Melting temperature measurements of six minerals (stishovite (SiO2), corundum(Al2O3), diopside (CaMgSi2O6), and three perovskites (MgSiO3, CaSiO3, Mg3Al2Si3O12)) at high pressures were carried out in a YAG laser-heated diamond anvil cell with rhenium meta

Measurement of melting temperatures of some minerals under lower mantle pressures

SUBDUCTED slabs are less dense than the surrounding mantle near the base of the transition zone (~660 km depth) because of the survival of garnet in former basaltic crust: by this depth mantle peridotite has transformed to denser perovskitite1'2. The buoyancy of the former basaltic crust may contribute to the observed accu-mulation or horizontal displacement of many slabs at the base of the transition zone3. Here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology, stable in the lower mantle. Synthetic mid-ocean-ridge basalt (MORE) glass subjected to pressures of 45, 80 and 100 GPa in a laser-heated diamond anvil cell transforms to an assemblage of aluminous Mg,Fe silicate perovskite, non-quenchable CaSiO3 perovskite, stishovite and a sodic, aluminous phase with the Ca-ferrite structure (Fig. 1). Per-ovskititic MORE is about 0.06 g cm-3 more dense than a model lower mantle (PREM) derived from seismological data. Thus even thermally equilibrated perovskititic slabs should encounter no sig-nificant hindrance to subduction and convection in the lower mantle.

Mineral chemistry and density of subducted basaltic crust at lower-mantle pressures

The main challenge of dual-frequency precise point positioning (PPP) is that it requires about 30 min to obtain centimeter-level accuracy or to succeed in the first ambiguity-fixing. Currently, PPP is generally conducted with GPS only using the ionosphere-free combination. We adopt a single-differenced (SD) between-satellite PPP model to combine the GPS and GLONASS raw dual-frequency carrier phase measurements, in which the GPS satellite with the highest elevation is selected as the reference satellite to form the SD between-satellite measurements. We use a 7-day data set from 178 IGS stations to investigate the contribution of GLONASS observations to both ambiguity-float and ambiguity-fixed SD PPP solutions, in both kinematic and static modes. In ambiguity-fixed PPP, we only attempt to fix GPS integer ambiguities, leaving GLONASS ambiguities as float values. Numerous experimental results show that PPP with GLONASS and GPS requires much less convergence time than that of PPP with GPS alone. For ambiguity-float PPP, the average convergence time can be reduced by 45.9 % from 22.9 to 12.4 min in static mode and by 57.9 % from 40.6 to 17.7 min in kinematic mode, respectively. For ambiguity-fixed PPP, the average time to the first-fixed solution can be reduced by 27.4 % from 21.6 to 15.7 min in static mode and by 42.0 % from 34.4 to 20.0 min in kinematic mode, respectively. Experimental results also show that the less the GPS satellites are used in float PPP, the more significant is the reduction in convergence time when adding GLONASS observations. In addition, on average, more than 4 GLONASS satellites can be observed for most 2-h observation sessions. Nearly, the same improvement in convergence time reduction is achieved for those observations.

https://www.tpbin.com/Uploads/Subjects/3ca65e13-65c7-479a-8bbc-79d961952948.pdf

Integrating GPS and GLONASS to accelerate convergence and initialization times of precise point positioning

