. Any elevated feature (typically tree cover) that gives rise to underestimated altimeter readings causes the canopy effect. Half‐space models obtained from conventional, half‐space procedures together with formal numerical inversion techniques form the basis of the study. Both theory and survey data are used to assess the significance of the canopy thickness on the resistivity models. The pseudolayer half‐space method has a stated immunity to altitude errors. The method is found to be highly effective in returning resistivity estimates unbiased by altimeter errors. The associated positive apparent depths provide realistic estimates of canopy thickness while, elsewhere, negative values may be returned. Published numerical inversion schemes do not discuss any corresponding requirement to reduce canopy effect bias. Underestimated altitude measurements introduce false high‐resistivity zones with high wavenumber content unless an equivalent pseudolayer concept is used. The study indicates a requirement for a formal pseudolayer (an at‐surface perfect resistor of variable depth) to be included in the model when canopy zones are present.

An assessment of inversion methods for AEM data applied to environmental studies

Airborne electromagnetic (AEM) surveys are currently being flown over populated areas and applied to detailed problems using high flight line densities. Interpretation information is supplied through a model of the subsurface resistivity distribution. Theoretical and survey data are used here to study the character and reliability of such models. Although the survey data were obtained using a fixed-wing system, the corresponding associations with helicopter, towed-bird systems are discussed. Both Fraser half-space and 1D inversion techniques are considered in relation to their ability to distinguish geological, cultural and environmental influences on the survey data. Fraser half-space modelling provides the dual interpretation parameters of apparent resistivity and apparent depth at each operational frequency. The apparent resistivity was found to be a remarkably stable parameter and appears robust to the presence of a variety of at-surface cultural features. Such features provide both incorrect altitude data and multidimensional influences. Their influences are observed most strongly in the joint estimate of apparent depth and this accounts for the stability of the apparent resistivity. Positive apparent depths, in the example data, result from underestimated altitude measurements. It is demonstrated that increasingly negative apparent depths are associated with increasing misfits between a 1D model and the data. Centroid depth calculations, which are a transform of the Fraser half-space parameters, provide an example of the detection of non-1D influences on data obtained above a populated area. 1D inversion of both theoretical and survey data is examined. The simplest use of the 1D inversion method is in providing an estimate of a half-space resistivity. This can be undertaken prior to multilayer inversion as an initial assessment. Underestimated altitude measurements also enter the problem and, in keeping with the Fraser pseudo-layer concept, an at-surface highly resistive layer of variable thickness can be usefully introduced as a constrained parameter. It is clearly difficult to ascribe levels of significance to a ‘measure’ of misfit contained in a negative apparent depth with the dimensions of metres. The reliability of 1D models is better assessed using a formal misfit parameter. With the misfit parameter in place, the example data suggest that the 1D inversion methods provide reliable apparent resistivity values with a higher resolution than the equivalent information from the Fraser half-space estimates.

Reconstruction of RHESSI Solar Flare Images with a Forward Fitting Method

We describe a forward-fitting method that has been developed to reconstruct hard X-ray images of solar flares from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), a Fourier imager with rotation-modulated collimators that was launched on 5 February 2002. The forward-fitting method is based on geometric models that represent a spatial map by a superposition of multiple source structures, which are quantified by circular gaussians (4 parameters per source), elliptical gaussians (6 parameters), or curved ellipticals (7 parameters), designed to characterize real solar flare hard X-ray maps with a minimum number of geometric elements. We describe and demonstrate the use of the forward-fitting algorithm. We perform some 500 simulations of rotation-modulated time profiles of the 9 RHESSI detectors, based on single and multiple source structures, and perform their image reconstruction. We quantify the fidelity of the image reconstruction, as function of photon statistics, and the accuracy of retrieved source positions, widths, and fluxes. We outline applications for which the forward-fitting code is most suitable, such as measurements of the energy-dependent altitude of energy loss near the limb, or footpoint separation during flares.

