Satellite transmitters have been used to track wildlife since the 1980’s (Hart and Hyrenbach 2009). In that time, technological advances have expanded satellite transmitters from a tool that could once only be used on large terrestrial mammals to a tool for tracking smaller animals at increasingly higher resolutions (Kays et al. 2015). Several reviews have summarized the decades of tracking data that have accumulated for species ranging from birds to mammals to reptiles (Hart and Hyrenbach 2009, Hammerschlag et al. 2011)
Platform Transmitting Terminals (PTT)
PTTs are transmitters that send user-defined periodic messages to the Argos system, a global satellite-based location and data collection system dedicated to studying and protecting our planet’s environment. The polar-orbiting satellites making up Argos fly at an orbit of 850 km above the earth. They pick up the signals, store them on-board, and relay them in real-time back to earth. Receiving stations then relay data from satellites to processing centers. These processing centers collect all incoming data, process them and distribute them to users.
The Argos system calculates locations by measuring the Doppler Effect on transmission frequency (401.650 MHz ± 30 kHz). The Doppler Effect is the change in frequency of a sound wave or electromagnetic wave that occurs when the source of vibration and observer are moving relative to each other. Argos users receive data directly in their office or on-site, depending on their choice (email, web, or directly on mapping software).
Global Positioning System (GPS) Data Loggers
The Global Positioning System is a freely accessible global navigation satellite system, maintained by the United States government, which is used to provide location and time information in all weather anywhere on earth. So long as a GPS receiver has unobstructed line-of-sight to four or more GPS satellites, it can use the information (time of transmission, precise orbital information of the satellite, and general system health/rough orbits of all GPS satellites) transmitted in order to calculate accurate time and location.
GPS data loggers do not maintain continuous location data, but instead calculate location at user-defined intervals, leaving a “breadcrumb” view of the path taken by the logger. In order to access this data, the animal carrying the GPS logger must be recaptured and the data downloaded, usually from an internal memory drive similar to a USB drive.
GPS/PTT transmitters, also known as “data pushers,” use the GPS or PTT system in order to determine location, then “push” (send) this data to a determined server at certain intervals. GPS/PTT transmitters are very useful because it is unnecessary to re-capture the animal in order to collect data. However, they are heavier than GPS data loggers, and thus cannot be used on smaller animals (such as songbirds). While GPS/PTT transmitters are usually used to track the location of an animal, when vaginally implanted in a pregnant animal they can be used to determine the site at which the animal gives birth.
The primary limitation in using PTT tags is their size, which is often too large for many smaller migratory species. Satellite tags require large batteries to power the tag and send satellite signals (Bridge et al. 2011). Technological advances are pushing the size smaller, and creating opportunities to track species that previously were too small (Scarpignato et al. 2016).
As the tracking technology improves, higher resolution data will also increasingly become available for more species. This will require analytic tools to keep pace with technology to make use of the growing tracking data that become available (Hebblewhite and Haydon 2010, Kays et al. 2015).
Another limitation to satellite telemetry is that the tags require clear signal path to transmit data. This can be affected by environmental characteristics that obstruct the signal, species traits, satellite coverage, and other factors (Christin et al. 2015, Hofman et al. 2019). Previously functioning tags may also stop transmitting, which can be caused by battery exhaustion, antenna loss, and a variety of other reasons (Hays et al. 2007). These are considerations that researchers should take into account when designing studies, deploying tags, and analyzing data.
Edited by: Matthew Johnson (USGS Forest and Rangeland Ecosystem Science Center, email@example.com), 2014.
Updated by: Allison Huysman (Migratory Connectivity Project, firstname.lastname@example.org), 2020.
- Bridge, E.S., K. Thorup, M.S. Bowlin, P.B. Chilson, R.H. Diehl, R.W. Fléron, P. Hartl, R. Kays, J.F. Kelly, W.D. Robinson, and M. Wikelski. 2011. Technology on the move: Recent and forthcoming innovations for tracking migratory birds. BioScience 61(9): 689-698.
- Christin, S., M.-H. St-Laurent, D. Berteaux. 2015. Evaluation of Argos telemetry accuracy in the High-Arctic and implications for the estimation of home-range size. PLoS ONE 10(11): e0141999.
- Hammerschlag, N., A.J. Gallagher, and D.M. Lazarre. 2011. A review of shark satellite tagging studies. Journal of Experimental Marine Biology and Ecology 398(1-2): 1-8.
- Hart, K.M. and K.D. Hyrenbach. 2009. Satellite telemetry of marine megavertebrates: the coming of age of an experimental science. Endangered Species Research 10: 9-10.
- Hays, G.C., C.J.A. Bradshaw, M.C. James, P. Lovell, and D.W. Sims. 2007. Why do Argos satellite tags deployed on marine animals stop transmitting? Journal of Experimental Marine Biology & Ecology 349: 52-60
- Hebblewhite, M. and D.T. Haydon. 2010. Distinguishing technology from biology: a critical review of the use of GPS telemetry data in ecology. Philosophical Transactions of the Royal Society B 365: 2303-2312.
- Hofman, M.P.G., et al. 2019. Right on track? Performance of satellite telemetry in terrestrial wildlife research. PLos ONE 14(5): e0216223.
- Kays, R., M.C. Crofoot, W. Jetz, and M. Wikelski. 2015. Terrestrial animal tracking as an eye on life and planet. Science 348(6240): aaa2478-1-aaa2478-9.
- Scarpignato, A.L., A.-L. Harrison, D.J. Newstead, L.J. Niles, R.R. Porter, M. Van den Tillaart, and P.P. Marra. 2016. Field-testing new miniaturized GPS-Argos satellite transmitter (3.5 g) on migratory shorebirds. Wader Study 123(3).