- Email: [email protected]
Wildlife telemetry by satellite
Wildlife telemetry by satellite John French No branch of sciencetakes up new technology with more enthusiasmor greater expectationsthan those in the field of wildlife conservation. It is a subject of great diversity and rapid advance. One of the major concernsis how animals use their habitat, greater knowledge directing scarceresources,usually for protection against a man-made effect or pollution. Tracking wildlife by satellite is here briefly reviewed by looking back at VHF techniques and forward to what the future may bring. Those familiar with wildlife programmeson television will have noticed that small VHF radio tracking collars are now quite commonly used. Smallnesshas always been a problem for the designer and the tiniest transmitters (radio tags) have a mass of just 0.8 g, including the battery. Radio frequency (RF) power radiated from such devices is very low, usually a few tens of nanowatts. Typical applications might be small birds, amphibians, or snakes.Many have suggestedthat the advantagesof silicon integration would makeit possibleto realise ultra-small and smarterradio tags. This has yet to happen and the smallest still use simple technology evolved over about the last three decades.Circuits usually consist of two or three transistorsand an integrated circuit timer. Devising a good method of attaching the transmitter to the animal is in itself a considerable task, but with care, impressive results can be obtained. Small creatures live in relatively small areas, often less than 1 km square. It is therefore not too difficult to follow the animal on foot using a VHF receiver and a directional antenna. Although this is laborious, the results are accuratebecause the ultimate location is effected by sighting the animal. Thesetechniquesare of limited usefulness with larger animals. Polar bears, whales, turtles, albatrosses,and many other species travel great distances in a short time in places inhospitable to man. As all the applicable earth-based radio tracking systemsare limited to the radio horizon, a meansof long rangetracking was a problem waiting for spacetechnology to solve. This was the challenge that stimulated the leap Eur. Ing. John MIEEE.
French,
Ph.D.,
C.Eng.,
WEE.,
Has 40 years experience in electronic design, the first 25 involved with radio-telephones, echosounders, autopilots, and radar for small craft. Currently he runs a small company, Mariner Radar Ltd, and for some 15 years he has designed and built wildlife telemetry equipment including miniature satellite tags and radar transponders. He is an Honorary Research Fellow at the University of Aberdeen in this field.
Endeavour, New Serler, Volume 18, No. 1, 1994. CopyrIght 0 1984 Elwvler Soianca Ltd. PrInted In Qmat Bfitaln. All rlghtr mserved. 0160.9327/94 M.00 + 0.00. @
Pergamon
32
in complexity from very simple low-cost tags to highly sophisticated and efficient transmitters tracked by satellite. GPS Many people associate satellite tracking with the GPS navigation system, probably becauseits wondersrightly attractheadlines. Enough satellites are now in place for three or four to be visible most of the time. This results in a fix accurateto about 50 m RMS over most of the earth’s surface. Off the shelf availability of the receiving equipment, and Gulf War publicity, has rapidly multiplied the number of users. Prices have droppedand it has becomepossibleto think in terms of quality positioning using yachting equipment costing a few hundred pounds. However, GPS is a navigation systemdesignedto show the user’s position and not that of a remote object. Recovery of this information using a separateradio link consumespower and may reintroduce the radio horizon. Tracking technology as distinct from navigation has taken another route and attracts few headlines, but much is being achieved.
could be kept below 100 g. A systemclosely matching theserequirementswas to be part of the NIMBUS programmeplannedby the National Oceanic and Atmospheric Administration (NDAA). CLS Argos In 1978,ajoint operationby CentreNational d’EtudesSpatiales(CNES) in Franceandthe National Oceanic and Atmospheric Administration (NOAA) in the U.S.A. created the ARGOS system using NOAA satellites, making it available for approved experiments, including wildlife telemetry. It is on this sytem that all our present knowledge has been built. This includes operating the systemand establishingusers’ requirements,designof transmitters,design of animal packsfor a wide rangeof species, design of software for plotting and manipulating data, and a host of other support tasks.
