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Re: [questions] Re: GPS+PPS vs NTP server, why a huge offset ?



Daniel O'Connor <darius@xxxxxxxxxxx> wrote:
> 
> 
>> On 17 Jun 2022, at 00:07, David Taylor <david-taylor@xxxxxxxxxxxxxxxx.invalid> wrote:
>> 
>> On 16/06/2022 10:00, Thiebaud HUMBERT wrote:
>>> To do the inversion, I just changed the "Pulse Mode" parameter to "Falling edge" from "Rising edge".
>>> The offset induced by the "pulse length" has disappeared.
>>> But there is still an offset of around 10.3ms, which I think is induced by USB as explained in this article about other chipsets (https://lists.freebsd.org/pipermail/freebsd-usb/2019- August/016078.html)
>> 
>> Yes, Thiebaud, USB is not good enough for PPS signals!
> 
> This is absolutely false.
> 
> If you are using it for NTP then GPS+PPS over USB is quite adequate (from personal experience).

As USB is a two wire interface, there is no such thing as PPS over USB.

You of course can get the ASCII data over USB, but to get a PPS signal
you in general have to hack a USB GPS and add a signal wire for PPS then
hack some interface on the computer to accept PPS.

If all you need is accuracy in the 2 millisecond range, most recent USB
GNSS dongles will achieve that without PPS.
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