The Chip-Scale Atomic Clock (CSAC) fits the bill with the low power

A portable atomic clock is just the ticket for many UAVs, and the more SWaP-optimized the better. The Chip-Scale Atomic Clock (CSAC) fits the bill with the low power draw and accurate performance inherent in its design.

 

Unmanned Aerial Vehicles (UAVs) began as tools for military surveillance. As their capabilities expanded, they found usage in civilian applications such as border patrols and drug interdiction, while on the military side the expanded capabilities led to missions using armed UAVs.Throughout their use, accurate clocks have been required for UAVs to carry out their missions. A principal need has been navigation; UAVs typically use a clock that has been synchronized to Global Positioning System (GPS) for very accurate timing. However, when the GPS signal is lost, the clock is used to provide a “holdover” function that integrates with a backup navigation system, usually some form of an Inertial Navigation System (INS). The clock’s holdover performance is important because, in military applications, GPS signal loss is sometimes due to intentional jamming, which can persist for long periods of time.Accurate clocks are also needed in UAV communications. As UAV sensor payloads have advanced from still photos to video, to video integrated with infrared and other sensor data, high-density encrypted waveforms have been employed to transmit this data, as well as to receive vehicle control data. These waveforms can only stay synchronized with stable, accurate clocks.

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refer to
http://smallformfactors.com/articles/chip-scale-swap-design-challenges/#at_pco=cfd-1.0