Preface
Biologists use animal radio tags for two main purposes: to locate study animals in the field, and to transmit information about the physiology or behaviour of wild or captive animals. These uses can be described, respectively, as "radio tracking" and "radio telemetry," the latter term being derived from the Greek words for distance and measurement.
Wildlife radio tags were first used for telemetry. One of the earliest projects, inspired by the use of physiological telemetry on U.S. Navy test pilots, resulted in an implanted transmitter to monitor chipmunk heart- rates (Le Munyan et al., 1959). This study closely coincided with a publication from Norway describing an externally-mounted transmitter for telemetering heart and wing beats from mallard (Eliassen, 1960). The construction of these first tags, in the late 1950s, was crucially dependent on the development of the transistor.
The first radio tags transmitted a continuous signal, with physiological changes being indicated by slight changes in the signal frequency. Similar frequency modulation (FM) of a continuous carrier signal is still widely used for medical telemetry of animals in laboratories, because it is a very accurate way of conveying subtle changes in muscle potentials, neural activity, joint pressures, and other physiological parameters.
For wildlife radio tags, however, the cell life can be greatly extended if the signal is transmitted as brief pulses of the carrier frequency. In theory, at least, one 25 ms pulse could be repeated every second for 40 times as long as a continuous signal from the same cell. Moreover, a faint pulsed signal is easier for the human ear to detect, against the continuous background noise from wideband FM broadcasts or cosmic radiation, than a continuous whine. Wildlife radio tracking has therefore, since its start in the early 1960s (e.g. Cochran and Lord, 1963; Marshall and Kupa 1963), been based almost exclusively on tags built for pulsed signals.
During the last two decades, radio tagging has become an important biological technique. A great deal of useful information has been published on the subject, in the proceedings conferences in Europe (e.g. Amlander and MacDonald, 1980; Cheeseman and Mitson, 1982), from the proceedings of the International Conferences on Wildlife Biotelemetry held in North America, in technical notes from research organizations or equipment suppliers, and in hundreds of other scientific papers. Indeed, there is so much published material that the beginner hardly knows where to start looking for simple advice. Even those with experience have difficulty deciding which equipment to use, or how best to collect and analyze their data.
This book is a general guide to radio tracking and activity monitoring with pulsed-signal radio tags. The most elementary tags have constant pulse rates and are used for radio tracking, either to sample an animal's position with single fixes or for radio surveillance: using the radio tag to find the animal so that it can be watched, captured or monitored in other ways. Such tags can also have their pulses modulated by a variety of simple sensor subcircuits to telemeter temperature, posture, movement, compass orientation and other aspects of animal activity. These tags are normally worn externally, to simplify replacement and to obtain the strongest signals. I have included a brief review of implantation, which is the best way to tag some species and is now the rule for physiological telemetry.
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[Chapter 2, Basic Equipment]
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Complex circuits are required for tags which can telemeter information from more than one sensor, for instance by modulating pulse duration and pulse interval, or by sending a train of pulses in which each is modulated by a different sensor (Smith, 1974; Standora, 1977; Lotimer, 1980). Since it is no simple matter to assemble receiving equipment which can interpret these signals, the use of multiplex tags has so far remained the province of groups working with electronics engineers. However, suitable wildlife tags and receiving packages will probably become available commercially in due course. This could also apply to frequency-modulated tags which contain microphones, to transmit calls or feeding noises (Greager et al., 1979; Gautier, 1980).
F. Data Storage
Work on marine animals and some diving seabirds, from which radio tags can transmit a signal very infrequently, has been a strong incentive for developing tags which can store information on dive depths, durations and physiology for transmission the animal surfaces. A crude tag of this type, containing sensors, analogue-to-digital conversion, and logic circuitry, clock and memory costs about L300 to build (Robinson, 1986). Present designs transmit relatively small quantities of data in repeated signal streams, which are triggered by a pressure sensor as the animal surfaces or by a master transmission (i.e. in a transponding mode). Future designs could store large quantities of behavioral data, perhaps dumping these once a day to automatic receiving stations at bird or bat roost sites or near carnivore dens.
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VI. Tags for Tracking by Satellite
Tags for tracking by satellite have been used since the early 1970s, on wapiti (Buechner et al., 1971), polar bears (Kolz et al., 1980; Schweinsburg and Lee, 1982), turtles (Timko and Koltz, 1982) and basking sharks (Priede, 1980). The early tags weighed 5-11 kg, for location by the Nimbus 3 and Nimbus 6 weather satellites.
Satellite tracking works on the Doppler principle. A frequency shift in each received signal indicates the satellite's speed relative to the tag, and the tag's direction is computed from the ration of this speed to the satellite's true ground speed. For example, a ground-speed/relative-speed ratio of 1:2 would give a tag bearing of 45[degrees] to the satellite's track. To estimate a fix, at least two uplinks are needed during each pass, which may take as little as 10 min (horizon to horizon). Since the tag's could then be on either side of the satellite's track, it can only be located unambiguously (i) if it can be recorded again from the different track on another orbit, or (ii) with reference to a recent previous fix, or (iii) if one of the two computed positions is impossible (e.g. for a whale on dry land!).
Since 1978 the Argos system has been made available for animal tracking. This equipment, designed and operated by CNES in Toulouse, is carried on two Tiros satellites of the U.S. National Oceanic and Atmospheric Administration. Argos charges only $10 per day per tag, and tag power requirements are reasonable: a 360-920 ms pulse train, of which the first 160 ms must be constant carrier (at 401.65 MHz) for the Doppler vectoring, at 1-2 W. However, the intended location accuracy of within 5 km requires a very high frequency stability. Signals are rejected if they shift more than 2 Hz during a satellite pass, or 24 Hz between orbits (Priede, 1986). Compare this with the drift of perhaps 50 Hz per C[degree Celsius] in normal tracking tags. Moreover, the Argos system requires at least four uplinks over an interval of at least 7 min during a satellite pass.
Tracking by satellite is the most economic technique for wide-ranging marine mammals, although the uplink requirements mean that few fixes are obtained for species which are rarely on the surface. This is because the number of satellite passes per day ranges from only seven, at the equator, to 15 at higher latitudes. Satellite tracking also has considerable potential for monitoring migration routes and important feeding areas for large raptors, cranes, and seabirds. This potential is likely to be realized, thanks to the recent development of suitable 160 g solar-powered tags, the first of which have been used in the satellite tracking of bald eagles and giant petrels (Fuller et al., 1984).
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