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A Critique of Wildlife Radio-tracking
and its Use in National Parks

Details of Radio-tracking Technology


Conventional VHF Radio-tracking

Conventional VHF radio-tracking systems consist of transmitting and receiving systems.

Transmitting Systems

Basic transmitting systems include a transmitter, power supply, transmitting antenna, and material to protect the electronic components and other material to attach the system to the animal. The size and mass of the total transmitting package, type and strength of signal sent, and life of the unit vary considerably.

Transmitters. — Each radio transmitter consists of electronic parts and circuitry, usually including a quartz crystal tuned to a specific frequency. Crystals come in different degrees of shock-resistance, and for animals such as wolves that lead aggressive lifestyles, high-shock resistant crystals are usually used.

Signals can be either continuous, which sounds through a speaker like a high-pitched whine, or pulsed, which sounds like a series of "beeps." Pulsed signals are usually used at rates of 30-120 per minute. Lower pulse rates yield longer transmitter life. Pulse widths can also vary, with 18 milliseconds being the minimum that is easily tracked. The narrower the pulse, the longer the life.

Transmitting Frequency. — Frequencies used in wildlife telemetry usually range from 27 MHz to 401 MHz. VHF transmitters typically give a ground-to-ground range of 5-10 km which is increased to 15-25 km when received aerially (Rodgers et al. 1996). Lower frequencies propagate farther than higher frequencies since they reflect less when traveling through dense vegetation or varying terrain (Cederlund et al.1979; Mech 1983). However, lower-frequency signals (e.g., 32 MHz) consist of longer wavelengths, which increase the size of the transmitting and receiving antennas necessary for detecting them. This has implications for feasibility and receiver portability (see below).

The commonest frequency ranges used for VHF tracking are 148-152 MHz, 163-165 MHz, and 216-220 MHz. The higher frequencies bounce more (e.g. off mountains) but have the advantage of requiring smaller antennas. Whatever frequency is selected, individual transmitters are usually tuned ≥10 KHz apart to allow distinctiveness despite signal drift (1-2 KHz) due to temperature and battery fluctuations (Mech 1983).

Frequency selection is regulated by law in the United States (by the Federal Communications Commission, FCC) and internationally as well. Thus authorization is necessary from local and federal agencies to avoid operating at prohibited frequencies.

An additional frequency-choice consideration involves proximity of other research projects using similar frequencies. Coordination among projects is necessary in order to avoid duplicating frequencies for individual study animals that may use the same areas. One famous example highlighting the importance of this consideration involved a radio-tracker homing in on what he thought was an elk. Upon closing in on the animal, he discovered that it was some other researcher's grizzly bear!

Power Supply. — Complete transmitter packages can weigh as little as 0.8 g (Samuel and Fuller 1996). However, many transmitter packages weigh much more, with the principal weight determined by the battery used and the collar and protective material. Since both the total weight and the life of the transmitter are determined by the battery, its selection is critical. Clearly, there are species-specific limits to the weight an animal can carry as a transmitter package without significantly affecting its survival or behavior. Therefore, a compromise must be made by using batteries heavy enough to meet the study objectives but light enough to minimize effect on the animal.

Lithium batteries (2.9 - 3.9 V) are generally employed in conventional systems because they have longer shelf life and an energy capacity-to-volume ratio twice that of mercury or silver oxide batteries. Silver oxide batteries are often used with subcutaneous transmitters to minimize bulk. However, these transmitters usually only last 6-120 days. (Although zinc-air batteries have about the same energy density as lithium batteries, they are not widely applicable in wildlife telemetry because they can only be used in situations where dirt or moisture will not enter the required vent hole [Samuel and Fuller 1996]).

Photovoltaic or solar cells have also been utilized in transmitter packages (Aucouturier et al. 1977; Snyder et al. 1989). These cells theoretically allow indefinite signal output until other components of the transmitter fail. However, the signal is only transmitted during daylight when using solar cells alone. Attempts to achieve round-the-clock signal transmission while extending battery life have included the combined use of solar cells and rechargeable batteries. During the day, the transmitter pack uses the solar battery to operate and to store additional energy in the NiCd rechargeable batteries. At night, the unit is powered solely by the NiCd battery. While this system initially allows round-the-clock signal output, eventually the signal would only be broadcast during the day since rechargeable batteries can only be recharged a limited number of times.

