Sensors_**2012**, _12_, 1930-1949; doi:10.3390/s120201930  
_Sensors_  
**ISSN 1424-8220**  
www.mdpi.com/journal/sensors  
_Article_  
**Sunlight Intensity Based Global Positioning System for Near-Surface Underwater Sensors**  
**[PERSON] \({}^{1}\), [PERSON] \({}^{2,\star}\) and [PERSON] \({}^{3}\)**  
\({}^{1}\) Department of Systems and Automation, Carlos III University of Madrid, Avda. de la Universidad 30, 28911 Leganes, Madrid, Spain; E-Mail: [EMAIL_ADDRESS]  
\({}^{2}\) Institute of Information Technology, Faculty of Technology, Art and Design, Oslo and Akershus University College of Applied Sciences, P.O. Box 4, St. Olavs plass, N-0130 Oslo, Norway  
\({}^{3}\) Department of Computer Science, Rey Juan Carlos University of Madrid, Calle Tulipan s/n, 28933 Mostoles, Madrid, Spain; E-Mail: [EMAIL_ADDRESS]  
\({}^{\star}\) Author to whom correspondence should be addressed; E-Mail: [PERSON]el. : +47-22-45-30-23. _Received: 15 December 2011; in revised form: 1 February 2012 / Accepted: 7 February 2012 / Published: 10 February 2012_  
**Abstract:** Water monitoring is important in domains including documenting climate change, weather prediction and fishing. This paper presents a simple and energy efficient localization strategy for near surface buoy based sensors. Sensors can be dropped randomly in the ocean and thus self-calibrate in terms of geographic location such that geo-tagged observations of water quality can be made without the need for costly and energy consuming GPS-hardware. The strategy is based on nodes with an accurate clock and light sensors that can regularly sample the level of light intensity. The measurements are fitted into a celestial model of the earth motion around the sun. By identifying the trajectory of the sun across the skies one can accurately determine sunrise and sunset times, and thus extract the longitude and latitude of the sensor. Unlike previous localization techniques for underwater sensors, the current approach does not rely on stationary or mobile reference points. **Keywords:** outdoor; localization; light; intensity; global positioning; sunlight; underwater Introduction  
Most of the Earth's surface is covered in water which affects us in numerous ways. We depend on the oceans as a vital source for food. Moreover, the conditions in the oceans greatly affect the weather on land. Increasingly, we have become aware that our actions also affect the world's oceans and there is currently much focus on climate change. For instance, variations in ocean salinity are important as it is a vital indicator of water cycle and used for climate forecasting [1]. Several ocean-monitoring systems have been developed for collecting data about the upper ocean salinity and temperature data using ships and buoys. Not only are the measurements themselves important, but also the geographic location where the measurement is made. In addition, underwater or near surface sensors and sensor networks have been used for several other civilian and military applications such as salmon detection and tracking [2]. There is a vast body of research on underwater sensors and sensor networks. Issues addressed include energy consumption [3], image analysis [4; 5; 6], optics [7], routing protocols [8; 9; 10], sound [11] and acoustic localization through measurements [12]. In fact, localization is a challenging problem for several reasons. The commonly used GPS technology is dependent on energy consuming hardware, long satellite locking times and a satellite infrastructure that is approaching the end of its lifetime [13]. Moreover, another problem with GPS technology is that GPS signals do not propagate through water [14], and this is the reason existing research on underwater sensor networks focuses on acoustic communication. However, underwater conditions are often characterized by harsh physical conditions resulting in low bandwidth, large propagation delays and high bit error rates. Another challenge is the speed of sound due to water currents.