Very Accurate Time Sync please

How accurate is the Time Sync of Windows?

I need a way to sync a group of computers' time as accurate as possible to the millisecond, and if possible to the microsecond.

We are all scathered across the world and thus far, relies on Window's built in time sync to tock.usno.navy.mil or tick.usno.navy.mil. Im not sure if we are sync to the second, millisecond or microsecond...

 

I think as long as the priority is set high you are good to go. I mean that is the US Navy? It has to be based on an atomic clock.

 

It doesn't matter what priority you set your system clock to. The sync is passive, and only happens every-so-often.

The RTC (realtime clock) in your computer is not capable of that type of accuracy. I think the Intel RTC is rated for +/- 3 seconds per day. You're going to need to use another piece of hardware to get accuracy to the microsecond.

 

The protocol is called NTP. Wikipedia is your friend Relevant quote:

 

But, as lithius said, the service running the sync is not constant. It checks periodically and syncs if it can (network traffic and server timeouts can prevent the sync from occurring), or resets and tries again later.

So, if he needs them to be completely in sync he needs to try something else.

Even if he runs a time sync via command line with task scheduler, I think the most he can run it is every five minutes.

As for syncing these computers to the millisecond...wow. You must be spy.

 

There are also USB/serial radio clocks which pick up low frequency time signals, you'd need to check if all of you are in range of such broadcasts.

GPS also carries time information, but not all receivers (or accompanying software) may be suited for really time critical applications.

 

You will need a different method for that kind of precision. NTP synchronization is reliably precise to about 10 ms over the internet. However banging someone's NTP server at that rate will surely get your IP banned quickly. As Nebelwand suggests, for precise time synchronized in multiple locations GPS will be your best method.

It's not just a problem with the RTC and sync schedule.

http://support.microsoft.com/kb/939322
http://technet.microsoft.com/en-us/library/cc773013(WS.10).aspx

Furthermore the W32Time API precision is only about 10 ms. If you need ms or µs precision you are going to need a high precision time API on the Windows platform.

http://www.bytefusion.com/products/ntm/ptnt/highresolutiontime.htm

For ms/µs resolution, synchronization and precision you are going to need to:

1) Change your remote synchronization method -- GPS instead of NTP
2) Change your local synchronization software -- a quality GPS-NMEA time reader
3) Change your time API for a high precision API

--L.

 

There are also NTP servers that rely on the time signals sent by the GPS satellites. Horribly expensive though.

 

You will require external clock generators to get that kind of accuracy to the microsecond--but the main problem is how to sync them across the world; which is, afaik, not economically feasible...

 

The next question is WHY? ebay sniping, video editing on a shared pool of 'cloud' disk or ???

Not a lot at the consumer level requires time sync at an accuracy or more than 1 or 2 seconds.

 

It is feasible, since GPS satellites carry time signals that are much more precise than what the OP needs. However, as lbohn has pointed out, you also need high-precision synchronization software. If you want microsecond accuracy, then I'm not even sure this is feasible in standard Windows. You may need a real-time OS to achieve this, since Windows cannot, by design, guarantee execution of commands at specific times, and with fixed delays, and neither does any other general-purpose OS. On the other hand, some clever software design may get around these issues, using some smart mathematics. It's unlikely that anybody in a newsgroup like this has the kind of specialized expertise to help you with this. I certainly don't.