Though the level of precise timekeeping for military satellite communications (milsatcom) applications may not be as stringent as that required for satellite navigation, milsatcom poses its own unique timkeeping problems. For example, milsatcom timekeeping must be precise without putting an undue burden on a ground station's workload. Further, milsatcom timekeeping must be robust, with the ability to autonomously detect and correct timekeeping problems during protracted periods when the ground control station is either unavailable or burdened with other pressing tasks. Here, we discuss three different space-segment timekeeping systems that could be employed in milsatcom, and our numerical simulations investigating their various attributes. These systems include a Master/Slave system (similar to present day Milstar), an Ensembling system (based on NIST's AT1 algorithm), and a Kalman-Filter system (similar to GPS when it goes to crosslink ranging). The timekeeping performance of the three systems is characterized by the median time interval between ground station updates, given a somewhat arbitrary 2-musec requirement for space-segment timekeeping. Among other effects, our simulations include: satellite temperature variations, satellite clock random noise, satellite clock frequency aging, time-transfer noise between satellites, as well as time-transfer noise between satellites and the ground station. As we will show, milsatcom timekeeping involves a complicated interplay between satellite timekeeping hardware and the space-segment timekeeping system. Judicious choice of the hardware and space-segment system can allow weeks between ground station updates of the constellation's timekeeping.
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