Another recent advance in VHF telemetry which extends battery life is duty-cycling which allows the transmitter to turn on and off at regular intervals (e.g., on 8 hr, off 16 hr). This feature minimizes the number of times a radio-tagged animal must be recaptured to replace a dead battery.

Transmitter Protection. — Transmitters are usually coated with "potting", a resin-like material used to seal the electronic components. Transmitter packages are coated to minimize damage done both by the animal (chewing, scratching, etc.) and by the environment (moisture, mechanical damage, etc.) to the sensitive circuitry. The most common reason for transmitter failure is a dysfunctional battery attributed to moisture exposure or shelf deterioration (MacDonald 1978).

Many types of coating are commercially available such as beeswax, dental acrylic, epoxy resin, electrical resin, silastic, and Plasti-Dip (Mech et al. 1965; Macdonald 1978; Donaldson 1980; Jansen 1982; Kuechle 1982). Transmitting units can also be hermetically sealed (Tomkiewicz 1996). Particular sealants provide specific advantages for different situations. Selecting the appropriate sealant is especially important when working with aquatic organisms. Morris (1992b) described a method of waterproofing transmitter packages for depths up to approximately 100m.

Transmitting Antennas. — Transmitting antennas are critical components of radio-transmitter packages since they project the signal for capture by receiving antennas. Along with power supply, the antenna's orientation, construction, and length determine the effective power radiated (EPR), and therefore, the range at which the signal can be received. Ideally, antennas should be oriented perpendicular and away from the animal's body. However, such antennas become entangled in vegetation, break off or are torn off. Thus often they are embedded between layers of a collar, thus protecting them but compromising their range.

Two basic types of transmitting antennas are whip antennas and loop antennas. Whip antennas consist of a wire with one free end and the other end attached to the transmitter. The antenna's entry point must be thoroughly sealed to prevent moisture damage to the transmitter. Whip antennas are usually shorter than the ideal length, so sometimes they include additional components to help compensate for this decrease in EPR (Kenward 1987:32). An additional decrease in EPR is due to the whip antenna's close contact with the animal's body (Cochran 1980).

While whip antennas incorporated into a collar are often used on large mammals, simple loop antennas (connected on both ends to the transmitter) are usually used on small mammals (Anderka 1987). Loop antennas consist of copper, brass, or coated wire which fits around the animal's neck. The diameter of the loop is adjusted to tune the antenna to match the frequency of the transmitter. Therefore, if the loop collars are tuned prior to use on an animal, the diameter must not be altered. Otherwise, the loop collars can be tuned in the field when placed on an animal.

Transmitter-Attachment Methods. — Mech et al. (1965) suggested five guidelines in selecting the ideal transmitter package and attachment for a particular project: 1) minimum weight, 2) minimum effect on the animal, 3) maximum protection for the transmitter, 4) permanence of the attachment, and 5) maximum protection of transmitter from animal mortality factors such as predation and accident. Various attachment methods show varying effects on animals (Garrott et al. 1985; Marcstrom et al. 1989) (see below).

Radio-Tagging Mammals. — Collars have traditionally been used to fit transmitter packages on mammals (Pouliquen et al. 1990) with prominent necks, large ears, or horns/antlers since these structures help prevent the collar from slipping over and off the head of the animal. Standard collars cannot be used with animals such as boars (Jullien et al. 1990), which have temporarily enlarged necks during the rutting season, and polar bears (Anderka and Angehrn 1992), which undergo large seasonal weight changes. Jullien et al. (1990) used an expandable collar to allow for seasonal neck circumference changes in the boar, while Kolz et al. (1980) fitted polar bears with a loose collar attached to a secure back harness. Because the necks of male ungulates swell during the rut, researchers apply foam rubber inside the collars to allow for the swelling.

Expandable collars allowing for growth of young animals have also been applied successfully to mountain lions (Garcelon 1977), bobcats (Jackson et al. 1985), pronghorns (Beale and Smith 1973), black bears (Strathearn et al. 1984), caribou calves (Adams et al. 1995), elk calves (Singer et al. 1997), white-tailed deer fawns (Kunkel and Mech 1994), and coyotes. These collars remain securely fitted on the animal as sewn pleats or foam-rubber inserts give way with the growth of the animal. Break-away collars, in contrast, are designed to fall off the animal following degradation of weaker materials purposely incorporated into the collar. Break-away collars with cotton fabric have been used on black bears (Hellgren et al. 1988), while rubber or wire breakaway components have been used on sea otter collars (Loughlin 1980).

