OCEANOGRAPHIC DRIFT BUOYS POSITIONING THROUGH SATELLITES

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1 Série Arquimedes, Volume 2, Anais do DINCON 2003, pp º Congresso Temático de Aplicações de Dinâmica e Controle da Sociedade Brasileira de Matemática Aplicada e Computacional (SBMAC). São José dos Campos, SP, Brasil, Agosto de Editores: J. M. Balthazar, G. N. da Silva, M. Tsuchida, M. Boaventura, L. S. Góes e J. D. S. Silva. OCEANOGRAPHIC DRIFT BUOYS POSITIONING THROUGH SATELLITES Cristina Tobler de Sousa / INPE National Institute of Space Research-INPE/DSE, Cx.P.515 S JC/ SP: , Brazil Cristina.tobler@dss.inpe.br Hélio Koiti Kuga /INPE National Institute of Space Research-INPE/DMC, Cx.P.515 S JC/ SP: , Brazil hkk@dem.inpe.br 1. ABSTRACT The main goal of this work is to present an application in positioning of drift buoys, using the GEOLOC (GEOgraphic LOCation of transmitters by satellites) system. The system developed at INPE was applied in positioning of drifting buoys launched in the ocean for scientific research. These buoys were deployed and launched by researchers of the University of Sao Paulo (USP) Oceanographic Institute whose goal is the research of meteorologic data collected over the ocean. Having the buoy geographical position along the time makes possible a rescue in case these buoys drift from their original positions. For the first time, INPE's Brazilian system of geographical location was operationally exercised; being obtained as a final product the buoys geographical positions in near-real-time. To obtain the positioning results, the Brazilian Data Collecting satellite (SCD-2) and the China-Brazil Earth Resources Satellite (CBERS-1) were used. The buoys data collection campaign took place from January to May The buoys position location was satisfactory with 67% of the results being around 3.0km from the reference position. 2. KEYWORDS Geo-Positioning; Transmitters; Doppler shift.

2 3. INTRODUCTION In Brazil, geographic location of transmitters is applied in several areas as in biology research, by fixing mini-transmitters in wild animals, to monitor their displacements and habits as an example in Figure 1: Figure 1 Fixed Transmitter in a seal. In oceanography research, drift buoys are launched in the ocean to monitor oceanic streams and to track their displacement as in Figure 2: Figure 2 Monitoring drift buoys. The geographical location has also played roles in location and rescue of aircrafts and ships in emergencies situations. It is also useful to monitor and rescue people in remote areas, for example, in the Brazilian Antarctic Program - PROANTAR using Mini Remote Transmitters MTR, which are manufactured at INPE [6]. Transmitters are also used in marine tasks to position buoys from PETROBRAS, and in hydrologic activities collecting data for meteorological science. In this work, to obtain location results two Brazilian satellites were used: the Data Collection Satellite (SCD-2) and the China-Brazil Earth Resources Satellite (CBERS-1) as Figure 3:

3 Figure 3 Data Collection Satellite (SCD-2) and China-Brazil Earth Resources Satellite (CBERS-1). The geographic location system offers the users the possibility of obtaining their transmitters position, in near-real-time, independent of any external factor. This means that if the user acquires a portable reception station, which is manufactured in Brazil, he just depends on the geographic location algorithm and the satellites in orbit to get his location data. Thus he can own his location system. There is a similar French system that also provides location position. It is called Argos system [3], which uses the NOAA satellites and supply location position after a few hours. Besides the cost for using this service the users remains dependent of such tecnology. The set of data for the analysis were collected from January to May Statistical results are presented in the sequel. The location precision for the analysed buoys was satisfactory for the foreseen purpose. Assuming a Gaussian distribution, 67% of results have obtained a mean error location value of 3.5km for Buoy #32338 and 2.9km for Buoy # ACRONYMS AND ABBREVIATIONS SCD-2 - Data Collection Satellite CBERS-1 - China-Brazil Earth Resources Satellite GEOLOC - Geographic Location of Transmitters by Satellites INPE - National Space Research Institute PROANTAR - Brazilian Antarctic Program MTR - Mini Remote Transmitters 5. LOCATION SYSTEM DESCRIPTION The location system determines the geographic location by measuring the Doppler shift of the transmitted signal frequency due to the relative velocity between the satellite and the transmitter [7-8]. During the satellite

4 pass UHF signals from the transmitters are received on board the satellite, which are relayed in real time to the ground reception Station. The signals are then processed right after the satellite pass [7] and in less than a minute the location result using the algorithm for geographic location of transmitters is obtained. This occur when the satellite visibility includes at the same time the transmitter and the Reception Station. The Doppler data is modeled and processed using a robust Householder orthogonalization for the least squares processing procedure [2, 4, 9]. The satellite velocity relative to the transmitter (v cos α) in vacuum conditions, denoted by ρ is given by the Doppler effect equation [5] as follows: ( f f ) ρ = r t c, (1) f t where: - f r is the frequency value as received by the satellite; - f t is the reference frequency sent by the transmitter; - (f r - f t ) is the Doppler shift due to the relative velocity satellite-transmitter; - c is the speed of light; - α is the angle between the satellite velocity vector v and the transmitter position relative to the satellite. A characteristic Doppler curve is shown in Figure 4 where b o and b 1 are constants associated with each Doppler curve [1]. Figure 4 - Doppler curve

