Although the word 'geolocator' is non-specific, this is often used to describe a device that primarily and periodically records ambient light level (solar irradiance) to determine location. A geolocator, light-level logger, GLS or bird logger is essentially a lightweight, electronic archival tracking device, usually used in bird migration research to map migration routes, identify important staging areas, and sometimes provide additional ecological information.
Animal tracking using light level data appears to have been first carried out on elephant seals. Although not described until 1992, the first device to be developed was reportedly in 1989 as an adaptation of a TDR (time-depth recorder) and called a geographic location, time-depth recorder (GLTDR) weighing 196g. Also in 1992 was a publication by a different group with a similar design (weight 113g) who called it a global location sensor (GLS) though no field use was mentioned.
The use of dedicated light level recorders for tracking birds was pioneered in the 1990s by engineers and scientists at the British Antarctic Survey (BAS) which first developed a device in an attempt to record the movements of juvenile Wandering Albatross during the many years between fledging and returning to their colony to breed. From albatrosses and other seabirds the use of geolocators has been extended to other migratory species, including waders, wildfowl, raptors and songbirds as designs have become gradually smaller and more energy efficient.
Geolocators record changes in light levels. The smallest are archival types that do not use satellite or radio telemetry and recapturing the bird is necessary to retrieve the device to download the data to a computer for analysis. The disadvantage of having to recapture is offset by the miniature size to which archival loggers can be made. By using low power design techniques and data compression they can record data for long periods of time.
Recording light levels over time produces data that can be used to calculate latitude and longitude readings of a bird’s long-distance movements. The traditionally used 'threshold analysis method' requires only twilight data accurately time-stamped. Typically, daylight length (the time between dawn and dusk) is used to determine latitude, while the mid-time between a dawn and dusk is used to determine longitude. In this way, two position fixes can be obtained daily. Other analysis techniques can include analysis of the dawn and dusk curve (rate of change of light), or use the noon light level to attempt cloud compensation. The location data so derived is not as accurate as that from GPS or PTT (platform transmitter terminal) tracking involving satellites, but the devices can be made considerably lighter and cheaper. Other sensors, such as for recording temperature, or whether the logger is wet or dry, may be used in conjunction with the light-level logging in order to provide further ecological information. The devices may be attached to the bird being tracked by a harness, or to the band on the bird’s leg. The weights of geolocators range from about 0.6g to 2.5g, with a battery life of 1–5 years.
The main accuracy limitation of light level geolocation is due to the uncertainty caused by shading. Shading, or the attenuation of ambient light, can have many causes e.g. cloud, feathers, foliage. Because of this, the quality of the resulting location calculations can vary with species, habitat and behaviour.
- "Theory of Geolocation by Light Levels by Roger D. Hill, (Le Boeuf, Burney J., and Richard M. Laws, editors Elephant Seals: Population Ecology, Behavior, and Physiology. Berkeley: University of California Press, c1994 1994)".
- Documenting Migrations of Northern Elephant Seals Using Day Length, Delong R.L., Stewart B.S., Hill R.D., Marine Mammal Science 8:155-159, 1992
- Estimation of location: global coverage using light intensity, Wilson R.P., Ducamp J., Rees W.G., Culik B.M., Niekamp K., in Wildlife Telemetry: remote monitoring and tracking of animals, Priede I.M., Swift S.M. (eds), Ellis Horward, Chichester, p131-134, 1992
- "Migrating Bird Tracking Logger".
- "A miniature daylight level and activity data recorder for tracking animals over long periods".
- "Geolocator manufacturers".