Some mammals do not retain collars well since they do not have prominent necks. For example, mammals such as hedgehogs (Morris 1966), badgers (Kruuk 1978; Cheeseman and Mallinson 1980), dolphins (Jennings and Gandy 1980), and harbor seals (Broekhuizen et al. 1980) have instead been fitted with backpack harnesses. Tail harnesses have also been used to fit animals with short stocky necks such as manatees (Priede and French 1991).

Some alternatives to applying collars or harnesses on mammals include ear-tag transmitters (Servheen et al. 1981), transmitters fixed with adhesive directly onto the mammal such as bats (Stebbings 1982) or bears (Anderka 1987), and transmitters making use of an animal's special structures. One study embedded a transmitter into the horn of the black rhinoceros with a loop antenna fastened around the base of the horn (Anderson and Hitchins, 1971).

The above alternatives are not appropriate for highly social animals such as wolves. Instead, very rugged or sometimes even armored collars must be used since extensive grooming and collar chewing occurred that can result in damage or removal of the transmitter, antenna, or entire collar (Thiel and Fritts 1983).

Surgically implanted transmitters such as subcutaneous transmitters, abdominal transmitters, or rumen transmitters represent other attachment alternatives. Transmitters have been implanted in mammals such as beavers (Guynn et al. 1987), river otters (Melquist and Hornocker 1979; Davis et al. 1984), sea otters (Garshelis and Siniff 1983; Williams and Siniff 1983; Ralls et al. 1989), yellow-bellied marmots (Van Vuren 1989), lions (McKenzie et al. 1990); grizzly bears (Philo et al. 1981), and black bears (Jessup and Koch 1984).

Two considerations when using implanted transmitters are greatly reduced signal range (sometimes < 50%; Samuel and Fuller 1996) and increased invasiveness to the animal that may result in greater data bias and potentially require subsequent veterinary procedures (Morris 1980). See below for assessment of effects of implanted transmitters.

Radio-Tagging Birds. — Attachment methods for fitting transmitters to birds vary widely. Examples include transmitters with whip antennas fitted to backpacks with attachment loops under the wings (Dwyer 1972, Hardy 1977); loops meeting near the breast (Amlaner et al. 1978), or loops under the legs (Rappole and Tipton 1991); loop-antenna harness-chest packs (Nicholls and Warner 1968; Siegfried et al. 1977); whip antennas adhered directly to tail feathers (Dunstan 1973; Kenward 1976, 1978; Raim 1978; Giroux et al. 1990); collars, neck band mounts, or necklaces (Amstrup 1980; Shields and Mueller 1983; Montgomery 1985; Marcstrom et al. 1989); leg-band transmitters (Melvin et al. 1983).

Other methods include suture-only attachments (Martin and Bider 1978; Mauser and Jarvis 1991); adhesive-only attachments (Jackson et al. 1977; Raim 1978; Harrison and Stoneburner 1981; Perry 1981; Perry et al. 1981; Heath 1987; O'Conner et al. 1987; Wanless et al. 1989; Wilson and Wilson 1989; Sykes et al. 1990; Johnson et al. 1991); suture and adhesive attachments (Wheeler 1991); patagial band mounts; and surgical implants (Klugman and Fuller 1990; Anderka and Angehrn 1992; Olsen et al. 1992).

Radio-Tagging Other Animals. — Other animals such as reptiles, amphibians, and fish have also been radio-tagged. Sea turtles, for example, have been tracked using transmitting harnesses (Ireland 1980). Snakes have been fitted with internal transmitters that still allow the snake to move through small openings and to shed its skin (Speake et al. 1979; Anderka and Weatherhead 1983; Weatherhead and Anderka 1984). Ingested implants have also been used in snakes (Lutterschmidt and Reinert 1990). Transmitters have been surgically implanted in endangered hellbenders (Stouffer et al. 1983) and amphibians such as toads (Smits 1984). Both internal and external radio-tags have been applied to fish as well (Haynes and Gray 1979). Different tags, or a combination of tags, must be utilized when considering freshwater vs. mixed or salt water because of the differences in signal propagation due to varying degrees of conductivity (Priede 1992).

Other Uses Of Transmitters In Wildlife Research. — Transmitters have been used for a variety of special purposes beyond individually marking and tracking animals. Some have been placed in tranquilizing darts to allow researchers to track darted animals until they succumb to the drugs (Lovett and Hill 1977).