How the system works From a polar orbit in the region of 850 km above the earth, at any instant the Argos satellites view a circular area 2500 km in diameter. The plane of the orbit is set to Early experimental work maintain their motion synchronouswith the Soon after the launch of the Sputnik and Sun and as a result they will be visible at Vanguard satellites in 1957 and 1958, roughly the samelocal time eachday. Figure respectively, scientistsbeganto considerthe 1 shows an overall view of the system. possibility of tracking animals from space. Normally two satellites are operational, It was the Elk that had the honour to be first although othersmay be orbiting in ‘standby’ [l] using the early IRLS system. The mode to support day-to-day operational distance between satellite and elk was requirements. determined by measuring the propagation A small transmitter is attached to the time of a radio frequency pulse in much the animal or object to be tracked; this is called sameway as radar operates. Knowing the a Platform Terminal Transmitter (PTT), position of the satellite relative to the earth beacon, or tag. Individual PTTs have a allowed the location of the elk to be unique code number embedded in each transmission identifying it to the user’s calculated. Almost as soonas the first satellites were programme. At the satellite these timed launched NASA was looking seriously at transmissionsare stored by tape recorders. what could be done within their earth When the satellites pass over a ground resources role. In 1972 NASA report station, the tape recorders are commanded TR-1424 [2] consideredall the experimental by a radio uplink to replay and transmit the satellite systems, real and proposed, that data recorded on the previous orbit. From might be used for wildlife tracking and the this a location can be calculated by CLS requirementsof a numberof potential users. Argos, who compile the information into a It also looked closely at the available computer database. Figure 2 shows the technology pointing the way towards underlying principle. Once processed,this random-access Doppler location. With some data can be accessed round the clock limitation of performance, this principle anywhere in the world by the simple could work with just a single satellite. Also expedient of a modem and a personal with care, the massof the animal transmitter computer. To makelife even simpler, CLS
temperature,ice velocity, andposition, there is no other viable method of obtaining this information. World Ocean Experiments On an even larger scale, the international World Ocean Circulation Experiment (WOCE) [4] and Tropical Ocean Global Atmosphere (TOGA) programmes aim to makemixed-layer velocity and temperature measurements on basin-wide scales, ultimately to map surfacecurrents and their variances on a global basis. It is a five-year programmewith deployments managedby scientistsat the Global Drifter Centresat the Scripps Institution of Oceanography and NOAA. Each year between 1991and 1993 over 500 ocean drifters were employed, giving some idea of the scale of the operation. Such projects have become possible only with spacetechnology. North Sea Project The Natural Environmental Research Council (NERC)‘s North Sea Project was a programme on a shorter time-scale but very important. One of the experiments carried out by RVS Challenger involved the deployment of databuoys (see figure 3). These relatively small buoys (18 kg) were unique in that they also carried DECCA receiversgiving tracksof greataccuracy[5]. A DECCA location was stored in memory every 10 min and the Argos systemallowed recovery of the buoy. Thus pollution outflows, long-distance current flows, or sources of food-like plankton can now be mappedin near real time. An important step in targeting scarceresourceswith certainty. In addition to the Argos PTT a VHF beacon was included to aid local recovery, making possible extensive reuse of the buoy.
Figure 1
The Argos system.
Argos haveproducedsoftwarecalled ELSA. With the aid of a personal computer and a modem, it handles communication with the main computer in Toulouse and plots the results graphically on a world map. What the system can do By its very nature the Argos system can handle location on a world scale, yet with a typical resolution of less than a kilometre it has great flexibility. In someinstancesit is helping to unravel some of the world’s great unsolved puzzles. In others it is
Satellite PTT design improvements While the foregoing examples show that CLS Argos can easily handle location and data collection on a world scale, resultsjust asdramaticare emergingfrom the effort put into PTT designs. When the systembegan operation in 1980 there were no miniature PTTs; typical mass for transmitter, 12 v battery, and housing was l-2 kg. The driving force for the dramatic size reduction now achieved, has come from biologists whoseaim is to havePlTs suitablefor direct attachment to ever-smaller species [6]. PTTs are transmitters of rather special design and must conform to a specification laid down by CLS Argos. Only a handful bringing difficult work into the realm of of manufacturers worldwide produce sensible economics for the first time. equipment specifically for animal tracking. The specification itself is unusual because Arctic monitoring it also gives details of parameters that Many nations have scientific study groups improve performancealthough not essential operating in the polar regions participating for certification of the transmitter. This is in very large programmes.For example,the in sharp contrast to VHF biotelemetry Polar Science Center at the University of transmitters, where the few specifications Washington, in collaboration with NOAA that exist are intended solely to prevent and the Norwegian Polar ResearchInstitute interference with other services. Two has collected data every day from 350 principal problems had to be addressed; buoys, with an expected life of two years first, very high frequency stability and, [3]. This experiment measures pressure, second, high power efficiency.
33
90 s. As the overpass period averages about 10 min and the orbital period is about 100 min, it follows that 90 per cent of the power payload serves no useful purpose. A timer built into the PTT can be used to limit the active periods so that the package life cycle is longer, if such a compromise is acceptable. It is also possible to use a command receiver to activate the PTT when the satellite rises above the horizon. Power economy circuits must be exceptionally efficient to effect any real overall improvement. Such effort has gone into reducing the power consumption of the PTT, that quiescent current is now only a few microamps and mean current for continuous operation is typically 3 mA.
Problems of power and size
Pl TO P6 FRE SiTELLITE POSITICNS
When RF power is limited, fewer locations are obtained and the quality falls (see figure 5). If this lowered performance is acceptable the batteries can be smaller and in consequence so can the housing. With equipment for creatures up to about 5 kg in body weight, the battery mass dominates the weight budget. Thus the PTT performance is closely linked to that of the batteries. With heavy creatures - an elephant for example - the total added mass is determined by the harness and counterbalance required to position a transmitter on the top of the animal. Because the batteries can be bigger, the radiated power can be of the order of 1 W. In this case the electronics package, which will almost certainly be microprocessor-controlled, might have a mass of about 300 g, but the housing, collar, and fittings needed to make it suitable for the animal add about another 6 kg. Between 100 g and 6 kg is a wide range of applications, most of which are designed with great care so that the natural activities of the animal are not affected. In this context it is worth noting what our present knowledge indicates [7,8], that in common with man, animals have no sensation of local RF transmissions.
