Tropospheric Refraction Research Articles

Use of Radar Inferometry for Study of Dynamics of the Earth's Surface and Troposphere

The results of interferometric processing of survey data for the northern shores of the Caspian Sea by synthetic aperture radar (SAR) are presented. The survey was made in May 1996 from near-earth orbits of the ERS-1 and ERS-2 satellites, respectively, on successive days in the C range with vertical polarization. A map of the coherence of the radar images was compiled and an interferogram was obtained. The types of surface cover classified and the changes occurring during the time between the surveys are described, including shifts of the scattering surface and changes in phase path length of radio waves attributable to fluctuations of the tropospheric refraction coefficient.

The Troposphere Refraction Estimation by Attenuation Factor of Radiowave beyond-the-Horizon Propagation

The Troposphere Refraction Estimation by Attenuation Factor of Radiowave beyond-the-Horizon Propagation

3‐D refractivity field from GPS double difference tomography

GPS meteorology can be successfully used to model the tropospheric refraction effect on radiowave signals. We describe an approach to estimate and model the spatial distribution of the tropospheric water vapor. We refine the tomographic software package AWATOS which is based on the assimilation of GPS double difference observations. The observations are allocated to a voxel model, which is defined according to the distribution of the GPS stations. Performing a least‐squares adjustment, the refractivity of each voxel is determined. Tests of the software are performed, based on simulated and real data. The latter were obtained during a dedicated field campaign on the Big Island of Hawaii. The tomographic profiles are compared with radiosonde data. The results obtained for continuous atmospheric conditions fit well, whereas larger differences occur in discontinuous atmospheric weather conditions. Overall, an accuracy of 5–20 ppm (refractivity units) has been achieved.

Comparison of tropospheric gradients determined by VLBI and GPS

The estimation of geodetic parameters from GPS and VLBI requires the modeling of tropospheric refraction in order to account for the path delays of radio signals being emitted by extragalactic radio sources and satellites, respectively. Precise models consider the deviation from the azimuthal symmetry by introducing horizontal tropospheric gradients. While estimation of gradients had been included in the GPS software package GIPSY several years ago it was implemented in the BERNESE GPS software package (version 4.2) and in the VLBI software OCCAM (modified version 4.0) quite recently. We present a comparison of tropospheric gradient estimates derived with the three software packages mentioned above. The parameters have been estimated for the fundamental station Wettzell (Germany) for four VLBI experiments in the first half of 1999 (Euro48, Euro50, Iris 136, Iris 139) and for simultaneous GPS observations. The gradients obtained by the different software packages show a similar behaviour although offsets between the solutions can be as large as two millimeters. Astonishingly there is no better agreement between the two gradient estimates derived by GPS (BERNESE compared with GIPSY) than between gradients from the different techniques (OCCAM compared with GIPSY or BERNESE). This is probably due to the application of different estimation strategies by the GPS solutions.

GPS measurements of present-day uplift in the Southern Tibet

Wuhan Technical University Surveying and Mapping (WTUSM) carried out three GPS campaigns in the Tibetan Plateau since 1993. To improve the vertical component precision of GPS measurement, we estimated the remaining tropospheric refraction using the piecewise linear method after the SAASTAMOINEN model and corrected the tropospheric refraction by selecting one parameter for every two hours model to consider the variationof tropospheric refraction. Using the IGS combined ephemeris and corresponding EOP Series for 1995 and 1997 GPS campaigns, all baseline vectors were processed by GAMIT (Ver. 9.71) software, and network adjustment was carried out in ITRF97 frame by PowerAdj software (Ver. 3.3) which was developed by WTUSM considering the baseline vectors’ full covariance matrix. The resulting vertical velocity field determined from GPS campaigns in 1995 and 1997 shows that the average uplift rate in ITRF 97 frame is about 7.8 ± 7.6 mm/yr for all 14 repeated surveying stations in Tibet, and the average uplift rate is 16.2 ± 7.9 mm/yr in southern Tibet, which are compared with the results from the gravity measurements, leveling data and fission-track dating (FT).

Influence of the ocean loading effect on GPS derived precipitable water vapor

For meteorology the precipitable water vapor becomes a more and more important product in the data analysis of the Global Positioning System (GPS). It is derived from the wet part of the tropospheric refraction obtained as zenith path delay in the data analysis. However, the estimates of the zenith path delay and of the station height are strongly correlated. The ocean loading effect has a periodic influence on station heights with major frequencies in the diurnal and semidiurnal band and with amplitudes up to several centimeters. If one ignores this effect it affects the estimates of the zenith path delay and thus the precipitable water vapor. This will be demonstrated by some examples.

