Since 2011, this server is operated as a free public service to the citizens of the internet by USSHC.
It is hosted at USSHC on a multicore/multi CPU dedicated Dell server using 1Gbit uplinks, and auto start generator and uninteruptable power supplies in a climate controled server room to prevent too much local osilator drift.
Its only purpose is time, and to serve the web page with the explanation.
As of 2016, it receives about 10,000 NTP queries per second.
tock.usshc.com ntpq -p output
tock:~$ ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
xSHM(0) .BAD. 10 l 9 16 377 0.000 -20.911 9.965
*SHM(1) .GPS. 0 l 8 16 377 0.000 0.000 0.001
+tick.usshc.com .GPS. 1 u 8 64 377 0.123 -0.001 0.002
-clock.isc.org .GPS. 1 u 49 64 377 59.039 -2.141 0.119
+clock.nyc.he.ne .CDMA. 1 u 3 64 377 23.389 -0.007 0.013
-tick.usnogps.na .PTP. 1 u 57 64 377 100.385 9.694 0.130
-time-c.timefreq .ACTS. 1 u 8 64 377 42.906 9.764 0.014
-tik.cesnet.cz .GPS. 1 u 7 64 377 112.136 0.537 0.018
-andromeda.cs.pu .CDMA. 1 u 42 64 377 14.086 -0.161 0.208
Explanation:
This server's time source is a WAAS enabled Garmin GPS receiver with serial and PPS output.
By default this particular GPS receiver will continue to provide PPS output, even if it has no GPS lock!
Disseminating an undisciplined free running clock as it it were GPS time would be a very bad thing!
To prevent this, the Garmin receiver has been set to stop sending PPS immediately upon loss of GPS lock.
The serial output on this GPS receiver is also not terribly accurate, but since it provides PPS
output, it doesn't need to be terribly accurate. It only needs to provide date and time. As long as
it's within a few tenths of a second, PPS will take care of the rest.
SHM(0) is the not super accurate GPS serial output.
SHM(0) is labeled ".BAD." and set to stratum 10 for a reason. It's labeled that way because we only have
space for four characters and ".BAD." fits, whereas "Not as good as the 1 PPS source, but better than
nothing, and perfectly fine for bootstrapping a server that has lost its clock" won't fit.
As you can see, because it's serial, and because of the design of the GPS receiver not using fixed outputs
for the date and time, it's more than 100,000 times (!!!!) less accurate than the SHM(1) PPS output.
So the stratum is set to 10 on purpose. This way, it does not introduce timing inaccuracy during normal operation.
And as a matter of fact, it is ignored as a false ticker most of the time by NTP!
However, if all other sources of time are gone, this server can be "boot strapped" from the GPS date/time, and
it will set itself to stratum 10 to get a 'course' date and time. In this case, anyone using this NTP
server will see the .BAD. during this time. And their NTP client should heed the stratum 10 setting. Then
the 1PPS .GPS. will quickly take over, within about 60 seconds, and the server will show .GPS. as the timing source.
SHM(1) is 1PPS (One Pulse Per Second) output from the same GPS receiver accurate to about 200 nanoseconds.
This is overkill in the other direction, and is about 5X more accurate than the NTP server's hardware/kernel can read.
tick.usshc.com is another nearly identical NTP server being fed from a nearly identical GPS receiver.
The other stratum 1 servers listed are used as backups/sanity checks, in case of GPS issues.
Since GPS is so widespread, and to help prevent a collapse in the uninterrupted dissemination of NTP time from this
server if the GPS constellation were to go offline, the sources are chosen to favor NON GPS time sources. We realize that
CDMA time is derived from GPS, but we also realize that CDMA time typically has holdover oscillators in the timing chain.
In the even of total GPS loss to this server (this has never happened outside of us testing it) this server simply obtains stratum 1 time
from other NTP servers, and will set it's stratum level to 2 so NTP clients can make an informed decision on picking the
best time. In the event that all other NTP sources are lost, this server will assume stratum 16.
This has also never happened outside of our testing.
NOTE:
A few times a year we get a phone call from a very confused person that asks us "WHY IS YOUR SERVER CONNECTED TO OUR DVR/ALARM/SECURITY SYSTEM!?"
They are usually upset, and demand an answer.
Because your DVR/ALARM/SECURITY system is using pool.ntp.org for it's NTP time source, and USSHC contributes to the pool as a public service.
No, we aren't hacked in to your security system.
No, we can't hear your private conversations.
No, we can't view your videos.
No, we aren't employing mind control.
No, you don't need to wear a tin foil hat.
Seriously... you'd think these would be funny conversations, but they're not.