:
Cl TO C6 FKE Ci..RVESCF UWSTFWT DOPPLER FREEUDJCT Ll /WI L2 FE TfWE LOCATICN Fwo IPEE
Figure 2
Doppler
location
principle.
Sea mammals Frequency stability
Power efficiency
Doppler location requires a time-ordered set of frequency measurements. Clearly, the frequency stability of the PIT itself must be of a high order and Argos specify this for short, medium, and long term conditions. The actual figures are: short term frequency stability < 1 part in lo9 over 100 ms; medium term < 4 Hz over 20 min; and long term less than 4 kHz over the period of deployment. In a complete transmitter, the oscillator is affected by many factors including vibration, temperature, and electrical isolation from the antenna. That satisfactory operation has been attained with the equipment deployed on a bird, elephant, wild hunting dog, lioness (figure 4), turtle, basking shark, seal, whale, polar bear, caribou, and many other species and does indeed represent considerable endeavour on the part of many people.
A well-designed satellite PTT converts battery energy to RF power with an efficiency of about 30 per cent and the antenna radiates some 95 per cent of the power applied to it. This efficiency is crucial, as the range to the satellite varies between 850-2500 km. Again, this is in marked contrast to VHF animal-tag technology where quite frequently the comparable figures are 5 per cent and C 1 per cent, respectively. The two technologies are a quantum leap apart at present, but the differences in efficiency are so marked that questions are at last being asked.
34
Battery limitation It is the power source that most seriously limits the performance of all animal tracking systems. For example, a typical satellite PTT transmits a 360 ms pulse once every
The first work in the U.K. was on basking shark (Cetorrhinus manimus) when transmitter design was still at an early stage. Basking shark swim just below the surface for long periods when they are feeding. To keep the antenna above the water and visible to the satellite, a buoyant enclosure was required. This not only had to be capable of withstanding intense pressure in deep water, but to be self-righting on a possibly turbulent surface to keep the antenna facing upwards. Add to this factors like positioning the towing point, extra buoyancy for the tow line, fins to prevent spinning when towed underwater, mechanical protection for the antenna, etc and the real difficulty of the project becomes apparent. The results justified the work [9], additionally demonstrating the technique of simultaneous infrared mapping of the sea surface by sensors on the same satellite. This project at the University of Aberdeen directly led
Figure 4 Lioness with satellite location collar (Courtesy Dr M. Gorman, University of Aberdeen, U.K.)
Fiaure 3 Data buov beina deoloved from RVS Challenger NkC, U.K.) ’ -
my own research into small PTT design [lo]. The work continues and conservation projects such as that on swans [l 11, wild dogs [12], sea mammals [13], houbara bustards [14] and data buoys [S] have followed. Albatrosses In one of the more imaginative projects, French scientists[ 151succeededin tracking the wandering .albatross using 180 g Toyocom PT’Ts. These birds can spend as much as 90 per cent of their life at sea. As a consequence,the developmentof satellite transmitters small enough for birds was a crucial stepin the discovery of their foraging habits. It was known that male and female birds alternate their attendanceon the nest during incubation and brooding. This made it possible for a number of birds to be fitted with harnessedbackpackPITS and for their ultimate removal. It was found that during
(Courtesy R.V.S. Barry,
the incubation period the birds move mainly during the daytime with flights in long loops of between 2000 and 15,000 km. Several complete flights were recorded, discovery indeed. Current state of technology This is a particularly good time to takestock of where satellite tracking technology stands.Experience with designing PTTs of ever-decreasingsize for the Argos system and analysis of results over 15 years has shown what can be achieved. Application of tiny microprocessorsabout 7 mm square and timed using a watch crystal, hasreduced the quiescentcurrent of the control functions to a few microamps. As mentioned above, batteries are the most conspicuouslimiting factor as they place a lower limit on output power, mass, and longevity. Regrettably, there is no sign of a new high-energydensity battery on the horizon and the lithium
thionyl chloride cell is the presentbest. For birds and small animals that can tolerate a mass of about 100 g, an operating life of about 30 days can be obtained with radiated RF power of about 0.5 W. Of course, this activity can be spread out over perhaps a year or lower output power can be used, but there is always a trade-off in overall performance. With the presentsystemof two satellites, the rate of update is limited to the orbital period of about 101 min between overpasses.This means that where the animal PTT is not directly visible to the satellite for the whole of the overpass period, there is potential for location to fail. For example, a sea mammal might miss an overpass completely by submergence. Although there are a lot of factors involved, provided the PTT is continuously visible to the satellite, presentperformance can be summarisedwith a worst and best caseas follows. In a typical PTT day near the equator, a 250 mW PTT will give one to two locations per day with an accuracy of about 1 km. At higher latitude, ca 70”, a 1 W PTT will give about 12 locations per day, many within 0.5 km. Continuing enhancementsare plannedfor the Argos system for some time into the future. From 1996the replacementsatellites NOAA-K/L/M will have the number of data-recovery units increased from four ,to eight. It will virtually double the simultaneous handling capability of the satellite receivers; this, together with emergencycapacityin the standbysatellites, will see the system into the next century. New satellite systems The availability of Argos hasgiven impetus to a developing technology which is bound to influence the services offered in the
35
Locations per&Y
The condition of the indigenous wildlife is a good indicator of the environment, indeed it was so even as the miner’s candle and canary gave way to Davy’s safety lamp.
