Re: [time-nuts] U-blox teaser

Fri, 26 Feb 2021 18:31:23 -0800

A while ago I tried doing a decidedly non-anechoic measurement with a VNA exciter going to a 1500 MHz ground plane and the receiver connected to the antenna (with a known delay cable) and I got a similar result, but there was enough noise that I didn't think I could nail it down to within 10 ns.
I've also measured GPS antenna splitters and they tend to have 20-ish ns delays, mainly due to the SAW filters. I did surgery on an HP splitter to remove the filters so it could be used for L1 and L2 and that dropped the delay down to only 1 or 2 ns. 


So there's definitely lots of stuff to calibrate if you want to get 
accurate time transfer.


John
----

On 2/26/21 8:02 PM, Michael Wouters wrote:

Typical L1 antenna delays range from 20 to 70 ns.
I know of only one antenna for which a delay is given by the vendor and the
technique used was just to measure the electronic delay ie by injecting a
signal into the circuit. To do it properly, you need a setup in a microwave
anechoic chamber with transmitting antenna etc. The practical difference
may be small though, 1 or 2 ns ( sample of one antenna!).

Cheers
Michael

On Sat, 27 Feb 2021 at 11:42 am, John Ackermann N8UR <[email protected]> wrote:


They're claiming "even better than" 5 ns for relative time, which given
the 4 ns jitter seems at least sort-of reasonable.  But until someone
shows me otherwise, I'm still thinking that getting better than 25 ns
absolute accuracy is a pretty good day's work.

John
----

On 2/26/21 5:26 PM, Bob kb8tq wrote:

Hi

I can’t think of many antennas (multi band or single band) that claim to

know their

delay to < 5 ns. Simply having a *differential* delay spec of < 5 ns is

quite good.

Same thing with delay ripple, you see specs out to around 15 ns on a lot

of antennas.

None of this is getting you to the actual total delay of the antenna.

It’s a pretty good

bet that number is a bit larger than either of these.

Some of the ripple probably comes out in the standard modeling. I’m not

sure that

the differential delay is taken out that way. Total delay, not taken out

in any obvious

fashion ( at least that I can see). If the F9 has a built in antenna

database, that’s not

mentioned in the doc’s. Any benefit from the corrections would have to

be part of

post processing.

No, that’s not the same as talking about the F9 it’s self doing X ns,

but it would be part

of any practical system trying to get close to 5 ns absolute accuracy.

5 ns *relative* accuracy between two F9’s? I probably could buy that if

the appropriate

one sigma / on a clear day / with the wind in the right direction sort

of qualifiers are

attached.

Bob


On Feb 26, 2021, at 4:27 PM, John Ackermann N8UR <[email protected]> wrote:

It's interesting that they talk about the F9 receivers offering 5 ns

absolute time accuracy.  Does anyone know of tests confirming that, and
what sort of care was required in the setup to get there?


John
----

On 2/26/21 9:34 AM, Robert LaJeunesse wrote:

FWIW. No detailed content, and a rather quick read. "Five key trends

in GPS".

https://www.u-blox.com/en/blogs/insights/five-key-trends-gps
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