Radio signals have been employed with special capture collars (Mech et al. 1984; Chapman et al. 1985a, 1985b; DelGiudice et al. 1990; Mech et al. 1990; Kunkel et al. 1991; Mech and Gese 1992). Upon receiving the activating signal, the collar injects the drug contents of a dart into the animal's neck. Should the dart fail or not provide sufficient drug, the researcher can address the collar again - this time to release the contents of a second (back-up) dart into the animal. Transmitter-fitted anesthetic darts have also been placed in penguin nests for remote activation to capture the birds (Wilson and Wilson 1989).

Transmitters have also been used both to monitor trap status and also to remotely activate a release system on traps (Hayes 1982; Nolan et al. 1984). Transmitters have served to mark important locations (Nicholls et al. 1981). For example, before hand-held GPS units were available, researchers doing aerial surveys marked wolf kill-sites by dropping a transmitter protected by sponge rubber; then later, using a portable hand-held receiving system, they could hike to the site for further investigation. Transmitters have also been attached to prey that may be taken to dens or nests, allowing scientists to find the den or nest (Samuel and Fuller 1996). Implanted transmitters can also help lead authorities to poachers (Samuel and Fuller 1996).

Researchers have used vaginal transmitters in female deer to mark fawning sites (Bowman and Jacobson 1998). These transmitters emit a changed pulse when expelled from the doe (expected during fawning) because they are temperature-sensitive. Researchers can then home in (see Tracking Methods) on the transmitter and find the fawn nearby.

Receiving Systems

Receiving systems detect and identify signals from transmitters. A basic receiving system consists of a battery-powered receiver, a receiving antenna, cables, a mechanical or human recorder, and a human interpreter with accessories such as a speaker or headphones (useful for reducing external noise when detecting the transmitted signal, Mech 1983; Samuel and Fuller 1996). Other accessories include devices for mounting receiving antennas to vehicles and aircraft, scanners to enhance searching for numerous signals, specialized recorders to aid in data collection, downloading and data-interpreting devices, and various types of software.

Receivers. — Receivers must be able to detect and distinguish signals of specific frequencies. Standard receiver controls include a three-position power switch (internal or external power and off), dials for gain and channel, band, and fine frequency adjustments, jacks for an external antenna (UHF or BNC), headphones, a recorder, and external power. Some receivers also have a volume control. Volume differs from gain in that an increase in gain increases signal sensitivity (up to a point beyond which the sensitivity does not increase) whereas increasing volume affords no greater signal sensitivity (Mech 1983).

Some receivers require frequencies to be entered by dials while others are digitally programmable. Many receivers also include two needle gauges; one indicates remaining battery power and the other the strength of the signal received. The latter can be especially useful in aerial tracking where the signal strength changes rapidly (Mech 1983). Most portable receivers are powered by standard alkaline batteries (i.e. 8 AA, 1.5 V penlight cells) (Cederlund et al. 1979) and will function for 8-12 hours. (With rechargeable batteries, the unit functions 5-8 hours [Samuel and Fuller 1996].) Generally, receiving units can also be powered externally from vehicle cigarette lighters or separate larger batteries such as motorcycle or marine batteries.

Some receivers include a sweep option that allows the unit to search within 10 KHz of the tuned signal. This is useful since signal drift can occur in the field due to temperature and battery fluctuations (Mech 1983). Other receivers are programmable with memory functions and can automatically scan for several frequencies at user-defined intervals from as little as second to as long as 10 minutes (Samuel and Fuller 1996). The researcher presets the search time and can stop the scanning to home in (see Tracking Methods) on a particular signal (Kuechle 1982). This time-saving feature allows the researcher to locate many animals in a short time. Hand-held walkie-talkie-size receivers (weight 352 g) that can store up to 999 frequencies are now available.

Receiving Antennas. — Frequency determines the size of receiving antennas. In general, the higher the frequency, the smaller the antenna. For example, a receiver tuned for 150 MHz (wavelength 2 m) with an accompanying wavelength multi-element Yagi antenna (see below) is available as a 1-m, hand-held unit, while a receiver tuned for 27 MHz (wavelength 11 m) necessitates a wavelength multi-element Yagi antenna much larger (5.5 m) and thus is not practical as a hand-held device (Cederlund et al. 1979) (unless a loop antenna is used; see below).

