Is anyone using GPS for time sync?

Question

I've come across a handful of GPS NTP servers, as well as some inexpensive solutions using off-the-shelf receivers and software. Right now I'm just using NTP with a list of servers over the Internet. What is the advantage to using GPS instead (considering the alternative is free)?

Answer 1

I've used them at a gambling operation so we could keep accurate time but still have a true air gap between the production network and the internet/corporate network.

With the revenue generated per hour and that on one occasion a single remote sales rep came into the office and plugged in his infected laptop to the corp network and brought it down within minutes (production was unaffected). The effort of using things like gps time to allow for an air gap was worth it.

Answer 2

With a GPS ntp server you'll be getting a stratum 0 clock source. the advantage is accuracy and lower jitter. The downside is cost. There are very few circumstances that require this level of accuracy. in general you want to make sure your systems are in sync with each other. So syncing all your servers against a pair of internal ntp boxes that sync against external lower stratum ntp servers is usually sufficient.

The only time I've been involved in using a GPS ntp box (for a PKI system), we never actually used them. We couldn't get permission to mount the antenna on the roof of the datacentre and get the cabling back to the ntp box.

Answer 3

We do have a pair of stratum 1 time servers using GPS for NTP.

Does it make a difference in our day to day business vs when we had no NTP servers? A little, for example, on the client support side, the phones, both digital and voip and all the desktops now have the exact same time. Which cuts down on all the why do all these clocks have different times and can I fix it questions.

On the server side, when we aggregate logs we know that our timestamps are all the same. So if something happened at 12:37:15.34 on two servers we know that it really was at the SAME TIME, not 30 seconds to a minute appart.

Did we need GPS based NTP to do this? Probably not, we could have used any NTP source. We know everything is synced perfect and the time never drifts. Also if our internet connection is down, or laggy it's still perfect. Any NTP would have been an improvement over the lack of NTP we had though.

Answer 4

While not used for time sync so that all devices are using the same time for logs, GPS time sync is commonly used in carrier class wireless transmission hardware. It allows access points to sync and all transmit/recieve simultaneously, so that they won't be causing interference or raising background noise when listening for transmissions from clients.

Answer 5

The cost is not small (relatively) and the benefits are few. The only places I've seen running their own stratum-0 time source had extremely high expectations for the quality of the input, and a lack of trust in external time sources.

Answer 6

Great answers so far! I can say that I've seen GPS time sources used on multiplexers, and certain types of crypto ... it was used for NTP, just because it was already there.

The advantage for using a local GPS NTP server would be

• High Accuracy
• Not dependent on an Internet connection

How much does one of these things cost anyway? It shouldn't be that expensive. GPS + Linux + NTP ...

Wow, $1000 - $3000, do you need it that bad?

Answer 7

A couple years back I looked at whether it was possible to use a cheap GPS (~$200-$300) to get accurate time on a server. I'm sure that at the time Garmin or one of the other companies had software you could run on Windows to read the time on a computer to read the time from a GPS connected to a serial port. I was going to ask if anyone has used this as a cheap alternative to a purpose-built NTP server. It should be accurate enough and only a few hundred bucks.

Answer 8

A neutrino detection experiment I worked on used GPS for the primary time signal to our Data Acquisition system. They used a fairly expensive system supplied by someone in Japan.

This as part of a world-wide effort to provide high precision information about the different arrival times for the neutrino and photon wave-fronts from big supernovas. Lots of cool physics in that, if you can get precise enough data.

Answer 9

We use them because of their "deadman's" feature (won't correct more than a few seconds so that if something goes wrong your whole infrastructure does not move 12 hours into the past) and security; NTP is totally insecure; spoofing, man-in-the-middle, etc.

Answer 10

There are several points to consider:

• What accuracy do you need actually?
• What type of Internet connection do you have? For example DSL lines are renowned for being highly asymmetric, especially when saturated.
• What would happen if the external servers are unavailable for a longer time (like hours or days)?
• GPS (or GNSS (Global Navigation Satellite System) today) may be very accurate (as received), but to have the precise time locally, you need a high-quality oscillator (basically the time is just a by-product when calculating the current position (GPS' main purpose).
• To get the precise time from the GPS module to the computer clock, you would need a special connection named PPS (Pulse Per Second), because only few receivers send out the time messages exactly on start of the second.

So using GPS efficiently is complicated (read: expensive), and the main advantage is precision. Depending on the hardware of a local GPS solution, reliability may also be an issue, specifically as there are known attacks to the NTP protocol that could cause your time to drift away from the actual time.

Also when the public server changes its IP address (we had that once), your client may stop to receive valid time.

Despite of that it's being considered to be anti-social to query high-quality public servers by many clients that don't need that quality (e.g. one router manufacturer had hard-coded the IP address of a high-quality time server in its home routers, which was not considered to be nice if ten thousands of clients "hammer" on a public server). The recommended way is to have a local server if you have more then (say) 10 clients. Some companies also want you to register when using their services. Microsoft, Facebook, Apple (and many more) run their own time services, for example.

Answer 11

The advantages are you can get real seqence of events logging.

An example is in critical infrastructure. All electrical utilities should be using stratum 1 time for their databases. Stratum 1 time is critical to working out what made something fail on the scale of local utility downtime, to figuring out who caused the Northeast Blackout of 2003. NTP is often passed up in favor of PTP (IEEE 1588) for synching to the microsecond level.

Answer 12

At my old job we used GPS for timing a lot of different equipment. each piece of equipment had their own GPS receiver, which allowed perfect sync where latency was not an issue. Using GPS practically eliminates network-factors in the accuracy.

Answer 13

It gives you a Stratum 0 NTP server, under your control. You can then configure a few machines as Stratum 1 servers, syncing from your GPS-clocked server and have slightly less traffic needing to leave your network.

It's been around 10 years since I last had one of those (that I know of) in my network, but at that point, we didn't have a good antenna position and while it mostly saw a GPS signal, it did occasionally lose the signal, requiring a reset of the receiver equipment. With a better antenna placement, that would most probably not have occured.