GPS receivers for georeferencing of spatial variability of soil attributes

A caracterização da variabilidade espacial dos atributos do solo é indispensável para subsidiar práticas agrícolas de maneira sustentável. A utilização da geoestatística para caracterizar a variabilidade espacial desses atributos, como a resistência mecânica do solo à penetração (RP) e a umidade gravimétrica do solo (UG), é, hoje, prática usual na agricultura de precisão. O resultado da análise geoestatística é dependente da densidade amostral e de outros fatores, como o método de georreferencimento utilizado. Desta forma, o presente trabalho teve como objetivo comparar dois métodos de georreferenciamento para a caracterização da variabilidade espacial da RP e da UG, bem como a correlação espacial dessas variáveis. Foi implantada uma malha amostral de 60 pontos, espaçados em 20 m. Para as medições da RP, utilizou-se de penetrógrafo eletrônico e, para a determinação da UG, utilizou-se de trado holandês (profundidade de 0,0-0,1 m). As amostras foram georreferenciadas, utilizando-se do método de Posicionamento por Ponto Simples (PPS), com de (retirar) receptor GPS de navegação, e Posicionamento Relativo Semicinemático, com receptor GPS geodésico L1. Os resultados indicaram que o georreferenciamento realizado pelo PPS não interferiu na caracterização da variabilidade espacial da RP e da UG, assim como na estrutura espacial da relação dos atributos.

The wavelet method as an alternative for reducing ionospheric effects from single-frequency GPS receivers

The ionospheric effect is one of the major errors in GPS data processing over long baselines. As a dispersive medium, it is possible to compute its influence on the GPS signal with the ionosphere-free linear combination of L1 and L2 observables, requiring dual-frequency receivers. In the case of single-frequency receivers, ionospheric effects are either neglected or reduced by using a model. In this paper, an alternative for single-frequency users is proposed. It involves multiresolution analysis (MRA) using a wavelet analysis of the double-difference observations to remove the short- and medium-scale ionosphere variations and disturbances, as well as some minor tropospheric effects. Experiments were carried out over three baseline lengths from 50 to 450 km, and the results provided by the proposed method were better than those from dual-frequency receivers. The horizontal root mean square was of about 0.28 m (1 sigma).

Mitigating systematic errors for single frequency GPS receivers employing a penalized least squares methodology

Among the positioning systems that compose GNSS (Global Navigation Satellite System), GPS has the capability of providing low, medium and high precision positioning data. However, GPS observables may be subject to many different types of errors. These systematic errors can degrade the accuracy of the positioning provided by GPS. These errors are mainly related to GPS satellite orbits, multipath, and atmospheric effects. In order to mitigate these errors, a semiparametric model and the penalized least squares technique were employed in this study. This is similar to changing the stochastical model, in which error functions are incorporated and the results are similar to those in which the functional model is changed instead. Using this method, it was shown that ambiguities and the estimation of station coordinates were more reliable and accurate than when employing a conventional least squares methodology.

Extended ionospheric amplitude scintillation model for GPS receivers

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Metodologia para avaliação do desempenho de receptor de GPS de uso agrícola em condição cinemática

Activities that use Global Navigation Satellite System (GNSS) are countless and the most used one is the Global Positioning System (GPS) developed by the United States. In precision agriculture there are demands for static and cinematic positioning with distinct levels of accuracy for different applications; nevertheless cinematic performance data are not available as manufacturers of GPS receivers present only static performance information. For this reason it was developed an instrumented vehicle to test a methodology of performance evaluation of GPS receivers in kinematic conditions, which is representative to agricultural operations. A set of instrumentation was composed and used for collecting data under variable speed and rotation direction. Tests were conducted showing that the methodology allows to measure accuracy and precision, but improvements have to be implemented on the instrumentation equipment for long term tests.

