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

David Taylor <david-taylor@xxxxxxxxxxxxxxxx.invalid> wrote:
> On 17/06/2022 03:03, Daniel O'Connor wrote:
>>> 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).
>> Ian Lepore (RIP) who worked for Micro Semi and worked on FreeBSD did a bunch of tests on a PPS over USB setup and found it more than
>> acceptable for keeping a PC in (good) time. Here's the thread:https://lists.freebsd.org/pipermail/freebsd-arm/2019-August/020263.html
>>> See if your motherboard has a true serial port - perhaps just as a header but not a back connector.  If not, just set the offset of the PPS to ~10.3 milliseconds  (10.3 - IIRC the offsets are in milliseconds but please check). Plus or minus 10.3, try it and see!   Not perfect, but better than nothing.
>>> You might find better results using that GPS/PPS with a Raspberry Pi as a stratum-1 server and offering that as a server on your LAN.
>> The next level would be something where you can do an input capture on the PPS I don't think there are any pre canned solutions. I made one with a Beagle Bone Black and a uBox GPS module but it's not exactly turn key. Or for a server then you would need a fancy (ie $$$$) internal card.
>> The Raspberry Pi does not have an input capture timer, but I believe you can do better with DMA hackery (I haven't tried though).
> If a 125 us uncertainty in the PPS is something you can tolerate, so be it.  If 
> you are bothering with PPS then presumably you want better accuracy than can be 
> achieved without it.
> No need for DMA hackery.  Standard NTP with the Raspberry Pi can handle PPS on 
> a GPIO signal with a couple of edits to allow the PPS support already built 
> into the kernel to be attached to the appropriate GPIO pin.  Not out of the 
> box, but very little effort required.
> The Raspberry Pi can act as a server for hundreds of clients.  If you mean a 
> PC-based Windows server, that's not something I would immediately recommend, 
> but if you must a £20 serial card may be all you need to add.

The Pi4 with a current GNSS model $29.95 Adafruit Ultimate GPS HAT will
provide an accuracy of about 1 to 2 microseconds.

If you plot loopstats you can see the offset vary with the heat/AC going
on and off and the error varying by about .5 microseconds.

If you want better than that, you need a GNSS receiver with a managed
oscillator such as an OCXO or rubidium.

However actually achieving in NTP anywhere near the +/- 1 nanosecond
accuracy of an OCXO will require careful selection of both hardware
and software.
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