![]() ![]() ![]() So in this example if the system reports that is is 13:52:34.00234 that time might not be exactly the same time as reported on the GPS satellite due to the delay between the GPS sat transmitting the time, and the system receiving and processing that time signal. An imprecise clock might read 23:59:59.9998 one day and 00:00:01.0123 the next.Īccuracy is how close the time reported by the system is to the ‘correct’ time. Precision, in this case, is how well the system can measure time, eg, after 24 hours a precise clock has measured exactly 24 hrs, 00 minutes, 00 seconds, 00 milliseconds etc. A cheap GPS+RPi system can be very *precise*, but might not be *accurate*. J_B is talking about the difference between precision and accuracy. To me, it is highly unlikeable this “accuracy”, not “precision”, can be reached with a simple Raspberry Pi and a $12 GPS receiver connected over USB… to ensure time delays caused by software processing are constant.) That’s were the specialized and expensive hardware comes into the picture, or, in case of software, you need to use a realtime O.S. If you want to forward time information into another component, you also need to minimize delay in your data processing, or to ensure a constant delay and account for this delay. ![]() This last part alone makes it already unsuitable for use while simultaneously making accuracy claims. The chipset used in this project (GT-U7) is only capable of 1 PPS, which is way to low for high accuracy timekeeping and there is no data regarding the actual internal delays. These chipsets also output a high frequency PPS signal to keep everything in sync between the absolute time mesages that are outputed serially. There is also time needed for decoding the GPS signal (in the GPS chipset), there is time needed to receive it from the hardware (Serial over USB in this case), time needed to do something with the data in software, time needed to feed the data it into NTP, etc…įor time critical applications, you need a GPS chipset that accounts for satelite distance (time of flight) of the time data. Reception delay varies based on the distance (i.e.: the position of the satellite relative to the receiver). The time information available in the GPS signal is valid at the moment it was sent from the satellite. The reason why “real” GPS based NTP servers are expensive, is because you need to account for the processing time to be “accurate”. I doubt this is microsecond “accurate”, probably only microsecond “precise”. Posted in clock hacks Tagged accuracy, clock, gps, microsecond, raspberry pi, real time clock, rtc, time Post navigation On the other hand, the Raspberry Pi has no built-in real time clock of its own, so this might actually be a cost-competitive way of timekeeping even when compared to something more traditional like a DS3231 RTC module. While this is an excellent way to self-host your own NTP server if you don’t have Internet access (or just want to do it yourself), does note that this is probably overkill on timekeeping as far as accuracy goes. There are only two parts to this build, a cheap GPS receiver and a Raspberry Pi, but goes into great detail about how to set up the software side as well including installing PPS, GPSd, and then setting up the actual NTP server on the Pi. This build simply teases out that time information from the satellite network and ignores the location data. GPS works by triangulating position between a receiver and a number of satellites, but since the satellites are constantly moving an incredibly precise timing signal is needed in order to accurately determine location from all of these variables. In this case it’s using the precise timekeeping abilities of the satellites to build a microsecond-accurate network time protocol (NTP) server. There are plenty of ways to take advantage of some of the ancillary tools that the GPS uses to determine location. Finding the position of things is one of many uses for GPS, though. Cheap GPS units are readily available nowadays, which is great if you have something that needs to be very precisely located.
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