Satellite Sea Turtle Tracking

ROBERT E. TIMKO and A. LAWRENCE KOLZ

Introduction

For years, marine biologists haveaspired to develop a means of recordingthe daily movements and migrationroutes of far-ranging animals such asthe various species of sea turtles. Suchknowledge would define preferred turtlehabitats, locate nesting sites, provideforaging and behavioral data, and directadministrators toward sound manage­ment decisions.

Historically, marine turtles have beentracked with flipper tags (Caldwell,1962), tethered floats (Carr et aI., 1974),balloons (Carr, 1962), and radio beacons(Carr et aI., 1972). Flipper tag returnsrely on the chance returns by others andmake a study very unreliable. Tetheredfloats and balloons are extremely ex­pensive for they require a continuoussupport vessel to maintain visual contact,and they are restricted to fair-weatheroperation. Tracking with radio beacons,also costly, has been done with somesuccess, but the experiments were limitedto short duration and restricted range.

Robert E. Timko is with the National SpaceTechnology Laboratories. National MarineFisheries Service. NOAA. NSTL Station, MS39529. A. Lawrence Kolz is with the DenverWildlife Research Center. U.S. Fish and WildlifeService, Denver, CO 80225.

ABSTRACT- A female loggerhead seaturtle. Caretta caretta, weighing 96 kg wasinstrumented with a buoyant cylindrical radiotransmillerdevelopedjoint~v by the u.s. Fishand Wildlife Service and the National MarineFisheries Service. The transmiller was at­tached to the rear edge ofthe turtle scarapace

April 1lJ82. 44(4)

Our research study provides an alterna­tive: Tracking sea turtles with a satellitereceiving system.

Methods

Background

The feasibility of tracking wild animalsby satellite was originally demonstratedby Craighead et al. (1972), when an elkwas tracked for 2R days with the NIM­BUS 3 weather satellite. In 1977. theU.S. Fish and Wildlife Service (FWS)successfully tracked polar hears (Kolz etaI., 1980) using the NIMBUS 6/RandomAccess Measurement System (RAMS)(Cote et al.. 1973).

The transmitter used for tracking thepolar bears was constructed by theHandar Company', Santa Clara, Calif. Itconsisted of a transmitter module (14 X14 x 5 cm) and a 5 kg plastic attachmentcollar which contair.ed the battery pack.The module contained the temperature­controlled oscillator, radio transmittercircuits, timing controls, and antenna.The coplanar antenna was contained in

IMention of trade names or commercial firmsdoes nOl imply endorsement hy the NationalMarine Fisheries Service. NOAA.

with a nylon tether; and the wrtle was trackedbv the NIMBUS 6satellite svstem ror8monthsduring 1979and 1980. The'turtle'was releasedabout 20 miles olfMississippi into the GullorMexico and traveled west and south to withina few miles ol Brownsville. Tex. The totaltracking distance exceeded 1.400 miles.

the lid of the transmitter module. Lithiumbatteries encapsulated in the collarserved as the power supply with sufficientcapacity for 1 year's operation. Theinstrument was designed to transmitpulses to the satellite for an 8- hour periodevery fourth day. This low-duty cyclewas necessary to conserve the batteries,extend the transmitter operating life to1 year, and reduce the total packageweight.

This transmitter-collar unit was at­tached to a polar bear near Point Barrow.Alaska, and tracked by the satellitesystem for 390 days and over 1,000 miles.Although there are drastic physical andbehavioral differences between polarbears and turtles, the NIMBUS satellitesystem and the hardware developed forthe bear transmitter offered an approachwhich could be adapted for marine turtletracking.

Satellite Tracking Equipment:The NIMBUS System

The NIMBUS 6/RAMS satellite sys­tem is a data collection and transmitterlocation system designed to collectmeteorological and oceanographic datatransmitted from randomly located mo­bile transmitters. The system is capableof handling the random transmissionsfor 200 transmitters within its field ofview with a maximum of eight signalsoccurring simultaneously. The satellitestorage capacity allows for 1,000 trans­mitters worldwide.

