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 “metrology”
18 pages · all tags
Topics In Timekeeping
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.
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.
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.
Questions to ask when benchmarking clocks
You have the hardware and you want to compare it. Most of the traps are in the comparison, not the clocks.
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.
How big is a nanosecond?
A nanosecond is about a foot. Once the units are physical rather than numerical, most timing arguments get easier to have.
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.
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.
Visualizing precision, trueness, and accuracy
The most-drawn diagram in metrology, with the axis everyone mislabels put right. Trueness and precision are the two axes; accuracy is the corner where you have both, not a third thing.
Measurement resolution and accuracy
Coarse resolution does not merely limit precision — it counterfeits it. A grid too coarse to show scatter returns the same confident number every time, and stops being able to warn you.
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.
Do timestamp errors average out?
No. The archer gets ten arrows and can average them; a network packet is timestamped once. That difference decides which errors you can tolerate — and it moves all the work to before the event.
Should I prioritize clock agreement or accuracy to UTC?
Usually a false choice. The two are not opposed, most improvements buy both, and the real tradeoff only appears at the very last nanoseconds — where it is a genuine engineering preference rather than a confession of low standards.
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.