Northern Prairie Wildlife Research Center
Satellite telemetry utilizes a platform transmitter terminal (PTT) attached to an animal which sends an ultra high frequency (401.650 MHz) signal to satellites. The satellites calculate the animal's location based on the Doppler effect and relay this information to receiving/interpreting sites on the ground. PTTs are attached by collars, harnesses, subdermal anchoring, harpooning with a connected float, or by fur bonding (Taillade 1992).
PTTs are programmed to transmit every 50-90 seconds with a pulse width of about 0.33 seconds (Howey 1992, Samuel and Fuller 1996). (This narrow pulse is very important when working with animals such as dolphins because they have a mean surfacing time of only 0.75 seconds during which the signal must be sent [Taillade 1992]). When a satellite passes overhead, there is a 10-12-minute window during which a PTT's signal can be received. Two satellites are needed to obtain location information (Taillade 1992).
Since PTTs must be powerful enough to transmit a signal to satellites orbiting 800-4,000 km away (Howey 1992), their radiated power ranges from about 250 mW to 2 W (compared with 10 mW of radiated power in a typical conventional VHF animal-tracking transmitter) (Taillade 1992). A standard PTT collar, for example, requires three D-size lithium batteries which last 3-12 months, depending on specific duty-cycling (Fancy et al. 1988). For example, to prolong PTT life, some researchers program the transmitter to turn on for 1 day each 3. This duty cycle would yield three times the life of a PTT transmitting every day.
Early PTTs were designed to work with NIMBUS satellites (Kolz et al. 1980; Schweinsburg and Lee 1982; Timko and Kolz 1982). The next generation of PTTs were coordinated with the Argos Data Collection and Location System carried on Tiros-N weather satellites (Fancy et al. 1988; Rodgers et al. 1996). Currently, only the US/French Argos system is functional. The receiving systems are positioned on two NOAA (National Oceanic and Atmospheric Administration) polar-orbiting satellites resulting in complete global coverage (Taillade 1992). The location signal sent from the PTTs is relayed to receiving stations where researchers record and interpret the data (Priede 1992).
The rate of data collection by satellites varies according to topography and latitude. Keating et al. (1991) found location fix probability to be 11% of attempts in valley bottoms and 56% on mountain peaks. Also, since the satellites are in polar orbits, more overhead satellite passes occur, yielding more data at higher latitudes (Ancel et al. 1992).
Satellite telemetry was first used to track animals in the early 1970's (Buechner et al. 1971). Because early PTTs weighed several kilograms, satellite telemetry was only useful for large animals such as bears (Craighead et al. 1971) and elk (Craighead et al. 1972; Lentfer and DeMaster 1982). By the 1990's, improvements in PTT technology, reductions in weight of both batteries and housing, and use of solar cells have enabled satellite tracking of a wide variety of animals. The primary advantage of satellite telemetry is its ability to track animals over long distances and in remote areas.
Therefore, the best application of satellite telemetry is for far-ranging species such as migratory birds and marine mammals that are difficult or impossible to track with conventional VHF radio telemetry. Examples include dugongs (Marsh and Rathbun 1987), manatees (Mate et al. 1986, 1987; Rathbun et al. 1986), dolphins (Jennings and Gandy 1980; Woods and Kemmerer 1982), harp and hooded seals (Folkow and Blix 1992), humpback whales (Mate et al. 1983), sea turtles (Stoneburner 1982; Timko and Kolz 1982; Byles 1987; Hays 1992), basking sharks (Priede 1984), and polar bears (Kolz et al. 1980; Schweinsburg and Lee 1982; Fancy et al. 1988; Garner et al. 1989).
Satellite telemetry has also been useful in tracking the long-migrating albatross (Jouventin and Weimerskirch 1990; Weimerskirch et al. 1992) and otherwise elusive birds such as bald eagles, golden eagles, gyrfalcons, vultures, penguins, and various swans (Fuller et al. 1984;Strikwerda et al. 1985; Strikwerda et al. 1986; Priede et al. 1988; Higuchi et al. 1990; Ancel et al. 1992; Griesinger et al. 1992; Howey 1992).
Rosenberg and Petrula (1998) conducted studies involving surgically implanted satellite transmitters placed in surf and white-winged scoters from 1998 through 2000. The transmitters were designed to transmit weekly for about one year; however, the weekly signals were not always received.
