[time-nuts] U-blox teaser

[John Ackermann N8UR]

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 <jra at febo.com> 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 <jra at febo.com> 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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