Encounternet tags are
microprocessor controlled and have radio transceivers that can detect
and log the transmit pulses of other tags. Each tag pulses a
unique ID number so logs of received pulses, along with their date and
time, can be used to study associations among animals in great detail.
Additionally, tags log the received signal strength (RSSI) of ID
pulses, allowing researchers to estimate distance between individuals.
Tags collect association data continuously as "encounter logs",
even when researchers are not present. Tags can store as many as 8000
encounter logs to non-volatile memory.
The second critical
component of Encounternet are wireless base stations, which are
placed out in the environment where tagged animals roam. Base stations
also detect and log tag ID pulses, and will automatically download the
encounter logs stored on any tag. With base stations in the field
to collect tag logs at all hours, a researcher only needs to
periodically visit the field site to wirelessly download tag
encounter logs stored on base stations.
hand-held device called the master node is used to downlod logs from
base stations, and tags. The master node is also used to configure tag
and base station parameters, and for conventional tag tracking.
Encounternet proximity logging tag network
Data flow in an Encounternet system configured for proximity
logging: tags monitor and log ID pulses of other tags, base stations
collect the logs from tags when they are within radio range, and a
master node connected to a PC is used to download logs from base
stations (and tags, if required) and store them onto the PC. Once logs
are transferred from one device to another, the are deleted from the
original device, to free memory.
Low power tag mode
In addition to proximity logging mode,
Encounternet tags can operate in a very low power radio mode. The low
power mode operates similarly to a conventional radio tag (transmit
pulses but do not receive), except that all tags transmit on the same
frequency and each tag transmits a unique digital ID code with every
radio pulse. These features allow all base stations to continuously detect and log
any tag pulses transmitted in range. Tags configured to run in low
power mode can use smaller batteries and so can be made smaller and lighter
(currently >= 0.7g), and have a longer lifespan. Although the low
power tag mode does not allow tag-to-tag proximity logging, base
stations can be used instead to monitor tag locations.
Using signal strength to monitor animal locations
coordinates can be calculated if base stations are placed in a grid in
the habitat such that at least three base stations can detect and log
each tag pulse along with its RSSI value. The differential RSSI values
of tag pulses received by different base stations can be used to
estimate tag distance to each
base station, and tag position can be
then determined using triangulation.
The advantage of
over conventional methods is that tagged animals can be continuously,
and autonomously located as long as they are in range of three or more
base stations. Furthermore, this system can be used with less accuracy
in more sparsely distributed base station placements: the detection of
a tag by a single base station places that tag within the radio range
of that base station.
Encounternet tag location monitoring network
A grid of six base stations are used to record the position of tag 1 (any number of tags can be monitored in this system).
In the figure, the leftmost base stations all detect and log tag 1's
pulses. If tag 1 moves, then the differences in pulse signal strength
received by base stations can be used to locate it. A typical location
deployment will have many more base stations covering a much larger