GPS Diving Computer for Underwater Tracking and Mapping

doi:10.3723/ut.30.189 International Journal of the Society for Underwater Technology, Vol 30, No 4, pp 189–194, 2012

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 per 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 position 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 coordinate 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). One type was an emergency (XML) notation for expressing geographic anno- SMS including the actual depth and GPS coordi- tation and visualisation data and had been spe- nates of the diver at the time of sending, and the cifically designed for use with the Google Earth other was the actual depth and GPS coordinates. application. Google Earth was the chosen platform To be able to handle all necessary data tasks for displaying the recorded data, as it already pro- (acquisition, storage, visualisation and computation) vides a framework by which to visualise 3D objects simultaneously, the firmware contained a scheduling inside a map without further programming neces- mechanism that handled time-critical tasks within sary. Irrespective of that, KML files can be imported interrupt routines. Depth measurements and display to 3DS or Blender with some minor re-formatting. updates were made within a timer interrupt occur- Since only viewed parts of the ocean are mapped ring in 250ms intervals. Reception of GPS/GSM in Google Earth, the PC software converted the tasks was done in the USART (universal syn- depth profile into an altitude profile (Fig 2). chronous/asynchronous receiver/transmitter) inter- Although Google Earth can display a depth profile, rupt. Position calculations, data storage and SMS there is no method to correct the anomaly that transmission were done in the main loop. the profile was showing above sea level. The dive

Fig 2: KML-generated profile of an example dive using the experimental unit and opened in Google Earth

191 Kuch et al. GPS diving computer for underwater tracking and mapping

data were also converted into a comma separated value (CSV) file, which can be easily analysed using Microsoft Excel. The final feature of the PC software permitted the addition of GPS, date/time and depth information into a photograph taken during a dive. Before a dive was undertaken, the internal clocks of the diving computer and the camera were time- synchronised. Most digital image file formats con- tain, beside the image itself, a metadata container for image information. Metadata files are defined in the exchangeable image file (EXIF) format and the IPTC standard (IPTC, 2012). The EXIF metadata container contains data about the basic image that Fig 4: GPS diving computer prototype mounted on a was taken (camera manufacturer, camera model, cable reel image resolution, ISO ratings, aperture, exposure, etc.). The IPTC metadata container contains addi- to 300m. The buoy-system was housed in a modi- tional image information (photographer, copyright, fied drybox (Dive Egg: inside diameter of 5cm; image description, keywords, etc.). height of 10cm; Mares specified to 50m). The diving Typically GPS data were stored as a metadata computer and buoy-system were connected to each attribute in the EXIF format. However, there were other via a 50m-long four-wire cable. To be able to no fields available for additional information like recharge the buoy-system battery, a waterproof depth or water temperature. To avoid dividing the (IP68 rated) charging connector was placed at the storage of position, depth and temperature data in side of the buoy-system. different locations and formats, all the data were The final system was small and compact, with the stored together as a defined string into the IPTC cable between diving computer and buoy-system caption feature. If the picture timestamp matched having a diameter of 3mm and thus not thicker than the timestamp stored inside the KML file by± 2sec, a regular buoy-line. the IPTC metadata container was updated (Fig 3). The prototype was tested in the Mediterranean Sea, where several dives in Labin, Croatia, and Livorno, Italy, were tracked and visualised in Google 3. Results Earth, including the depth profile. The positional One prototype was manufactured (Fig 4). The hand- accuracy of the system was estimated (using charted set was housed in an aluminium case and encapsu- features such as underwater cliffs) to be less than lated with silicone gel to ensure water resistance up 5m. The IPTC data of the photographs taken during