Análise da ionosfera usando dados de receptores GPS durante um período de alta atividade solar e comparação com dados de Digissondas

Cada vez mais é crescente o uso do GPS (Global Positioning System ) em estudos da atmosfera terrestre. Neste artigo, a atmosfera superior da Terra, denominada ionosfera, foi estudada durante um período de alta atividade solar (ano de 2001) usando dados de receptores GPS de dupla freqüência localizados na região brasileira, pertencentes à RBMC (Rede Brasileira de Monitoramento Contínuo). A partir dos dados GPS foram calculados os valores de TEC (Total Electron Content - Conteúdo Total de Elétrons) da ionosfera. Adicionalmente foram incluídos no estudo dados de freqüência crítica da camada F2 advinda de Digissondas localizadas em São Luís/MA (3ºS; 44ºW) e Cachoeira Paulista/SP (22ºS; 45ºW), para fins de comparação. de uma forma geral, os resultados mostraram maiores valores de TEC durante os meses próximos aos equinócios e menores durante os meses de inverno. Quanto à variação diária do TEC, os menores valores foram verificados por volta das 4-6 HL (Hora Local) e os maiores durante o período da tarde, com valores um pouco maiores para São Luís. O segundo pico da anomalia equatorial foi verificado em Cachoeira Paulista nos meses próximos aos equinócios e verão. Na maioria dos meses, uma alta correlação linear foi verificada quando realizada a comparação entre os valores de freqüência crítica da camada F2 e os de TEC, principalmente para São Luís.

Modelagem em tempo real do erro sistemático das observáveis GPS devido à ionosfera

Single frequency GPS receivers have been many used in GPS surveys. Among the several applications, one can mention those that are to obtain the receiver's antenna coordinates in real time. One of the main error sources to these applications is the ionosphere systematic error. In the FCT/UNESP a regional ionosphere model (Mod_Ion) was developed. It has been implemented to execute after collecting of GPS data. At real time application two improvements in the Mod_Ion were introduced, consisting of an alteration of the function of modeling and implementation of the Kalman Filter. The results of the experiments showed that the modifications were the most effective in the ionosphere systematic effect's corrections, providing a improvement in the accuracy of point positioning, of 90,75%, in period of the highest ionosphere activity.

Real time orbit determination using GPS navigation solution

This work analyses a real time orbit estimator using the raw navigation solution provided by GPS receivers. The estimation algorithm considers a Kalman filter with a rather simple orbit dynamic model and random walk modeling of the receiver clock bias and drift. Using the Topex/Poseidon satellite as test bed, characteristics of model truncation, sampling rates and degradation of the GPS receiver (Selective Availability) were analysed. Copyright © 2007 by ABCM.

Avaliação de funções para modelagem do efeito da refratação ionosférica na propagação dos sinais GPS

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Nivelamento através do GPS: avaliação e proposição de estratégias

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Implicações do uso de recptores GPS de navegação sem conhecimento de suas limitações e configurações básicas

Pós-graduação em Agronomia (Energia na Agricultura) - FCA

Estimativas do IWV utilizando receptores GPS em bases terrestres no Brasil: sinergia entre a geodésia e a meteorologia

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Avaliação do posicionamento plano-altimétrico com receptor gps em diferentes tipos de uso do solo

Pós-graduação em Agronomia (Energia na Agricultura) - FCA

Inverse Modeling of GPS Multipath for Snow Depth Estimation-Part II: Application and Validation

GPS multipath reflectometry (GPS-MR) is a technique that uses geodetic quality GPS receivers to estimate snow depth. The accuracy and precision of GPS-MR retrievals are evaluated at three different sites: grasslands, alpine, and forested. The assessment yields a correlation of 0.98 and an rms error of 6-8 cm for observed snow depths of up to 2.5 m. GPS-MR underestimates in situ snow depth by 10%-15% at these three sites, although the validation methods do not measure the same footprint as GPS-MR.

Web Service para geração de VRS no contexto de posicionamento GPS

Internet is fully inserted in contemporary society, specially in relation to entertainment services and trading. Its reach has transposed the traditional desktop computer models coming to mobile devices like cell phones and GPS receivers. Likewise, the scientific community takes its benefits, both for publication of studies and for communication between clusters processing information, such as at LHC, located in Switzerland. Concerning geodetic positioning, researches in the area present the concept of Virtual Reference Stations - VRS, in which is necessary a communication way between the real reference stations and a central system as well as between central system and a service requester. In this work, we analyze the current solutions for generation of VRS with regard to data delivery for the service requester and present a solution based on Web Services as an alternative to the model being developed by Spatial Geodesy Study Group – GEGE/FCT/UNESP. Comparing solutions, it was verified the potential of Web Services to aid in researches of geodetic positioning using VRS. Using such technology, it is obtained interoperability, providing greater flexibility to develop client applications, both development carried out by researchers of the university or by any person or enterprise wishing to use the service