Nicholas F. Quackenbush, John P. Janis, Michael R. Jones, L3Harris Technologies

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For decades Global Navigation Satellite System (GNSS) payloads have incorporated multiple Atomic Frequency Standards (AFS) to provide both redundancy and technology diversity. Space borne AFS have demonstrated exceptional reliability as the technology has matured, but still experience occasional anomalous frequency and phase shifts that require control segment intervention. In fact, the reliability is sufficient such that there is a potential opportunity for the operation of multiple AFS on each Space Vehicle without significantly impacting mission life. Operating multiple AFS not only provides the ability to monitor each of the clocks with respect to the others and autonomously correct for these anomalies, but additionally enables clock ensembling for increased overall stability. To realize these two capabilities the payload must be equipped with hardware to perform precise onboard phase measurements and a mechanism for controlling the clock used for navigation signal generation. For the common case of three identical AFS an improvement to the stability, and thus the clock component to User Range Error (URE), of a factor of 1.7 at one day is realized.