The Brazilian National Institute for Space Research (INPE) is operating the Brazilian Environmental Data Collection System that currently amounts to a user community of around 100 organizations and more than 700 data collection platforms installed in Brazil. This system uses the SCD-1, SCD-2, and CBERS-2 low Earth orbit satellites to accomplish the data collection services. The main system applications are hydrology, meteorology, oceanography, water quality, and others. One of the functionalities offered by this system is the geographic localization of the data collection platforms by using Doppler shifts and a batch estimator based on least-squares technique. There is a growing demand to improve the quality of the geographical location of data collection platforms for animal tracking. This work presents an evaluation of the ionospheric and tropospheric effects on the Brazilian Environmental Data Collection System transmitter geographic location. Some models of the ionosphere and troposphere are presented to simulate their impacts and to evaluate performance of the platform location algorithm. The results of the Doppler shift measurements, using the SCD-2 satellite and the data collection platform (DCP) located in Cuiabá town, are presented and discussed.
The geographical locations of data collection platforms (DCP) in the Brazilian Environmental Data Collection System are obtained by processing Doppler shift measurements between satellites and DCP. When the signals travel from a DCP to a satellite crossing the terrestrial atmosphere, they are affected by the atmosphere layers, which generate a delay in the signal propagation, and cause errors in its final location coordinates computation. The signal propagation delay due to the atmospheric effects consists, essentially, of the ionospheric and tropospheric effects. This work provides an assessment of ionospheric effects using IRI and IONEX models and tropospheric delay compensation using climatic data provided by National Climatic Data Center. Two selected DCPs were used in this study in conjunction with SCD-2 satellite during high and low solar activity periods. Results show that the ionospheric effects on transmission delays are significant (about hundreds of meters) in equatorial region and should be considered to reduce DCP location errors, mainly in high solar activity periods, while in those due to tropospheric effects the zenith errors are about threemeters. Therefore it is shown that the platform location errors can be reduced when the ionospheric and tropospheric effects are properly considered.
The current Brazilian System of Environmental Data Collection is composed of several satellites (SCD-1 and 2, CBERS-2 and 2B), Data Collection Platforms (DCPs) spread mostly over the Brazilian territory, and ground reception stations located in Cuiabá and Alcântara. An essential functionality offered to the users is the geographic location of these DCPs. The location is computed by the in-house developed “GEOLOC” program which processes the onboard measured Doppler shifts suffered by the signal transmitted by the DCPs. These data are relayed and stored on ground when the satellite passes over the receiving stations. Another important input data to GEOLOC are the orbit ephemeris of the satellite corresponding to the Doppler data. In this work, the impact on the geographic location accuracy when using orbit ephemeris which can be obtained through several sources is assessed. First, this evaluation is performed by computer simulation of the Doppler data, corresponding to real existing satellite passes. Then real Doppler data are used to assess the performance of the location system. The results indicate that the use of precise ephemeris can improve the performance of GEOLOC by reducing the location errors, and such conclusion can then be extended to similar location systems.
ABSTRACT. The use of the Doppler shift measurements to locate a transmitter has been studied and tested within the Brazilian Environmental Data Collection System.To improve the location of Data Collection Platforms (DCPs) with their transmitters, some effects such as corrections due to the ionosphere and the troposphere have been considered in a previous work (Celestino et al., 2007) using simulated data and some initial error evaluation for one existing DCP. This work presents an evaluation of the impacts on the geographical location due to the ionospheric and the tropospheric effects, however using two meteorological DCPs and covering a longer test period (one year). The seasonal and day-night effects are also analyzed during this test period. The results of the analysis indicated that correction of the ionospheric and tropospheric effects can, on the average, reduce the location errors to the scale between ten and hundred meters for the test conditions independent of the DCP location that was used. Due to the complexity of modeling sources of errors in the location of a platform, still a considerable work need to be done to reduce location errors in the system. Initial analysis indicates that the knowledge of the satellite position can be another significant source of platform position error.Keywords: geographical location, Data Collection Platforms, Doppler shift, tropospheric and ionospheric effects. RESUMO. O uso de medidas do desvio Doppler para localizar um transmissor foi estudado e testado considerando o Sistema Brasileiro de Coleta de Dados Ambien-tais. Para melhorar a localização de plataformas de coleta de dados (PCDs), com os seus transmissores, alguns efeitos como a correção devidoà ionosfera eà troposfera, foram considerados no trabalho de Celestino et al. (2007) usando dados simulados e algumas avaliações dos erros iniciais de umaúnica PCD. Este trabalho apresenta uma avaliação dos impactos sobre a localização geográfica, devido aos efeitos da ionosfera e da troposfera, entretanto usando duas PCDs meteorológicas e cobrindo um período mais longo de testes (um ano). Os efeitos, sazonal e dia-noite, também são analisados durante este período experimental. Os resultados das análises indicam que a correção dos efeitos da ionosfera e da troposfera pode, em média, reduzir os erros de localizaçãoà escala entre dez e cem metros para as condições de testes independente da localização da PCD que foi utilizada. Devidoà complexidade da modelagem das fontes dos erros na localização de uma plataforma, um trabalho considerável ainda precisa ser feito para reduzir o erro de localização no sistema. A análise inicial indica que o conhecimento da posição do satélite pode ser outra fonte significativa do erro na posição da plataforma.Palavras-chave: localização geográfica, plataformas de coletas de dados, desvio Doppler, correção dos efeitos da ionosfera e da troposfera. INPE/DSE,
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