Data are transmitted to the satellite as401.2 MHz radio frequency (rf) pulses.These pulses have a duration of about 1second and occur at a rate of approxi­mately one pulse per minute. Each pulse

I I)

contains a period of unmodulated carrierand a phase-modulated section for datasynchronization, transmitter identifica­tion, and sensor data (if included). Theunmodulated portion of rf carrier is usedto measure the doppler frequency shiftand calculate the latitude and longitudeof the transmitter. The system is capableof determining a position to within ± 5km. This accuracy is a function of thenumber of messages received by thesatellite in a given orbit. A minimum offour messages per orbit must be receivedin order for a position to be computed.

Preliminary Technical Considerations

The National Marine Fisheries Service(NMFS) and the FWS agreed to modifythe hardware developed for polar bearsatellite tracking into an instrumentpackage suitable for attaching to a largesea turtle. Technical questions concernedthe quality of signal that could be trans­mitted from a coplanar antenna floatingdirectly on the ocean's surface becauseof possible signal distortions caused bysurface reflections. Therefore, one testobjective was to prove the feasibility forsatellite tracking under these conditions.

Although the transmitter module forthe polar bear was not of optimum sizeor shape, it was both expedient andeconomical to repackage for marine use.We therefore considered various enclo­sures for housing this basic module withan appropriate battery pack. The prob­lem of attaching any package to a turtlewas of real concern. Little is known aboutmarine turtles, so the following guidelineswere used.

1) The enclosure had to be watertightand capable of withstanding externalwater pressure. We arbitrarily selected adesign specification of 200 pounds persquare inch (psi) since no scientific dataexisted on expected pressure.

2) Radio signals could not be sent tothe satellite from underwater. Therefore,the transmitter design had to maximizeantenna exposure.

3) The package had to be smooth withno sharp edges or protrusions whichcould snag or catch on marine growthsor other obstructions.

4) The package had to be constructedof low-loss dielectric material to allowradio signals to penetrate.

20

5) The package should have a smallcross-sectional area to minimize dragand have minimal effect on a turtle'snormal activity.

After considering these restraints, weconcluded that a small cylinder towedby a lanyard offered the best solution interms of available materials, strength, andsimplicity. Prior research discouragedattachment of a rigid package directly tothe turtle's carapace since turtles fre­quently surface with only their headsexposed. Therefore, the towed cylinderoffered increased antenna exposure timeand greater probability of reception. Acylinder 25 cm long and 15 cm in diam­eter was calculated to satisfy our require­ments for weight versus flotation volume.

Captive Behavioral Studies

We intended to develop a packagethat would not substantially interferewith a turtle's nornlal behavior, and wedevised tests to evaluate the degree otinterference (obviously there would besome). Therefore, a cylindrical model ofa transmitter was constructed, usingplastic pipe fittings. The complete cylin­der weighed 3.1 kg in air and floated 40percent exposed or approximately 1.8kg positively buoyant. This model wasfastened by a nylon tether to the rearedge of the carapace of a 91 kg logger­head turtle, which was the minimumsize turtle considered for this trackingexperiment. The turtle was released intoa large aquarium at Marine Life Park,Gulfport, Miss., and observed for anyabnormal behavior. Aside from someinitial problems with the attachmentmethod, no significant problems wereobserved. However, we decided to use amore precise evaluation method.

A cylindrical recording device (6.4 cmin diameter and 7.6 cm long) was thendeveloped by the FWS electronics staffto quantify the surface behavior of a seaturtle. This recorder sensed the electricalresistance between two external elec­trodes that were either exposed to air asthe cylinder floated on the surface orsurrounded by conductive seawaterwhen the turtle sounded. Electroniccomponents tallied each time the elec­trodes were exposed to air, as well asdata on the cumulative time the cylinderfloated on the surface. We hypothesized

that surface time would be one of themost effective measures to monitor theturtle's behavioral responses to the addedbuoyancy of the satellite transmitterpackage. We assumed that the small sizeof the recording device would have neg­ligible effects on a large turtle.

Two 3-day rests were conducted atthe Marine Life Park with the recordingdevice: First, with only the small record­ing cylinder attached to the 91 kg seaturtle, and then with both the transmittermodel and recording cylinder attached.The results indicate the turtle decreasedthe "number of surfacing" times from5.357 to 3,489 and increased its totalsurface time from 16.5 percent to 29.7percent when the transmitter modelwas attached. These absolute surfacingcounts are suspect because of errorscaused by wave action, but the per­centage of surface time is consideredquite accurate. We concluded that thetransmitter package caused the turtle tospend about twice as much time at thesurface.

