Re: [avr-chat] GPS based digital clock

[Royce Pereira]

Hi Chris,
On Fri, 13 Jul 2007 11:03:11 +0530, Chris Albertson <address@hidden> wrote:

>
> How long did it take for the message to get from the guts of the
> GPS to your terminal?  And is it always the same amount of time?
> The message does not get to you at the exact "turn of the second"
>
> The GPS does not tell you the time it is now. It tells you the
> time it was when it formatted the message.
>
> The clock will be slow by some number of milliseconds.
> At 4800b each character takes 1/600th of a second and the GPS
> may spend 1/2 second sending text each second,
> Maybe  for casual use you don't care? Half a second off is OK.
> Or you could substract a fudge factor of 1/4 second from what
> the GPS tells you.  But some people do want precise
> time at the micro or nano second level.
>
> Some GPSes allow you to control the order of which lines come
> out first.  Just make it so the time comes out first if it
> doesn't aleady and maybe this is good enough.  Some GPSes can
> do 9600 baud.  So the delay is cut in half
>
Yes, I get your point. However, am I right in saying that, although there is a 
delay from the actual tick to the time the time is emitted, this delay is more 
or less fixed & non cumulative? If so, then it will serve my purpose as a clock 
showing the time of the day with a resolution of 1 sec.

Also, from my short research, most GPS receivers emit a 1 Hz pulse which is 
'spot on' and may be used to calibrate any delay/error...right?

I've shortlisted Trimble/Lassen iQ, Garmin GPS18 LVC & Motorola FS Oncore GPS 
modules, but I would welcome any suggesations.

Thanks,
--Royce.
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