  To mitigate spoofing attacks targeting global navigation satellite systems
(GNSS) receivers, one promising method is to rely on alternative time sources,
such as network-based synchronization, in order to detect clock offset
discrepancies caused by GNSS attacks. However, in case of no network
connectivity, such validation references would not be available. A viable
option is to rely on a local time reference; in particular, precision hardware
clock ensembles of chip-scale thermally stable oscillators with extended
holdover capabilities. We present a preliminary design and results towards a
custom device capable of providing a stable reference, with smaller footprint
and cost compared to traditional precision clocks. The system is fully
compatible with existing receiver architecture, making this solution feasible
for most industrial scenarios. Further integration with network-based
synchronization can provide a complete time assurance system, with high short-
and long-term stability.
