Syncing Clocks with Satellites: How NTP Actually Works

2025-03-31 10:03:34 - No Comments

I wanted to understand why the NTP system has so accurate clocks and what an accurate clock actually means and this is a rabbit hole that you might get stuck in for a while. I don't want to reiterate the resources I've seen so I will link them in this write-up.

Time is dependent on gravity and speed, one that you might understand, and the other one broke my brain.

In 1976, they sent a rocket up and down in a known parabolic trajectory and measured the difference in time sent back to Earth. This was the most accurate test they had for gravity's impact on time until they messed up a satellite launch The Best Test of General Relativity

Next we have speed and one of the things we sync up time with is the amount of time it takes to send light from one space to another. But we can't really measure light in one direction Why No One Has Measured The Speed Of Light this is pretty crazy to me and after discussing it with a GPT I understand why it's impossible but I was not convinced at first.

In this repository accurate-time I write a small application to fetch time from one of the time servers in the network. This will probably be from a secondary source but for second accuracy it's more than enough.

My program sends a package and receives one with the same bits below. I ignore the leap indicator and set's the version and mode. Then I will read the Receive Timestamp and Transmit Timestamp in order to calculate the right offset for my clock.

This video was prompted by Jeff Geerling's video It's about time..., where he talks to people who keep even more accurate time than my small project will. A really interesting view.