Measurements of Low-Flying Target Altitudes in Standard Three-Dimensional Surveillance VHF-Band Radars

Measurements of Low-Flying Target Altitudes in Standard Three-Dimensional Surveillance VHF-Band Radars

Killer Whales (Orcinus orca) Sighted West of Ni'ihau, Hawai'i

A rare sighting of five killer whales (Orcinus orca), including one juvenile, occurred on 20 March 2000 during an aerial survey of waters west of the Hawaiian island of Ni'ihau. The sighting occurred ca. 11 km west of Kamalino Bay on Ni'ihau, at 21049' N, 160020' W. Killer whales are not unknown in Hawaiian waters, but the most recent confirmed sighting on record for Hawaiian waters was in 1979. ON 20 MARCH 2000 during an aerial survey of marine mammals in Hawaiian waters, four large delphinids and one juvenile were sighted approximately 11 km west of Kamalino Bay on Ni'ihau (21049' N, 160020' W) in waters approximately 1830 m (1000 fathoms) deep. The animals were circled for 7 min at an altitude of 244 m, during which time two photographs (Figure 1) were obtained through a belly window using a Pentax 645 camera with a 206-mm lens. Simultaneously, altitude measurements were obtained with a radar altimeter (Collins 50A). Positive identification of species at the time of sighting was hampered by glare. One observer (M.W.N.) commented that the animals appeared unusually robust. In the absence of positive identification, the sighting was recorded as unidentified delphinids. Subsequent inspection of the photographs revealed clear diagnostic features of killer whales (Orcinus orca), including evidence of white eye patches on three individuals, white anus patch on two individuals, a relatively tall dorsal fin on one individual, and light gray saddle marking behind the dorsal fin of one 1 Manuscript accepted 30 October 2000. 2 Social Sciences, University of Hawai'i-West O'ahu, 96-129 Ala Ike, Pearl City, Hawai'i 96782. 3 National Marine Mammal Laboratory, 7600 Sand Point Way NE, Seattle, Washington 98115-0070. 4 Kewalo Basin Marine Mammal Laboratory, 1129 Ala Moana Boulevard, Honolulu, Hawai'i 96814. 522371 Hartman Drive, Los Altos, California 94024. Pacific Science (2001), vol. 55, no. 3:301-303 © 2001 by University of Hawai'i Press All rights reserved individual. The clearest diagnostic feature evident in the photographs was the characteristic wide, paddle-shaped flippers that are almost as wide as they are long (Leatherwood et al. 1988). Leatherwood et al. (1988) noted that killer whales can sometimes be confused with false killer whales (Pseudorca crassidens) and Risso's dolphins (Grampus griseus). However, the paddle-shaped flippers distinguish them from both of these species, and the dark coloration and larger body size (~9 m) distinguish them from Risso's dolphins (~4 m). By knowing the precise altitude via the radar altimeter, combined with the use of a calibrated camera system, we were able to apply standard photogrammetric techniques (Baker 1960, Perryman and Lynn 1993) to the two photographs. We estimated that the adults ranged in length from approximately 4.7 to 6.5 m, and the juvenile was approximately 3.8 m in length. In the first photograph, the four adults were arranged two abreast, with the juvenile trailing behind. In the second photograph, taken several minutes later, the adults were spread farther apart and the juvenile appeared to be swimming either canted to one side or inverted (based on the greater proportion of white coloration) just behind and below the right pectoral flipper of the largest adult. It is likely that this adult was a male, due to the larger flippers and flukes (Dahlheim and Heyning 1999). Killer whales are not unknown in Hawaiian waters, but are sufficiently rare to be worthy of note. The earliest record appeared in the Honolulu Star-Bulletin of 26 February 1938 (cited in Tomich 1986), which described a sighting off Olowalu, Maui. Because

Radar measurements of the reentry of the ariane 504 EPC

A new VHF radar system has been integrated onto a KC-135 aircraft to provide a mobile platform for observing reentry of the Ariane 5 Main Cryogenic Stage (EPC). This system was flown successfully in performing measurements of reentry of the EPC from the Ariane 504 launch occurring on December 10, 1999. The primary requirement was to obtain the altitude of breakup of the EPC. Secondary requirements were to characterize the breakup, including searching for debris that might survive reentry, and to provide data on the reentry trajectory. Breakup was observed to begin somewhat above 70 km altitude, with evidence that fragmentation of the main structure continued over an 8 second interval, making the process somewhat higher in altitude than observed for Ariane 503. Multipath interference was observed in the long pulse transmission mode, and these data were used to obtain a direct measurement of EPC altitude as a function of time. This paper discusses the underlying concepts that contributed to planning the data acquisition and presents results of those measurements. This paper also discusses the data and data processing, leading up to a discussion of the results and lessons learned during the data reduction and analysis process.

