08 Apr Where am I? Understanding the Resolutions of Locations from Wildlife Computers Tags

When selecting the most suitable tag for your wildlife study, it is important to consider the resolution of location data. Here, we provide an overview of the three types of location resolutions available with Wildlife Computers Tags. For detailed information about which tags offer specific technologies, we recommend reaching out to a Technical Sales Consultant.

Light-based Geolocation: A Matter of Degrees

For animals that do not frequently surface, light-based geolocation can be used to estimate their positions. This type of geo-positioning provides lower-resolution location estimations, making it ideal for tracking large-scale movements, such as across oceans, basins, large gulfs or bights and location will vary depending upon season and location. Wildlife Computers geolocation processing software (GPE3) utilizes observations of twilight, with the addition of sea surface temperature, and depth to improve location accuracy. The outcome is a time-discrete gridded distribution of location likelihoods throughout the tag’s deployment.

The tag filters the ambient light to isolate the blue wavelengths of interest and corrects for light attenuation with depth when underwater. Dawn and dusk curves are then identified in the corrected light data. And finally, locations are estimated using dawn and dusk light curves, bathymetry data, and satellite derived sea surface temperature. Ideal deployments will be on fish that remain in the top 200 m (i.e., epipelagic) in open, unobstructed areas with clear views of the horizon.

Argos Doppler Resolution: 100s of Meters

Argos tags send radio transmissions to Argos low-earth-orbiting (LEO) satellites when the tagged animal surfaces. When multiple transmissions are received by an overhead satellite, the satellite records the Doppler-shifted transmissions and relays the information to an Argos ground station. This information is then forwarded to the CLS Processing Center. Through their post-processing analysis, the most probable locations are determined. Location accuracy improves with an increased number of transmissions received during a satellite pass. The transmissions can also include data messages about the animal’s behavior, the environment, or tag diagnostic data. A satellite pass typically lasts around 10 minutes, and for good accuracy, the satellite should receive at least four uplinks from the tag during spread across the duration of that pass. With more uplinks received by the satellite, the quality error of the location estimate decreases increases. The transmissions can also include data messages about the animal’s behavior, the environment, or tag diagnostic data. Naturally, for this system to work, a satellite must be within view when the animal surfaces.

GPS/Fastloc^® Resolution: 10s of meters 

Traditional GPS systems are not always suitable for animals that spend time underwater and surface briefly. Traditional GPS receivers require downloading satellite ID codes, Ephemeris data, and Almanac data, and can take several minutes to process a location. Fastloc technology addresses this issue by employing a unique approach to acquiring location data.

When a tag on a marine animal breaks the water’s surface, it captures a snapshot of signals from the GPS satellite constellation in a fraction of a second. These signals are processed and compressed onboard the tag, optimized for transmission over Argos, or stored in the tag’s archive. The processing and compression take less than 12 seconds and continues even after the animal has submerged. Up to 8 GPS satellites can be processed if they were in view at the time of the snapshot, providing location accuracies ranging from 20 to 75 meters. The snapshot of signals are transmitted or uploaded to the Wildlife Computers Portal. Approximately 95% of location errors fall within 55 meters, with 50% of errors within 20 meters of the true location.

By understanding the resolution capabilities and considerations of each tag type, researchers can make informed decisions when selecting the appropriate Wildlife Computers Tags for their specific study requirements.