Navigation signal integrity is paramount for aviation and safety of life services. Hitherto, GPS signal anomaly alerting has been provided primarily by ground-based augmentations. Significantly improved navigation signal integrity and quality may be accomplished by on-board detection and correction, within stringent time-to-alert limits. In this way, most GPS signal errors, of which the timekeeping system anomalies are the major source, could be eliminated. It would then be possible to provide signal integrity innately from the source constellation to specified service category levels that are enhanced to meet the integrity metrics of hazardously misleading information, time-to-alert, availability, continuity, and accuracy. The method is to continuously monitor multiple atomic frequency standards with time-difference measurements against each other on-board the satellites, using existing components present in GPS architecture in a method similar to that routinely done in timing labs throughout the world. We focus in this paper on the issue of detecting and alleviating GPS clock anomalies by use of actual data illustrating frequency breaks in GPS clocks. These frequency breaks are derived from data taken in ground tests of Block IIR and Block IIF clocks. We also include some frequency breaks derived from on-orbit data. Using these data, we discuss how on-board measurements could be used to detect and mitigate problems, while also meeting stringent time-to-alert limits, such as 6 s or shorter.
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