doi:10.3723/ut.30.189 International Journal of the Society for Underwater Technology, Vol 30, No 4, pp 189–194, 2012 per GPS diving computer for underwater tracking and mapping 1 1 2 2 3,4 Benjamin Kuch* , Giorgio Buttazzo , Elaine Azzopardi , Martin Sayer and Arne Sieber Pa Technical 1Scuola Superiore Sant’Anna – RETIS Lab, Italy 2Scottish Association for Marine Science, UK 3Institute of Micro and Nanotechnology (IMEGO AB), Sweden 4Seabear Diving Technology, Austria Abstract underwater vehicles computing relatively basic tri- Obtaining accurate and affordable geo-referencing is not angulations between the vehicle-mounted transducer straightforward for divers because there is a lack of through- and an array of seabed transponders (e.g. Scheirer water penetration by global positioning systems (GPS). et al., 2000). Coupling USBL with an inertial naviga- Although a number of commercially available systems exist, tion system (INS) can enhance vehicle position and few are low-cost or operationally flexible enough for use in orientation estimates (Morgado et al., 2006). scientific diving. The present paper details a new GPS diving Another way to navigate underwater is to use an computer that supports navigation and Global System for acoustic tracking beacon and a receiver station Mobile Communications (GSM) underwater. The unit displays (which may have GPS positioning) with a set of the distance and heading information to set points and tracks hydrophones (underwater microphones). The bea- the dive in three dimensions (position, depth and time). con continuously sends out an acoustic signal, When downloaded, the tracked dive profile can be visualised which is received by the hydrophones. Distance is in 3D in Google Earth. By synchronising the dive computer clock to external recording devices (for example, cameras), then calculated by the signal runtime, and the tar- any recording event can be geo-referenced with attached get direction is measured by the phase shift between data relating to GPS position, temperature and depth. the different hydrophones (Gamroth et al., 2011). Most of these systems require preparation time, Keywords: underwater navigation, diver positioning, global since specific hardware needs to be deployed before positioning system (GPS), Global System for Mobile Commu- the dive. In addition, inaccuracies can occur since nications (GSM), diving computer acoustic signals may get disturbed by environmental noise, or may get blocked or reflected by larger objects like wrecks or big rocks. Furthermore, the 1. Introduction velocity of sound changes with temperature, salinity A major challenge for occupational divers is acquir- and pressure, all of which reduces accuracy as well. ing accurate navigation and positioning under- Positioning of unmanned underwater vehicles water mainly because the diver is moving in a typically makes use of an inertial measurement unit three-dimensional space and, quite often, there are (IMU) in combination with a Doppler velocity log limitations to being able to visualise reference (Lee et. al., 2005; Willumsen et. al., 2006; Huang points in conditions of low water clarity. Advanced et. al., 2010; Miller et al., 2010). The IMU computes technical solutions to this problem exist in the orientation and heading using a three-axial gyro- form of ultra-short baseline (USBL; sometimes scope. Since a gyroscope drifts over time, the values called super-short baseline (SSBL)) and long base- are corrected by a three-axial accelerometer and a line (LBL) acoustic positioning systems. USBLs allow three-axial magnetometer values using advanced for multiple subsea targets to be accurately posi- signal processing techniques like the Kalman tioned relative to a surface vessel (e.g. remotely or Complementary filter (Marins et. al., 2001; operated vehicle (ROV), tow fish), while LBLs pro- Madgwick, 2010). If the direction is known, the vide a method of accurate positioning over a wide position can be calculated by multiplying the veloc- area using transponder separations from 100m to ity provided by the Doppler velocity log. Such a several kilometres. Invariably, USBL or LBL technol- navigation system also exists for divers (Hartman ogies are used to position ROVs or unmanned et al., 2008), but is bulky (31 × 37 × 33cm), heavy (12kg) and expensive (several tens of thousands * Contact author. E-mail address: [email protected] of euros). 