GPS 时钟的准确度如何？
在我的公司，我们确实有需要准确时间的关键系统。
因此，我们有一个带有室外 GPS 天线的 NTP 服务器设备，用于从 GPS 卫星接收时间。
我的问题是：
- 时钟的准确度如何？
- 保持这种方式还是使用其他外部 NTP（US-GOV、NASA 等）是否值得？
谢谢，
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,
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根据此参考，时间应该精确到 40 ns 以内，这比将该信息传输到另一个系统。
该参考文献还解释了 GPS 时间与 UTC 时间有何不同，因为自 GPS 部署以来添加了闰秒。两者之间的差异被编码在 GPS 信号中，但由 GPS 设备在其显示时间中包含偏移量。据推测，专用于计时的设备可以正确地完成这部分工作。
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.
我们必须非常小心地理解这个时间精度问题。光在 3 纳秒内传播约 1m。因此，如果我们想要将自己精确定位到 1m 左右，那么我们将需要一个至少可以确定大约 3 纳秒或更短时间的接收器。然而，低成本 GPS 接收器并不那么准确（通常约为 30 纳秒），但它们利用了一个技巧。 GPS 卫星经过校准，可发送高度准确的时间和位置测量结果......远优于 30 纳秒，并且不断对其进行监控和校正以保持准确性。但即使 GPS 接收器正在测量 4-9 个不同的信号，且精度较差，但它是同一接收器测量来自不同卫星的信号。因此，如果存在内置误差，则可以收集并调整测量结果，以便每个卫星的接收器上的偏移误差基本上相同（不是那么简单，但这就是想法）。因此，如果接收器可以看到足够多的卫星，它可以执行到达时间差测量，从而消除大的误差。然后，系统可以使用元数据来估计和校正单个静态偏移。您可能已经注意到，在您的汽车中，如果您在导航系统未主动跟踪的位置打开 GPS 系统，例如您在停车场，系统将需要一些时间来获取任何信号，然后它经常会在地图上向您显示错误的位置，并且需要几分钟才能确定您所在的位置。该系统将不断检查不同的解决方案，并最终将它们与车辆中的地图和惯性导航探测器进行比较，直到最终找到适合地图和车辆运动的解决方案。
我的车有 GPS 系统。然而，我曾经把车从华盛顿乘火车运到洛杉矶，我的 GPS 系统花了近 30 分钟才弄清楚。
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.
很可能极其精确（假设您有一个不错的室外 GPS 天线）。 GPS 距离计算涉及以接近光速移动的信号，因此时钟必须极其精确，否则您会显得与实际位置相距许多英尺（每纳秒误差约一英尺）。就准确性而言，您可能需要根据您想要准确的时间标准来检查该时间。但由于精度的原因，准确度不应随时间而变化（除非您要比较的时间标准不精确）。
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).
GPS 接收器有自己的内部时钟，与 GPS 卫星上可能更准确的原子钟相比，它有一个时钟偏差。当 GPS 接收器固定了 4 个以上卫星信号时，它可以使用该信息来计算其偏差值，并将其时钟与卫星时钟同步。因此，有两个主要因素会导致不准确：
因此 GPS 同步时钟的误差将在 10- 1030ns，平均误差130ns。该制造商建议<用于 NTP 的典型 Stratum 0 参考时间服务器的精度为 50 纳秒；这是预期的，因为他们可能使用最高质量的 GPS 接收器，给您带来 <= 30 + 10ns (40ns) 的误差。您的特定 GPS NTP 服务器的制造商很可能能够为您提供有关时间准确度的规格。
另一方面，NTP 同步时间不太准确，而是在毫秒误差范围内： <本地网络为 1 毫秒，互联网上约 10+ 毫秒。假设您拥有持续、高质量的 GPS 定位，则现场 GPS NTP 服务器将为您提供比与其他外部服务器同步更高的准确性。
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:
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.