Built for research, GNSS-SP will also contribute to improving the application of GNSS technology in areas such as precision agriculture and weather forecasting.

São Paulo welcomes new Global Navigation Satellite System

July 17, 2013

By Elton Alisson

Agência FAPESP – São Paulo scientists who use Global Navigation Satellite Systems (GNSSs) can now count on improved infrastructure to use this technology in research areas such as geodesy (the study of the Earth’s shape, dimensions and gravity field), cartography and modeling both the ionosphere (the atmospheric layer that covers the Earth and is formed by ions and electrons) and troposphere (located between the surface of the Earth and the ionosphere).

Researchers from Universidade Estadual Paulista (UNESP) in Presidente Prudente, in partnership with colleagues from Universidade de São Paulo’s Luiz de Queiroz Agriculture School (ESALQ-USP), Universidade de São Paulo’s Polytechnic School (Poli-USP) and the National Institute for Space Research’s Center for Weather Forecasting and Climate Research (INPE - CPTEC), installed the first network of active GNSS stations in the state of São Paulo.

Named GNSS-SP, the network was built under the auspices of a Thematic Project funded by FAPESP.

“Now we have a network of GNSS real-time receivers in São Paulo that has been created for research but that should also contribute to improve the application of satellite navigation systems in sectors like precision agriculture, land, air and offshore positioning and weather forecasting, among other things,” said João Francisco Galera Monico, a professor at UNESP Presidente Prudente and coordinator of the project, to Agência FAPESP.

The project’s participating researchers held the Thematic Project’s third workshop during MundoGEO#Connect LatinAmerica 2013, where they overviewed the main results.

According to Monico, the GNSS-SP network currently has 20 active stations spread throughout various São Paulo municipalities.
In each of these stations, there is a GNSS receptor connected to the Internet that tracks a set of GNSS satellites in operation – such as GPS (United States) and Glonass (Russia) – and captures real-time electromagnetic signals that are sent to the Earth.

The satellite signals received are sent to a data processing and storage center located at the UNESP campus in Presidente Prudente and are available via an online platform that registered users can utilize in their research.

Data from some of the stations are also submitted to the Brazilian Geography and Statistics Institute (IBGE), which makes the data available to the general public through the Brazilian Continuous Monitoring Network (RBMC).

Additionally, the satellite data provided by the GNSS stations can also be used as a reference for relative positions – in effect, a user with a static or mobile GNSS receptor near one of the stations can obtain their coordinates with good accuracy.

“Utilization of active GNSS networks, such as GNSS-SP, for relative positioning is a trend that should grow,” estimates Monico.

“Today, cellular devices with GPS provide positions with a 12-meter accuracy. But, in the future, when they begin to receive corrections from GNSS stations, they will provide positions with an accuracy within half a meter,” the researcher commented, explaining that the shorter the distance of the position provided by the GNSS, the better the accuracy of the coordinates of the point of interest.

Effects of the ionosphere

According to Monico, the network also allows better monitoring of the ionosphere and a better understanding of the effects of signals emitted by satellites, which face interference when passing through the atmosphere and reach the Earth altered in various ways, ranging from attenuation of potency to changes in the propagation direction and the speed of the electromagnetic waves.

When crossing the ionosphere, for example, satellite signals collide with electrons, which change the signal speed. When crossing the troposphere, the signals are affected by water vapor; the extent of this effect can be estimated and utilized to improve models of weather forecasting because stations in the GNSS-SP network are integrated with temperature, pressure and humidity monitors.

The GNSS station receptors measure electromagnetic signals captured from satellites and decode them into data that can be gathered by researchers to evaluate the effects on the signals during passage through the atmosphere.

“To us, who work with geodetic positioning [determining the position of the Earth’s surface through a system of coordinates], these atmospheric interferences with satellite signals degrade the position and are errors that we want to eliminate to improve the accuracy of positioning,” said Paulo de Oliveira Camargo, professor at UNESP Presidente Prudente and one of the project’s main participating researchers.