Antennas serve to both increase the gain (signal gathering capacity) of a receiver and to assist the operator in determining the direction from which a signal is coming. Larger antennas (lower frequencies) generally yield greater gain and directionality but at the expense of portability. Selection of the appropriate antenna is important since even under ideal situations, half of the signal's power captured by the antenna is actually delivered to the receiver while the other half is re-radiated (Samuel and Fuller 1996).

The signal's power is transferred from the antenna to the receiver by coaxial cables which shield the electronic signal against extraneous noise and help minimize signal power loss (Samuel and Fuller 1996). If signal loss along the transmission line is too great, preamplifiers can be incorporated between the antenna and the transmission line (Kenward 1987; Howey et al. 1989).

The simplest kind of receiving antenna is a straight wire or "dipole" (one half the wavelength of the transmitted frequency) attached to the receiver's antenna jack. Dipole antennas are omni-directional and therefore, are most appropriate for presence /absence studies (Mech 1983). These antennas are often used at a stationary reception site in an automatic tracking system or as part of a portable unit mounted on vehicles (see Mounting Antennas).

Loop antennas can be a circle, an oval, or diamond-shaped, with, like other antennas, dimensions dictated by the signal frequency. Loop antennas are especially useful for minimizing the size of lower-frequency antennas so they can be used as hand-held portable units (Cederlund et al. 1979). Although loop antennas are bi-directional (the signal can be received equally strong from two different directions simultaneously), by merely moving a few hundred meters perpendicular to the bearings, and taking a second bearing, once can determine the direction of signal origin.

A more complicated antenna, the multi-element Yagi, is the most commonly used antenna in North America (Kuechle 1982). It consists of a horizontal length of metal (usually aluminum) with 3-17 vertical lengths attached to it, all in one plane. The length of the vertical elements and their spacing depend on signal frequency. Yagi antennas are directional with shorter elements at the distant end of the antenna. The signal's origin can be determined by swinging the antenna and determining the direction of the strongest signal when the tip of the Yagi (the shortest element) is farthest from the user (Mech 1983). Twin Yagi systems can be set up for greater range and more precise directionality. However that requires careful spacing of the antennas 1 or wavelength apart (Amlaner 1980; Anderka 1987).

Another type of receiving antenna is the Adcock, or 'H' antenna, often used for hand-held applications because it is smaller than the Yagi. However, this antenna has reduced gain and receives the signal equally strong from two directions, so the true direction of signal origin must be confirmed in the same way as with a loop antenna (Samuel and Fuller 1996).

Recent developments in hand-held receiving antennas include size and weight reductions and increased portability. Morris (1992a) described a copper antenna for 173 MHz that is appreciably smaller than the corresponding three-element Yagi but retains similar gain. Bosak (1992) illustrated a pocket-size receiver with two telescopic monopole antennas built into the casing. Extended, these monopoles form a dipole that has been used to track lizards. To detect underground lizards, one monopole from the receiver was stuck into a hole in the ground. For very short ranges, the system worked even when the monopoles were only extended 5-10 cm.

Bosak (1992) also described a flexible Yagi antenna with three, five, and ten elements constructed from flexible wire mounted on a nylon or kapron cord frame. The obvious plus to this antenna is its portability. The author stated that "When not in use the antenna is rolled up into a ball. If required in the field the antenna is stretched into a T-shaped frame lashed together from whatever material (e.g. flexible green twigs) is available locally" (Bosak 1992). A new precise method of close-up homing uses ultra high frequency antennas with short elements tuned to a transmitter's harmonic frequency to pinpoint a transmitter (Cochran and Pater 2001).

Mounting Antennas. — Some radio telemetry projects require antennas too large to be hand-held. These antennas must be mounted either on stationary receiving stations or on portable vehicles such as an automobile. Stationary systems can be fixed (non-rotating) to record presence and absence data or rotated to determine the direction of the signal (Smith and Trevor-Deutsch 1980; Mech 1983). White and Garrott (1990) described ideal positioning for antennas mounted at stationary receiving stations for increased accuracy and coverage. Samuel and Fuller (1996) discussed the positioning of antennas with respect to vegetation, large buildings, obstructive terrain, etc. and its effect on the range of signal reception.

Mobile systems are especially useful for covering large ranges. When in heavily roaded areas, antennas mounted to vehicles can serve as presence/absence indicators (e.g. omni-directional antennas mounted with a magnet to the roof of a car) (Bray et al. 1975; Kolz and Johnson 1975; Mech 1983) or as direction-finding antennas (e.g. rotating stacked Yagi antennas attached to a compass inside the vehicle) (Cederlund and Lemnell 1980; Mech 1983; Hegdal and Gatz 1987).