Frequency transfer using GPS carrier phases: influence of temperature variations near the receiver

We have used carrier phase data from geodetic GPS receivers, driven by atomic clocks, and belonging to the IGS network, to demonstrate the capabilities of the GPS carrier phases for frequency transfer. On a regional baseline, we obtained frequency stabilities of 2.10(-16) over 4 h through a proper modeling of the tropospheric refraction and by keeping ambient temperature variations under control.

4D tropospheric tomography using GPS slant wet delays

Abstract. Tomographic techniques are successfully applied to obtain 4D images of the tropospheric refractivity in a local dense network of global positioning system (GPS) receivers. We show here how GPS data are processed to obtain the tropospheric slant wet delays and discuss the validity of the processing. These slant wet delays are the observables in the tomographic processing. We then discuss the inverse problem in 4D tropospheric tomography making extensive use of simulations to test the system and define the resolution and the impact of noise. Finally, we use data from the Kilauea network in Hawaii for February 1, 1997, and a local 4×4×40 voxel grid on a region of 400 km2 and 15 km in height to produce the corresponding 4D wet refractivity fields, which are then validated using forecast analysis from the European Center for Medium Range Weather Forecast (ECMWF). We conclude that tomographic techniques can be used to monitor the troposphere in time and space.Key words: Radio science (remote sensing; instruments and techniques)

Studies of Super-Refractivity and Ducting of Radiowaves in Nigeria

Studies of tropospheric refraction of long and short wavelength radio signals have been carried out in the coastal and savannah areas of Nigeria using meteorological data collected by radiosondes in Lagos (30 45' E, 60 28' N) and Kano (80 30' E, 120 2'N). Daily mean values of the initial refractivity gradients, dN/dh, were computed for the two zones and the results obtained have been used to classify the refractivity profiles as sub-refractive, normal, super-refractive and ducting. The refraction of radiowaves in the lower atmosphere is considered to be normal whenever the refractivity gradient is —40N/km. Between about — 41N/km and —156.9N/km, the atmosphere is super-refractive and when the refractivity gradients become equal to, or more negative than —157N/km ducting occurs. The modified refractive index was also computed for the two zones. A negative gradient of M is a useful indicator as to the occurrence of ducting. The refractivity values obtained show that the propagation conditions are super-refractive at the coastal region, especially during the rainy season. At Oshodi, the surface layer is super-refractive 75% of the time with 38% probability of occurrence of ducting. The surface layer at Kano, on the other hand, is sub-refractive 88% of the time with the probability of occurrence of ducting being only 3.5%. Nigeria Journal of Pure and Applied Physics VOLUME 1, AUGUST 2000, pp. 5-10

Efeito da refração troposférica no posicionamento geodésico com GPS

Para se corrigir a refração troposférica, que consiste em uma das principais fontes de erro que afetam as observações do Sistema de Posicionamento Global (GPS), usam-se modelos troposféricos e funções de mapeamento que estimam o valor do atraso nos sinais GPS provocado por esta refração. Tais modelos foram desenvolvidos usando dados meteorológicos coletados predominantemente no hemisfério norte. Este artigo apresenta um estudo comparativo sobre o efeito, no posicionamento geodésico com o GPS, provocado por modelos troposféricos, mais precisamente os de Saastamoinen, Hopfield e Ifadis, combinados às funções de mapeamento de Ifadis, Hopfield, Saastamoinen, Herring e Lanyi. Dados GPS coletados pela Rede Brasileira de Monitoramento Contínuo de Sistema GPS (RBMC) foram utilizados. As estações da RBMC estão distribuídas de modo que cada uma das regiões geográficas do Brasil tenha pelo menos uma delas. Visto que também a RBMC cobre as cinco regiões climáticas do país, foram estabelecidas três linhas de base configuradas de acordo com a região climática em que se localizam, tendo cada uma dessas bases características específicas quanto ao seu comprimento, variação altimétrica, dentre outras. Os dados GPS utilizados correspondem a quatro diferentes épocas do ano de 1997, e foram tratados com o programa DIPOP, de posicionamento diferencial. A análise do resultado do processamento indica uma maior homogeneidade com a utilização das funções de mapeamento de Ifadis, Herring e Lanyi independentemente dos modelos.