In spaceborne synthetic aperture radar (SAR), a single-polarization on-transmit offers twice the swath width compared to full polarization. This is linked to SAR system design issues, and, without getting into the technical details deserving by themselves a full paper, we can just mention the swath characteristics of ALOS PALSAR (the Advanced Land Observing Satellite, Phased Array L-Band Synthetic Aperture Radar), reducing from 70 km for the dual-pol mode to 30 km for the full polarization mode. The reduced coverage in the full polarization mode has a harmful impact on the revisit time, which is always a major drive for the Earth-observing community. The options chosen up to now for dual-pol system designs (or single-polarization on-transmit) rely on a linear polarization on-transmit [either horizontal (H) or vertical (V)], with two orthogonal polarizations on-receive. Souyris and Raney in earlier papers proposed more pertinent alternatives for the selection of the transmit polarization leading to a better characterization of the scattering mechanisms. In this paper, the analysis is pursued in more depth by including the effect of the ionosphere on the wave propagation and extending the applications to polarimetric interferometry SAR (PolInSAR). A compact mode is developed where the transmit polarization is circular, whereas the only constraint on the two receiving polarizations is independence. Indeed, the choice of the polarizations of the two receive channels does not matter, as any polarization on-receive can be synthesized from these two measurements. This is, however, not the case for the unique transmit polarization. At a low frequency, where the ionosphere has a significant effect, the circular transmit polarization is the only sensible option, as it provides an effective constant polarization as seen by the scattering surface. This is an essential condition for a meaningful multitemporal analysis. Both the polarimetric SAR applications and the PolInSAR applications in the context of this compact polarimetry (CP) mode are explored. A pseudocovariance matrix can be reconstructed following Souyris' proposed approach for distributed targets and is shown to be very similar to the full polarimetric (FP) covariance matrix. The reconstruction of the cross-polarized Sigma0 is shown to be reliable and to have very low sensitivity to Faraday rotation. A PolInSAR vegetation height inversion for P-band is presented and applied to the CP data with a level of performance that is similar to the one derived from FP (a 1.2-m root-mean-square height error on the ONERA Airborne radar (RAMSES) data over the Landes Forest). A procedure is developed to correct for the ionospheric effects for the PolInSAR acquisition in the FP or CP mode and is assessed on the data simulated from an airborne acquisition. The results demonstrate that the technique is efficient and robust. The calibration of CP data is identified as an important challenge to be solved, and some clues are provided to address the problem.

The Compact Polarimetry Alternative for Spaceborne SAR at Low Frequency

An analysis on the optimal combination of geoid, orthometric and ellipsoidal height data

This text is an updated English translation of Satellitengeodasie, a book that was published in German in 1989. The text is the first in many years that attempts to cover the broad spectrum of methods, applications, and systems, both classical and current, that have developed in the field of satellite geodesy.

The material is presented in a structure that follows the major observational methods used in satellite geodesy: classical techniques, Doppler, GPS, laser, altimetry, and special methods including satellite-to-satellite tracking, satellite radiometry, and VLBI. Before introducing these observational techniques in detail, the author provides the fundamentals on reference frames, time signal propagation, orbital mechanics and basic applications of satellite geodesy in positioning, gravity field modeling, navigation, marine geodesy, kinematics, and geodynamics. An excellent reference list completes the text.

Satellite Geodesy—Foundations, Methods, and Applications

Richard J. Anderle died on January 28, 1994, at age 68, while hospitalized near his home in Valley Forge, Penn. Anderle became an AGU member in 1965, was elected a Fellow in 1981, served as President of the Geodesy Section from 1980 to 1982, and was a Life Supporting Member. He also devoted much of his personal time to supporting the International Association of Geodesy, where he served on many scientific advisory committees and held several offices. He served as secretary of IAG's space techniques section from 1979 to 1983 and as president of the Section on Advanced Space Technology from 1983 to 1987. Dick was also active within the International Astronomical Union and the Committee on Space Research.

Richard J. Anderle (1926–1994)

The evaluation of a geopotential model can be accomplished using several procedures which have been described in the literature Standard methods include the estimation of precise satellite ephemerides and the recovery of the surface anomaly field and altimetric geoid This paper provides some preliminary results using the first procedure in which satellite ephemerides are estimated from an extensive collection of Doppler observations collected by a worldwide tracking network The motivation behind this work was to develop comparisons of performance between the World Geodetic System 1984 (WGS 84) geopotential model developed several years ago and more recent gravity models based on increased sources of satellite data and improved collections of surface anomaly data

Patrick Fell

Papers

Satellite Geodesy—Foundations, Methods, and Applications

Richard J. Anderle (1926–1994)

Performance of Recent Gravity Field Models in Precision Orbit Determination Using Doppler Observations