189 Kuch et al. GPS diving computer for underwater tracking and mapping INS uses just an IMU to calculate the actual posi- tion via dead reckoning, whereby accelerations and orientations are assessed in fixed time intervals ti( ). INS calculates orientation using three-axial gyro- scopes, accelerometers and magnetometers (as described earlier), which is then used to project the local acceleration vector of the IMU local coordi- nate frame into the global earth coordinate frame. The global velocity vector can be computed using a gravity-compensated acceleration vector multiplied by ti; the final global position is the product of the global velocity vector, multiplied again by ti. Unfor- tunately, dead reckoning purely based on IMUs works just for short time periods and only on fast accelerating objects like rockets or airplanes. Dead reckoning of a diver in 3D and purely based on inertial navigation is extremely challenging, mainly because diver accelerations are small compared to gravity (Kuch et al., 2011). Current underwater navigation systems that have been developed or adopted for diving applications either are not cost-effective for relatively small and mobile operations, or are based on evolving technol- ogies. The main objective of this study is to develop Fig 1: Hardware diagram an easy-to-use, lightweight and cheap underwater navigation system based on GPS, since GPS is not available underwater because the electromagnetic The diving computer comprised mainly of a micro- waves from its satellites do not penetrate water. controller, a pressure sensor, a flash memory chip Therefore, for any low-cost option, it is necessary to and a display (Koss and Sieber, 2011). The compu- have the GPS receiver on surface, floating above ter had one serial port for communication with the diver. Previous studies have used GPS receivers external hardware as described by Kuch et al. (2010). mounted in surface marker buoys above divers in The diving computer was mounted on a reel isolation. Matching the time code of the GPS down- (diameter 12cm), which carried the cable connect- loads to specific underwater ‘events’ generated ing the GPS transmitter to the diving computer. approximate geo-referencing (e.g. Collins and The transmitter was the Telit GM862-GPS module, Baldock, 2007). which also included a Global System for Mobile The present study aims to further develop this Communications (GSM) modem. approach through developing an in situ dive compu- Communication between the diving computer ter that is able to communicate with the GPS receiver and buoy-system was via serial communication and record the whole dive track and depth profile. (four-wire cable: received data (RXD), transmitted In doing so, it is intended that the diver would be data (TXD), mass, on/off switch). The wire was not able to enter GPS coordinates as set points, show dis- specially shielded because of the lack of any elec- tance and angle to these preset GPS set points while tromagnetic compatibility issues underwater. The also providing a tool to incorporate GPS coordi- communication speed was 9600 baud and the over- nates, temperature and depth into the International all low power consumption permitted the whole Press Telecommunications Council (IPTC, 2012) unit to be powered by a single 3V AA battery. The metadata files of any photographs taken during the buoy-system was powered by a rechargeable lithium- dive. Finally, the present study will provide a tool by ion mobile phone battery (1800mAh). which downloaded dive profiles from the computer can be visualised in 3D. 2.2. System software 2.2.1. Firmware The firmware of the device was developed in the 2. Methods programming language C in the IAR Embedded 2.1. System hardware Workbench (IAR Systems) development envi- The hardware consisted of a scuba buoy-system ronment. The diving computer acquired and dis- with an attached modified diving computer (Fig 1). played data, and depth, temperature and time were 190 Vol 30, No 4, 2012 recorded. In addition, the GPS data were received 2.2.2. PC software by the computer unit over the serial interface. A The PC software was developed in Java 1.6 under the menu-based user interface could be used to set up Eclipse SDK 3.4.1 and the Eclipse Standard Widget time/date, enter the PC transmission mode and Toolkit to keep it platform independent. RXTX 2.1 handle GSM/GPS actions. was chosen as the serial communication library. The basic GPS function of the diving computer The software provided a management and configu- was to store GPS data in the flash memory in 1sec ration suite consisting of three major features. The intervals throughout the dive. In addition, event first was configuration of the navigation system positions were stored via the GPS menu. An inte- which permitted all adjustments, such as GSM set- grated homing function allowed the user to choose tings and time synchronisation, to be set. preset points forming reference points to which The second feature of the software converted the diver could reverse navigate, with distance and dive data transfer into an output file that could sup- direction to that set point being shown on the dis- port visualising the data in 3D. The dive data would play (Kaplan, 2005). then need to be downloaded via the PC software The short message service (SMS) supported two and converted afterwards. Dive profiles were con- different kinds of pre-defined message to be sent to verted into the keyhole mark-up language (KML) a pre-defined mobile phone number (configured format. KML is the extensible mark-up language within the PC software).