The problem with relying exclusively on GPS to do time distribution is the ease with which one can spoof the GPS signals.

With a budget of around $1K, not including a laptop, anyone with decent technical skills could convince a typical GPS receiver it was at any position and was at any time in the world. All it takes is a decent directional antenna, some SDR hardware, and depending on the location and directivity of your antenna maybe a smallish amplifier. There is much discussion right now in the PNT (Position, Navigation and Timing) community as to how best to secure the GNSS network, but right now one should consider the data from GPS to be no more trustworthy than some random NTP server on the internet.

In order to build a resilient NTP server infrastructure you need multiple sources of time distributed by multiple methods - typically both via satellite (GPS) and by terrestrial (NTP) methods. NTP does a pretty good job of sorting out multiple time servers and discarding sources that are lying. But to do this you need multiple time sources. A common recommendation is to run a couple/few NTP servers which only get time from a GPS receiver and only serve time to a second tier of servers that pull from both those in-house GPS-timed-NTP servers and other trusted NTP servers. I'd recommend selecting the time servers to gain geographic diversity, i.e. poll NIST servers in Maryland and Colorado, and possibly both.

Note that NIST will exchange (via mail) a set of keys with you to talk encrypted NTP with you. See https://www.nist.gov/pml/time-and-frequency-division/time-services/nist-authenticated-ntp-service .

On Sun, Aug 6, 2023 at 8:36 PM Mel Beckman <mel@beckman.org> wrote:

GPS Selective Availability did not disrupt the timing chain of GPS, only the ephemeris (position information). But a government-disrupted timebase scenario has never occurred, while hackers are a documented threat.

DNS has DNSSec, which while not deployed as broadly as we might like, at least lets us know which servers we can trust.

Your own atomic clocks still have to be synced to a common standard to be useful. To what are they sync’d? GPS, I’ll wager.

I sense hand-waving :)

-mel via cell

On Aug 6, 2023, at 7:04 PM, Rubens Kuhl <rubensk@gmail.com> wrote:

On Sun, Aug 6, 2023 at 8:20 PM Mel Beckman <mel@beckman.org> wrote:

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.

BTW, the stratum-0 source you suggested is known to have been manipulated in the past (https://www.gps.gov/systems/gps/modernization/sa/), so you need to bet on that specific state actor not returning to old habits.

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