Latitude RF power Watts
0.25 t
References [l]
Craighead, F.C., Craighead, J.J., Cote, C.E. and Buechner, H.K. Satellite and
ground radiotracking of elk. Animal orientation and nagivation, NASA report Sp262, 1972.
Figure 5
Relationship
between
PTT power and location
[2] Garvin, L.E., Hem&en, S.W., Liskov, N.A., Pascucci, R.P. and Ziolkowski, F. Satellite wildlife research program, final tedmical repmt. NASA report no. TK-1424, 1972.
quality.
[3] Rigor, I. Arctic ocean buoy program. Argos Newsletter, 44, 1992.
future. It has also shown that there is a need for a commercially based location service. At the WARC conference in Seville in 1992 it emerged that nine American companies have proposed new low-orbit satellite systems. One such scheme proposed by Loral Qualcomm is called Globalstar and has a fleet of 48 satellites. It primarily covers latitudes from 0 to 60”, giving continuous visibility for 82 per cent of the time. With such a system the transmitter could be powered only when the animal was most visible to the satellite, and for a minimal period to suit the study criteria. FTTs could thus transmit less often but with higher power, improved signal-to-noise ratio at the satellite and consequently more accurate locations. Another attractive feature is that the higher frequency would allow efficient antennas to be smaller than at present. Also that if the system uses Doppler location the rate of change of frequency during the overpass would be higher, leading to higher accuracy.
[4] Paduan, J.D. and Niiler, P.P. WOCE/TOGA surface velocity program (SVP), Argos Newsletter, 42, 1991.
been suggested that wings flapping in the earth’s magnetic field have something to do with it. But that cannot explain how the seal (figure 6) made the journey from Donna Nook to the Fame Islands and back, mostly out of sight of land. When we find the answer there is sure to be another question. But it is heartening that so much effort world-wide is being put into conservation. 56”30’N
1,.
!
I,
!
,
,
[5] Roberts, G., Last, J.D., Roberts, E.W. and Hill, A.E. Position logging buoys using DECCA navigator and Argos for high resolution spatial sampling. Atmospheric and Oceanic Science, 8, 718-728, 1991. [6] Priede, I.G. and French, J. Tracking of marine animals by satellite. European Symp. !
,
1. Enough satellites to achieve continuous visibility - this would reduce FTT energy requirements by between 10 and 100 times. 2. Satellite receiver/antenna systems gain being high enough to allow good location with 100 mW of uplink power, this would be a closer fit to semiconductors and battery technology. 3. An uplink frequency of about 1 GHz allowing efficient PTT antennas with dimensions of less than 100 mm. When we have a system meeting these requirements, we shall be ready with the next imperatives. As ever in the field of research, greater knowledge produces new questions. Knowing where the albatross has been for the last 10,000 km poses the question - how does it find its way? It has
36
!
,
,
!
I,
!,
i
”
i
,
56”O’N i \
Fame Islands
&
iZO-Jul-89 iy5-Sep-89 55”30’N
--
00:28 Ol-Oct.89
Conclusions principally, satellite tracking has demonstrated extreme usefulness in determining the large scale movements of animals with greater than 3 kg body weight. I have had space to mention but few but the effort has been very international, with many groups achieving success. For the future the most conspicuous factors to address performance improvement are:
,
114:3728-A”;-:9 55”O’N
--
54”30’N
--
54”O’N
-*
53”30’N
-DonnaNook
53”O’N
i
”
i 2”O’W
”
i
”
i
”
i
”
Figure 6 Track of grey seal in North Sea (Courtesy NERC, U.K.)
” 1”O’E
1"O'W
Sea Mammal
Research
Unit,
on rhe role of Aerospace technology in Oceanography, Int. J. Remote Sensing, 12, 667-680, 1991. [7] French, J. ‘Possible Behavioural Effects of RF Emissions from Argos PTT’s. 1st Animal Tracking Seminar’. Museum d’Histoire Naturelle, Marseilles, 23-24 June 1986. [S] Houghton, E.W. and Laird, A.G. A preliminary investigation into the use of radar as a deterrent of bird strikes on aircraft. RRE memorandum, 1967. [9] Priede, I.G. A Basking Shark (Cetorrhinus muximus) tracked by satellite together with
simultaneous remote sensing. Fisheries Res., 22, 1983. [lo] French, J. Satellite technology for tracking birds and sea mammals. Ph.D. Thesis, University of Aberdeen, 1986. [ll] Priede, LG., Sigtitsson,A.Th. andDunnet, G. M . Tracking birds using a new ARGGS PTT powered by lithium primary batteries. Proc. Int. Conj Radio Telemetry for Tracking Terrestrial Vertebrates. Monaco 12-13. [12] Gorman, M.L., Mills, M. and French, J. Satellite tracking of the African Wild Dog (Lycaon pictus) Proc. 4th European Int.