Differential toroidal resonator transducer of absolute pressure

The microwave-resonator transducer is a high-Q oscillatory circuit for measurement of high aircraft altitudes. Resonator-based transducers have high sensitivity. A symmetrical system ensures high thermal, vibration, and shock stability. Hysteresis is eliminated due to insignificant deformation of the elastic system.

Improvement of walking speed prediction by accelerometry and altimetry, validated by satellite positioning

Activity monitors based on accelerometry are used to predict the speed and energy cost of walking at 0% slope, but not at other inclinations. Parallel measurements of body accelerations and altitude variation were studied to determine whether walking speed prediction could be improved. Fourteen subjects walked twice along a 1.3 km circuit with substantial slope variations (-17% to +17%). The parameters recorded were body acceleration using a uni-axial accelerometer, altitude variation using differential barometry, and walking speed using satellite positioning (DGPS). Linear regressions were calculated between acceleration and walking speed, and between acceleration/altitude and walking speed. These predictive models, calculated using the data from the first circuit run, were used to predict speed during the second circuit. Finally the predicted velocity was compared with the measured one. The result was that acceleration alone failed to predict speed (mean r = 0.4). Adding altitude variation improved the prediction (mean r = 0.7). With regard to the altitude/acceleration-speed relationship, substantial inter-individual variation was found. It is concluded that accelerometry, combined with altitude measurement, can assess position variations of humans provided inter-individual variation is taken into account. It is also confirmed that DGPS can be used for outdoor walking speed measurements, opening up new perspectives in the field of biomechanics.

Aerosol particle chemical characteristics measured from aircraft in the lower troposphere during ACE-2

During the Aerosol Characterization Experiment (ACE-2), filter samples were collected aboard the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Pelican aircraft near Tenerife in June and July of 1997. The flights included constant altitude measurements in the boundary layer as well as profiles up to 3800 m providing detailed chemical information about the composition of the aerosol distribution in the lower troposphere. Three cases with different air mass origins—clean marine air, anthropogenically-influenced air from the European continent, and dust-laden air from the Sahara—were identified. The samples were analyzed by ion chromatography (IC) for ionic species, by combined thermal and optical analysis (TOA) for organic carbon, and by total reflection X-ray fluorescence (TXRF) for elemental composition. Particle composition and size distributions for the range of air masses encountered illustrate links in the chemical and microphysical characteristics of aerosol from different sources. Clean marine air masses were characterized by low particle number and mass concentrations with no detectable metals, while anthropogenically-influenced and dust-laden air had high number, mass, and trace metal concentrations. Anthropogenic sources were characterized by high concentrations of submicron particles and some Fe and Cu, whereas dust particle loadings included a significant mass of micron-sized particles and significant loadings of Fe, in addition to small amounts of Mn, Cu, and Ni. These results showed similar tracers for air mass origin as those found in other measurements of oceanic and continental air masses. Aerosol optical properties were estimated with a simplified model of the aerosol based on the measured compositions. The real and imaginary refractive indices and single scattering albedos differed significantly among the three types of aerosol measured, with clean marine aerosol properties showing the least absorption and dust-containing aerosols showing the most. There were only small differences in optical properties for the two different cases of clean marine aerosol, but some significant differences between the two dust cases. Since measurement uncertainties affect these calculations, we studied the type of mixing and the fraction of absorbing species and found the calculation was sensitive to these variations only for the dust-containing aerosol case, probably due to the small amount of water present. While the optical properties varied little with composition for clean marine and anthropogenically-influenced cases, they showed a strong dependence on variations in particle composition and mixing state for the dust-containing cases.

