William Unruh wrote:
> I for one would trust time from a gps pps a lot lot more
> than that from even 3 other servers. And yes, I could be
> fooled. Just as you might trust advice from David Mills
> much more than any 5 other posters here
> ( and would probably be very resistant to a "majority vote"
> determination that David is a "falseticker") but that does
> not give a guarentee that you could not be led astray sometime.
Trusting the GPS (so far) might be the better choice.
However, as governments (Militarys') are occasionally
testing GPS spoofing {As opposed to plain Jamming};
Some day the GPS receiver may be picking up four
or more things it thinks are satellites,
but are really stronger / closer transmitters,
that are providing false data;
Perhaps based on the real data, but e.g. slowly slewing
the positions off from their true positions.
There is no reason they might not e.g. mess with the
L1 C/A PRN Subframe 1 handover word (time)
or GPS time offset, while messing with
the Subframes 2/3 ephemeris data orbital information;
They might even do the same for the L1C (since its on
the same band).
They would have to block the receiver(s) from getting
and real sat data, as it might still get the correct
time (and other data) from e.g. L2C and L5 messages,
e.g. Jamming L2 & L5 frequencies ...
I suppose a modern GPS receiver might determine something is wrong,
based on not getting valid L2 &/or L5 messages,
and indicate it through a fix quality message
and then NTP might consider how much it trusts that receiver,
based on the fix quality value
(as opposed to just using the fix good / bad)
but that is not yesterdays GPS receiver or NTP.
A Local External PPS Rubidium / Cesium RefClock,
could also make it easier for NTP to sort out.
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