5 6. RESULTS To obtain location results, two buoys were considered. Their approximate reference position is plotted in Figure Latitude Buoy Buoy Longitude Figure 5 Buoys reference positions To obtain the set of data the SCD-2 and CBERS-1 satellites are used. Besides, Cuiaba Reception Station that acquires Doppler data to be sent via FTP to INPE Mission Center located in Cachoeira Paulista is considered. The Mission Center provides position results via our location algorithm in less than one minute after the satellite pass and forwards them to users via FTP. The data were collected from January to 13 May For Buoy #32338, 216 satellites passes were acquired and for Buoy #32339, 150 pass. The latitude and longitude distribution for both Buoys are represented in Figures 6 to 9. The x-axis represents longitude in degrees in East direction in relation to Greenwich and y-axis indicates latitude in degrees in South direction from the Equator. Observing Figures 6 and 8, we can note that the black points represent the positions determinated by the location algorithm for each satellite pass, the triangle indicates the approximate reference position when Buoys were deployed in the ocean, the losangle providse the mean position of all calculated positions and the rectangle contain the cloud of 67% of the positions considering a Gaussian distribution.

6 Latitude (S) Longitude (E) Figure 6 Position distribution for Buoy To visualize better the rectangle behavior from Figure 6 it is enlarged in Figure 7, which follows. Latitude (S) Longitude (E) Figure 7 Distribution of 67% of the best positions for Buoy We can note that the positions distributions for Buoy in Figure 7 are concentrated near the triangle and the losangle symbols. The location mean error considering these 67% of the results from reference position is 3.5km and its standard deviation is 2.2km.

7 Making the same analysis for Buoy #32339, we note a similar distribution considering the 150 satellites passes as shown in Figures 8 and 9. Latitude (S) Longitude (E) Figure 8 Position distribution for Buoy Considering 67% of the best results for Buoy #32339, from reference position as Figure 9, the location mean error is 2.8km and its standard deviation is 1.4km. Latitude (S) Longitude (E) Figure 9 Distribution of 67% of the best positions for Buoy 32339

8 7. CONCLUSIONS This paper has shown geo-location results for two anchored marine Buoys identified by #32338 and #32339 and located in the following approximate latitudes and longitudes: 23.7 S and E, and 22.9 S and E respectively. Assuming that the cloud of location results follows a Gaussian distribution, an analysis of 67% of the best results provide the location errors of 3.5±2.2km and 2.8±1.4km for both buoys respectively, which are quite suitable to the applications described herein. The location system has achieved its expectances and is satisfactory to the user, who can promptly monitor the buoy and prepare an immediate rescue in case of drifting. Nowadays, the users can obtain up to twelve position locations a day for each transmitter considering both satellites SCD-2 and CBERS-1. If the user acquires a portable reception station, witch is manufactured at INPE and uses the Geolocation algorithm; he can operate his own positioning system depending only on the Brazilian satellites in orbit. The French system called Argos also provides the location service for users but not in near-real-time, at a certain cost, and becomes the user dependent of it. With the INPE s Geolocation system certification it could be extended for national and international users. 8. ACKNOWLEDGEMENTS The authors thank the fellowship CNPq/PCI # /00-9, and Dr. Luiz Nonnato from USP for the interest and confidence in using experimentally the INPE s location system. 9. REFERENCES 1. K. AKSNES; H. P. ANDERSEN; E. HAUGEN "A precise multipass method for satellite Doppler positioning." Celestial Mechanics, v. 44, p , Sep BIERMAN, G. J. Factorization methods for discrete sequential estimation. New York: Academic Press, p. 3. Colletion and Location System (CLS). Service Argos: guide to the Argos System. Toulouse: Sept., v C. L. LAWSON, L. C.; HANSON, J. R. Solving least Squares Problems. Englewood Cliffs, Prentice, 1972

9 5. RESNICK, R. Introdução à relatividade especial. New York, Wiley, SETZER, A. Manual do Sistema de Dados Remotos. (Instituto Nacional de Pesquisas Espaciais de São José dos Campos, 1997). Comunicação pessoal. 7. SOUSA, C. T. Geolocalização de transmissores com satélites usando desvio Doppler em tempo quase real. São José dos Campos, Tese (Doutorado em Engenharia e Tecnologia Espaciais). Divisão de Mecânica e Controle. Instituto Nacional de Pesquisas Espaciais. 8. SOUSA, C. T.; KUGA, H. K.; SETZER, A. W. Geo-Location of transmitters using real data, Doppler shifts and Least Squares. Acta Astronautica, v. 52, n. 9, p , WERTZ, R. J. Spacecraft attitude determination and control. Dordrecht: D.Reidel, p.