  Mobile wireless sensors are increasingly recognized as a valuable tool for
monitoring critical infrastructures. An important use case is the discovery of
leaks and inflows in pipe networks using a swarm of floating sensor nodes.
While passively drifting along, the devices must track their individual
positions so critical points can later be located. Since pipelines are often
situated in inaccessible places, large portions of the network can be shielded
from radio and satellite signals, rendering conventional positioning systems
ineffective.
  In this paper, we propose a novel algorithm for assigning location estimates
to recorded measurements once the sensor node leaves the inaccessible area and
transmits them via a gateway. The solution is range-free and makes use of a
priori information about the target pipeline network. We further describe two
extended variants of our algorithm which use data of encounters with other
sensor nodes to improve accuracy. Finally, we evaluate all variants with
respect to various network topologies and different numbers of mobile nodes in
a simulation. The results show that our algorithm localizes measurements with
an average accuracy between 4.81% and 7.58%, depending on the variability of
flow speed and the sparsity of reference points.
