[ntp:questions] Cheap ($29.90) GPS receiver

Rob nomail at example.com
Mon Apr 19 20:17:26 UTC 2010

John Hasler <jhasler at newsguy.com> wrote:
> Terje writes:
>> It would still be limited to the resolution of the rs232 clock,
>> i.e. ~1ms, instead of the ~1us we usually get from PPS receivers.
> The documentation claims 1us (again, that just what I read.  I don't
> have one.)  The RS232 could be synchronized to the PPS internally.

Often the datasheet does not even specify which byte of the message
is synced to PPS (the first one? the last one?).

And even if you knew that, the typical serial chip (or worse: serial
to USB converter) has an unknown delay between reception of a byte
and issueing an interrupt, which depends on how many bytes were received
before and after that specific byte.

Timing on receiving an NMEA message is never going to be very accurate,
even when the message would be sent with some accuracy.  And even THAT
is usually not true in a GPS receiver.

In fact, it can sometimes be troublesome to guess which PPS pulse belongs
to what second (when looking at the NMEA messages).

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