We recognize that sea turtles in thewild may react differently, but this testseems to indicate that the turtle adaptedto the added buoyancy without radicalbehavioral change. Within our limitedtime, we could conceive of no other sim­ple test to provide a better indication ofthe turtle's adaptation to the transmitter.

Prototype Transmitter

Structural Tests

Once assured that a large turtle couldreasonably tolerate our transmitter pack­age, we began to develop a strong struc­tural design. Heavier PVC plastic pipecomponents than those used for thebehavioral test model were obtained fordestructive testing. This new modelconsisted of two pipe caps and a 10 emlength of 15 em diameter plastic pipe.The end caps were PVC cemented tothe pipe and then "welded" together,using a process in which a PVC filler rodis actually melted into the base materialwith a heat gun similar to metal welding.A T-shaped bracket made of 0.6 cmPVC flat stock was also welded to one ofthe end caps as an attachment point forthe towing lanyard.

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PIPE

TRANSMITIER MODULE

~MAGNETIC SWITCH

(POWER)

Figure I. - Transmitterconstruction details.

This assembly was subjected to ahydrostatic pressure test and mechanicaltension test. For the pressure test, theassembly was placed in a hydrostatictest vessel and subjected to increasingpressure until the end caps deformed ata pressure of 200 psi (equivalent to about122 m depth). While this did not causeany internal leaks, we decided to regardthis as the operational depth limit.

The mechanical tension test deter­mined the strength of the attachmentbracket. The transmitter model wassecured in a test fixture and a shacklewas attached to the bracket through an0.6 cm hole. Increasing tension wasapplied to the bracket with a hydraulictest apparatus until the shackle was tornfrom the bracket. This occurred at atension of approximately 264 kg whichwas far in excess of our requirements.

Construction Details

The construction details of the actualtransmitter prototype are shown in Fig­ure 1. All of the electronic componentsare attached to the 0.5 cm flat PVCplate. This plate was guided into the

pipe by the mounting rails and was heldin position with epoxy. Weight and bal­ance were carefully checked to assurethat the coplanar antenna surfacefloated level in the water. The center ofgravity for the package was adjusted bypositioning the five organic lithium bat­teries (Mallory Battery Company TypeLO 26), which were mounted under thetransmitter module. This was sufficientto power the unit at the original timingcycle for a 1- year period. The completedpackage was sealed in exactly the samemanner as the test unit.

Two magnetically operated reedswitches provided the on/off control andtimer circuit initialization without theneed for access inside the transmitterpackage. One switch applied power toall the electronic circuits while the sec­ond switch initialized the basic 4-daytimer. The transmitter module also con­tained a secondary radio beacon whichcontinuously radiated 165 MHz rf pulsesat a peak power level of -10 dBM. Thisfull-time tracking transmitter operatedindependently of the satellite transmitterand allowed the turtle to be located atany time by search aircraft.

Final Preparations

Functional Transmitter Tests

The completed transmitter was thor­oughly tested for compatibility with theNIMBUS system by using two independ­ent signal reception modes: The space­craft system and the Local User Termi­nal (LUT) (Schmid and Lynn, 1978) atNASA's Goddard Spaceflight Center.Actual conditions were simulated forthese tests by floating the activatedtransmitter in a pool of seawater. Bothreceiving systems provided positivetracking data, and these results com­pleted our testing program for the trans­mitter package.

The Captive Turtle

A 96 kg female loggerhead turtle wasobtained from a Mississippi shrimp boatcaptain before the final hardware tests.This turtle, nicknamed "Dianne," wascaptured off Mississippi's barrier islandsand placed in a reef tank aquarium atMarine Life Park. The completed trans­mitter was now attached to Dianne and

April 1982. 44(4) 2/

Figure 2.- Dianne swims with allached Iransmiller

-'

she was released into an outdoor pondfor observation (Fig. 2), where sheseemed oblivious [0 the attached trans­mitter.

