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Regular
readers of Sagren and followers of wildlife tracking
efforts will be familiar with the term ‘Satellite
Tracking’ which is thought to be the most advanced means
of following animal movements remotely over long
distances. However, it may not be the Holy Grail of
tracking that some think and it seems that few people
really know what it involves, how it works or what
information it provides. As such this is a basic guide to
the science and wizardry of satellite tracking.
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| Diagram
of basic satellite transmission and relay system |
Basic
Principles
There are three basic components to the system: the
transmitter, known as a “Platform Terminal
Transmitter” or PTT; a receiver on-board a satellite to
receive and relay the signal; and the receiver on earth to
acquire and process the transmitted data.
The
animal to be studied is fitted with a PTT that transmits a
low output Ultra High Frequency radio wave signal. All
PTTs transmit on the same frequency and so transmissions
are very short (less than 400 micro seconds) but are
repeated at regular intervals of between 40 to 60 seconds.
The data transmitted is essentially an identity code for
the tag concerned plus whatever data the tag is configured
to transmit from its on-board sensors.
The
signals are picked up by “Argos” receivers on orbiting
satellites of the NOAA weather programme. This is a joint
venture with the Centre Nationale pour l’Exploration
Spatial based in Toulouse, France.
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NOAA
12 Satellite pass prediction over Seychelles for
Aug 17 2003, image created by “J-PASS”
courtesy NOAA.
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These satellites are able to monitor over 200 PTTs at any
one time. They record when they receive each PTT
transmission, this information is then relayed
to a receiving station on earth along with the original
transmission.
A
satellite can only pick up a transmission if it is
‘visible’ to the PTT concerned. Near the poles there
may be as many as 28 satellite passes a day, however there
are currently only four Satellites that track over the
Indian Ocean, (NOAA 11, 12, 13 & 15), with a total
of eleven passes lasting between 5 to 20 minutes. As
such your PTT has to be transmitting when one of these
satellites is overhead for a signal to be received,
relayed and processed.
If a
satellite picks up several transmissions from the same
tag, even if only a few seconds apart, the differences in
orbit location will allow the position of the tag to be
calculated on the surface of the earth.
However, even if a PTT was transmitting continuously for
24 hours in the Seychelles region, the Argos system would
only receive data on the eleven passes when the satellites
are visible and so a continuous 24 hour track of a tag is
simply not possible. Add to this the fact that radio waves
do not travel well through water and that our particular
subject animal is a large shark that spends a lot of time
under water and you soon find that the likelihood of a
transmission being received begins to diminish!
Having said that, the system capabilities are still pretty
astounding. The satellites re-transmit the received
transmission to the earth receiving stations that then
process the data from the PTT transmissions. Based on the
time and position of the satellites the system is able to
generate a prediction of the position of the PTT on the
earth’s surface along with an indication of the accuracy
of the prediction. Essentially, the more successive
transmissions recorded the higher the degree of accuracy
of the prediction. Once processed the information is then
sent to the owner of the PTT by one of various means that
are chosen by the owner, in the case of the MCSS tags this
is done by a daily e-mail message containing all
transmission from MCSS registered PTTs.
This
is the essence of using satellites for tracking. If you
have an animal that spends long periods of time on the
surface then you can build up impressive tracks of their
movements from satellite derived data. If, however, you
study animal spends long periods of time below the surface
of the sea as in our case, then the location data can
become erratic.
How
to Increase Transmissions
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SPOT2
tag deployment used by MCSS programme
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The key with marine animals is how to increase the number
of transmissions your PTT can make and this is done in a
number of ways. First of all there is the consideration of
power: allowing your tag to transmit while it is submerged
is a waste of battery power and as such limits the life of
your PTT once deployed on the animal. Tag manufacturers
thus include sensors in their tag designs so that the PTT
will not be powered up until it is actually on the
surface.
The
next strategy is how to increase the amount of time your
tag is on the surface. With marine mammals like whales and
seals, the fact that they are air breathers obviously
assist this aspect as the animal has a biological need to
surface regularly to breath. Sharks, however, have gills
and have no such requirement. Whale sharks do spend
very large amounts of the time close to the surface and so
the method of choice by several research groups is to
configure the tag into a buoyant module that is attached
to the shark by a long tether. That way whenever the shark
is close to the surface, the tag will float to the surface
and thus be able to transmit. In our studies we have used
a 10 metre tether which will allow the tag to reach the
surface when the shark is less than 7 metres deep and
swimming at their normal swimming speed (around 1.2km/hr).
Tag
Design
The
tags themselves have undergone some significant changes in
design and construction since the beginning of marine
animal monitoring. At first almost all tags were custom
built by individual research organisations using PTTs and
sensor components from various sources. This meant that
there was a lot of variety and experimentation and also a
lot of ‘empirical testing’… suck it and see!
However, the need for reliability and efficiency prompted
the specialist tag manufacturing companies to develop
products that would suit a variety of applications.
Marine study tags are now mainly ‘potted’ in a
synthetic resin that protects them from pressure to depths
of up to 1500 metres. These extreme depths also pose a
problem for buoyant designs as most synthetic foams do not
recover well from exposure to such pressures and as such
the tag loses its buoyancy. Consequently most of the
floating designs use synthetic foam which is pressure
resistant.
There are currently two common forms of marine tag
configuration, the towed buoyant module as previously
described and the fin-mount which is a non buoyant module
that is attached directly to the dorsal fin of the shark
and so will break the surface when the shark is right at
the surface. This latter type requires being able to
restrain the shark sufficiently long enough to attach the
tag securely which is difficult with a 6 metre free
swimming shark!
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Resin
‘potted’ SPOT2 satellite transmitter and
syntactic foam body
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For
the last three years we have deployed SPOT2 tags
manufactured by Wildlife Computers of Seattle, USA. The
SPOT2 PTT radiates 0.5W of output and when powered by a
single C-cell lithium battery it can make between
80-120,000 transmissions. The battery life is conserved by
the conductivity sensor that permits transmissions only
when the tag is ‘dry’ at the surface. The tags could
also be configured to limit the total
number of transmissions
per day and to times when satellites were known to be in
the area. However, as it was not known which area the tags
would end up in and thus the correct satellite regime to
select, this control was not enabled in our studies. A
constant-transmission limit was set to shut down the tag
after 23 hours of being dry (i.e. transmitting
continuously); this would shut down the tag should it
become detached and floating free at the surface. However,
as a safeguard, the tags were set to reactivate after
being submerged for 10 seconds or longer, just in case our
study animal was in a very shallow area, or simply basking
for a day.
Reports
on the tracks recorded by tags in our study have already
been described in Sagren
Vol1, Issue 2, June 2003 Oceanic Migrations.
But
there is a lot more to satellite tags than just the
location of the tag, the sensor packages integrated into
the tag are becoming more and more sophisticated as the
second part of this article will unveil!
To be
continued…...
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