Engineers help biologists' dreams
to collect data bounty
be a wildlife biologist, one must also be a bit of a sneak.
Creeping up on a group of animals, hiding in the tall grasses,
hoping that the click of the camera, the hushed whispers,
and the sound of a pen scribbling notes on a pad won't frighten
them off. Because if it happens that an alarm goes off and
the whole herd goes sprinting across the savanna, there's
no telling how long it will take to find them again, or what
intriguing data will be missed in the meantime.
It's not easy being a biologist. If only
there were a way to observe animals closely, without invading
Soon there will be. Princeton scientists
and engineers are now developing a sophisticated system to
track animals--specifically zebras--in the wild: a wireless
sensor network dubbed ZebraNet.
ZebraNet will be established in the Mpala
Research Centre in Kenya. Mpala is a biology field station
administered by Princeton University, the Kenya Wildlife Service,
the National Museums of Kenya, the Mpala Wildlife Foundation,
and the Smithsonian Institute.
by Frank Wojciechowski
EE Professor Margaret
Martonosi is conducting research that will one day help
biologists with their studies of animal behavior.
ZebraNet will collect round-the-clock data
on zebras over the course of one year through the use of a
sophisticated sensor network.
A sensor network like ZebraNet consists
of several data collecting locations, called nodes, which
communicate with a base station that compiles the data from
all nodes. In the case of ZebraNet, the base station will
Scientists will just pack the computer
into a truck or a plane. Each node is a tracking collar carried
by an animal in the wild. These nodes contain a global positioning
system (GPS), storage cells, a wireless transceiver, a CPU,
solar cells, and batteries.
These nodes will periodically collect and
store information about the animal's location, heart rate,
body temperature, and feeding frequency. This information
will be transmitted back to the base station when the researchers
fly or drive by within a certain distance from the nodes.
The plan is to collar 30 to 50 animals and use the data to
paint a clearer picture of their migration patterns, eating
habits, and inter- and intra-species interactions as well
as the impact of human action on their activity. Because the
collars will collect data all day and night, scientists can
enjoy their observation efforts without disturbing the animals.
The ZebraNet project is a collaborative
effort between Princeton's Departments of Electrical Engineering
(EE) and Ecology and Evolutionary Biology (EEB).
"It's a great Princeton story,"
said Principal Investigator Margaret Martonosi, associate
professor of electrical engineering.
A few years ago, Professor Martonosi--along
with Professors Perry Cook, David Dobkin, and Stephen Lyon--was
advising a pair of undergraduates working on a senior project.
Andrew Steiner '00 of EE and Daniel Davenport '00 of CS created
an interactive campus tour by using a GPS hooked to a palm
pilot. The GPS could detect a tourists' precise location and
provide information about the buildings and the sculptures
in that place.
by EE graduatestudent Pei Zhang, the ZebraNet II card
is smaller, lighter-weight, and lower-power than the
first hardware prototype.
The buzz about this project made its way
to Professor Daniel Rubenstein, professor and chairman of
the EEB department.
"Dan e-mailed me and said 'I should
tell you about zebras some time,'" Professor Martonosi
said. "I thought he was going to give me some tourism
To her surprise, Professor Rubenstein wanted
to discuss the possibility of applying this senior project
prototype to wildlife research. The idea was that if the contraption
could track people roving around campus perhaps it could track
animals in the wild as well.
"Normally, when you're studying animals
you have to find them before you can watch them, and they
often run away from you," said Professor Rubenstein,
one of the project's coprincipal investigators. "We'll
be able to get a sense of how a whole population is using
Intrigued by their conversations, Professor
Martonosi stopped by the Mpala Research Centre during a vacation
she made in Africa in the summer of 2001. During this visit
she got a feel for the challenges inherent in the task Professor
Rubenstein was suggesting and became inspired.
"This project is novel in
an engineering sense, and novel in a biological sense. We
scored big with NSF because we are solving needs in two very
-- Daniel Rubenstein, professor and chair, ecology and evolutionary
When it's up and running ZebraNet will
be a biologist's dream. Over the course of one year the collars
will take GPS samples every three minutes and collect detailed
activity logs over a three-minute span once every hour.
After the animals are captured, collared,
and released, the researchers will only require scant contact
with them as they drive or fly by the base station.
From this extensive, 24-7 data, biologists'
will make many inferences about zebras and their ecosystem.
Zebras are good animals to begin the project
with be-cause the social structure of certain zebra species
makes the engineers' job easier. Several animals move in a
tightly knit group called a "harem," consisting
of one male, several females, and their offspring. Since the
harem is a long-term union and all the animals in a harem
move together, the engineers only need to collar one animal
in that particular group.
Periodically many harems will meet up,
forming a large, temporary group termed a "herd."
Scientists wonder: Why do the zebras do this? How do they
collectively choose when to form larger groups and when to
migrate? Are there any social or biological changes that occur
in preparation for this event?
ZebraNet will help answer these questions.
It will give the scientists better information about migration
patterns, which will lend insights into these intricate herd
dynamics, as well as give them information about speciation,
and the mixing of gene pools.
Better migration pattern information will
also help biologists observe how human action and expansion
impacts the ecosystem. Professor Rubenstein hopes to eventually
be able to collar more species throughout the food chain to
learn about other interspecies interactions.
"There is a whole richness of biological
questions we could answer," Professor Rubenstein said.
While a biologist's dream, this project
could be an electrical engineer's nightmare.
