[time-nuts] Re: What GNSS module to buy for a good time reference?

[Lux, Jim]

On 7/2/23 11:22 PM, Ed Marciniak via time-nuts wrote:
> Navsparq has the NS-T but that's single band, so no further value discussing in this context.
>
> A look at the Trimble NetRS reveals it's PPS output has 40 ns resolution and says several external factors limit accuracy to +-1 microsecond. While 40ns would come from a 25 MHz clock, it seems to be common reference frequencies are 19.2,26,38.4 MHz. Perhaps it's not exactly 40ns. Elsewhere the manual claims position 5mm+1ppm horizontal and 10mm+1ppm vertical accuracy, which probably represents a fairly decent internal receiver frequency reference. That doesn't seem directly useful for time and frequency but it's interesting for other purposes.
>
> Ah, well...thanks for the interesting reading material the conversation generated.
>
A lot of receivers choose a clock rate so that when the signal is 
sampled at that rate, it aliases down to a convenient place.

1575.42 is a bit higher than 41 * 38.4, and so forth.

What you want is the alias to be far enough from zero that the largest 
possible Doppler keeps the carrier at a positive frequency.


5mm + 1 ppm of distance is a typical spec for differential GPS surveying.