How to Track Turtles
by Satellite

Tracking sea turtles via satellite is a powerful tool, but it is easier said than done.

NOAA's Tiros weather satellite with ARGOS instruments (ARGOS)

Every day, weather satellites of the US National Oceanographic and Atmospheric Administration (NOAA) circle the earth in their daily routine of taking pictures for the meteorologists. Their polar orbits take them around the Earth, from pole to pole, so that they pass over a certain part of the planet at predictable times every day.

Three of the satellites (NOAA-11,14, and 15) have ARGOS instruments on board that can receive ultra-high frequency (UHF) messages sent from so-called satellite platforms on earth. As the satellite comes over the horizon and in view of a site on earth from which a message is being transmitted, say from one of our project's turtles in the Sulu Sea, it begins receiving the signal. The turtle's transmitter sends a new message about every 40 seconds. Each message contains the identification code of the transmitter and some technical data, like how deep the turtle's last dive was, how often she has dived since the last transmission, etc. The satellite continues to receive signals until it disappears over the opposite horizon. Such a satellite pass can take between 2 and 12 minutes, depending on how high above the horizon the satellite passes. The satellite stores all the information, and as soon as it comes within view of an earth station, it re-transmits or downloads it to the station. From there it reaches first the ARGOS system computers and then our project computers.

This animation of the ARGOS data transfer was produced by the BBC for another one of our projects.
(You need the VIVO plugin installed on your browser to view the movie)

Using a physical phenomenon, the Doppler Shift, the ARGOS computers then estimate the location of the turtle. In general, the more messages the satellite has received during a single pass over the turtle, the more accurate the computers can calculate her location. In our tables and in our animations of the turtle movements, we encoded the quality of the location in different colors.  Red is top quality (LC 1-3) and blue is mediocre (LC 0); poor quality positions are not shown. In the Location Class (LC) column of our data tables you can see the quality of the individual locations.

But what determines how many messages the satellite receives during a pass? Sea turtles live in the ocean, and spend most of their time swimming, eating and resting in the water. The transmitter's signals can only be received by the satellite when the turtle is at the surface and the antenna is out of the water and in the air. But turtles rarely come to the surface; they do so only to breathe or maybe to float for a while as they rest or sun. For a good quality location to be calculated, several things must all happen at once: the turtle must be at the surface, the antenna must be in the air, and the satellite must be above the horizon. All these have to take place long enough for at least 4 messages to be received during one pass. Only then can the computers calculate at least a blue quality location.

Green turtle with satellite transmitter leaves the nesting beach. (Stuewe, Oct. 1998)

Tough chance! Even when a turtle surfaces while a satellite is overhead, one small wave or wave splash that smothers the tip of the antenna at just the wrong time is enough to interrupt the transmission. It is not easy to get good locations for a sea turtle!! That's why our tables with the raw data have so few good locations. We received many more messages, but most of them had to be thrown away because they were useless for plotting accurate positions of the turtles.

Telonics ST14 transmitter with whip antenna. To avoid having algae, barnacles and other marine animals trouble the turtle, the transmitter has been painted with a green anti-fouling paint. (Stuewe, Oct. 1998)

But there are more problems in getting signals from our turtles.
Loss of power is one. Batteries keep the sensors and the electronics working, and they power the transmissions that go out to the distant satellites. Actually, most of the space in the transmitter case is filled up with batteries. When the batteries are drained, the turtle is off the air. That's why many energy saving features are built into each transmitter. The most important one is a saltwater switch which turns off the transmitter whenever it is under water and signals would not reach the satellite anyway.

Damage or loss of the antenna is another serious problem. Sea turtles, especially the green turtles we study in the Turtle Islands, often live around hard corals, reefs or rocks. They frequently have specific sites to which they retire for resting. Also, they often rub their hard shells against corals or rocks, either by accident or to scrape off barnacles or other epizoans, animals and plants that live attached to the turtle's shell. Sometimes this rubbing can be so vigorous that it can destroy corals and make scratches in the shell. If there is a foreign object attached to the shell -- like a satellite transmitter -- the turtle is likely to scrape it vigorously. The transmitter's "vital parts" are encased inside a resistant epoxy case, but the antenna sticks out, fully exposed. All this scraping and bumping can easily break the antenna. No matter how much power is left in the battery, or how often the turtle comes to the surface, once the antenna breaks, there can be no more communication between the transmitter and the satellite. Bye, turtle! We won't hear from her again until she comes back to the Turtle Islands after several years. If we manage to see her when she lays a new clutch of eggs, and if she has not lost her transmitter with all this scraping on rocks and corals, we can remove the transmitter and send it back to the manufacturer for repair.


Here are some more details about the analysis of the satellite telemetry data.

Doppler Shift

Because the satellite is moving rapidly at about 26,640 km per hour, the frequency of the signals sent from the transmitter to the satellite is influenced by the Doppler shift. That is similar to what happens when a speeding train or car moves towards you and then passes by -- the sound suddenly changes from high-pitched to low-pitched. The ARGOS computers make use of the Doppler shift, by analyzing the frequency change of the incoming signals and, with a complex series of formulas, they calculate two predicted positions for the transmitter for each over pass. The position that is too far away to be possible then has to be discarded.

Location Class

In general, the more messages the satellite has received during a single pass, the more accurately the computers can calculate the location of the transmitter. But even under the best of conditions, there is always some error in the prediction of the location. ARGOS calculates the predicted location along with the size of the error, called Location Class or "LC". LC 3 is the best, it means that the position that ARGOS has calculated should fall 95 out of 100 times inside a circle with a radius of 150m. For LC 2, the location should fall inside a circle with a radius of 350 m, and for LC 1, the circle will have a radius of 1000m or one kilometer.

At least four messages per pass must be received to achieve LC 3, LC 2 or LC 1 quality. On our maps we have indicated these high quality positions in red. If at least four messages are received, but for some reason the conditions are poor, the calculation may be LC 0. This means that a position is calculated, but there is no estimate for how big the error is.

If only 3 messages are received during a pass, a position can be calculated, but there will be no estimate of how big the error is: this is LC A.

When only 2 messages are received during a single pass, ARGOS can still calculate a position, but it is not possible to calculate how much error is involved: this is LC B. Finally, if only one message is received during a pass, LC Z is reported; there is no prediction of position.

On our map we coded the mediocre quality positions (LC 0) in blue; the poor quality positions are not shown. Likewise, in our analysis, we only use the red and blue locations. Only when we have no other information from the turtle at all do the poor quality positions give us at least an indication of where she might be.

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