Or one can read recent research papers that thoroughly document the incredible fragility of the existing NTP hierarchy and soberly consider their recommendations for remediation:
The paper suggests the compromise of critical infrastructure. So, besides not using NTP, why not stop using DNS ? Just populate a hosts file with all you need.
OTOH, 4 of the 5 servers I suggested have their own atomic clock, and you can keep using GPS as well. If GPS goes bananas on timing, that source will just be disregarded (one of the features of the NTP architecture that has been pointed out over and over in this thread and you keep ignoring it).
Rubens