Whenever a sensor gets localized using the
TPSS algorithm, it will broadcast its own location and help other sensors for
position detection. In other words, it can work as a beacon node.
4 TPSS: A Time-Based Positioning Scheme with Short
Range Beacons
In this section, we propose TPSS, a time-based positioning scheme for sensor
networks with short range beacons. TPSS consists of three steps. In the first
step, a sensor collects all the signals from the neighboring beacons, and groups
them according to the sources of the signals. The next two steps work on the
signals belonging to the same group: the range di?®erences from beacon nodes
to the sensor are computed and then the coordinates are resolved.
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Fig. 1. An Example Sensor Network.
4.1 Step 1: Signal Collection
Assume each beacon node initiates a beacon signal once every T seconds.
This signal contains the beacon??™s location and a TTL (Time To Live) field
with an initial value ?? 3. The format of the message is demonstrated in
Figure 2. A beacon node hearing a beacon signal with TTL > 0 will broadcast
it again after decreasing the TTL value by 1 and after attaching both its own
location and the time di?®erence between when the signal is received and when
it is re-broadcasted. This is indicated by the relay and delay fields in the
message format shown in Figure 2. Each sensor with unknown location listens
passively for the beacon signals and groups them according to the initiators
of the messages.
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