As I understand there is no much difference. GPS should receive a time from a sat and then it can keep it in its RTC. My uderstanding, the main battery is connected to AT6558 backup power supply. When you turn off GPS power AT6558 will keep time in RTC and ephemerides in memory.
But the quartz is external, not in the chip, so if it is not good enough it can drift similar way as a "main" watch quartz. In normal operation GPS always receives time from sats. So RTC is just a backup, when no reception. Because it is not that important, they may put a cheap quartz there.
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As I understand there is no much difference. GPS should receive a time from a sat and then it can keep it in its RTC. My uderstanding, the main battery is connected to AT6558 backup power supply. When you turn off GPS power AT6558 will keep time in RTC and ephemerides in memory.
But the quartz is external, not in the chip, so if it is not good enough it can drift similar way as a "main" watch quartz. In normal operation GPS always receives time from sats. So RTC is just a backup, when no reception. Because it is not that important, they may put a cheap quartz there.