New consideration of atmospheric refraction in laser ranging data

In this paper we reconsider the formulae of tropospheric refraction correction for the Satellite Laser Range technique. From the expansion of the complementary error function, a new continued fraction form of the mapping function at optical frequencies is derived. The correction terms related to the operation frequency of the laser beam are considered in both the zenith delay and the mapping function, The correction for low-elevation satellites is briefly reviewed. The theoretical accuracy of the new mapping function has been analysed via the ray tracing integrals under the standard atmospheric profile. With respect to the radiosonde data, the deviations of the new mapping function are investigated in an elevation range down to near 1 degrees, which is comparable with the results of the Marini-Murray formulae.

Modeling weather fronts to improve GPS heights: A new tool for GPS meteorology?

The precision of vertical position and atmospheric water vapor content determined by the Global Positioning System (GPS) is limited by errors due to tropospheric delay. One factor is the spatial and temporal variability in tropospheric refractivity caused by passing weather fronts. We can explain some of the temporal characteristics of estimated tropospheric delay in terms of a simple path delay model as a function of frontal parameters. These results suggest that GPS could be used to estimate the geometry and passage time of a frontal zone. We have developed indices which detect tropospheric variability from GPS data alone; the detection rate of fronts with this approach is up to 70%. Once detected, we eliminated days affected by fronts or other tropospheric variability from the time series of station height estimates, resulting in improved long‐term repeatability. The additional variance attributable to fronts is estimated to be up to 10 mm2 at Herstmonceux, England, where fronts occur every 2–3 days. The effect of fronts on the horizontal station component is up to 80% smaller than for the vertical. Studies in the field of GPS meteorology may be improved by estimating frontal parameters.

Theoretical performance limits on tropospheric refractivity estimation using point-to-point microwave measurements

Ducted propagation above the ocean surface can seriously impact shipboard radar and communications. Point-to-point microwave measurements have been proposed as a means of estimating tropospheric refractivity for the purposes of characterizing surface-based ducts. This paper addresses the theoretical performance of refractivity estimates that can be made by combining field measurements at different frequencies with prior statistics of refractivity variation. Parameterizing the refractivity profile using empirical orthogonal functions derived from a historical database, both Cramer-Rao performance bounds and the maximum a posteriori (MAP) estimate are discussed using coherent or incoherent signals. Results obtained using a realistic model of refractivity conditions off Southern California suggest that multifrequency propagation measurements can significantly improve the estimation of refractivity and propagation loss profiles.

The ancient and modern history of EM ground-wave propagation

Radio-wave transmission over the surface of the Earth is a subject of enquiry going back to the beginning of the century. In this review, an attempt is made to describe the ground-wave mechanism that is omni-present. We first call attention to the early analytical contributions of Zenneck and Sommerfeld, based on a flat-Earth model. The subsequent controversies, particularly with regard to the role of the Zenneck surface wave, are outlined. Further developments by other pioneers, such as van der Pol, Fock, Bremmer, Norton, and Millington, are reviewed, and an attempt is made to put these in a modern context. We also show that the trapped surface wave can be a significant contribution to the total ground-wave field, when the Earth boundary is sufficiently inductive. Mixed-path theory and confirming model tests by Ray King are described briefly, along with calculated propagation curves for twoand three-section paths. The bibliography includes references to related topics, such as tropospheric refraction and topographic influences.

Field strength variations of LF radio waves prior to earthquakes in central Italy

The electric field strength of the LF radio broadcasting RMC (Principality of Monaco) which operates at 216 kHz has been recorded since January 1991 by two receivers in central Italy. During the monitoring period we observed two evident attenuations of the field strength in one receiver, with durations of 6–10 days. The geomagnetic and ionospheric observations carried out in the same time interval do not seem able to explain the attenuation of the radio signal. An analysis of the seismic activity occurring in the area between transmitter and receiver has revealed that some days after the attenuations the energy released by earthquakes reaches a maximum. The observed attenuation might therefore be precursors of earthquakes. We also checked meteorological conditions and found that advections of warm air occurred during both the two anomalous periods. It seems possible that these conditions can help the action of preseismic effects in generating irregularities in the vertical gradient of the tropospheric radio refractivity able to produce defocusing of LF radiobroadcast propagation.