The Release

Intense shrimping activity preventedDianne's return to her capture site.Therefore, Dianne was released fartheroffshore into the Gulf of Mexico (Iat.:mo5'N, long. 88°46'W) at 1230 hourson 16 October 1979. This position isabout 20 miles south-southeast of Biloxi,Miss. As soon as the turtle was over­board, both visual and radio contactwere lost for the rest of the day.

Tracking

During the entire experiment. therewere three sources of position for theanimal: Aircraft, spacecraft, and LUT(Schmid and Lynn, 1978). The aircrafttracking was accomplished using a char­tered, fixed-wing aircraft equipped withdirectional wing-mounted antennas.Position fixes were obtained using stan­dard radio direction finding (RDF) tech­niques and an onboard loran-C receiver.

Two types of satellite fixes were avail­able. The first type, the spacecraft sys­tem, used the following approach. Thetransmitter data were collected by the

22

spacecraft during an entire orbit andwere recorded on an onboard tape re­corder. This information was teleme­tered to a ground station at Fairbanks,Alaska. and was subsequently relayed tothe Goddard Spaceflight Center forcomputer processing. Position fixes werethen available on computer tab formsafter approximately 1 week.

The second type of satellite fix wasavailable from the LUT. which is a real­time system and uses the satellite as atransponder (receiver/ retransmi tter).The transmitter data are reflected by thesatellite to the LUT at Goddard which,using the self-contained minicomputer,calculates the platform position. Dataare available by phone within 2-4 hoursafter satellite overpass.

Results and Discussion

The days immediately following therelease were thought to be critical interms of the operation of the transmitter,as well as Dianne's acceptance of it.Thus. we felt special concern when fewradio transmissions to the satellite werereceived. Table 1 summarizes the datesDianne was located by the three trackingsystems: Aircraft tracking, NASA's LUT,or the spacecraft. As indicated, only (wesatellite positions were obtained duringthe last 2.5 months of 1979. Dianne's

position was determined from aircraftthree times in October and three timesin November. There was great concernin November when no satellite recep­tions were obtained. Then, for unex­plained reasons. good satellite trackinginformation began in January 1980 andrecorded until 15 June, when the trans­mitter was recovered from the beach 30miles west of Port Arthur. Tex. Duringthe full 8-month period. satellite fixeswere obtained on 35 days out of 60 dayson which the transmitter operated (58percent successful). Dianne's positionwas obtained during the same day byboth the spacecraft and the LUT ononly three occasions. so the two systemsseem to complement each other withminimal duplication.

Dianne's migratory route across thenorthwestern Gulf of Mexico is chartedin Figure 3. She remained in the vicinityof Chandeleur and Breton Islands duringOctober and November. In mid- Decem­ber. she moved southward around themouth of the Mississippi River and thenwestward approximately following the10- fathom depth contour into Texaswaters. She paused briefly offshore fromGalveston during late January or earlyFebruary. On 11 February, with the aidof a U.S. Coast Guard helicopter, visualcontact was made with Dianne and pho­tographs were taken of the turtle withthe attached transmitter (Fig. 4).

Dianne continued her track south­ward to an area off Corpus Christi, Tex.,where she remained from mid- Februaryto mid- April. Her southernmost locationwas within about 30 miles of Browns­ville. Tex. After April, she began to move

Table 1.-Dates on which loggerhead turtle "Dianne"was located by the three tracking systems.

Date Aircraft LUT Spacecraft

1979October 17 18,25 20,28 24November 5,15,29December 15,31

1980January 12.28 4,12,16.24February 1 11 1,5,13. 1,9.21

17.21.29March 8.20,24.28 12.16April 1.5,13 17,21,29May 7,23 19,31June 4,8

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... A.rc:r.ftLocuion

• s.tenit. Locftion

<)Visu.IIHelol

93°97° 96°

250--U:...:..._..--__.,--__-,-__--, ,--__..,.__....__-,-__--,,---1

KANSAS

27°

TEXAS

26°

30°

29°

28°

31°-,-----------+--,------------,,----.::-----,

northeast to a position about 10 milesoff Port Arthur, Tex. Two consecutivesatellite fixes indicated that the trans­mitter was on the beach about 30 mileswest of Port Arthur. After a II/rweekperiod, during which no positions wereobtained, we began to receive data indi­cating that the transmitter had movedinland approximately 500 miles to asmall town named Galena, Kan. It waslater learned that the transmitter hadbeen found on the Texas beach by a manvacationing there. He had returnedhome to Kansas with the transmitter andlater returned it to us with the reportthat the attaching lanyard had "obvious­ly been cut" and there was no indicationwhat might have happened to the turtle.

