GPS network monitors the Western Alps' deformation over a five-year period: 1993–1998

  The Western Alps are among the best studied collisional belts with both detailed structural mapping and also crustal geophysical investigations such as the ECORS and EGT seismic profile. By contrast, the present-day kinematics of the belt is still largely unknown due to small relative motions and the insufficient accuracy of the triangulation data. As a consequence, several tectonic problems still remain to be solved, such as the amount of N–S convergence in the Occidental Alps, the repartition of the deformation between the Alpine tectonic units, and the relation between deformation and rotation across the Alpine arc. In order to address these problems, the GPS ALPES group, made up of French, Swiss and Italian research organizations, has achieved the first large-scale GPS surveys of the Western Alps. More than 60 sites were surveyed in 1993 and 1998 with a minimum observation of 3 days at each site. GPS data processing has been done by three independent teams using different software. The different solutions have horizontal repeatabilities (N–E) of 4–7 mm in 1993 and 2–3 mm in 1998 and compare at the 3–5-mm level in position and 2-mm/yr level in velocity. A comparison of 1993 and 1998 coordinates shows that residual velocities of the GPS marks are generally smaller than 2 mm/yr, precluding a detailed tectonic interpretation of the differential motions. However, these data seem to suggest that the N–S compression of the Western Alps is quite mild (less than 2 mm/yr) compared to the global convergence between the African and Eurasian plate (6 mm/yr). This implies that the shortening must be accomodated elsewhere by the deformation of the Maghrebids and/or by rotations of Mediterranean microplates. Also, E–W velocity components analysis supports the idea that E–W extension exists, as already suggested by recent structural and seismotectonic data interpretation. Received: 27 November 2000 / Accepted: 17 September 2001     

LightSquared effects on estimated C/N 0, pseudoranges and positions

We present experimental results showing the impact of the proposed LightSquared (LS) Long-term Evolution (LTE) signals on both GPS and Galileo civil modulations in the L1/E1 band. The experiments were conducted in radiated mode in a large anechoic chamber. Three Galileo enabled receivers were chosen for the tests, and a state of the art GNSS signal generator was used to simulate both GPS and Galileo signals. The LTE signals were generated by an Agilent Programmable Signal Generator with a license to generate the signals according to the 3GPP LTE FDD standard. The interference impact was measured in terms of a Carrier-to-Noise power spectral density ratio (C/N ₀) degradation, in accordance with the methodology which the LS/GPS Technical Working Group (TWG) established by mandate of the FCC. A model for determining the impact of the LS signal on the considered GNSS signals is provided and is validated against experimental data. It is shown that the Galileo E1 Open Service (OS) signal is marginally more susceptible to this form of interference than the GPS L1 C/A signal due to its greater proximity to the lower edge of the L1 band. The impact of LS interference was further analyzed in terms of pseudorange and position errors. Despite its relevance for most GNSS users, this aspect was not considered by the TWG. Measurement and position domain analysis along with the study of the LS impact on the Galileo OS signals are the major contributions. The analysis confirms the results obtained by the TWG and shows that the receiver front-end plays a major role in protecting GNSS signals against RF interference. While it appears that, for now, the LS network will not be deployed, the approach taken and the results obtained herein can be readily adapted for any future terrestrial mobile network that may take the place of LS.     

Use of the L2C signal for inversions of GPS radio occultation data in the neutral atmosphere

Results from processing FORMOSAT-3/COSMIC radio occultations (RO) with the new GPS L2C signal acquired both in phase locked loop (PLL) and open loop (OL) modes are presented. Analysis of L2P, L2C, and L1CA signals acquired in PLL mode shows that in the presence of strong ionospheric scintillation not only L2P tracking, but also L1CA tracking often fails, while L2C tracking is most stable. The use of L2C improves current RO processing in the neutral atmosphere mainly by increasing the number of processed occultations (due to significant reduction in the number of L2 tracking failures) and marginally by a reduction in noise in statistics. The latter is due to the combination of reduced L2C noise (compared to L2P) and increased L1CA noise in those occultations where L2P would have failed. This result suggests application of OL tracking for L1CA and L2C signals throughout an entire occultation to optimally acquire RO data. Two methods of concurrent processing of L1CA and L2C RO signals are considered. Based on testing of individual occultations, these methods allow: (1) reduction in uncertainty of bending angles retrieved by wave optics in the lower troposphere and (2) reduction in small-scale residual errors of the ionospheric correction in the stratosphere.     

ANALYSIS OF THE CHEMICAL POLLUTION OF BUFFER ZONES BY MODELING AND SUB-SATELLITE EXPERIMENTS

Principles are outlined for the use of sub-satellite soil, air, water, and vegetation samples as an auxiliary source of information in remote sensing studies of vegetation stress caused by air pollution. More specifically, procedures for satellite-synchronous aerial overflights, vegetation transects, water quality measurements, and soil tests are described as a basis for delineating boundaries between areas of vegetation stress and unpolluted areas in an industrialized area in the central Urals. Translated from: Kosmicheskiy monitoring biosfery, Yu. A. Izrael', V. V. Bugrovskiy, and Yu. V. Novikov, eds. Leningrad: Gidrometeoizdat, 1985, pp. 93-99.     

