Performance of GPS in surveys within the coast of Paraná state (Brazil)

ABSTRACT
The objective of this article is to present the performance of GPS in surveys within the coast of Parana State (Brazil). Such surveys began in 1996 because of the lack of previous available data and the need of better control over the coast of Parana State and its conformations. Among the GPS surveys that have ever been accomplished within such area it should be mentioned: the monitoring of the shoreline, in some parts in retreat or in progradation; the precise positioning of vessels, of artificial reefs, buoys and floating platforms; the location of artificial reefs; the cadastral surveys of ports and the establishment of a local geodetic network. GPS, Differential GPS (DGPS) and Precise DGPS (PDGPS) have been used at such sites because there is a need of precision and for the purpose of the survey. With the aid of GPS along with multidisciplinary actions in this area, a systematic view of the environmental issue will be provided as well as better information for its environmental monitoring.

Keywords : GPS, Surveys in the Marine Ambit, Surveys within the Coast of Parana State, DGPS, PDGPS.

1. INTRODUCTION

Since 1985, in the marine environment, the GPS-Global Positioning System- has been successfully used. Its initial conception was one of a navigation system. Initially, the accuracy achieved with stand-alone P-code receivers was of 10 and 15 m; even so, with the introduction of safety techniques Selective Availability -SA- and Anti-Spoofing -AS, the accuracy has become of 50 m to 100 m, which is not satisfactory in many marine geodetic applications. In order to achieve an improvement in accuracy, Differential GPS was launched (DGPS). With it there can be a relative accuracy between 10 m and 1 m, depending on the used observables.

Nowadays within such environment new application fields for GPS are being discovered. For example: precise navigation in coastal areas; mapping of bottom of the sea, precise hydrographic surveys, access to ports; monitoring of dredging control; support of the coastal engineering; attitude control in vessels, buoys and flotation platforms; continuous and accurate control of heights and positioning of sensors below water. Today, there is a growing interest in real-time applications and in their integration with other fields which rely on accurate coordinates (e.g.: GIS); and systems (e.g.: ecobathymetry). For such applications, it is necessary a more accurate precision than a meter. Such accuracy is achieved with the use of Precise DGPS (PDGPS).

The objective of this article is to present the performance of GPS in surveys carried out within the coast of Parana State. Such surveys began in 1996 because of the lack of previous available data and also because of the need of better control over the coastal areas of Parana State and their conformations.

2. DIFFERENTIAL GPS (DGPS) AND PRECISE DGPS (PDGPS)

Differential DGPS can be seen in Figure 01. It comprises the positioning of a mobile station through the corrections generated in the reference station. Those corrections are sent in real-time via data links (transmission radio, phone line, or communication satellites) and in accordance to an appropriate format, set forth by the Radio Technical Commitee for Marine Services (RTCM) (RTCM, 1994; KRUEGER, 1996).

In order to carry out a survey with a differential system, two receivers are at least necessary. One is positioned on a point of well-known coordinates, which represents the reference station where the corrections are generated. The other receiver is positioned on the vehicle where one whishes to navigate, called, "mobile" station from now on. The corrections consist of the difference between the true pseudoranges and those computed for each one of the satellites in the reference station (KRUEGER, 1996).

The application of this system facilitates a decrease in the influence of errors of the satellite clock, of the ephemerides and of the atmospheric signal propagation. If those errors are the same to the reference stations and the mobile station, it can be said, they will be eliminated by the corrections. The observables in that system are the phase of the code or the carrier phase smoothed code.

Figure 01 – Differential GPS; range corrections are transmitted to the mobile station

According to HEIMBERG and SEEBER (1993), the relative accuracies obtained in that system with the use of the code solutions fall in the order of 1 to 10 m; for the carrier phase smoothed code measurements, it is possible to obtain an accuracy between 1 to 3 m, as it is observed in the Figure 02. It can still be influenced by some problems as the distance between the reference station and the mobile station, as well as the quality of the communication system (KRUEGER, 1996).

Figure 02 – Accuracies in Kinematic Positioning with GPS.
(Source: SEEBER et al., 1993)

In the navigation there is a need of working with DGPS in real-time; then, it is necessary to look more closelly at some factors of great importance in the setting of the precision, e.g.: computation of the corrections; age of the data and data transmission (KRUEGER, 1996).

