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 “GNSS”
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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.
Datacenter GNSS Time Best Practices
Eight things worth doing, from the closing slide of a talk on getting the last nanoseconds to UTC. Most cost nothing but attention, and most of the error they remove is invisible until somebody checks.
What makes an accurate GNSS timing receiver?
Six traits, in the order they start to matter. The last one separates a box that wants to be your master clock from a box that knows you already have a better one.
How does GNSS holdover work?
When GNSS goes away your clock coasts on the frequency it was last told. How far it drifts is decided by your room more than your oscillator — which is why a holdover measurement and a holdover specification are different objects, and only one of them is a promise.
Is my clock right?
Timing failures are usually quiet, and nothing inside a wrong clock knows it is wrong. The only detector is a second clock — and the boundary between what the two share and where they are independent decides what you can learn by comparing them.
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.
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.
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.
How big is a degree?
Each decimal place of latitude and longitude buys a factor of ten. Knowing where the useful digits stop saves arguing about the ones that cannot mean anything.
How to configure antenna position for datacenter GNSS receivers?
If you don't know your exact location, you can't know the exact time — about a nanosecond of error for every foot you are off vertically. Here is how to get it right.
How to configure feedline length for datacenter GNSS receivers?
An uncalibrated feedline is likely your largest source of error to UTC — tens of nanoseconds from a cable nobody measured. Here is how to find the number and configure it.
The limits of averaging
Averaging buys you improvement that stops. What is left when it stops is bias — and the same story plays out in position, in time, and in frequency.
From the blog
Sub-Nanosecond at Home, Take Four
The same talk, four slides longer — and almost everything new in it came out of writing the timekeeping pages rather than building the clock
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.
Sub-Nanosecond at Home, with GNSS and AI
Building a sub-nanosecond clock from a $1000 parts list, and what four months of AI-assisted research actually looked like
The Last Nanoseconds to UTC
Is sub-nanosecond UTC sync possible? Literally no. Practically yes — and the difference is the whole point