How accurate is the GPS Clock ?

Question

In my company we do have critical systems that require an accurate time.

As so, we have an NTP server appliance with an outdoor GPS antenna that receives the time from the GPS satellites.

My questions are:

• How accurate is the time clock?
• Is it worth it to keep this way or use another external NTP (US-GOV, NASA, etc) ?

Thanks,

Answer 1

Most likely extremely precise (assuming you have a decent outdoor GPS antenna). GPS distance calculations involved signals moving close to the speed of light, so the clock has to be extremely precise or you will appear to be many feet from your actual location (approximately one foot error for every nanosecond off). As far as accuracy, you would probably need to check that time against whatever time standard you want to be accurate against. But because of the precision, the amount of accuracy should not change over time (unless your time standard you're comparing to is not precise).

Answer 2

I found a good writeup here....http://www.ehow.com/about_5073608_accurate-gps-time.html

the answer is: very, very accurate.

Answer 3

According to this reference, the time should be accurate to within 40 ns, which is much less than the time it would take to transmit that information to another system.

The reference also explains how GPS time might differ from UTC because of leap seconds that have been added since the deployment of GPS. The difference between the two is encoded in the GPS signal, but it is up to the GPS device to include the offset in its displayed time. Presumably an appliance that is dedicated to keeping time would do this part correctly.