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. 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. 

For detailed information about which tags offer specific technologies, we recommend reaching out to a Technical Sales Consultant. 

Light-based Geolocation

Resolution: A matter of degrees

For animals that do not regularly surface thus precluding the use of satellite tracking, light-based geolocation can be used to estimate their positions. This type of geo–positioning provides lower-resolution location estimations, making it applicable for tracking large-scale movements, such as across basins, large gulfs or bights. Wildlife Computers geolocation processing software (GPE3) utilizes observations of twilight dawn and dusk to estimate longitude and latitude. The tag filters the ambient light to isolate the blue wavelengths of interest and corrects for light attenuation with depth. Dawn and dusk curves are then identified using the corrected light data. Finally, locations are estimated using dawn and dusk light curves, bathymetry data, and satellite–derived sea surface temperature. The addition of surface temperature and depth reduces location error. Location error will vary depending upon season and position on the globe. The outcome is a time-discrete gridded distribution of location likelihoods throughout the tag’s deployment. 

Ideal deployments will be on fish that remain in the top 200 m (i.e., epipelagic) during dawn and dusk, in open, unobstructed areas with clear views of the horizon. 

Argos Doppler Locations Overview

Resolution: 100s of meters

Argos tags send radio transmissions to Argos low-earth-orbiting (LEO) satellites when the tagged animal surfaces. When a transmission is received by an overhead satellite, it is recorded and relayed to an Argos ground station. This information is then forwarded to the CLS Processing Center where the Doppler shift of the transmissions is calculated, and the most probable locations are determined. A satellite pass typically lasts less than 10 minutes. The satellite needs to receive at least four uplinks from the tag spread across the duration of that pass to calculate a location based only on Doppler shift. Locations can be calculated with fewer than four uplinks, but the error may be quite large. Generally, the more uplinks received during a pass, the lower the error of the location estimate. 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^® Overview

Reolution: 10s of meters

Traditional GPS systems are not always suitable for animals that surface briefly and infrequently. 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 estimating 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 10 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 processed snapshot of signals is 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.