Antennas mounted on aircraft generally consist of two Yagis or H types, one attached to each wing strut. The antennas can be positioned pointing forward or off to the side on either fixed-wing aircraft or helicopters (Whitehouse and Steven 1977; Gilmer et al. 1981; Inglis, 1981; Mech 1983).

Most aerial systems are set up with the antennas pointing to the side for fine-point location estimates when circling. Initially, the operator switches between the two antennas to determine coarse signal direction by finding the bearing where the signal from each antenna is equally strong, heads in that direction, and circles around the peak signal location to pinpoint the animal. This method of tracking is particularly useful for areas difficult to access by foot or automobile such as wilderness and remote mountainous areas. More complete descriptions of the process of aerial tracking can be found in Whitehouse and Steven (1977) and Mech (1983).

Recorders, Counters, And Decoders. — Ultimately, a human is needed to interpret signals in wildlife studies. However, many automated devices can aid the researcher by performing a majority of the recording, counting, and decoding necessary for data analysis. These devices are very useful for presence/absence, activity, or physiological studies.

Recorders range from simple to complex. Kenward (1987) used a tape recorder to register transmitted signal pulses. Other devices such as a simple paper strip-chart recorder can register the presence or absence of signals in the receiving area of the antenna (Licht et al. 1989), along with time and date (Gillingham and Parker 1992). These recorders are especially useful for gathering physiological data (Kuechle et al. 1987; Althoff et al. 1989; Schmidt et al. 1989; Stohr 1989).

More complicated computerized systems can be programmed to scan for various signals, record their parameters, decode and process them for error and mean values, and store them for later retrieval (Howey et al. 1987; Janeau et al. 1987; Kuechle et al. 1989; Schober et al. 1989).

Counters and decoders are used in automatic recording systems primarily to count and decode signal pulses associated with physiological telemetry. These devices can receive signal pulses, measure their properties (i.e. amplitude, interval, etc.) and convert them into user-defined outputs (e.g. Schmidt et al. 1989 converted analog signals into digital signals to obtain EEG readings).

Tracking Methods

Researchers can track animals in the field through two main methods, homing in (either by ground or aerial tracking) and triangulating. Passive remote tracking is accomplished through automatic tracking systems (Cochran et al. 1965).

Homing. — Homing consists of following a signal toward its greatest strength. As the researcher closes in on the animal, the signal increases and the receiver gain must be reduced to further discriminate the signal's direction. The process of proceeding forward and continually decreasing the gain is repeated until the researcher sees the animal or otherwise estimates its location when sufficiently near (Mech 1983). Homing can introduce bias to the data as the animal may be disturbed and behave differently as a result (White and Garrott 1990).

Triangulating. — Triangulating involves obtaining two signal bearings from different locations (preferably at angles of about 90° to one another) which then cross at the animal. In practice, it is better to take three or four bearings because antenna directionality is imprecise. When more than two bearings are plotted, the bearings form an error polygon on a map (Heezen and Tester 1967; White and Garrott 1990). This polygon theoretically contains the animal's location.

Significant error can be introduced if the bearings are not taken in a relatively short period, since the more time that passes, the greater the probability that the animal has moved. This problem can be avoided by researchers simultaneously taking bearings each from a different location. Triangulation locates an animal with minimal disturbance since the researcher can be far from the animal while obtaining a bearing. The farther away, however, the greater the error. Mech (1983) more completely describes the process of triangulation.

Automatic Tracking. — Automatic radio-tracking differs from the above techniques because the researcher is not required to be in the field to obtain the animal's location. The obvious advantage is reduced human presence in the field. Also, since bearings are recorded automatically, measurements should be without subjective error (Angerbjorn and Becker 1992). A significant disadvantage to this method is the high initial investment in equipment and also its maintenance.

The first complete automatic radio-tracking system was the Cedar Creek (University of Minnesota) system (Cochran et al. 1965). This system recorded locations of 52 animals (reception range from approximately 100 m to 10 km) every 45 seconds through the use of two rotating stacked-Yagi antennas on 20-m and 30-m towers.

Since that pioneering system, other types of automatic tracking systems have been described (Angerbjorn and Becker 1992). Nicholas et al. (1992) have used an inexpensive, low-maintenance system that automatically records signals from free-ranging seals even if the signal is weak or the background noisy.


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