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I am using QSTARZ 818XT device for my app. I am using GPGGA sentences to get the GPS fixes, but I have noticed that timings are always rounded like xxxxxx.000 - for 1Hz GPS frequency and xxxxxx.x00 for 10 Hz GPS frequency.
Why is this happening? Is that something device specific? Can I trust these timings or should I make some corrections? My app does not function in real time, but it really needs those milliseconds.
It's unlikely that the times are being rounded. By their nature GPS receivers have an extremely precise notion of time and almost always align their observations to the measurement edge of the GPS second. For example, see section 6.2 of the u-blox receiver protocol manual, which indicates the observations are constrained to be within 1 ms of the requested interval.
This is often not documented in consumer devices, so I suppose there are some that may not be precise, but I would feel safe assuming they are accurate to within the number of significant digits of the NMEA time.
In the same PDF manual that J Hayward cited also says that "In NMEA messages it is always reported rounded to the nearest hundredth of a second". That is in section 6.4. So the milliseconds are always rounded from what this says. But im not sure if that means all NMEA sentences that report milliseconds are rounded before being sent to the device on the ground no matter what.
