[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Magnus Danielson]

Erik,

The NEO-7M may have sawtooth correction output, have you checked that 
and made compensations?

Since the oscillator is not steered and free-floating, the 
cycle-assignment of the PPS may be less than optimal so just measuring 
that without the compensation can cause a wider range of PPS than the 
actual receiver time stability represents.

In particular, check chapter 12 and the TIM-TP message of [1].

[1] 
https://content.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf

Do notice that the TIM-TP message is documented to be issued before the 
(PPS) pulse it report on.

The variations you report is consistent with what the datasheet report 
for the pulse assignment, which may not be representative of the 
receivers performance.

Cheers,
Magnus

On 2022-05-30 13:00, Erik Kaashoek via time-nuts wrote:
> Further evaluation did shown the time differences between the 3 GPS 
> modules was due to difference in the trigger level setting of the 
> timer/counter and difference in length of GPS antenna cables.
> After removal of the phase drift due to Rb frequency offset the 
> attached image shows the phase differences of the 3 modules versus a 
> Rb reference.
> The two ATGM modules are very consistent over a 2.8 hours period. The 
> NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to 
> a somewhat less optimal antenna position.
> It seems phase variations over time in the order of 10-20 ns are 
> indeed unavoidable, even with a good antenna.
> Erik.
>
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