COMPUTATION AND MAPPING OF THE ELEMENTS OF THE WATER-ICE BUDGET OF GLACIERS

Two general approaches for compiling maps of components of the water-ice budget in regions of glacier nourishment are compared and the resulting maps are analyzed. The compilation of raw data is based on field measurements rather than remote sensing information. Translated by Edward Torrey, Alexandria, VA 22308 from: Materialy glyatsiologicheskikh issledovaniy, 1991, No. 72, pp. 94-102.     

A Microwave Tomographic Approach for Nondestructive Testing of Dielectric Coated Metallic Surfaces

A microwave imaging method for nondestructive testing of perfectly conducting surfaces beyond a layered media is presented. The method is an adaptation of the surface reconstruction approach by Yapar et al. to the present problem. It is based on the analytical continuation of the measured data to the surface under test through a special representation of the scattered field in terms of Fourier transform and Taylor expansion. Then the problem is reduced to the solution of a nonlinear equation which is solved iteratively via the Newton method and regularization in the least squares sense. Numerical simulations show that defects as small as lambda/500 can be recovered through the presented algorithm.     

Spin rate estimation of sounding rockets using GPS wind-up

Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna. In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas. Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR.     

GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea

The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward electric field, and are not associated with the increase of the O/N₂ ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically coupled upward to generate ionospheric perturbations and oscillations.     

VLBI-derived troposphere parameters during CONT08

Time-series of zenith wet and total troposphere delays as well as north and east gradients are compared, and zenith total delays (ZTD) are combined on the level of parameter estimates. Input data sets are provided by ten Analysis Centers (ACs) of the International VLBI Service for Geodesy and Astrometry (IVS) for the CONT08 campaign (12?C26 August 2008). The inconsistent usage of meteorological data and models, such as mapping functions, causes systematics among the ACs, and differing parameterizations and constraints add noise to the troposphere parameter estimates. The empirical standard deviation of ZTD among the ACs with regard to an unweighted mean is 4.6?mm. The ratio of the analysis noise to the observation noise assessed by the operator/software impact (OSI) model is about 2.5. These and other effects have to be accounted for to improve the intra-technique combination of VLBI-derived troposphere parameters. While the largest systematics caused by inconsistent usage of meteorological data can be avoided and the application of different mapping functions can be considered by applying empirical corrections, the noise has to be modeled in the stochastic model of intra-technique combination. The application of different stochastic models shows no significant effects on the combined parameters but results in different mean formal errors: the mean formal errors of the combined ZTD are 2.3?mm (unweighted), 4.4?mm (diagonal), 8.6?mm [variance component (VC) estimation], and 8.6?mm (operator/software impact, OSI). On the one hand, the OSI model, i.e. the inclusion of off-diagonal elements in the cofactor-matrix, considers the reapplication of observations yielding a factor of about two for mean formal errors as compared to the diagonal approach. On the other hand, the combination based on VC estimation shows large differences among the VCs and exhibits a comparable scaling of formal errors. Thus, for the combination of troposphere parameters a combination of the two extensions of the stochastic model is recommended.     

Predicting plant water content in Eucalyptus grandis forest stands in KwaZulu-Natal,South Africa using field spectra resampled to the Sumbandila Satellite Sensor

The measurement of plant water content is essential to assess stress and disturbance in forest plantations. Traditional techniques to assess plant water content are costly, time consuming and spatially restrictive. Remote sensing techniques offer the alternative of a non-destructive and instantaneous method of assessing plant water content over large spatial scales where ground measurements would be impossible on a regular basis. In the context of South Africa, due to the cost and availability of imagery, studies focusing on the estimation of plant water content using remote sensing data have been limited. With the scheduled launch of the South African satellite SumbandilaSat evident in 2009, it is imperative to test the utility of this satellite in estimating plant water content. This study resamples field spectral data measured from a field spectrometer to the band settings of the SumbandilaSat in order to test its potential in estimating plant water content in a Eucalyptus plantation. The resampled SumbandilaSat wavebands were input into a neural network due to its ability to model non-linearity in a dataset and its inherent ability to perform better than conventional linear models. The integrated approach involving neural networks and the resampled field spectral data successfully predicted plant water content with a correlation coefficient of 0.74 and a root mean square error (RMSE) of 1.41% on an independent test dataset outperforming the traditional multiple regression method of estimation. The best-trained neural network algorithm that was chosen for assessing the relationship between plant water content and the SumbandilaSat bands was based on a few points only and more research is required to test the robustness and effectiveness of this sensor in estimating plant water content across different species and seasons. This is critical for monitoring plantation health in South Africa using a cheaply available local sensor containing key vegetation wavelengths.