What??™s more, the localization scheme
should not raise the construction cost of sensor nodes. Finally, the positioning
scheme should be robust enough to provide high precision even under noisy
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environments. In this chapter, we present TPSS, a time-based scheme that
meets many of the requirements mentioned above.
TPSS is di?®erent from TPS [8] and iTPS [38], even though all three rely on
TDoA measurements to calculate a sensor position through trilateration. The
beauty of TPSS lies in that there is no requirement for base stations to cover
the entire network by powerful long-range beacons. Only short-range beacon
nodes with known positions need to be deployed. A beacon node could be a
typical sensor mounted with a GPS. Recall that TPS (iTPS) requires three
(four) long-range beacon stations with each being able to cover the entire network.
TPSS releases this restriction while retaining many nice features of the
other two. For example, all these three schemes require no time synchronization
among sensors and beacons. In TPSS, each sensor listens passively for
signals from the beacons in its neighborhood. A sensor computes the range
di?®erences to at least three beacons and then combines them through trilateration
to obtain its position estimate. This procedure contains only simple
algebraic operations over scalar values, thus incurs low computation overhead.
Since a beacon signal is transmitted within a short range only, the communication
overhead is low, too.
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