Con& on Wildlife Telemetry. University of Aberdeen, 1991. [13] Maconnell, B.J., Chambers, C., Nicholas, K.S. and Fedak, M.A. Satellite tracking of Grey Seals (Halichoerus grypus). J. Zool., 226, 271-282, 1992. [14] French, J. and Goriup, P. Design of a low mass/volume PTT for the Houbara Bustard Proc. 4th European Int. Con& on Wildlife Telemetry, University of Aberdeen, 1991. [15] Jouventin, P. and Weimerskirsch, H. Satellite tracking of wandering albatrosses. Nature, 232, 265.
37
French,
Ph.D.,
C.Eng.,
WEE.,
Has 40 years experience in electronic design, the first 25 involved with radio-telephones, echosounders, autopilots, and radar for small craft. Currently he runs a small company, Mariner Radar Ltd, and for some 15 years he has designed and built wildlife telemetry equipment including miniature satellite tags and radar transponders. He is an Honorary Research Fellow at the University of Aberdeen in this field.
Endeavour, New Serler, Volume 18, No. 1, 1994. CopyrIght 0 1984 Elwvler Soianca Ltd. PrInted In Qmat Bfitaln. All rlghtr mserved. 0160.9327/94 M.00 + 0.00. @
Pergamon
32
in complexity from very simple low-cost tags to highly sophisticated and efficient transmitters tracked by satellite. GPS Many people associate satellite tracking with the GPS navigation system, probably becauseits wondersrightly attractheadlines. Enough satellites are now in place for three or four to be visible most of the time. This results in a fix accurateto about 50 m RMS over most of the earth’s surface. Off the shelf availability of the receiving equipment, and Gulf War publicity, has rapidly multiplied the number of users. Prices have droppedand it has becomepossibleto think in terms of quality positioning using yachting equipment costing a few hundred pounds. However, GPS is a navigation systemdesignedto show the user’s position and not that of a remote object. Recovery of this information using a separateradio link consumespower and may reintroduce the radio horizon. Tracking technology as distinct from navigation has taken another route and attracts few headlines, but much is being achieved.
could be kept below 100 g. A systemclosely matching theserequirementswas to be part of the NIMBUS programmeplannedby the National Oceanic and Atmospheric Administration (NDAA). CLS Argos In 1978,ajoint operationby CentreNational d’EtudesSpatiales(CNES) in Franceandthe National Oceanic and Atmospheric Administration (NOAA) in the U.S.A. created the ARGOS system using NOAA satellites, making it available for approved experiments, including wildlife telemetry. It is on this sytem that all our present knowledge has been built. This includes operating the systemand establishingusers’ requirements,designof transmitters,design of animal packsfor a wide rangeof species, design of software for plotting and manipulating data, and a host of other support tasks.
How the system works From a polar orbit in the region of 850 km above the earth, at any instant the Argos satellites view a circular area 2500 km in diameter. The plane of the orbit is set to Early experimental work maintain their motion synchronouswith the Soon after the launch of the Sputnik and Sun and as a result they will be visible at Vanguard satellites in 1957 and 1958, roughly the samelocal time eachday. Figure respectively, scientistsbeganto considerthe 1 shows an overall view of the system. possibility of tracking animals from space. Normally two satellites are operational, It was the Elk that had the honour to be first although othersmay be orbiting in ‘standby’ [l] using the early IRLS system. The mode to support day-to-day operational distance between satellite and elk was requirements. determined by measuring the propagation A small transmitter is attached to the time of a radio frequency pulse in much the animal or object to be tracked; this is called sameway as radar operates. Knowing the a Platform Terminal Transmitter (PTT), position of the satellite relative to the earth beacon, or tag. Individual PTTs have a allowed the location of the elk to be unique code number embedded in each transmission identifying it to the user’s calculated. Almost as soonas the first satellites were programme. At the satellite these timed launched NASA was looking seriously at transmissionsare stored by tape recorders. what could be done within their earth When the satellites pass over a ground resources role. In 1972 NASA report station, the tape recorders are commanded TR-1424 [2] consideredall the experimental by a radio uplink to replay and transmit the satellite systems, real and proposed, that data recorded on the previous orbit. From might be used for wildlife tracking and the this a location can be calculated by CLS requirementsof a numberof potential users. Argos, who compile the information into a It also looked closely at the available computer database. Figure 2 shows the technology pointing the way towards underlying principle. Once processed,this random-access Doppler location. With some data can be accessed round the clock limitation of performance, this principle anywhere in the world by the simple could work with just a single satellite. Also expedient of a modem and a personal with care, the massof the animal transmitter computer. To makelife even simpler, CLS
temperature,ice velocity, andposition, there is no other viable method of obtaining this information. World Ocean Experiments On an even larger scale, the international World Ocean Circulation Experiment (WOCE) [4] and Tropical Ocean Global Atmosphere (TOGA) programmes aim to makemixed-layer velocity and temperature measurements on basin-wide scales, ultimately to map surfacecurrents and their variances on a global basis. It is a five-year programmewith deployments managedby scientistsat the Global Drifter Centresat the Scripps Institution of Oceanography and NOAA. Each year between 1991and 1993 over 500 ocean drifters were employed, giving some idea of the scale of the operation. Such projects have become possible only with spacetechnology. North Sea Project The Natural Environmental Research Council (NERC)‘s North Sea Project was a programme on a shorter time-scale but very important. One of the experiments carried out by RVS Challenger involved the deployment of databuoys (see figure 3). These relatively small buoys (18 kg) were unique in that they also carried DECCA receiversgiving tracksof greataccuracy[5]. A DECCA location was stored in memory every 10 min and the Argos systemallowed recovery of the buoy. Thus pollution outflows, long-distance current flows, or sources of food-like plankton can now be mappedin near real time. An important step in targeting scarceresourceswith certainty. In addition to the Argos PTT a VHF beacon was included to aid local recovery, making possible extensive reuse of the buoy.
