Venu Gopal wrote: > David J Taylor wrote: >> Venu Gopal wrote: >>> David J Taylor wrote: >>>> Venu Gopal wrote: >>>>> I have a couple of similar experiences ! >>>>> I observed that the NMEA sentences are not generated in sync with >>>>> PPS. Theres lot of jitter in these sentences resulting in 1 second >>>>> offsets. Its fine if the jitter is within few milliseconds. But >>>>> sometimes it exceeds a second and thats really painful. >>>>> >>>>> This observation was discussed earlier and the solution is to go for >>>>> a better GPS receiver! >>>> Use the shortest GPS sentences, and the highest baud rate, to keep >>>> the total message time as short as possible. >>>> >>> The issue is not with the length of the messages and the baud rate. >>> >>> Venu >> >> Well, I found that if you had a bad configuration (sentences too long >> or baud rate too low), the sentences could exceed one second, and >> therefore be useless for NTP. >> >> However, I have not made any measurements showing jitter versus >> sentence length, so I would appreciate any pointers to measurements >> you have made or know about. >> >> Cheers, >> David >> > I almost scratched my head for a week on this issue. > I tested with multiple sentences, single sentences with baud rate of > 4800 and 9600. Ultimately I understood that the problem is not with the > sentence length and baud rate. The problem is with the scheduling > process/techniques used within the receiver that generate the sentences. > > For example, we use Accord GPS clock which generates GPGGA, GPGLL, > GPZDA, GPZGD(custom sentence). Only GPZDA and GPZDG are generated in > sync with PPS and the rest are unusable as they result in 1 second offset. > > I also had a chance to experiment with u-blox GPS receiver. Tried all > possible combinations but the problem occurred frequently. It was worse > than Accord. > > I modified the NMEA driver to log the sentences(all of them) with > timestamps. Then wrote scripts/programs to parse these huge logs to > check for time offsets. It was decided not to use it with NTP. 'it' here means "u-blox" receiver! > > In fact I had few ideas to solve this problem. But at the end of the day > the problem seemed difficult to solve. > > Venu
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