<[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
>I have a GPS device on my box and would like to use its PPS (I believe
> it's the $GPRMC message) as a reference clock for ntpd? I assume it's
> possible? Currently, there is no driver for the GPS device, I'm
> getting position updates by opening the ttyS0 comm port and reading
> $GPGLL messages.
>
> Any ideas?
A PPS can't be a message, it has to be a pulse. What GPS device are you
using? Does it have a PPS output or just the RS232 output?
I strongly urge against using a GPS that has no PPS output and no
guaranteed relationship between its RS232 output and its internal timebase.
You might get it to "just happen to work", but that's not a good way to
build a reliable timeserver.
First choice is a GPS with a PPS output, which is an actual wire that
pulses at the second boundary. Second choice is a GPS with some specific
timing guarantee between its RS232 output and the GPS secound boundary. A
timestamp output every second is not useful if it isn't output exactly on
the secound boundary (or with some reliable relationship to it).
You may find, for example, that the GPRMC output is delayed if the GPS
unit gets 'busy' processing something. Generally, the RS232 output is
considered a very low priority task. You want a receiver that's designed (or
at least specified) for timing applications.
DS
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