Observation of fractal scattering from ground surfaces using millimeter wave radar altimeter

A millimeter-wave altimeter using an FM-CW system in the radio navigation frequency band is installed on a helicopter. The estimation results and their application are described for a surface coating condition based on the measurement of the altitude and the scattering from the ground surface near the angle of normal incidence. It is shown that the spectral spreading of the beat frequency of the received signal, the fractal dimensions of the scattering coefficient with respect to the flight direction in a straight flight, and the variation of the surface roughness are millimeter-wave characteristic quantities that can classify the surface cover conditions. Also, it is shown that there is a positive correlation in the fractal dimensions of both. By estimating the surface cover conditions in real time during flight and reducing the radiation power to the minimum suited to the surface scattering characteristics, these findings can be applied to prevent interruption of altitude measurement by suppressing the variations of the received level. Also, a procedure enabling calculation of local fractal dimension even when no sufficient data from the identical ground coating can be acquired or when white Gaussian noise is mixed with fractal normal noise is proposed. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 83(12): 25–36, 2000

Results from the 1995 Stratospheric Ozone Profile Intercomparison at Mauna Loa

In August 1995, multiple instruments that measure the stratospheric ozone vertical distribution were intercompared at the Mauna Loa Observatory, Hawaii, under the auspices of the Network for the Detection of Stratospheric Change. The instruments included two UV lidar systems, one from the Jet Propulsion Laboratory and the other from Goddard Space Flight Center, electrochemical concentration cell balloon sondes, a ground‐based microwave instrument, Dobson‐based Umkehr measurements, and a new ground‐based Fourier transform infrared instrument. The Microwave Limb Sounder instrument on the Upper Atmosphere Research Satellite provided correlative profiles of ozone, and there was one close overpass of the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument. The results show that much better consistency among instruments is being achieved than even a few years ago, usually to within the instrument uncertainties. The different measurement techniques in this comparison agree to within ±10% at almost all altitudes, and in the 20–45 km region most agreed within ±5%. The results show that the current generation of lidars is capable of accurate measurement of the ozone profile to a maximum altitude of 50 km. SAGE II agreed well with both lidar and balloon sonde down to at least 17 km. The ground‐based microwave measurement agreed with other measurements from 22 km to above 50 km. One minor source of disagreement continues to be the pressure‐altitude conversion needed to compare a measurement of ozone density versus altitude with a measurement of ozone mixing ratio versus pressure.

Annual mass balance of blue glacier, usa: 1955–97

Mass changes of Blue Glacier, USA are calculated from topographic maps made from vertical aerial photography in late summer of 1939, 1952, 1957, and 1987, along with laser altimetry flown in June 1996. Changes in elevation between maps were adjusted for seasonal variations in the snow cover, and to account for the ablation between the date of photography and 1 October. Topography obtained from the laser altimetry was adjusted for snow thickness and glacier motion to estimate topography of 1 October 1995. The mass of Blue Glacier has changed less than 7 m (water equivalent) during this 56 year period which is minor compared with other glaciers in the region and elsewhere in the world. Glacier‐average annual mass balances, beginning in 1956, have been calculated either from stake measurements and probing of late‐season snow, or from a regression analysis using late‐season measurements of the equilibrium line altitude. A comparison with the changes derived from surface maps shows values obtained from field measurements are too positive by about 0.4 m a−1 , indicating that considerable caution is needed when interpreting time series of mass balance. Two alternative time series of mass balance consistent with the long‐term mass changes are created by making simple adjustments: (1) a single constant is subtracted from each value so that the series is consistent with the 1957–95 mass change; (2) one constant is subtracted from each value over 1957–87 and another is subtracted from each value over 1987‐95 so that the series is consistent with both the 1957–87 and 1987–95 mass changes.The mass balance of Blue Glacier was generally positive until the mid‐1970s and negative since. The fluctuations of mass balance closely resemble those of snowfall on the glacier as estimated from the joint distribution of temperature and precipitation. The climate in western Washington was cooler and wetter during the decade before the mid‐1970s, but the trend since has been towards warmer and drier conditions.

A new technique for processing airborne gamma ray spectrometry data for mapping low level contaminations

A new technique for processing airborne gamma ray spectrometry data has been developed. It is based on the noise adjusted singular value decomposition method introduced by Hovgaard in 1997. The new technique opens for mapping of very low contamination levels. It is tested with data from Latvia where the remaining contamination from the 1986 Chernobyl accident together with fallout from the atmospheric nuclear weapon tests includes 137Cs at levels often well below 1 kBq/m 2 equivalent surface contamination. The limiting factors for obtaining reliable results are radon in the air, spectrum stability and accurate altitude measurements.