Figure 1
The Argos system.
Argos haveproducedsoftwarecalled ELSA. With the aid of a personal computer and a modem, it handles communication with the main computer in Toulouse and plots the results graphically on a world map. What the system can do By its very nature the Argos system can handle location on a world scale, yet with a typical resolution of less than a kilometre it has great flexibility. In someinstancesit is helping to unravel some of the world’s great unsolved puzzles. In others it is
Satellite PTT design improvements While the foregoing examples show that CLS Argos can easily handle location and data collection on a world scale, resultsjust asdramaticare emergingfrom the effort put into PTT designs. When the systembegan operation in 1980 there were no miniature PTTs; typical mass for transmitter, 12 v battery, and housing was l-2 kg. The driving force for the dramatic size reduction now achieved, has come from biologists whoseaim is to havePlTs suitablefor direct attachment to ever-smaller species [6]. PTTs are transmitters of rather special design and must conform to a specification laid down by CLS Argos. Only a handful bringing difficult work into the realm of of manufacturers worldwide produce sensible economics for the first time. equipment specifically for animal tracking. The specification itself is unusual because Arctic monitoring it also gives details of parameters that Many nations have scientific study groups improve performancealthough not essential operating in the polar regions participating for certification of the transmitter. This is in very large programmes.For example,the in sharp contrast to VHF biotelemetry Polar Science Center at the University of transmitters, where the few specifications Washington, in collaboration with NOAA that exist are intended solely to prevent and the Norwegian Polar ResearchInstitute interference with other services. Two has collected data every day from 350 principal problems had to be addressed; buoys, with an expected life of two years first, very high frequency stability and, [3]. This experiment measures pressure, second, high power efficiency.
33
90 s. As the overpass period averages about 10 min and the orbital period is about 100 min, it follows that 90 per cent of the power payload serves no useful purpose. A timer built into the PTT can be used to limit the active periods so that the package life cycle is longer, if such a compromise is acceptable. It is also possible to use a command receiver to activate the PTT when the satellite rises above the horizon. Power economy circuits must be exceptionally efficient to effect any real overall improvement. Such effort has gone into reducing the power consumption of the PTT, that quiescent current is now only a few microamps and mean current for continuous operation is typically 3 mA.
Problems of power and size
Pl TO P6 FRE SiTELLITE POSITICNS
When RF power is limited, fewer locations are obtained and the quality falls (see figure 5). If this lowered performance is acceptable the batteries can be smaller and in consequence so can the housing. With equipment for creatures up to about 5 kg in body weight, the battery mass dominates the weight budget. Thus the PTT performance is closely linked to that of the batteries. With heavy creatures - an elephant for example - the total added mass is determined by the harness and counterbalance required to position a transmitter on the top of the animal. Because the batteries can be bigger, the radiated power can be of the order of 1 W. In this case the electronics package, which will almost certainly be microprocessor-controlled, might have a mass of about 300 g, but the housing, collar, and fittings needed to make it suitable for the animal add about another 6 kg. Between 100 g and 6 kg is a wide range of applications, most of which are designed with great care so that the natural activities of the animal are not affected. In this context it is worth noting what our present knowledge indicates [7,8], that in common with man, animals have no sensation of local RF transmissions.
:
Cl TO C6 FKE Ci..RVESCF UWSTFWT DOPPLER FREEUDJCT Ll /WI L2 FE TfWE LOCATICN Fwo IPEE
Figure 2
Doppler
location
principle.
