  We address the problem of localising a mobile terminal ("blind" node) in
unknown position from a set of "anchor" nodes in known positions. The proposed
method does not require any form of node synchronisation nor measurement (or
control) of the transmission times, which is difficult or anyway costly to
achieve in practice. It relies exclusively on reception timestamps collected by
the anchor nodes, according to their local clocks, that overhear packets
transmitted by the blind node and by (at least one) other transmitting node(s)
in known position, e.g., other anchors. The clock differences between the nodes
are not eliminated ex ante through clock synchronisation, as in traditional ToA
and TDoA methods. Instead, they are counteracted ex post, during the data
processing stage, leveraging the data redundancy that is intrinsic to the
multiple reception of the same packet by different (anchor) nodes. We validate
the proposed method in different experimental settings, indoor and outdoor,
using exclusively low-cost Commercial-Off-The-Shelf WiFi devices, achieving
sub-meter accuracy in full Line-of-Sight conditions and meter-level accuracy in
mild Non-line-of-sight environment. The proposed method does not require that
the blind node participate actively to the localisation procedure and can use
"opportunistically" any legacy signal or packet available over-the-air for
communication purposes. Considering the very minimal requirement on the system
- basically, only that anchors in known positions are able to collect and share
reception timestamps - the proposed approach can enable practical adoption of
opportunistic and/or cooperative localisation on top of existing radio
communication systems.
