  As the cost and technical difficulty of jamming devices continue to decrease,
jamming has become one of the major threats to positioning service.
Unfortunately, most conventional technologies are vulnerable to jamming attacks
due to their inherent shortcomings like weak signal strength and unfavorable
anchor geometry. Thanks to the high operational flexibility, unmanned aerial
vehicle (UAV) could fly close to users to enhance signal strength while
maintaining a satisfactory geometry, making it a potential solution to the
above challenges. In this article, we propose a UAV-assisted anti-jamming
positioning system, in which multiple UAVs first utilize
time-difference-of-arrival (TDoA) measurements from ground reference stations
and double-response two-way ranging (DR-TWR) measurements from UAV-to-UAV links
to perform self-localization as well as clock synchronization, and then act as
anchor nodes to provide TDoA positioning service for ground users in the
presence of jamming. To evaluate the feasibility and performance of the
proposed system, we first derive the Cramer-Rao lower bound (CRLB) of UAV
self-localization. Then, the impacts of UAV position uncertainty and
synchronization errors caused by jamming on positioning service are modeled,
and the theoretical root-mean-square error (RMSE) of user position estimate is
further derived. Numerical results demonstrate that the proposed system is a
promising alternative to existing positioning systems when their services are
disrupted by jamming. The most notable advantage of the proposed system is that
it is fully compatible with existing user equipment (UE) and positioning
methods.
