Defense in depth. Like the Space Shuttle that had four rad hardened processors. And a voting system to reject anomalous outputs. With two you don't know which one is right. With three you're OK. Four allows for one to permanently fail and still leave three.
This is analogous to having at least three NTP servers. Or filesystem level checksums on top of ECC RAM and LDPC in the SSD. Or TCP checksum on IPv4 header checksum on Ethernet/LTE/Wi-Fi checksum.
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Defense in depth. Like the Space Shuttle that had four rad hardened processors. And a voting system to reject anomalous outputs. With two you don't know which one is right. With three you're OK. Four allows for one to permanently fail and still leave three.
This is analogous to having at least three NTP servers. Or filesystem level checksums on top of ECC RAM and LDPC in the SSD. Or TCP checksum on IPv4 header checksum on Ethernet/LTE/Wi-Fi checksum.