Fig 3: IPTC metadata with the addition of GPS, depth and temperature information

192 Vol 30, No 4, 2012

Fig 5: Illustration of the difficulty in maintaining the buoy directly above the diver: (a) shallow dive with optimal sea surface conditions; and (b) deep dive with rough sea surface conditions the dives were all complemented by position, depth employing the added capabilities of navigating to and temperature information. fixed points and/or adding geo-referencing data to photographs. The integration of additional flow sensors on 4. Discussion the surface buoy could help compensate for some A prototype GPS diving computer was developed of the measurement error. This information could and proved to support underwater navigation and be further integrated with data of the diver’s depth GSM communication. Although not tested method- and the length of the unrolled cable, thereby giving ically in the present study, this fairly basic and low- an estimate the buoy drift during the dive. Although cost device provided a realistic 3D representation this solution would improve accuracy it would also of the dive profile while, at the same time, provid- require additional hardware and processing power. ing the capability to geo-reference still photographs with reasonable accuracy. Acknowledgement With any cable connected device, accuracy will always be dependent on the ability to maintain the Special thanks to Martin Henke for his contribu- buoy over the top of the diver as precisely as possible tion of the illustrations in Fig 5. (Schories and Niedzwiedz, 2012). However, there are numerous factors which make this difficult to References achieve. Under optimal conditions the accuracy of Collins KJ and Baldock B. (2007). Use of diving computers the GPS receiver is <2.5m, but in windy and rough in brittlestar surveys. Underwater Technology 27: 115–118. sea surface conditions or with current, it is difficult Gamroth E, Kennedy J and Bradley C. (2011). Design and to maintain the buoy’s position. That accuracy will testing of an acoustic ranging technique applicable for also be compromised as soon as the diver tries to an underwater positioning system. Underwater Technology move horizontally through the water column, as 29: 183–193. the effect of this movement (particularly the drag Hartman R, Hawkinson W, Sweeney K and Gurgaon H. (2008). Tactical underwater navigation system (TUNS). In: on the cable) will be multiplied as the diver moves Proc. Position, Location and Navigation Symposium. New York: to greater depths (Fig 5). Institute of Electrical and Electronics Engineers, 898–911. It is, therefore, suggested that this system, as it is Huang L, He B and Zhang T. (2010). An autonomous navi- presently configured, will provide relatively accu- gation algorithm for underwater vehicles based on iner- rate results in shallow water, in favourable weather tial measurement units and sonar. In: Proc. 2nd Intl Asia Conf Informatics in Control, Automation and Robotics, vol 6, conditions and at stationary locations. In their 311–314. study on the precision and accuracy of diver-towed International Press Telecommunications Council. (2012). underwater GPS receivers, Schories and Niedzwiedz IPTC Standards. Available at www.iptc.org, last accessed (2012) modelled and recorded GPS displacements <03 February 2012>. of 2.3m in 5m depth; 3.2m in 10m depth; 4.6m in Kaplan ED. (2005). Understanding GPS: Principles and Appli- 20m depth; 5.5m at 30m depth; and 6.8m in 40m cations, second edition. Boston: Artech House, 726pp. Koss B and Sieber A. (2011). Head-mounted display for diving depth. The unit in the present study gives the diver computer platform. Journal of Display Technology 7: 193–199. in situ and real-time information on positioning, so Kuch B, Haasl S, Wagner M, Buttazzo G and Sieber A. (2011). overall accuracy may not be quite as relevant when Preliminary report: Embedded platform for inertial based

193 Kuch et al. GPS diving computer for underwater tracking and mapping

underwater navigation. In: Proc. 9th Workshop on Intelligent Miller PA, Farrell JA, Zhao Y and Djapic V. (2010). Autono- Solutions in Embedded Systems, 101–108. mous underwater vehicle navigation. IEEE Journal of Kuch B, Koss B, Dujic Z, Buttazzo G and Sieber A. (2010). Oceanic Engineering 35: 663–678. A novel wearable apnea dive computer for continuous Morgado M, Oliveira P, Silvestre C and Vasconcelos JF. plethysmographic monitoring of oxygen saturation and (2006). USBL/INS tightly-coupled integration technique heart rate. Diving and Hyperbaric Medicine 40: 34–40. for underwater vehicles. In: Proc. 9th Intl. Conf. on Infor- Lee PM, Jun BH, Choi HT and Hong SW. (2005). An inte- mation Fusion. New York: IEEE, 8pp. grated navigation systems for underwater vehicles based Scheirer DS, Fornari DJ, Humphris SE and Lerner S. on inertial sensors and pseudo LBL acoustic transpond- (2000). High-resolution seafloor mapping using the ers. In: Proc. Oceans, vol 1, 555–562. DSL-120 sonar system: Quantitative assessment of sides- Madgwick SOH. (2010). An efficient orientation filter for can and phase-bathymetry data from the Lucky Strike inertial and inertial/magnetic sensor arrays. Technical segment of the Mid-Atlantic Ridge. Marine Geophysical report, University of Bristol, 32pp. Available at http:// Research 21: 121–142. sharenet-wii-motion-trac.googlecode.com/downloads/ Schories D and Niedzwiedz G. (2012). Precision, accuracy, list, last accessed <09 March 2012>. and application of diver-towed underwater GPS receivers. Marins JL, Yun X, Bachmann ER, McGhee RB and Zyda MJ. Environmental Monitoring and Assessment 184: 2359–2372. (2001). An extended Kalman filter for quaternion-based Willumsen AB, Hallingstad O and Jalving B. (2006). Integra- orientation estimation using MARG sensors. In: Proc. of tion of range, bearing and Doppler measurements from the IEEE/RSJ International Conference on Intelligent Robots transponders into underwater vehicle navigation systems. and Systems, vol 4, 2003–2011. Proc. Oceans 2006: 1–6.

SubSea Control and data aCquiSition 2010 Future Technology, Availability and Through Life Changes

Proceedings of the International Conference held in Newcastle, UK, 2-3 June 2010

ISBN 0 906940 52 4 ISBN-13 978 0906940525 Hardback edition 2010 Price: £95 Published by SUT *10% discount for 176 pages plus CD-ROM SUT members and booksellers

This long established international conference dealing exclusively with underwater instrumentation, control and communication technology for subsea oil and gas production has been structured to cover relevant experience and new thinking in subsea developments. Providing a unique forum for the supplier and operator of subsea technology to exchange views and experiences, its aim is to bring together the many diverse disciplines engaged internationally in this technology. Experience gained and current challenges, as well as new advances in technology, will be the main topics to meet the future challenges. Equal importance has been placed on reliability and global issues such as environment, decommissioning, deepwater problems and long distance offsets. These proceedings will feature contributions from professionals giving experience gained and new challenges to overcome and is therefore of interest to all in subsea engineering.

For more information or to purchase a copy, contact SUT Head Office 1 Fetter Lane London EC4A 1BR UK t +44 (0)20 3440 5535 f +44 (0)20 3440 5980 e [email protected] Publications Catalogue is available on the SUT website at www.sut.org

194