The task is to create a wireless network
that will range over an undetermined number of miles, in the
middle of the wilderness, in a region where no cellular network
exists, attaching each node to the neck of a wild animal that
will undoubtedly spend a considerable amount of its time standing
in the rain and running from predators.
by Denise Applewhite
Margaret Martonosi, center, and her students Hidekazu
Oki, front, and Philo Juang, back, have begun field
testing of wireless devices that will be hung from the
necks of zebras in Kenya and used to monitor the animals'
The engineers, however, are undaunted.
Their design choices are based upon the environment, the particular
needs of the biologists, and other situations that challenge
One of the biologists' considerations is
weight. If the collar is so heavy that a zebra couldn't wear
one while continuing his normal, daily behavior, it would
be of little use as a source of accurate data.
All components of the zebra collar must
add up to no more than five pounds, which limits the number
of weighty batteries that can be included in the node.
Thus, power becomes a major limiting factor
for the engineers. The goal is to have a power supply system
in which the battery will be able to operate for five full
days between recharges by solar panels that line the collar.
Another major consideration is spatial
range. This network must span the entire migration pattern
of several harems in a zebra herd over the course of one year.
Since there is no existing cellular network
established in the region that could be tapped into, a complete
network must be set up.
The region, however, cannot support a fixed
base station because of the harsh environment, as well as
the chances of vandalism.
The alternative is to design a peer-to-peer
network with a mobile, intermittent base station. What this
means is that each node will collect and store the data for
the individual animal wearing the collar. Each collar will
be able to store up to 100 days of information for one animal.
Intermittently, scientists will drive by
or fly over the region. When a node comes within a certain
proximity of the base station, it will transmit all of the
stored information. The scientists can then compile and observe
the data at their leisure back at the lab.
Another essential ingredient of the peer-to-peer
network is that peers can swap information. Occasionally,
all nodes in the network will perform what is called a "peer
At a given time, all nodes will search
for other nodes within a certain range and transmit their
own data to one another. This way, if one zebra who is particularly
shy around humans never comes close enough to the base station
to transmit his data directly, then a more gregarious friend
of his may transmit the data on his behalf. This increases
the data homing success rate.
There are two methods of swapping that
the engineers are considering. One is called "flooding,"
a method in which a peer sends his data to every single peer
within range. This is quite reliable, yet it does have a major
Data swaps use more power than any other
operation the nodes perform. Since power is a major concern
in ZebraNet, flooding may not be the best choice.
A more selective, power-efficient method
is called a history-based method.
In this method, a node "chooses"
which other nodes to send data to, based upon its track record
of communicating data back to base. It wouldn't waste energy
swapping data with a node worn by an animal that was notoriously
bad at getting within range of the base station.
Another concern for the engineers is that
changes may need to be made to the nodes' software if errors
are discovered and adjustments need to be made.
"One of the coolest developments of
the past four months," Professor Martonosi said, "is
a software system we call Impala. It allows for adaptation
between different protocols, depending on how they are performing
at that particular moment."
Ting Liu, a graduate student in the Department
of Computer Science, has been committing most of her time
to working on Impala.
"Since these collars are going to
be on wild animals and you can't just press a reboot button
when you want to upgrade the software, one of the things we've
been thinking about is the ability to send out wireless software
updates," Professor Martonosi said. "The Impala
software also allows a hardware node to accept software over
the radio so that we can do software updates or bug fixes
remotely, without physically touching the collar."
Wireless software updates, the peer-to-peer
network, data swapping, and a rechargeable power source will
make ZebraNet an almost entirely autonomous system.
Although ZebraNet is being designed for
the specific purpose of biological research, Professor Martonosi
believes that the engineering breakthroughs the team is making
will have implications in many other fields.
"The notion of an auton-omous network
system that needs little human intervention is really satisfying
to anyone who's tinkered extensively with a computer or a
home appliance," Professor Martonosi said. "These
things really should be more auton-omous, even if you can
easily reach the reboot button."
The attitude of the engineers is not that
ZebraNet is a nightmare, but that it is merely an exciting
set of challenges.
"The challenges have led to some really
interesting research thoughts," Professor Martonosi said.
"Wireless software updates are something we might not
have thought about if the realities of the system we're trying
to deploy didn't make us think about it.
"In fact, that's one of the neat things
I find in a lot of engineering research. When you set out
to build something there are a lot of corner cases that you
have to deal with."
In addition to being a novel source of
research ideas, ZebraNet is an enjoyable departure from the
norm for Professor Martonosi.
"From a personal standpoint, this
is a ton of fun," she said. "I've always enjoyed
being outside, but being a computer engineer is one of those
jobs that rarely lets you be outside. So the ability to combine
what I enjoy as a hobby and what I enjoy as a professional
is very exciting."
In addition to Professors Martonosi and
Rubenstein, the ZebraNet team includes EE Professors H. Vincent
Poor *76 *77 and Stephen Lyon, EE Assistant Professor Li-Shiuan
Peh, EE graduate students Hidekazu Oki, Chris Sadler, Pei
Zhang, and Philo Juang, and CS graduate students Ting Liu
and Yong Wang.
The team is currently working on prototypes.
They just recently completed the design of the second generation
of ZebraNet cards, which are smaller, more integrated, and
use less power than the first generation (see photo on page
They've got some collars able to communicate
across campus and they hope to test them on horses at a local
stable soon. The plan is to begin testing in Africa this summer
and install the final system next year.
Professor Rubenstein is understandably
anxious to put the system to use. He said his colleagues are
chomping at the bit to put ZebraNet collars on the animals
"So much of our work has always
been hidden from us biologists," he said. "ZebraNet
will give us a much larger window to peer through. The invisible
will become visible."
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