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doi:10.3723/ut.30.189 International Journal of the Society for Underwater Technology, Vol 30, No 4, pp 189194, 2012

TechnicalPaper

GPS diving computer for underwater tracking

and mapping

Benjamin Kuch*1, Giorgio Buttazzo1, Elaine Azzopardi2, Martin Sayer2 and Arne Sieber3,4

1

Scuola Superiore SantAnna RETIS Lab, Italy2Scottish Association for Marine Science, UK

3Institute of Micro and Nanotechnology (IMEGO AB), Sweden

4Seabear Diving Technology, Austria

Abstract

Obtaining accurate and aordable geo-reerencing is not

straightorward or divers because there is a lack o through-

water penetration by global positioning systems (GPS).

Although a number o commercially available systems exist,

ew are low-cost or operationally exible enough or use in

scientifc diving. The present paper details a new GPS diving

computer that supports navigation and Global System or

Mobile Communications (GSM) underwater. The unit displays

the distance and heading inormation to set points and tracks

the dive in three dimensions (position, depth and time).

When downloaded, the tracked dive profle can be visualised

in 3D in Google Earth. By synchronising the dive computer

clock to external recording devices (or example, cameras),

any recording event can be geo-reerenced with attacheddata relating to GPS position, temperature and depth.

Keywords: underwater navigation, diver positioning, global

positioning system (GPS), Global System or Mobile Commu-

nications (GSM), diving computer

1. Introduction

A major challenge or occupational divers is acquir-ing accurate navigation and positioning under-

water mainly because the diver is moving in a

three-dimensional space and, quite oten, there arelimitations to being able to visualise reerencepoints in conditions o low water clarity. Advancedtechnical solutions to this problem exist in theorm o ultra-short baseline (USBL; sometimescalled super-short baseline (SSBL)) and long base-line (LBL) acoustic positioning systems. USBLs allowor multiple subsea targets to be accurately posi-tioned relative to a surace vessel (e.g. remotelyoperated vehicle (ROV), tow sh), while LBLs pro-

vide a method o accurate positioning over a wide

area using transponder separations rom 100m toseveral kilometres. Invariably, USBL or LBL technol-ogies are used to position ROVs or unmanned

underwater vehicles computing relatively basic tri-angulations between the vehicle-mounted transducer

and an array o seabed transponders (e.g. Scheireret al., 2000). Coupling USBL with an inertial naviga-tion system (INS) can enhance vehicle position andorientation estimates (Morgado et al., 2006).

Another way to navigate underwater is to use anacoustic tracking beacon and a receiver station(which may have GPS positioning) with a set ohydrophones (underwater microphones). The bea-con continuously sends out an acoustic signal,

which is received by the hydrophones. Distance isthen calculated by the signal runtime, and the tar-

get direction is measured by the phase shit betweenthe dierent hydrophones (Gamroth et al., 2011).Most o these systems require preparation time,since specic hardware needs to be deployed beorethe dive. In addition, inaccuracies can occur sinceacoustic signals may get disturbed by environmentalnoise, or may get blocked or refected by largerobjects like wrecks or big rocks. Furthermore, the

velocity o sound changes with temperature, salinityand pressure, all o which reduces accuracy as well.

Positioning o unmanned underwater vehiclestypically makes use o an inertial measurement unit

(IMU) in combination with a Doppler velocity log(Lee et. al., 2005; Willumsen et. al., 2006; Huanget. al., 2010; Miller et al., 2010). The IMU computesorientation and heading using a three-axial gyro-scope. Since a gyroscope drits over time, the valuesare corrected by a three-axial accelerometer and athree-axial magnetometer values using advancedsignal processing techniques like the Kalmanor Complementary lter (Marins et. al., 2001;Madgwick, 2010). I the direction is known, theposition can be calculated by multiplying the veloc-

ity provided by the Doppler velocity log. Such anavigation system also exists or divers (Hartmanet al., 2008), but is bulky (31 37 33cm), heavy(12kg) and expensive (several tens o thousandso euros).* Contact author. E-mail address: benjamin.kuch@gmx.de

mailto:benjamin.kuch@gmx.demailto:benjamin.kuch@gmx.de

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INS uses just an IMU to calculate the actual posi-tion via dead reckoning, whereby accelerations andorientations are assessed in xed time intervals (ti).INS calculates orientation using three-axial gyro-scopes, accelerometers and magnetometers (asdescribed earlier), which is then used to project the

