Satellite
Tracking and Manatees |
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Tie a
Belt Around Your Peduncle!
All this
talk about "tags" and transmitters may have you wondering. What
do the tags look like, how are they attached to the Manatees, what kind
of signals are sent out or received, how long do they last, what is a
'tip' and what happens if the tag gets caught in something?
Journey
North students tracking the migration of the Bald Eagle know that the
eagle wears a "backpack" transmitter. Manatee tags, on the other
hand, are attached with a "peduncle belt" and a "nylon
tether." The belt goes around the manatee just in front of its tail,
in an area called its "peduncle", and the tether is a stiff
nylon rod (about 10 mm in diameter and 130 to 200 cm long) that connects
the belt to the tag. Each tether has a "weak link" built into
it, which will break and allow the manatee to swim free if the tether
or tag ever become snagged on something such as a dock, a boat, thick
vegetation, or even a hungry alligator!!
Credit Birmingham Zoo
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Take a look
at this video of scientists fitting a captured manatee with an transmitter
and the releasing it back into the water:
Radio Waves
Jim
Reid and Susan Butler from the Sirenia Project explained that most manatee
tracking has been accomplished through the use of very high frequency
(VHF) and Argos-monitored radio-tags. Both of these systems are usually
combined in one tracking tag, and both rely on radio transmitters to broadcast
signals from the tagged manatee. The tags are also usually
equipped with a third transmitter, which gives off sonic beacons.
Signals from the Argos transmitter are received by NOAA weather satellites
and relayed back to earth for processing; researchers then use computers
and the Internet to get the calculated locations from Service
Argos. To locate a Manatee, scientists will begin with location coordinates
from the Argos satellite system, and travel to that area.
VHF Receiver
Click image for a closer look
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Once in the
field, the next step is for researchers to use the VHF signals to narrow
down the location of the manatee. Researchers in boats do this by using
antennas and radio receivers tuned to the specific VHF radio frequency assigned
to that specific manatee's transmitter. When the VHF signal is very strong,
researchers will then narrow their search even more, and turn to listening
for the sonic beacons, using a hydrphone which they lower in the water.
When they hear the sonic beacons with the hyrdrophone, the scientists will
now know the direction the Manatee is located in. At this point using the
sonic beacons, the Manatee is usually within 50 to 100 feet away, and the
scientists watch for the Manatee's tag in the water.
The radio transmitters are contained inside of floating plastic cylinders
about 39 cm long and 9 cm in diameter. The cylinders have a 20 cm wire antenna
on the top of them, and every cylinder is color-coded with large identifying
letters and engravings on them.
Sirenia
Project tracking tag expert Susan Butler holds a completed VHF/PTT tag
which she built in the lab
Click image for a closer look
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As noted above,
each cylinder usually has three different transmitters inside it. One is
called a VHF transmitter, which sends specific radio frequency signals to
scientists in the field using a portable receiver and antenna. The other
is called a Platform Transmitter Terminal or "PTT", and it sends
out location signals to an orbiting satellite. The third is the sonic transmitter,
which sends a very localized signal which scientists listen for using a
hydrophone in the water.
Typically, the PTT will send from two to six different
location points for each tagged manatee every day. Besides "location"
information, data is also sent on manatee "activity" (# of dives,
duration of dive, # of times the transmitter "tips" greater
than 90 degrees, i.e. swimming, playing, etc) and temperature.
Disassembled
VHF/PTT tag components next to the cylinger which holds them.
Click image for a closer look
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According to
Cathy Beck, satellite tracking data can be inaccurate or be interrupted
for several different reasons. "The quality of the location plotted
by the satellite varies, depending on whether the Manatee (actually the
antenna) is at the surface when the satellite passes--the antenna must reach
the surface in order to broadcast the signal to the satellite. Also, data
quality can be affected when there are a lot of structures or vegetation
that may interfere with the signal. Finally, transmitters (and regrettably
manatees too) can be damaged if struck by boats"
"Beeping" or "Listening":
The Difference Between Radio-Tags and GPS Tags
Jim Reid pointed out that radio tracking and GPS tracking are very different
processes. "Unlike the radio tag which sends signals ("beeping"),
the GPS tag attached to the Manatee is a radio receiver ("listening")
that is tuned to signals being transmitted from NAVSTAR
satellites. The GPS receiver then calculates a location and stores it
to memory in the tag. These locations are normally very accurate and the
tag can be programmed to obtain many locations per day. Unfortunately,
the GPS tag must be recovered for us to download the stored locations
and plot where the manatee has been. New technology is being developed
to relay the stored GPS data to researchers, perhaps through an Argos-monitored
transmitter!"
For more details on GPS Technology go to:
Special thanks
to Service Argos
for their ongoing assistance.
Copyright
2001-2003 Journey North. All Rights Reserved.
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