Sea mammals Frequency stability
Power efficiency
Doppler location requires a time-ordered set of frequency measurements. Clearly, the frequency stability of the PIT itself must be of a high order and Argos specify this for short, medium, and long term conditions. The actual figures are: short term frequency stability < 1 part in lo9 over 100 ms; medium term < 4 Hz over 20 min; and long term less than 4 kHz over the period of deployment. In a complete transmitter, the oscillator is affected by many factors including vibration, temperature, and electrical isolation from the antenna. That satisfactory operation has been attained with the equipment deployed on a bird, elephant, wild hunting dog, lioness (figure 4), turtle, basking shark, seal, whale, polar bear, caribou, and many other species and does indeed represent considerable endeavour on the part of many people.
A well-designed satellite PTT converts battery energy to RF power with an efficiency of about 30 per cent and the antenna radiates some 95 per cent of the power applied to it. This efficiency is crucial, as the range to the satellite varies between 850-2500 km. Again, this is in marked contrast to VHF animal-tag technology where quite frequently the comparable figures are 5 per cent and C 1 per cent, respectively. The two technologies are a quantum leap apart at present, but the differences in efficiency are so marked that questions are at last being asked.
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Battery limitation It is the power source that most seriously limits the performance of all animal tracking systems. For example, a typical satellite PTT transmits a 360 ms pulse once every
The first work in the U.K. was on basking shark (Cetorrhinus manimus) when transmitter design was still at an early stage. Basking shark swim just below the surface for long periods when they are feeding. To keep the antenna above the water and visible to the satellite, a buoyant enclosure was required. This not only had to be capable of withstanding intense pressure in deep water, but to be self-righting on a possibly turbulent surface to keep the antenna facing upwards. Add to this factors like positioning the towing point, extra buoyancy for the tow line, fins to prevent spinning when towed underwater, mechanical protection for the antenna, etc and the real difficulty of the project becomes apparent. The results justified the work [9], additionally demonstrating the technique of simultaneous infrared mapping of the sea surface by sensors on the same satellite. This project at the University of Aberdeen directly led
Figure 4 Lioness with satellite location collar (Courtesy Dr M. Gorman, University of Aberdeen, U.K.)
Fiaure 3 Data buov beina deoloved from RVS Challenger NkC, U.K.) ’ -
my own research into small PTT design [lo]. The work continues and conservation projects such as that on swans [l 11, wild dogs [12], sea mammals [13], houbara bustards [14] and data buoys [S] have followed. Albatrosses In one of the more imaginative projects, French scientists[ 151succeededin tracking the wandering .albatross using 180 g Toyocom PT’Ts. These birds can spend as much as 90 per cent of their life at sea. As a consequence,the developmentof satellite transmitters small enough for birds was a crucial stepin the discovery of their foraging habits. It was known that male and female birds alternate their attendanceon the nest during incubation and brooding. This made it possible for a number of birds to be fitted with harnessedbackpackPITS and for their ultimate removal. It was found that during
(Courtesy R.V.S. Barry,
the incubation period the birds move mainly during the daytime with flights in long loops of between 2000 and 15,000 km. Several complete flights were recorded, discovery indeed. Current state of technology This is a particularly good time to takestock of where satellite tracking technology stands.Experience with designing PTTs of ever-decreasingsize for the Argos system and analysis of results over 15 years has shown what can be achieved. Application of tiny microprocessorsabout 7 mm square and timed using a watch crystal, hasreduced the quiescentcurrent of the control functions to a few microamps. As mentioned above, batteries are the most conspicuouslimiting factor as they place a lower limit on output power, mass, and longevity. Regrettably, there is no sign of a new high-energydensity battery on the horizon and the lithium
thionyl chloride cell is the presentbest. For birds and small animals that can tolerate a mass of about 100 g, an operating life of about 30 days can be obtained with radiated RF power of about 0.5 W. Of course, this activity can be spread out over perhaps a year or lower output power can be used, but there is always a trade-off in overall performance. With the presentsystemof two satellites, the rate of update is limited to the orbital period of about 101 min between overpasses.This means that where the animal PTT is not directly visible to the satellite for the whole of the overpass period, there is potential for location to fail. For example, a sea mammal might miss an overpass completely by submergence. Although there are a lot of factors involved, provided the PTT is continuously visible to the satellite, presentperformance can be summarisedwith a worst and best caseas follows. In a typical PTT day near the equator, a 250 mW PTT will give one to two locations per day with an accuracy of about 1 km. At higher latitude, ca 70”, a 1 W PTT will give about 12 locations per day, many within 0.5 km. Continuing enhancementsare plannedfor the Argos system for some time into the future. From 1996the replacementsatellites NOAA-K/L/M will have the number of data-recovery units increased from four ,to eight. It will virtually double the simultaneous handling capability of the satellite receivers; this, together with emergencycapacityin the standbysatellites, will see the system into the next century. New satellite systems The availability of Argos hasgiven impetus to a developing technology which is bound to influence the services offered in the
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Locations per&Y
The condition of the indigenous wildlife is a good indicator of the environment, indeed it was so even as the miner’s candle and canary gave way to Davy’s safety lamp.
