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,

Seaside

Answer 1

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.

Molokai

It does however talk about leap-second differences – Trude

@Jochem, I was busy adding that info as you made your comment - thanks. – Molokai

40 ns only the difference of GPS reference time from UTC time modulo 1 second. But as GPS's positional accuracy is within metres, the accuracy of the GPS receiver's clock has a similar error. If your fix has a 10 metre accuracy, your clock has a discrepancy in the range of the time the light takes to travel these 10 metres, or about 33 ns on top of those 40 ns. – Newell

"which is much less than the time it would take to transmit that information to another system." - or more usefully, it's much less than the accuracy of the error in timing how long it takes for the signal to go between both systems, as you can account for the vast majority of the time taken. – Viator

Answer 2

We have to be very careful how we understand this time accuracy issue. Light travels about 1m in 3 nanoseconds. Hence if we want to accurately locate ourselves to around 1m, then we are going to require a receiver that can at least determine time to ~ 3 nanoseconds or better. Low cost GPS receivers are not that accurate however (typically ~30nsecs), but they take advantage of a trick. The GPS satellites are calibrated to send highly accurate time and location measurements....a LOT better than 30 nsecs and they are constantly monitored and corrected to maintain the accuracy. But even though the GPS receiver is measuring 4-9 different signals with poor accuracy, it is the SAME receiver measuring the signals from different satellites. Hence if there is a built in error, the measurements can be collected and adjusted so that it is essentially the same offset error on the receiver for each satellite (not quite that simple, but thats the idea) . Hence if the receiver can see enough satellites it can perform Time Difference of arrival measurements which can remove the large error. The system can then use metadata to estimate and correct for the single static offset. You may have noticed in your car, if you turn on the GPS system at a location that your nav system was not actively tracking, and you are for example in a parking garage, the system will take some time to acquire ANY signal, and then it will often show you at the wrong location on the map, and will take a few minutes to figure out where you are. The system will be constantly checking different solutions and ultimately comparing them with map and inertial navigation detectors in your vehicle until it eventually finds a solution that appears to fit the map and your vehicles motion. My car has a GPS system. However, I once had the car taken by rail from DC to LA and it took almost 30 minutes for my GPS system to figure things out.

Serieswound

This is not a particularly accurate description of how GPS works and why, after large movements or long off-times, the GPS shows the wrong position and takes a while to figure out where you are. – Outstay

Answer 3

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).

Haggadah

this answer is about the same as the one downvoted below.. extremely precise.. this has vastly different meaning for a car navigation vs missile navigation vs CERN particle accelerator experiment.. – Providenciaprovident

Answer 4

A GPS receiver has its own internal clock, which has a clock bias compared to the potentially more accurate atomic clocks on the GPS satellites. When a GPS receiver has a fix on 4+ satellite signals, it can use that information to calculate what the value for its bias is and synchronize its clock to that of the satellites. So we have two main components contributing to inaccuracy:

• accuracy of the GPS satellite times; the satellites have their own atomic clocks which are synchronized to a ground station's reference time: according to gps.gov the error is <= 30ns
• how accurately clock bias can be estimated from GPS signals, which includes both the solution to the equations to calculate bias, and also the design of the GPS module's electronics/processing: according to this manufacturer, this manufacturer, and gps.gov this is around 100ns for an average receiver, but can range between 10-1000ns

So the error for a GPS synchronized clock will be between 10-1030ns, with an average error being 130ns. This manufacturer suggests < 50ns as an accuracy for a typical Stratum 0 reference time server used for NTP; this is expected, as they likely use the highest quality GPS receivers giving you <= 30 + 10ns (40ns) error. Its likely the manufacturer for your specific GPS NTP server will be able to give you specs on how accurate the times are.

On the other hand, NTP synchronized times are less accurate, instead being in the millisecond error range: < 1 ms for local networks, ~10+ ms over the internet. Assuming you have a constant, high quality GPS fix, an on-site GPS NTP server is going to give you better accuracy than synchronizing with another external server.

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