By precisely synchronizing with a satellite??™s clock, GPS
computes node position based on signal propagation time.
Compared to ToA, TDoA has an advantage as the former??™s processing
delays and non-LOS propagation can introduce larger errors [6]. Ref. [34] proposes
a TDoA based scheme (AHLos) that requires base stations to transmit
both ultrasound and RF signals simultaneously. The RF signal is used for
synchronization purposes. A sensor first measures the di?®erence of the arrival
times between the two signals, then determines the range to the base station.
Finally, multilateration is applied to combine range estimates and generate
location data.
RSSI computes distance based on transmitted and received power levels,
and a radio propagation model. RSSI is mainly used with RF signals [1, 17],
but the range estimation can be inaccurate due to multipath fading in outdoor
environments [34].
AoA-based methods first measure the angle at which a signal arrives at a
base station or a sensor, then estimates the position using triangulation. The
calculation is quite simple, but AoA techniques require special antenna and
may not perform well due to omni-directional multipath reflections. Further,
the signals can be di?±cult to measure accurately if a sensor is surrounded
178 Fang Liu et al.
Time-of-
Range-based localization relies on the availability of point-to-point distance or
angle information. The distance/angle can be obtained by measuring
Arrival (ToA), Time-Di?®erence-of-Arrival (TDOA), Received-Signal-
Chapter 7 TPSS
by scattering objects [6].
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