How do these SPOT tags work anyway?

Obtaining and sharing our precise locations is something we rarely think twice about these days. Our smart phones not only provide our current latitude and longitude, but can associate that information to the photos we take and the places we visit. Why then is it so difficult to track where sharks are going?

 

Attaching a SPOT tag to a Scalloped Hammerhead (Sphyrna lewini) we caught in 2014. Photography courtesy of David Hay Jones.

We are all familiar with the Global Positioning Satellites (GPS) described above, but for sharks (and other wildlife) that gets tagged and not recovered, we need a means of retrieving the data remotely. And to do that, we use a platform known as Advanced Research and Global Observation Satellite (ARGOS), a system that collects, processes and disseminates data from both mobile and fixed platforms. The ARGOS system estimates the position of a tagged animal using the Doppler Effect, the change in frequency of a wave when a two objects are in motion relative to each other (imagine the sound a train makes as it's approaching, then moving away). In the same way, as the satellite "approaches" the tagged shark, the frequency of the signal measured by the ARGOS satellite is higher than the actual transmitted frequency. The Doppler Effect is great for estimating positions in difficult environments like the ocean, and provides position estimates with an accuracy of up to 150 meters. These position estimates are then relayed (through the ARGOS system) to a network of nearly 70 land-based receiving stations, positioned to provide coverage across the entire planet. Finally, data are retransmitted from the receiving stations to one of two global processing centers. At this point, the data are verified, time tagged, classified, and then processed to calculate a location. Processed data can be accessed by the researcher online, or though fax, CD-rom, etc.

Check out this excellent video from the folks at ARGOS to better visualize how these tags work!