[time-nuts] usb gps devices

[dlewis6767]

I've purchased some VERY cheap ones from eBay (china/hongkong).  Some from 
Motorola/ Trimble/ etc, ....  Some with SiRF protocol, all with NMEA 
protocol.   All cheap.   :-)

Just read the spec sheet on each before you buy, ...they are often in the 
eBay auction, btw.

If it says the 1-pps is 'tied to' UTC time, then they will be accurate, 
...mine are within 15-50ns.

Otherwise, ...like David says, ...they are not tied to the GPS clock and can 
be off by microseconds.


-Don






--------------------------------------------------
From: "David" <davidwhess at gmail.com>
Sent: Wednesday, September 26, 2012 3:31 PM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Subject: Re: [time-nuts] usb gps devices

> The pulse per second output on the Garmin serial/CMOS interface units
> is only specified to be within 1 microsecond.  I guess they are
> commonly used to synchronize NTP servers where any extra precision
> would be lost in network transport anyway.
>
> I have been thinking of picking one up cheap to test since the
> specification is so vague.  1 microsecond makes me think that the
> pulse per second output is generated from a 1 MHz asynchronous clock.
>
> On Wed, 26 Sep 2012 14:11:10 -0600 (MDT), "Don Latham"
> <djl at montana.com> wrote:
>
>>Have any of the 'nuts hacked one of the very simple very cheap GPS USB
>>devices for your car top to see if there is an available 1 sec tick
>>inside one of them somewhere? I have one that I intend to look at, but
>>I'll have to get a scope and teenyweeny probe outside to do it, so, if
>>there is a readily available something, I'd like to know. Thanks!
>>Don
>
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