Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination

Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved from about 10 to 6 mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO positions.     

Effect of an exotic prey on the feeding pattern of a predatory snail

The mussel Brachidontes pharaonis, which invaded the Mediterranean from the Red Sea about 120 years ago, has recently become abundant in many midlittoral and some infralittoral rocky habitats along the Israeli rocky shore. We investigated the influence of B. pharaonis, as a novel prey, on the foraging patterns of the large whelk Stramonita haemastoma in the field, and examined food preferences in the laboratory. S. haemastoma has shifted from indigenous species to feeding on the novel mussel when abundant. The whelk prefers to prey upon the invasive mussel over all indigenous species offered (e.g. barnacles and mussels), probably due to its larger size. In the midlittoral zone, the foraging activity of S. haemastoma is considerably low even where refuges are readily available (incisioned-rocks) and food density is high (mainly B. pharaonis). Higher proportions of whelks are actively foraging in the infralittoral zone but usually on smaller prey, mostly barnacles. We suggest that this differential foraging activity in the two zones is related to the degree of exposure to wave action. The midlittoral is inherently more exposed to wave action than the infralittoral, where sea conditions are more benign and the whelks may enjoy longer activity periods.     

Epimenia arabica spec, nov., a Solenogaster (Mollusca) Feeding on the Alcyonacean Scleronephthya corymbosa (Cnidaria) from Shallow Waters of the Red Sea

Abstract. The body organization and some biological data of Epimenia arabica , a new species of Sotenogastres of the order Cavibelonia , are presented. The species is described based on four large-sized specimens (13–21 cm × 0.7-1 cm) from three different localities off the coast of the Sinai Peninsula (Red Sea). It feeds on Scleronephthya corymbosa V erseveldt & C ohen ( Octocorallia: Alcyonacea ), generally at depths of 2–5 meters.     

Theoretical amplitudes of body waves from a dislocation source in the Earth II. Core phases

Expressions are obtained for the ray-theoretical spectral amplitudes of body waves induced by a shear dislocation of arbitrary orientation and depth situated in a radially heterogeneous model of the earth. Account is taken of the azimuthal and colatitudinal radiation patterns of the source, the geometrical spreading, and the reflections and refractions at the free surface and at the mantle-core boundary.In this work spectral amplitudes are calculated for PKP, PKS, SKP and SKS. The results are presented in the form of tables for a source of strength U₀dS = 10¹⁵ cm³, where U₀ is the amount of the dislocation and dS is the fault area. Given the slip and dip angles of the source, the amplitudes of the four core phases can be obtained from these tables for all azimuths, for most of the epicentral distances at which a particular phase is observable, and for all the fourteen focal depths included in the Jeffreys-Bullen tables. It is found that the depth of the source has a strong effect on the amplitudes of the body-wave signals.     

Seismic observations at Lake Emosson,Switzerland

Since 1973 several seismic stations have been brought into operation at Lake Emosson, the last large conventional hydrodam in the Swiss Alps. The reservoir is situated in a seismically active zone which stretches from the Rhone valley in southwestern Switzerland into Haute Savoie, France. The monitoring program was started before the first impounding and continued during four full load cycles. A certain correlation is indicated between water level changes and the occurrence near the reservoir of weak seismic events (M_L<0) with high frequency content. A transient series of 30 tremors occurred in August 1974, during a period of rising water level. Local events of a different type frequently occur during the periods of decreasing water level. Their daily distribution indicates a mechanism which is caused by temperature variations.Contribution No. 131 of the Geophysical Institute, Karlsruhe University. Sonderforschungsbereich Felsmechanik of Karlsruhe University. Contribution No. 299 of the Institute for Geophysics, ETH Zürich.     

A simple failure criterion for rough joints and compound shear surfaces

The subject under investigation is the strength of a single shear plane which exhibits a regular, asymmetric roughness pattern. In the shear direction the asperities are so steeply inclined that the joint becomes mechanically non-effective with the result that the asperities are sheared off. Against the shear direction the asperities are only gently inclined. It is shown that this particular roughness pattern is of some general importance in geomechanics (examples: unconfined compression test; shear plane with secondary fractures).

Simple analytical considerations allow the formulation of a shear criterion, which is dependent on friction angle ø_m and cohesionc_m of the intact rock and on the inclination γ of the gently inclined parts of the asperities which are dipping against the shear direction. In the Mohr-diagram the criterion results in envelopes which converge at high normal stresses against the envelope of intact rock. Furthermore, the criterion expresses that both the slope of the envelopes and the dilation angle continuously decrease with increasing normal stress. Therefore the criterion adequately describes features which are regarded as most important when shearing rough joints or compound shear surfaces.     

Theoretical amplitudes of body waves from a dislocation source in the earth III. P,S and surface reflections

Expressions are obtained for the ray-theoretical spectral amplitudes of body waves induced by a shear dislocation of arbitrary orientation and depth situated in a radially heterogeneous model of the earth. Account is taken of the azimuthal and colatitudinal radiation patterns of the source, the geometrical spreading, and the reflections and refractions at the free surface and at the mantle-core boundary.In this work spectral amplitudes are calculated for P, SH, SV, PP, PPP, PS, PSS, SP, SPP, SSH, SSSH, SSV, and SSSV. The results are presented in the form of tables for a source of strength U₀dS = 10¹⁵cm³, where U₀ is the amount of the dislocation and dS is the fault area. Given the slip and dip angles of the source, the amplitudes of these surface reflections and direct phases can be obtained from these tables for all azimuths, for most of the epicentral distances at which a particular phase is observable, and for all the fourteen focal depths included in the Jeffreys-Bullen tables. It is found that the depth of the source has a strong effect on the amplitudes of the body wave signals.