Far-ranging terrestrial animals such as African wild dogs (Gorman et al. 1992), caribou (Pank et al. 1985; Craighead 1986; Curatolo 1986; Harrington et al. 1987; Fancy et al. 1988, 1989), muskoxen (Reynolds 1986), camels (Grigg 1987), wolves (Ballard et al. 1995; Merrill and Mech 2000), and elephants (Tomkiewicz and Beaty 1987) have also been tracked by satellite telemetry.
As noted above, satellite telemetry's greatest advantage is in tracking elusive and far-ranging species and minimizing the researcher's travel/field time requirements. Theoretically, an animal can be tracked anywhere by a researcher in an office. Satellite telemetry involves a one-time handling of the animal until the PTT battery expires without repeated field trips by researchers. Furthermore, in some situations, such as far-offshore animals, satellite telemetry may be the only feasible means of tracking. Without satellite telemetry it would have been impossible to track emperor penguins traveling across sea-ice since there were no flights during winter over Antarctica, and tracking the penguins by foot on sea-ice was too dangerous (Ancel et al. 1992).
However, satellite telemetry is far less accurate than either conventional VHF radio-tracking or GPS radio-tracking (below). Satellite telemetry frequently reports locations whose accuracy varies from within 150 m to many kilometers (Keating et al. 1991). Locations are categorized into 4 classes (0-3) based on estimated location accuracy prior to receipt by the researcher (Taillade 1992).
Fancy et al. (1989) found 90% of satellite-based location estimates to be within 900 m of the known location, with a mean error of 480 m. The large degree of error is tolerable when tracking far-ranging species such as African wild dogs (Gorman et al. 1992), migratory birds and marine mammals, and long-distance dispersers but not for small-scale habitat analysis or animals using a relatively small area.
Another disadvantage of satellite-based tracking is that it is almost impossible to track the animal from the ground unless a VHF transmitter is built into the PTT. Many workers do incorporate such a transmitter, if only to facilitate recapturing the animal and retrieving the expired PTT for re-use.
Satellite telemetry can be viewed either as costly or economical. The cost of a single PTT unit is usually $3,000-$4,500, some 10-20 times as high as that for a conventional VHF transmitter (White and Garrott 1990). (If the PTT can be retrieved, refurbishment costs $150-$300). Additionally, the researcher must pay for the data acquisition and processing which can cost $90-260 per month per animal (Wilson et al. 1992).
However, satellite telemetry may be cost-effective in certain situations (Craighead and Craighead 1987; Harrington et al. 1987; Fancy et al. 1989). For example, on a cost/data-point basis, conventional VHF telemetry can be 43 times more expensive than satellite telemetry (5 yr study; 10 animals; 1 location per day) (Fancy et al. 1989).
Also, when working with remote species difficult to track, the cost of following the animal through nearly inaccessible terrain or distant oceans is eliminated by using satellite telemetry (Gorman et al. 1992). Furthermore, costs associated with field staff salaries and travel/living expenses, and for purchasing and receiving equipment are saved (Taillade 1992).
Technological improvements have extended the life of transmitting units (Taillade 1992). Most PTTs last from 3 months to 1 year depending upon duty-cycles. Similar to the duty-cycling feature of conventional VHF transmitting units, PTTs can be programmed to cycle on and off at regular intervals thereby conserving battery life. Argos markets a complete solar-powered, 28.5-g PTT for birds with a life of 1 year when duty-cycled to activate every 5 days for 8 hours. Also, photovoltaic cells in combination with NiCd batteries have extended transmitter life. For sea mammals, a sea-water trigger switch can activate a PTT when an animal surfaces. This saves battery life since the unit "sleeps" while the animal is submerged.
Besides reporting location data, new PTT's can store a wide range of physiological, behavioral, and environmental data such as heart rate, dive depth, ambient temperature, etc. for later downloading to the satellite system (Tomkiewicz and Beaty 1987; Taillade 1992). Some PTT collars include a backup VHF beacon built in for locating the animal should the PTT fail, or for facilitating PTT retrieval for refurbishing. Gorman et al. (1992) used African wild dogs implanted with a VHF transmitter in order to facilitate later retrieval of the PTT satellite collar.
Differences among PTTs from various companies are also important to consider. Folkow and Blix (1992) compared the Toyocom T-2028 PTT and the Telonics SAT-103 PTT on harp and hooded seals. The transmission rate, power output, locations obtained, and location quality were all higher for the Telonics version. However, the Toyocom PTT withstood pressure at depths of 600 m while, the Telonics version was only reliable to depths of 400 m.