[time-nuts] U-blox teaser

Bob kb8tq kb8tq at n1k.org
Sat Feb 27 15:05:23 UTC 2021 

Hi



> On Feb 27, 2021, at 9:41 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> I've long understood that ionospheric delays and variations thereof lead to
> *position*
> uncertainties in GPS navigation receivers, to the tune of perhaps 10m
> (2DRMS IIRC).,
> and that these are said to constitute the single largest GPS error source.
> 
> Q1: Would this not imply timing errors of comparable magnitude (10's of
> nsec)
>       for a single band GPS?

Once all the signals “hit” the antenna, the delays are mostly common mode.
Instead of showing up as a position error, they show up as an error in the 
time estimate. Since time is one of the things you estimate in the solution 
(along with X,Y, and Z) it get’s it’s own independent solution. 
> 
> Q2: Why have I not seen this issue raised in connection with the present
> discussion
>      about achievable absolute timing accuracy?

GPS time transfer is often done to the sub-ns level. There are a number of 
papers on this. 

> 
> Q3: Do any of the "modern" timing GPS receivers available to civilians do
> dual-band
>      reception in a way that includes estimation of (and correction for)
> said delays and
>      their variations?  I know that Garmin, for one, is now selling L1/L5
> handheld GPS
>      receivers (GPSMAP66sr and GPSMAP65s), but I've seen no indication
> that these
>      units make any attempt at doing such corrections.

Yes, some receivers do an estimate of ionospheric delay based on the 
variation of that delay with frequency. This does not help with tropospheric 
delay or all of the various “common mode” issues we have been talking about. 
It is also unclear how the “unknown” timing variation between the bands
due to the antenna impacts these solutions…..

Bob

> 
> Dana
> 
> 
> 
> On Sat, Feb 27, 2021 at 7:43 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> The same 20 or so ns delay in a saw would also apply to the
>> saw (or tight filter) in some timing antennas. It also would apply
>> to the saw(s) in some modules. Even if the tolerance is “only”
>> a couple ns on each of them, you *could* have 3 or more in the
>> chain.
>> 
>> Lots of numbers to crunch to get to 5 ns “absolute”. One could go
>> grab a GPS simulator and start poking. First step would be to find
>> a simulator that is spec’d for a < 5 ns tolerance on the PPS into
>> GPS out. I do believe that rules out the eBay marvels that some
>> of us have lying around …..
>> 
>> Simpler answer would be a quick “clock trip” with your car full
>> of 5071’s …… hour drive over to NIST and then back home.
>> That sounds practical for most of us :) :)
>> 
>> Bob
>> 
>>> On Feb 26, 2021, at 9:29 PM, John Ackermann N8UR <jra at febo.com> wrote:
>>> 
>>> 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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