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

unruh unruh at wormhole.physics.ubc.ca
Mon Apr 19 21:12:48 UTC 2010

On 2010-04-19, Rob <nomail at example.com> wrote:
> 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.

I think that depends on how you set up the chip. I think you can set it
up so it will interrupt on each byte. This is not terribly efficient for
a serial port driver, but if it is timing you want, then that is
probably what you want. Well, actually you probably want the interrupt
to occur at the beginning of the first start bit of the first
character, which could in theory on a 4800bd give you  sub ms resolution
( and maybe much better) . But clearly one wants a dedicated PPS line,
and not rely on trying to set the serial port to do stuff it was never
really meant to do. 

> 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).

Well, the nmea message certainly should come in within one second of the

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