What are the limits of GNSS time accuracy?
The limits are not about radio or geometry. GNSS time is a prediction, so its accuracy is bounded by how good the corrections are — and corrections are forecasts of errors that have not happened yet.
Tagged “UTC”
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Topics In Timekeeping
Choosing a GNSS timescale for syncing datacenter clocks
You are not choosing a receiver so much as choosing whose timescale you end up on. There are three ways to do it, they differ by a factor of a hundred in cost, and most datacenters should take the cheapest.
Can I build my own link to UTC(NIST)?
The data you would need is public, free, and better than most people realize. You can build the measurement in a weekend. What you cannot build is the one thing that turns it into a number somebody else has to accept.
Comparing distant clocks
You cannot run a cable, and neither clock can be the source for the other. So you both watch a third clock in the sky — which drags its error into your answer. Every technique here is a different way of removing it.
Ways to get time into a datacenter
GNSS, a fiber from a national lab, a commercial delivery service, or NTP from the internet. Why GNSS usually wins below 100 ns, and why you want more than one.
Can I sync my datacenter clocks to UTC?
Literally no, but practically yes. UTC is computed weeks after the fact, so nothing on Earth can synchronize to it — and yet a well-built datacenter clock sits a few nanoseconds away. Here is how both of those are true.
What are timestamps and timescales?
A timestamp is a count, and a count means nothing until you say what you counted and from when. That is a timescale: tick length, origin, and count — and anything with all three qualifies, including some absurd ones.
How does GNSS time relate to UTC?
Not one relationship but two: a whole number of seconds you can look up, and a sub-nanosecond prediction that two control loops — one running in minutes, one in weeks — spend their lives maintaining.
Matching acquisition and distribution
Two distinct problems that both have to be solved. Their errors sum, so the accurate one is paying for accuracy the other cannot deliver.
Which GNSS constellation keeps the best time?
Folklore says Galileo. BIPM measures it daily, per constellation, and publishes the answer — and over fourteen months the folklore does not hold up.
UTC(k), and who defines yesterday
The realizations you can actually reach in real time — and why the laboratories with the best clocks get the most say in what UTC turns out to have been.
What does "traceable to UTC" actually mean?
It means an unbroken chain of comparisons back to UTC, each with its uncertainty stated. Not a logo, not a certificate, and not a GPS antenna — which is why most systems that claim it do not have it.
Accurately measuring elapsed time
A clock so bad the internet refuses to talk to it still measures a microsecond-scale interval to about a nanosecond. Short durations forgive both ways a clock can be wrong, and knowing when you are measuring one saves real money.
From the blog
Topics In Timekeeping Is Live
Thirty pages on precise time in datacenters — what limits GNSS accuracy, what traceability actually requires, and how you would know your clock was still right. Not blog posts. Pages I intend to keep correct.