The role of GPS in the study of global change

The Global Positioning System (GPS) may be used today as a mature technique in geodesy and geodynamics. Thanks to the orbits, the Earth orientation parameters, and the coordinates and velocities of about 100 IGS (International GPS Service for Geodynamics) stations, which are made available to the scientific community on a daily basis, GPS is a very powerful and serious contributor to all scientific questions related to high accuracy positioning on and near the Earth's surface. In this article we first give a short characterization of the GPS (as compared to other space-geodetic techniques like VLBI and SLR). The difficulties related to GPS-determined station heights are subsequently discussed, in particular in view of the necessity to model tropospheric refraction. In studies related to Global Change as, e.g., sea level monitoring, and postglacial rebound, station heights and their development in time are of particular interest. It is known for a long time, however, that GPS-derived station heights are not of the same quality as GPS-derived horizontal positions. It is thus much more delicate to extract vertical (as opposed to horizontal) movements from GPS time series. When looking at station heights on a global scale, the motion of the Earth's crust, defined by the tracking sites, relative to the center of mass of the Earth has to be taken into consideration in order to correctly interpret the relative movement of the crust and the sea level. This “motion of the geocenter” may have a size of up to a few centimeters. Solar radiation pressure acting on the GPS satellites is difficult to model due to the complicated shape of the spacecrafts and may affect the geocenter estimates derived from GPS data. GPS may contribute, however, to monitoring the time variations of the geocenter. The influence of the troposphere on the GPS signals is not just a nuisance, it may be used as a signal, too. GPS may be used as an accurate instrument to measure the integrated water vapor above the GPS sites. Long time series of water vapor values from a dense global network may eventually reveal trends in the water vapor content of the atmosphere. Last but not least, we discuss the importance of the IGS, its global products and the densification project, for Global Change investigations.

Likelihood estimation of tropospheric duct parameters from horizontal propagation measurements

A Bayesian estimation technique using an embedded electromagnetic parabolic equation model is implemented to solve the inverse problem of determining the atmospheric refractivity from measurements of beyond‐line‐of‐sight radio‐frequency propagation factors. The inverted refractivity structure is associated with the trapping layer that can be formed by the capping inversion of the stable marine boundary layer. Why this inverse problem is of interest is established by describing the refractivity structure associated with the stable marine boundary layer in the context of its effect on horizontal propagation and then describing the difficulty of obtaining representative estimates of the tropospheric refractivity structure using existing sensing methods. The implementation is then described. A three‐parameter model of the marine boundary layer is used; two of the three parameters are inverted, while the third is assumed to be a random variable. The inversion method is applied to radio data from the Variability Of Coastal Atmospheric Refractivity Experiment (VOCAR). A matrix of propagation factors corresponding to the two parameters to be inverted is calculated for the path geometry and the frequencies of VOCAR. Parameter likelihoods are determined for the entire parameter space, and mean estimates of refractivity parameters are computed from the distributions. The parameter estimates are compared with values obtained from radiosondes launched during VOCAR. The remotely sensed refractivity parameters for multiple‐frequency sensing show good agreement with directly sensed observations.

The Generator Function Method of the Tropospheric Refraction Corrections

view Abstract Citations (22) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Generator Function Method of the Tropospheric Refraction Corrections Yan, Hoojian ; Ping, Jinsong Abstract Both the astronomical angular refraction and the refractive delay of tropospheric atmosphere can be calibrated by means of the generator function method of mapping functions. Based on the continued fraction expression of the complementary error function, the new mapping function forms of the tropospheric delay have been derived for the standard atmospheric model and Hopfield atmospheric profile, respectively. The accuracies of these new models are insured to be better than 1 cm to an elevation angle lower than 2°.5 over a wide range of meteorological conditions in comparison with ray-trace solutions. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117574 Bibcode: 1995AJ....110..934Y Keywords: ATMOSPHERIC EFFECTS full text sources ADS |

Correcting VLBI observations for ionospheric refraction

Radio astronomical observations with local interferometers and very long baseline interferometers (VLBI) are affected by atmospheric (i.e. tropospheric and ionospheric) refraction. A procedure to correct VLBI observations for ionospheric refraction using simultaneous ionospheric measurements is outlined. If networks of ionosonde and satellite observing stations operate during the astronomical VLBI observations, pathlength corrections ranging from several times the VLBI observing wavelengths to a fraction of this wavelength using the simultaneously collected ionospheric data are possible, provided that the geographical distribution of these ionosphere stations is similar to the VLBI network.

Definition of Very Long Baseline Interferometry (VLBI) Group Delay and Relativistic Correction Model With Picosecond Precision

In this paper, the incorrect definition of VLBI coordinate time delay in solar system barycenter frame (B-frame) is clarified, thus correcting the error of sign of the individual term in VLBI relativistic time delay model of transformation published earlier. It is proved that the terms, i. e. the tropospheric refraction delay, the axis offset refraction delay, and the gravitational delay caused by the major planets which are respectively multiplied by the velocity and the acceleration vector of the geocenter, should be taken into account in the model mentioned above under the precision of 1 ps. Finally, the VLBI relativistic time delay model with precision of better than 1 ps is obtained.