Precise DGPS follows the same principle of DGPS system, except for the type of used GPS observables. That system uses the measure of the carrier phases that can achieve to a relative accuracy of better than 0.1 m down to several centimeters (Figure 02), because the carrier phase has a small noise (of 0.5 to 5 mm).
The main obstacle of PDGPS is data communication, because transmission and range depend on the distance between stations. Also, the effect of the ionosphere should be considered (WANNIGER, 1995). In this system the format RTCM 2.1 is used for the transmission of data (KRUEGER, 1996).

For more information about GPS, DGPS and PDGPS, as well as on the format RTCM and the communication systems refer to KRUEGER (1996), SEEBER (1993), HEIMBERG et al., (1993) and WÜBBENA et al., (1995).

3. DESCRIPTION OF THE STUDIED AREA

The coast of the state of Paraná, located in the South of Brazil, between the latitudes 25° and 26° South and between the longitudes 48° and 49° West of Greenwich, with an area of approximately 6,600 km2, has characteristics that make it different from other States of the Brazilian South and Southeast coast. The state limits are respectively with the State of São Paulo to the North, in the mouth of the Ararapira river and with the State of Santa Catarina to the South in the mouth of the Saí-Guaçu river. In the Figure 03, it is observed the main cities of the area litoranea being them: Paranagua, Morretes, Antonina, Guaraqueçaba, Praia de Leste, Pontal do Parana and Guaratuba.

Figure 03. Coast from Paraná State
Source: www.matinhos.com/imagens/mapapr.jpr (2005)

4. SURVEYS IN THE COAST OF PARANA STATE

In the coastal areas of Parana State an absence of reliable information is verified. There is a great interest however and continuous concern from research institutions (CEM/UFPR) and other state agencies (e.g.: Parana State Environment Secretariat) over this issue. Having in mind an appropriate support and an effective contribution for a better geodetic knowledge of the mentioned area, the development of a geodetic system is sought for environmental studies developed within this coastal area.

Among the GPS surveys that have been carried out within Parana State coast it is worth mentioning: the establishment of a local geodetic network; cadastral surveying of ports; the precise positioning of ships, artificial reefs, buoys and floating platforms; the surveys for the coastline; the monitoring of shoreline, in some parts in retreat or in progradation; and the location of artificial reefs. GPS, DGPS and PDGPS have been used in such sites aiming to a desired accuracy and for the purpose of such survey. Different processing GPS software packages have been used with data processing: commercial (PRISM) and scientific (GEONAP-K, GNRT, BERNESE); also in some cases the hydrography software package (HYPACK) is used.

4.1. Establishment of a local geodetic network

Brazil relies today on the so-called Brazilian Network for Continuous Monitoring of GPS (RBMC). It is made up of 20 permanent GPS stations, located in the some cities like Curitiba. They are part of the Network of the South American Geocentric Reference System (SIRGAS). The accuracy of the RBMC stations coordinates is of the order of ± 5 mm, being configured as one of the most precise networks of the world.

Along the Brazilian coast the establishment of a network can be verified with 11 stations and with a reach of 200 to 300 nautical miles. According to DHN, organization of the Navy of Brazil, responsible for this network, the reached accuracy is better than 9 m ((3s) + 2 ppm). This network supplies some supports like: marine navigation, hydrographics surveying, nautical road signs and dredging.

Having in mind the development of a geodetic system which will allow high accuracy, a local geodetic network is due to be implemented. It will be connected to RBMC stations. The stations that will be part of this local network will be located in key places of interest for entire technical scientific community that works in the area.

In Figure 04, 11 stations of this geodetic network can be seen. They were implanted in the coast of Parana.

Such network is made of 11 stations and has been tracked in one campaign so far. The CEM1 station (belongings this network), is located at headquarters of the Center of Studies of the Sea (CEM). This site was connected with PARA station (RBMC) through relative positioning. Its coordinates were determined with the data collected for 8 days of measurements, being post-processed and adjusted by the BERNESE software. In the future, this connection will be made to another station of RBMC that will be located in the city of Porto Alegre. For the other stations of this network the relative positioning with static technique was used. The base station was CEM1, the measurements were obtained every 15 s and the time of occupation in these stations was equal to 5 hours. In this campaign Ashtech Z-XII receivers were used with antennas receiving L1 and L2 signals.

