  The operation of critical infrastructures such as the electrical power grid,
cellphone towers, and financial institutions relies on precise timing provided
by stationary GPS receivers. These GPS devices are vulnerable to a type of
spoofing called Time Synchronization Attack (TSA), whose objective is to
maliciously alter the timing provided by the GPS receiver. The objective of
this paper is to design a tuning-free, low memory robust estimator to mitigate
such spoofing attacks. The contribution is that the proposed method dispenses
with several limitations found in the existing state-of-the-art methods in the
literature that require parameter tuning, availability of the statistical
distributions of noise, real-time optimization, or heavy computations.
Specifically, we (i) utilize an observer design for linear systems under
unknown inputs, (ii) adjust it to include a state-correction algorithm, (iii)
design a realistic experimental setup with real GPS data and sensible spoofing
attacks, and (iv) showcase how the proposed tuning-free, low memory robust
estimator can combat TSAs. Numerical tests with real GPS data demonstrate that
accurate time can be provided to the user under various attack conditions.