“But for other areas, such as spatial sciences, these errors are important signals through which it is possible to calculate the total number of electrons and generate ionosphere models, make inferences about irregularities and detect the causes of disturbances, such as ionospheric scintillation,” he said.

Characterized by an alteration in the magnetic field during the passage of a satellite signal through the ionosphere, ionospheric scintillation occurs with the highest intensity from 6:00 p.m. to 2:00 a.m. As a result, the phenomenon hampers the utilization of the GNSS in precision agriculture, where technology is used to guide harvesters equipped with GNSS receivers for the automatic piloting of machines day or night.

During periods of ionospheric scintillation, satellite signals captured by base stations and submitted to a mobile retransmitter, which then retransmits them to agricultural machines, are affected. As a result, the machines can have lower positioning quality and may not be able to adequately locate the planting area where they are operating at night, for example. “This is a problem we are trying to solve,” said Monico.

To analyze the effects of the phenomenon, increase understanding of its causes and develop new counter-measurement techniques to be implemented in GNSS receptors, at the beginning of 2012 researchers from UNESP, Petrobras and the University of Nottingham, among others, concluded the “Concept for ionospheric scintillation mitigation for professional GNSS in Latin America” (CIGALA) project.

Funded by the European Community, the project also resulted in the installation of a network of GNSS stations located in Manaus (AM), Palmas (TO), Macaé (RJ), Porto Alegre (RS) and, in São Paulo, Presidente Prudente and São José dos Campos.

Continuing CIGALA, in November 2012, researchers began the “Countering GNSS high accuracy application limitations due to ionospheric disturbances in Brazil” (CALIBRA) project. Also funded by the European Community, the project includes some of the institutions that participated in CIGALA. Among the project’s objectives are improving and developing new algorithms to mitigate the effects of ionospheric disturbances for high-accuracy GNSS positioning. The project should be concluded by the end of 2014. The project also envisages the installation of five more GNSS stations in different Brazilian states.

“These two projects, with the GNSS-SP, constitute infrastructure for monitoring the ionosphere that has been collecting data since 2011,” said Bruno Vani, who is pursuing his master’s degree at UNESP Presidente Prudente and is participating in three projects.

Data analysis

According to Vani, more than 13 terabytes of data have already been collected, along with 7.5 billion registers of ionospheric monitoring through the three projects.

Every minute, a receptor provides more than 60 different parameters of the ionosphere generated by a set of satellites, such as the variation of a signal within the last 60 seconds. The data are presented in archives with columns of information that are made available on an internet portal in Portuguese and English.

Through visualization and data-relation techniques utilized by the online tool, users can monitor the state of the ionosphere on any given day, for example, and use the information to conduct research in several areas and develop techniques that mitigate the effects of the ionosphere in positioning.

The database allows researchers to evaluate the peaks of ionospheric scintillation over the interval of a day or a week, for example, and to identify which satellite is most affected. However, although it is in a very advanced stage of development, the database still needs some improvement.

“Since we receive large volumes of data, it is important for us to have infrastructure to analyze and detect specific behaviors of the ionosphere,” stressed Vani.

“We have positioning difficulties, for example, in periods when the availability of data is jeopardized, when few satellites are being tracked and are exposed to high levels of scintillation. It could be that at these times we do not have positioning capacity,” he added.

To solve this problem, researchers from the CALIBRA and GNSS-SP project are developing a computer tool for data exploration and analysis based on freeware.

The program will allow researchers to exclude a given satellite that is more affected by ionospheric scintillation from the tracking for better positioning or to forecast which satellites are more susceptible to ionospheric disturbances.

“The ionosphere is a very unstable layer that experiences variations over several scales of time – during the day, throughout the seasons of the year and during solar cycles, which occur every 11 years - and it is difficult to know how this [the ionosphere] will be in a month,” said Marco Mendonça, another Master’s student who is participating in the project. “The software for data exploration and analysis will help us to respond to this question,” he affirmed.