High,
High Tech
The Science of Satellite Tracking
In order
to track an eagle by satellite, a transmitter must be fitted on the eagle's
back. The transmitter fits like the backpack you might wear to school.
Notice the black straps shown in the picture below. The straps go in front
of and behind both wings, and are stitched together in the front. An antenna
hangs down the eagle's back and sends radio signals.
Eagle
with satellite backpack. |
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These backpacks
are known as "PTT's," which is short for "platform transmitter
terminals." The eagle's PTT is powered by a battery. Because the
battery wears down each time it's used, the PTT can't constantly send
signals. To save battery life, the PTT is programmed to send radio signals
according to a schedule the scientist sets. For example, Peter Nye programs
the PTT to emit signals every 2 days. His eagles' batteries now last for
approximately 2 years.
The eagles'
signals are picked up by polar-orbiting satellites when they pass high
overhead. Just think: Zipping through space, the same satellites are picking
up signals from other Journey North species--from the manatees and whooping
cranes in sunny Florida all the way to the the caribou in the cold Arctic.
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NOAA-15
before launch |
According to
NOAA, in
addition to tracking migratory species, "The polar orbiters are able
to monitor the entire Earth, tracking atmospheric variables and providing
atmospheric data and cloud images. NOAA is operating three polar orbiters:
NOAA-14 launched in December 1994 and a new series of polar orbiters, with
improved sensors, which began with the launch of NOAA-15 in May 1998 and
NOAA-16 on September 21, 2000."
- The satellites
travel 833 km above the earth
- It takes
the satellite only 101 minutes to circle the earth!
- NOAA-15
(see photo) was launched May 13, 1998 Vandenburg Air Force Base, CA
- Weight
at liftoff: 2,231 kg (4920 pounds)
- Length:
4.2 meters (13.75 feet), Diameter: 1.88 meters (6.2 feet)
The satellites receive the message sent by the transmitter and the transmitter's
location (its latitude and longitude) are calculated by computers on
the ground. The locations are calculated using what's known as "the
Doppler shift." You've experienced the Doppler shift if you've
ever listened to the shift in sound a race car or train make when approaching
and then traveling past you. (With your voice, make the high-pitched
sound of an approaching race car, then the zooming sound it makes as
the car races past you, and its pitch quickly slides lower.)
According to Service Argos, the
company who provides the satellite-tracking service to scientists, "When
the satellite 'approaches' a transmitter, the frequency of the signal
measured by the satellite receiver is higher than the actual transmit
frequency, and lower when it moves away." This is the basis for
calculating location.
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2002-4 Journey North. All Rights Reserved.
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