 

Figure 04. Map of Location of the Geodetic Network of the Coast of Paraná and of the main cities.

4.2. Cadastral surveying of port wharfs

GPS is an effective component to support a marine Geographic Information Systems (SIG). It contributes to the formation of a uniform geometric base and for the geometric location of objects that enter into SIG (cradles, buoys, ports, canalizations of water and electricity, pipelines).

So far cadastral surveys have been carried out on port wharfs of Paranagua’s port  and on Capitania’s port.

Paranagua’s port is the largest exporter of grains of Latin America and the second in the Brazilian economic ranking. Its profile began to change with the Law of Modernization of Brazilian ports (Law no. 8,630/93) and the consolidation of MERCOSUR. The modernization program of the Administration of Paranagua and Antonina’s ports-APPA – has been carried out by works of the Center of Studies of the Sea for the port operation. Such works will include several types of surveys such as the design of a number of remarkable sites along the port wharfs.

24 remarkable sites were surveyed along the port wharfs which are depicted in detail in Figure 05. In these surveys, the relative positioning was used with the fast static technique, with a sampling rate of 2 s and with a time of ocupation in each point equal to 5 minutes. The base station was installed in the mark APPA, located in the administrative headquarters of Paranagua and Antonina’s ports. Ashtech Reliance receivers were used with antennas that receive L1 signal (Figure 06). The geodetic coordinates (WGS-84) were obtained with a post-processing of data through the software Reliance. They come with the respective deviations of internal accuracy, which reinforces indication of the quality of the results.

Figure 05.  Detail of some surveyed points along the port wharfs, in the Bay of Paranagua

 

Figure 06. Survey of some remarkable sites along of port wharfs of Paranagua, in the Bay of Paranagua, with Ashtech Reliance receivers.

4.3. Precise positioning of artificial reefs and continuous location

Accurate launching of artificial reefs is critical because of the need of frequent visits. The implantation of such reefs in short and medium term aim at: the conservation of the marine biodiversity; the creation of fishing runners and exclusion areas; the development of the local fishing and of the sustainable tourism; a better understanding of the ecological and hydrological processes in the continental platform of the Parana. Additionally, it suggests strategies for the implementation of programs of artificial reefs in Brazil. They are being relatively positioned with GPS and their coordinates have been obtained in the data post-processing.

Regular locations of the artificial reefs have been carried out through DGPS  having as reference station the mark located on top of CEM’s water container. This positioning has been done with Ashtech Reliance receivers and the corrections are sent to the mobile station (vessels) through a communication system (radios of 35 W in the reference station and 2 W in the mobile station). The format of the differential corrections has been the RTCM 2.0.

4.4. Monitoring of the coastline, in some parts in retreat or in progradation

Coastline is defined in this article as the boundary between the continent and the portion adjacent to the sea where there is not effective marine action beyond the maximum reach of the waves, which are identified by the cliffs, in the boundary between the vegetation and the seashore, or in the rocks, or for any other feature that determine the beginning of the continental area. Coastlines can suffer a retreat (erosion) or progradation (SOARES, C.R., 1995). The determination of these lines can be made through several methods. GPS has been used since 1996 (KRUEGER; et al. , 1996) in the detection of the variation of some coastlines in the coast of Parana. With those measures more information maybe obtained on the studied area consequently keeping update records of the coastlines.

In the surveys and monitoring of coastlines the method of relative positioning has been used with the cinematic continuous technique. The stations have been equipped with Ashtech Z-XII receivers. The GPS collected data have been post-processed with ASHTECH SOLUTIONS software. The reference station (base station) is installed in the landmark like CEM, implanted in the Center of Studies of the Sea, and PEDR, implanted in the Stone of Matinhos,  with a sampling rate of 3 s. The mobile station in these cases is determined by an individual traveling along the coast by foot with an average speed of 4km/h, with receiver and antenna installed in a backpack (Figure 07). First of all it is necessary to carry out an initialization, staying for 5 to 15 minutes in the initial point of the trajectory that will be followed. These surveys have been done using redundant satellites (³ 4 satellites), dual frequency data and code/carrier combinations. Those factors are extremelly important in Cycles Slips’s recovery, in the cinematic method (SEEBER, 1993).

