Precise GPS Positioning by Applying Ionospheric Corrections from an Active Control Network

In this article, initial results are presented of a method to improve fast carrier phase ambiguity resolution over longer baselines (with lengths up to about 200 km). The ionospheric delays in the global positioning system (GPS) data of these long baselines mainly hamper successful integer ambiguity resolution, a prerequisite to obtain precise positions within very short observation time spans. A way to correct the data for significant ionospheric effects is to have a GPS user operate within an active or permanently operating network use ionospheric estimates from this network. A simple way to do so is to interpolate these ionospheric estimates based on the expected spatial behaviour of the ionospheric delays. In this article such a technique is demonstrated for the Dutch Active Control Network (AGRS.NL). One hour of data is used from 4 of the 5 reference stations to obtain very precise ionospheric corrections after fixing of the integer ambiguities within this network. This is no problem because of the relatively long observation time span and known positions of the stations of the AGRS.NL. Next these interpolated corrections are used to correct the GPS data from the fifth station for its ionospheric effects. Initial conclusions about the performance of this technique are drawn in terms of improvement of integer ambiguity resolution for this baseline. ? 1999 John Wiley & Sons, Inc.     

Highly interstratified clay minerals in salt affected soils

Minerals consisting of a highly random interstratificasion of montmorillonite and montmorillonite interlayered with silicic acid-Al(OH)₃ polymers were found to occur in the separate of the surface of a solontchak-like solonetz soil in southern Bulgaria.The minerals were formed by interlayering of the above components in the original montmorillonite parent material because of the alkaline conditions promoting strong destruction and the intermittent extremely dry and wet periods prevailing in that region.     

A regular talc-saponite mixed-layer mineral from Ferriere,Nure Valley (Piacenza Province,Italy)

A regular mixed-layer talc-saponite mineral, contaminated with a certain amount of orthochrysotile, was identified by several methods in the weathering products of a serpentinite rock from Ferriere, Nure Valley (Piacenza Province, northern Italy) and belonging to the ophiolitic formation of the Italian Apennines. The structural formula of the mineral is: [Mg_2.80 Fe _0.09 ²⁺ Fe _0.20 ³⁺ ] [Fe _0.07 ³⁺ Al_0.45 Si_3.48] O₁₀ (OH)₂ Mg_0.006 Ca_0.02 Na_0.01 Electron microscopy enables the chrysotile fibres to be distinguished from the platy particles of the talc-saponite mineral. The infra-red method proved very suitable for identifying the talc and saponite constituents of the mixed-layer mineral. The thermal analyses (d.t.a. and d.t.g.) are valuable in revealing the mixed-layer character of the mineral.     

Marine muds along the Guyana coast and their origin from the Amazon basin

The mineralogical composition of clay samples from the Amazon basin and the Guyana coast, including samples collected inland and on the continental shelf, was investigated by X-ray diffraction. Also humus content, specific surface and degree of potassium fixation were determined. The muds from the Guyana coast have virtually the same composition as those of the Amazon but are very different from the Guyanese soils and river muds. This suggests that most Guyanese coastal muds come from the Amazon which is confirmed by a consideration of the quantities involved. The only measurable effect of the sea water on the mud particles moving from the Amazon to Venezuela, which takes from one month up to 1000 years, is a decrease in potassium fixation, indicating that some potassium has been taken up from the sea water. The presence of large amounts of soil chlorite in shelf samples may be due to a prolonged contact with sea water but most probably is the result of subaereal conditions during the Pleistocene.     

InSAR datum connection using GNSS-augmented radar transponders

Deformation estimates from Interferometric Synthetic Aperture Radar (InSAR) are relative: they form a ‘free’ network referred to an arbitrary datum, e.g. by assuming a reference point in the image to be stable. However, some applications require ‘absolute’ InSAR estimates, i.e. expressed in a well-defined terrestrial reference frame, e.g. to compare InSAR results with those of other techniques. We propose a methodology based on collocated InSAR and Global Navigation Satellite System (GNSS) measurements, achieved by rigidly attaching phase-stable millimetre-precision compact active radar transponders to GNSS antennas. We demonstrate this concept through a simulated example and practical case studies in the Netherlands.