Latitude RF power Watts
0.25 t
References [l]
Craighead, F.C., Craighead, J.J., Cote, C.E. and Buechner, H.K. Satellite and
ground radiotracking of elk. Animal orientation and nagivation, NASA report Sp262, 1972.
Figure 5
Relationship
between
PTT power and location
[2] Garvin, L.E., Hem&en, S.W., Liskov, N.A., Pascucci, R.P. and Ziolkowski, F. Satellite wildlife research program, final tedmical repmt. NASA report no. TK-1424, 1972.
quality.
[3] Rigor, I. Arctic ocean buoy program. Argos Newsletter, 44, 1992.
future. It has also shown that there is a need for a commercially based location service. At the WARC conference in Seville in 1992 it emerged that nine American companies have proposed new low-orbit satellite systems. One such scheme proposed by Loral Qualcomm is called Globalstar and has a fleet of 48 satellites. It primarily covers latitudes from 0 to 60”, giving continuous visibility for 82 per cent of the time. With such a system the transmitter could be powered only when the animal was most visible to the satellite, and for a minimal period to suit the study criteria. FTTs could thus transmit less often but with higher power, improved signal-to-noise ratio at the satellite and consequently more accurate locations. Another attractive feature is that the higher frequency would allow efficient antennas to be smaller than at present. Also that if the system uses Doppler location the rate of change of frequency during the overpass would be higher, leading to higher accuracy.
[4] Paduan, J.D. and Niiler, P.P. WOCE/TOGA surface velocity program (SVP), Argos Newsletter, 42, 1991.
been suggested that wings flapping in the earth’s magnetic field have something to do with it. But that cannot explain how the seal (figure 6) made the journey from Donna Nook to the Fame Islands and back, mostly out of sight of land. When we find the answer there is sure to be another question. But it is heartening that so much effort world-wide is being put into conservation. 56”30’N
1,.
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[5] Roberts, G., Last, J.D., Roberts, E.W. and Hill, A.E. Position logging buoys using DECCA navigator and Argos for high resolution spatial sampling. Atmospheric and Oceanic Science, 8, 718-728, 1991. [6] Priede, I.G. and French, J. Tracking of marine animals by satellite. European Symp. !
,
1. Enough satellites to achieve continuous visibility - this would reduce FTT energy requirements by between 10 and 100 times. 2. Satellite receiver/antenna systems gain being high enough to allow good location with 100 mW of uplink power, this would be a closer fit to semiconductors and battery technology. 3. An uplink frequency of about 1 GHz allowing efficient PTT antennas with dimensions of less than 100 mm. When we have a system meeting these requirements, we shall be ready with the next imperatives. As ever in the field of research, greater knowledge produces new questions. Knowing where the albatross has been for the last 10,000 km poses the question - how does it find its way? It has
36
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Conclusions principally, satellite tracking has demonstrated extreme usefulness in determining the large scale movements of animals with greater than 3 kg body weight. I have had space to mention but few but the effort has been very international, with many groups achieving success. For the future the most conspicuous factors to address performance improvement are:
,
114:3728-A”;-:9 55”O’N
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Figure 6 Track of grey seal in North Sea (Courtesy NERC, U.K.)
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Research
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on rhe role of Aerospace technology in Oceanography, Int. J. Remote Sensing, 12, 667-680, 1991. [7] French, J. ‘Possible Behavioural Effects of RF Emissions from Argos PTT’s. 1st Animal Tracking Seminar’. Museum d’Histoire Naturelle, Marseilles, 23-24 June 1986. [S] Houghton, E.W. and Laird, A.G. A preliminary investigation into the use of radar as a deterrent of bird strikes on aircraft. RRE memorandum, 1967. [9] Priede, I.G. A Basking Shark (Cetorrhinus muximus) tracked by satellite together with
simultaneous remote sensing. Fisheries Res., 22, 1983. [lo] French, J. Satellite technology for tracking birds and sea mammals. Ph.D. Thesis, University of Aberdeen, 1986. [ll] Priede, LG., Sigtitsson,A.Th. andDunnet, G. M . Tracking birds using a new ARGGS PTT powered by lithium primary batteries. Proc. Int. Conj Radio Telemetry for Tracking Terrestrial Vertebrates. Monaco 12-13. [12] Gorman, M.L., Mills, M. and French, J. Satellite tracking of the African Wild Dog (Lycaon pictus) Proc. 4th European Int.
Con& on Wildlife Telemetry. University of Aberdeen, 1991. [13] Maconnell, B.J., Chambers, C., Nicholas, K.S. and Fedak, M.A. Satellite tracking of Grey Seals (Halichoerus grypus). J. Zool., 226, 271-282, 1992. [14] French, J. and Goriup, P. Design of a low mass/volume PTT for the Houbara Bustard Proc. 4th European Int. Con& on Wildlife Telemetry, University of Aberdeen, 1991. [15] Jouventin, P. and Weimerskirsch, H. Satellite tracking of wandering albatrosses. Nature, 232, 265.
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