local acceleration vector o the IMU local coordi-nate rame into the global earth coordinate rame.The global velocity vector can be computed using agravity-compensated acceleration vector multipliedbyti; the nal global position is the product o theglobal velocity vector, multiplied again byti. Unor-tunately, dead reckoning purely based on IMUs

works just or short time periods and only on astaccelerating objects like rockets or airplanes. Deadreckoning o a diver in 3D and purely based oninertial navigation is extremely challenging, mainly

because diver accelerations are small compared togravity (Kuch et al., 2011).Current underwater navigation systems that have

been developed or adopted or diving applicationseither are not cost-eective or relatively small andmobile operations, or are based on evolving technol-ogies. The main objective o this study is to developan easy-to-use, lightweight and cheap underwaternavigation system based on GPS, since GPS is notavailable underwater because the electromagnetic

waves rom its satellites do not penetrate water.Thereore, or any low-cost option, it is necessary to

have the GPS receiver on surace, foating abovethe diver. Previous studies have used GPS receiversmounted in surace marker buoys above divers inisolation. Matching the time code o the GPS down-loads to specic underwater events generatedapproximate geo-reerencing (e.g. Collins andBaldock, 2007).

The present study aims to urther develop thisapproach through developing an in situdive compu-ter that is able to communicate with the GPS receiverand record the whole dive track and depth prole.

In doing so, it is intended that the diver would beable to enter GPS coordinates as set points, show dis-tance and angle to these preset GPS set points whilealso providing a tool to incorporate GPS coordi-nates, temperature and depth into the InternationalPress Telecommunications Council (IPTC, 2012)metadata les o any photographs taken during thedive. Finally, the present study will provide a tool by

which downloaded dive proles rom the computercan be visualised in 3D.

2. Methods2.1. System hardware

The hardware consisted o a scuba buoy-systemwith an attached modied diving computer (Fig 1).

The diving computer comprised mainly o a micro-controller, a pressure sensor, a fash memory chipand a display (Koss and Sieber, 2011). The compu-

ter had one serial port or communication withexternal hardware as described by Kuch et al. (2010).The diving computer was mounted on a reel(diameter 12cm), which carried the cable connect-ing the GPS transmitter to the diving computer.The transmitter was the Telit GM862-GPS module,

which also included a Global System or MobileCommunications (GSM) modem.

Communication between the diving computerand buoy-system was via serial communication(our-wire cable: received data (RXD), transmitted

data (TXD), mass, on/o switch). The wire was notspecially shielded because o the lack o any elec-tromagnetic compatibility issues underwater. Thecommunication speed was 9600 baud and the over-all low power consumption permitted the wholeunit to be powered by a single 3V AA battery. Thebuoy-system was powered by a rechargeable lithium-ion mobile phone battery (1800mAh).

2.2. System software2.2.1. Firmware

The rmware o the device was developed in the

programming language C in the IAR EmbeddedWorkbench (IAR Systems) development envi-ronment. The diving computer acquired and dis-played data, and depth, temperature and time were

Fig 1: Hardware diagram

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recorded. In addition, the GPS data were receivedby the computer unit over the serial interace. Amenu-based user interace could be used to set uptime/date, enter the PC transmission mode andhandle GSM/GPS actions.

The basic GPS unction o the diving computer

was to store GPS data in the fash memory in 1secintervals throughout the dive. In addition, eventpositions were stored via the GPS menu. An inte-grated homing unction allowed the user to choosepreset points orming reerence points to whichthe diver could reverse navigate, with distance anddirection to that set point being shown on the dis-play (Kaplan, 2005).

The short message service (SMS) supported twodierent kinds o pre-dened message to be sent toa pre-dened mobile phone number (congured

within the PC sotware). One type was an emergencySMS including the actual depth and GPS coordi-nates o the diver at the time o sending, and theother was the actual depth and GPS coordinates.

To be able to handle all necessary data tasks(acquisition, storage, visualisation and computation)simultaneously, the rmware contained a schedulingmechanism that handled time-critical tasks withininterrupt routines. Depth measurements and displayupdates were made within a timer interrupt occur-ring in 250ms intervals. Reception o GPS/GSMtasks was done in the USART (universal syn-

chronous/asynchronous receiver/transmitter) inter-rupt. Position calculations, data storage and SMStransmission were done in the main loop.