The transmitter was photographed onreturn and is shown in Figure 5. Thebraided lanyard had unravelled by thistime, but the transmitter was otherwisein perfect operating condition.

Figure 3.-Dianne·s track in the Gulf of Mexico.

Figure 4.-Dianne as photographed from (he U.S. Coast Guard helicopter.

April 11)(12. 44141 23

Figure 5. - Dianne's recovered transmitter.

Conclusions

The proximity of the coplanar anten­na to the surface of the water apparentlydid not distort or otherwise precludeadequate radio transmissions to the sat­ellite. However, no explanations areavailable for the lack of satellite mes­sages during 1979, other than the turtle'sactivity and sea state. There was turbu­lent, windy weather during this periodwhich could have caused the transmitterto be awash and thereby interfere withradio transmissions, but the turtle wasobviously at the surface part of the timebecause she was located from trackingaircraft.

We can offer no explanation as to why

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the spacecraft and the LUT receivingsystems seldom provided location dataon the same days. In fac!. "same-day"fixes from both systems occurred only 9percent of the time. A total of 19 posi­tions were obtained by the spacecraftand 18 by the LUT. In summarizing thedata, we find no apparent correlationbetween the reception time of day,number of received messages per satel­lite pass, or the number of successivelyreceived pulses. These data can be madeavailable to researchers upon request.

The success of this turtle trackingexperiment should provide an impetusfor researchers to develop satellite­compatible electronics for other marinelife. The obvious design problems for

many sea animals are that they spendlittle time near the surface, the timingfor the transmitted signals will be criticaLand it will take creative engineering todevelop suitable transmitting antennasand attachment methods. Certainly wewould project that the development ofsatellite tracking equipment for marineresearch will require a significant in­vestment in money and manpower.

Acknowledgments

We wish to acknowledge the cooper­ation of Thomas Moore and Gene Gil­bert of NASA-Goddard for their veryvaluable assistance throughout thetracking experiment.

Literature Cited

Caldwell. D. K. 1962. Comments on the nestingbehavior of Atlantic loggerhead sea turtles.based primarily on tagging returns. Q. J. Fla.Acad. SCI. 25.21\8-302.

Carr. A. 1962. Orientation problems in highseas travel and terrestrial movements of ma­rine turtles. Am. Sci. 50:359-374.

____ . 1972. Acase for long-range chemo­receptive piloting in chelonia. in S. R. Galler.K. Schmidt- Leonig. G. Jacobs. and R. Belle­vil1e leditorsl. Animal orientation and naviga­tion. p. 409-41\.1. Nat!. Aeronaut. Space Ad­min. SP- 202.

____ . P. Ross. and S. Carr. 1974. Inter­nesting behavior of the green turtle. Chelon­ia I11l'das. at a mid-ocean island breedingground. Copeia 1974:703-706.

Cote. C E.. J. F. Dubose. and J. L. Coates. 197.1.The NIMBUS F Random Access Measure­ment System. In The Fifth Annual South­eastern Symposium on System Theory. 5 p.

Craighead. F. C. Jr.. J. J. Craighead. C E. Cote.and H. K. Buechner. 1972. Satellite andground radio tracking of elk. in S. Galler.K. Schmidt- Leonig. G. Jacobs. and R. Belle­ville (editors), Animal orientation and naviga­tion. p. 99-111. Nat!' Aeronaut. Space Ad­min. SP-262.

Kolz. A. L.. J. W. Lentfer. and H. G. Fallek.1980. Satellite radio tracking of polar bearsinstrumented in Alaska. in C J. Amlaner.Jr.. and D. W. MacDonald (editors). A hand­book on biotelemetry and radio tracking. p.743-752. Pergamon Press. Oxford.

Schmid. P. E., Jr., and J. J. Lynn. 1978. Satellitedoppler-date processing using a microcom­puter.IEEE Trans. Geosci. Electron .. GE-16(4):340-349.

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