Northwest Fisheries Science Center

Satellite tags

Where do Southern Resident killer whales go when they leave Puget Sound? NWFSC scientists and their collaborators satellite tags on orcas to find location data that can reveal detail about about the winter migration of this endangered species and the extent of their coastal range.

How do satellite tags work?

Satellite-linked tags transmit a signal to a satellite, and position data is then relayed to the researcher. The Argos system functions very differently than the Global Positioning System (GPS) most people are familiar with. The transmitter on the whale emits a signal when the whale is at the surface and during the specific hours of the day when the transmitter is programmed to be on (to conserve battery life). The signal the transmitter emits is received by System Argos receivers on NOAA's polar orbiting weather satellites (the transmitter produces a quarter watt signal that has to be detected by a satellite 800 miles above). Weather satellites orbit overhead about every 90 minutes, are overhead for only about 8-14 minutes and are generally most directly overhead during the morning hours to give weather forecasters a first look at cloud cover and/or environmental data the satellites collect.

Assuming the satellite is overhead when the tag is turned on and the whale is at the surface, several signals are sent by the transmitter to the satellite receiver. When tag transmissions are received, these signals are then sent to a ground station which sends the entire transmitter ID and frequency information to Argos headquarters in France for processing.

Getting an estimated location requires the emission and receipt of a series of signals from a very stable frequency of the transmitter, and using a principle known as Doppler shift (this is what occurs when you hear a train horn sounding lower at the instant it passes by) a series of algorithms are applied to the signal data to estimate the transmitter signal's location.

Each location we receive has a location quality rating which estimates the amount of error associated with it. Argos has seven location quality ratings, four of which have no error estimate associated with it - in other words the location may be correct or may be off by dozens of miles. For the three ratings that have error estimates assigned to them the actual locations are generally accurate within a couple of miles.

Determination of the final set of locations requires the use of a filtering program to select those points that have the highest probability of being correct, based in part on the speed between consecutive locations. The result is a detailed and relatively accurate track of the whale's movements for the time the tag was on during the day.

We receive the location information once a day in the form of an e-mail from ARGOS, and can also access it online, although there is a delay (anywhere from 20 minutes to several hours) before the information is available online.

The median duration of signal contact for tagged killer whales is approximately 31 days with some deployments exceeding 3 months. We typically receive several reliable locations per day. Consequently, for each deployment scientists can expect to acquire several hundred new locations.