How Satellite Tags Work
Holding a satellite tag in the palm of your hand it is difficult to imagine that this little device is powerful enough to send a signal to satellites orbiting 1000kms above the surface of the earth. This signal is relayed by the satellite to a data processing computer back on Earth and can then be transferred electronically around the globe.
WhaleNet uses satellite transmitters that send signals to satellites maintained by the ARGOS System in Largo, Maryland and Talouse, France. A number of the U.S. National Oceanographic and Atmospheric Administration’s (NOAA) weather satellites, circling the earth, have ARGOS instruments attached. These instruments collect, process and disseminate environmental data relayed from fixed and mobile transmitters worldwide. What makes ARGOS’s system unique is the ability to geographically locate the source of the data anywhere on the Earth.
Data is collected by the tag while the marine animal is underwater and then transmitted when the animal surfaces. The tag has an antennae which is used to send a signal each time the animal surfaces. Information relayed includes time, date, latitude, longitude, dive depths, dive durations, amount of time at the surface in the last six hours and quality of the transmission. The ARGOS instruments detect the tag’s signal when the satellite passes overhead.
The location fix of the animal in relationship to the track of the satellites, with ARGOS instruments, affects how many satellites passes are made over the animal’s tag in a 24 hour period. Each pass may last between 2 and 12 minutes, depending on the location of the satellite in relation to the animal. The animal must be at the surface at the time of the pass for a successful transmission to take place. Therefore, each day there are a limited number of short opportunities, or maybe no opportunities, for a signal to be transmitted from an animal’s tag to a satellite.
The tag stores data for four to six hour time periods while collecting recent data. The information transmitted by the tag transmitter depends upon the programming of the tag and the data distribution system. The data is transmitted from the tag in the form of digital codes which must be deciphered. WhaleNet uses different types of tags, made by three different companies, with different programming set-ups. The data collected and the way the data is decoded can be different for each tag.
In order to get a fix or position reading it is necessary to receive two or more transmissions from the satellite tag. If at least two messages are received during one pass, computers at the earth station can calculate a location for the transmitter. However, locations based on only two messages are not very accurate. Ideally, locations should be based on three or more messages. In these cases there is a good chance the animals is actually within 1 km of the location calculated. The need for multiple readings to determine position can result in times when the location cannot be reported. You might get none, one or 12 fixes in a day. Other data, such as dive data, can be obtained in just one transmission.
The transmitter is programed to turn itself on and off to conserve battery life. It has a saltwater switch so that it only transmits while it is exposed to air. The signal is too weak to penetrate water. This limits the use of satellite technology to species that spend time at the water’s surface or come to the surface regularly. Therefore these devices are particularly appropriate for studying marine mammals that must come up to breathe. The tag is programmed to think it is dry when it sends five signals without wetting the saltwater switch (about seven minutes out of the water).
Battery life affects how much and the quality of data received from a tag. Battery life varies depending upon size, temperature, depth or pressure and corrosion by the saltwater. Also, data transmission uses up the battery. Therefore, the more transmissions the shorter the battery life.
WhaleNet has used various batteries with its tags in an effort to extend the life expectancy of the tag. Our tags have lasted up to nine months. Some tags stopped sending signals prematurely. Ideas about why this occurred include problems with salt water getting into the tag, the antennae breaking or the animal knocking the tag off.
|How Satellite Tags Are Attached|
Attachment of the tag varies depending upon the animal. In the case of seals and turtles the tag is glued to the fur or shell. When a seal molts the tag falls off. Some tags have been attached with suction cups or bolts.
With whales the tag is attached by partially implanting a barb into the blubber layer at a slight angle, to a depth of approximately 10 cm. Ideally it is placed high on the back of the whale, directly behind the blow hole. These tags are deployed using a compound crossbow. A study by the Minerals Management Society determined that this does not cause serious stress or pose a health risk to the whale. The tagging team goes out in a 4 meter rigid-hull inflatable equipped with an outboard motor in order to get close enough to the whale to implant the tag.
Many tags placed on whales have stopped transmitting in only a few days. It appears that some of the tags WhaleNet has placed on whales have been rejected, falling off and sinking to the bottom of the ocean shortly after they were attached. This may be caused by the whale’s immune system. We are working on ways to get a more secure attachment without harming the whale.
The durability of the tag attachment and antennae are still areas that need improvement. Tag designers are also working on developing more hydrodynamic shapes for the tags. For example, on turtles the tag slants on the front face creating less drag as the turtle swims.
Satellite tagging is very expensive. The cost of the tag itself ranges from $3,500 to $5,000. There are also the costs associated with attaching the tag. In the case of a whale these costs include a boat, crew, fuel, and travel to the tagging location. And there is a charge for the data transmission time. In order to recover some of this cost and equally important the tag, manufacturers place their name and address on each tag so that if it is recovered it can be returned.
|Other Information About Satellite Tags|
- Why use technology in scientific research?
- Why use satellite tags to study whales and marine animals?
- What precautions must be made in working with satellite positions?
- How to interpret satellite tag data
- Understanding and using dive data
- Movie of a satellite tag being attached to a seal
- Slide shows of seals being tagged and released
- Study guide questions to use with satellite tag data
- Other satellite tagging projects
- Argos Satellite System
- Tools to help you analyze satellite tag data
- Map Generator – Generate your own maps of any location.
- Distance Generator – Determine the distance and heading between any two fixes (latitude and longitude readings).
- Sea Surface and Ice Temperature – Satellite images.
- Tide Predictor
- National Buoy Center – Oceanographic data in real time.
- Additional Maps and Maps Index
|Lessons and Curriculum Connections|