In the Figure 08, am biggest difference between the coastlines 2002 and 2005 are seen with the surveys of the coastline to Matinhos beach.

Figure 07. Mobile station

Figure 08. Area with am biggest difference between the coastlines 2002 and 2005
 from Matinhos beach

5. CONCLUSIONS AND FUTURE PERSPECTIVES

With the aid of GPS and with a multidisciplinary performance in this area, a systematic view on the environmental issue and information for its monitoring is foreseen.

Surveys of remarkable points in port wharfs with GPS should be done preferably during periods of low marine traffic. The presence of anchor-ground’s vessels in the ports generate obstacles contributing to a poor reception of the GPS sign and an increment of the in coming errors due to multipath effects.

The determination of the coordinates of the ports vertices and some remarkable points will become important subsidies for the designs of accurate maps which is crucial to the coastal safe navigation.

Although the GPS is necessary and practical, it is not always possible to travel exactly along the coastline, due to the existent obstacles that contributet to losses of the GPS sign.

The densification of this local geodetic network will hopefully be extended in the near future in order to monitor possible displacement of marks. This control will be carried out every six months, and displacements will be verified starting from considered stable marks in other words say those located on rocky soil.

For the precise positioning of vessels, of artificial reefs, buoys and floating platforms PDGPS will be utilized with the help of GNRT/GNREF software.

In the municipality of Matinhos (PR), the monitoring of the shoreline has been done along 1,5  km  and  erosion effects  have been detected.

6. REFERENCES

HEIMBERG, F.; SEEBER, G. Kinematische Anwendungen von GPS – GPS-eine universelle geodätische Methode. Schriftehreihe, 11/1993, DVW e.V., Wittwer.

KRUEGER, C.P., SOARES, C.R., et. al. Levantamento com GPS da Linha de Costa na Área Erosiva da Ponta do Poço (PR). 2º COBRAC, 1996, Florianópolis, SC.

KRUEGER, C.P. Investigações sobre aplicações de alta precisão do GPS no âmbito marinho, 1996. Tese de doutoramento apresentada ao CPGCG – Universidade Federal do Paraná. Curitiba, Paraná, 267 p.

RTCM – RECOMMENDED STANDARDS FOR DIFFERENTIAL NAVSTAR GPS SERVICE. Version 2.1. January 3, 1994. Radio Technical Commission for Maritime Services, Washington, U.S.A

SEEBER, G. Satellite Geodesy: Foundations, Methods and Applications. Berlin – New York, 1993.

SOARES, C.R. Processos Costeiros, Erosão Marinha, Métodos de Monitoramento das Variações da Linha de Costa. Exame Geral de Qualificação apresentado ao Curso de Pós-Graduação em Geociências e Meio Ambiente – UNESP, para obtenção do título de doutor, 1995, São Paulo.

WANNINGER, L. Einfluss ionosphärischer Störungen auf präzise GPS-Messungen in Mitteleuropa. GPS-Leistungsbilanz’94, Beiträge zum 34. DVW-Seminar, p218 à 232, 05.-07. Oktober 1995, Geodätischen Institut, Universität Karlsruhe.

WÜBBENA, G. Zur Modellierung von GPS-Beobachtungen für die hochgenaue Positions-bestimmung. Tese, Nr.168, Institut für Erdmessung, Universität Hannover, 1991.

WÜBBENA, G., BAGGE, A  Präzises DGPS in Echzeit für Vermessung und GIS-An-wendungen. Beitrag zum DVW-Praxisseminar DGPS- Anwendungen, 19.-20. Sept. 1995, Hamburg, p.01 à 16.

Prof’-. Dra. Cláudia Pereira Krueger
Federal University of Parana
Curso de Pós-Graduação em Ciências Geodésicas
Departamento de Geomática, Setor de Ciências da Terra
Centro Politécnico, Jardim das Américas, CP 19011, CEP 81531-990
Curitiba – PR, Fone: (041) 3361-3155, Fax: (041) 3361-3161
e-mail: ckrueger@ufpr.br