2.2.2. PC sotware

The PC sotware was developed in Java 1.6 under theEclipse SDK 3.4.1 and the Eclipse Standard WidgetToolkit to keep it platorm independent. RXTX 2.1

was chosen as the serial communication library.The sotware provided a management and congu-

ration suite consisting o three major eatures. Therst was conguration o the navigation system

which permitted all adjustments, such as GSM set-tings and time synchronisation, to be set.

The second eature o the sotware converteddive data transer into an output le that could sup-port visualising the data in 3D. The dive data wouldthen need to be downloaded via the PC sotwareand converted aterwards. Dive proles were con-

verted into the keyhole mark-up language (KML)ormat. KML is the extensible mark-up language

(XML) notation or expressing geographic anno-tation and visualisation data and had been spe-cically designed or use with the Google Earthapplication. Google Earth was the chosen platormor displaying the recorded data, as it already pro-

vides a ramework by which to visualise 3D objectsinside a map without urther programming neces-sary. Irrespective o that, KML les can be importedto 3DS or Blender with some minor re-ormatting.

Since only viewed parts o the ocean are mappedin Google Earth, the PC sotware converted thedepth prole into an altitude prole (Fig 2).

Although Google Earth can display a depth prole,there is no method to correct the anomaly thatthe prole was showing above sea level. The dive

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

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data were also converted into a comma separatedvalue (CSV) le, which can be easily analysed usingMicrosot Excel.

The nal eature o the PC sotware permittedthe addition o GPS, date/time and depth inor-mation into a photograph taken during a dive.

Beore a dive was undertaken, the internal clockso the diving computer and the camera were time-synchronised. Most digital image le ormats con-tain, beside the image itsel, a metadata containeror image inormation. Metadata les are dened inthe exchangeable image le (EXIF) ormat and theIPTC standard (IPTC, 2012). The EXIF metadatacontainer contains data about the basic image that

was taken (camera manuacturer, camera model,image resolution, ISO ratings, aperture, exposure,etc.). The IPTC metadata container contains addi-

tional image inormation (photographer, copyright,image description, keywords, etc.).Typically GPS data were stored as a metadata

attribute in the EXIF ormat. However, there wereno elds available or additional inormation likedepth or water temperature. To avoid dividing thestorage o position, depth and temperature data indierent locations and ormats, all the data werestored together as a dened string into the IPTCcaption eature. I the picture timestamp matchedthe timestamp stored inside the KML le by2sec,the IPTC metadata container was updated (Fig 3).

3. Results

One prototype was manuactured (Fig 4). The hand-set was housed in an aluminium case and encapsu-lated with silicone gel to ensure water resistance up

to 300m. The buoy-system was housed in a modi-

ed drybox (Dive Egg: inside diameter o 5cm;height o 10cm; Mares specied to 50m). The divingcomputer and buoy-system were connected to eachother via a 50m-long our-wire cable. To be able torecharge the buoy-system battery, a waterproo(IP68 rated) charging connector was placed at theside o the buoy-system.

The nal system was small and compact, with thecable between diving computer and buoy-systemhaving a diameter o 3mm and thus not thicker thana regular buoy-line.

The prototype was tested in the Mediterranean

Sea, where several dives in Labin, Croatia, andLivorno, Italy, were tracked and visualised in GoogleEarth, including the depth prole. The positionalaccuracy o the system was estimated (using chartedeatures such as underwater clis) to be less than5m. The IPTC data o the photographs taken during

Fig 3: IPTC metadata with the addition o GPS, depth and temperature inormation

Fig 4: GPS diving computer prototype mounted on a

cable reel

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the dives were all complemented by position, depthand temperature inormation.

4. Discussion

A prototype GPS diving computer was developedand proved to support underwater navigation andGSM communication. Although not tested method-ically in the present study, this airly basic and low-cost device provided a realistic 3D representationo the dive prole while, at the same time, provid-ing the capability to geo-reerence still photographs

with reasonable accuracy.With any cable connected device, accuracy will

always be dependent on the ability to maintain thebuoy over the top o the diver as precisely as possible(Schories and Niedzwiedz, 2012). However, thereare numerous actors which make this dicult toachieve. Under optimal conditions the accuracy othe GPS receiver is

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