[time-nuts] U-blox teaser

Bob kb8tq kb8tq at n1k.org
Sat Feb 27 23:39:04 UTC 2021 

Hi



> On Feb 27, 2021, at 11:18 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> Thanks, Bob.
> 
> It seems to me that, depending on the positions of sats visible to one's GPS
> antenna and the spatial distribution of free electron density in the
> ionosphere,
> the ionospheric contribution to position errors could sometimes largely
> cancel.
> But that observation may (or may not) reflect strongly on one's ability to
> get
> accurate absolute time from GPS on "average" days.
> 
> During my Arecibo Observatory days we used NIST's TMAS service to keep
> our H-maser-based station clock synced with UTC.  Our user community
> (mainly VLBI and pulsar timing people) seemed pretty satisfied with +/-
> 100ns
> accuracy, so I tried to do better by keeping things well within +/- 50 ns
> during
> my reign.  IIRC, NIST was claiming that TMAS could produce results mostly
> within about +/- 20 ns.
> 
> To be honest I'm baffled by how time transfer much better than that could
> be achieved in practice.

One way (mentioned about a month back on the list) is a two way transfer 
via satellite. The “delay is equal in both directions” assumption is pretty good
in this case. Once you have that as a baseline, you can measure the performance
of other approaches. 

One of *many* starting points to rumble down this rabbit hole:

https://www.nist.gov/pml/time-and-frequency-division/time-services/common-view-gps-time-transfer <https://www.nist.gov/pml/time-and-frequency-division/time-services/common-view-gps-time-transfer>

I would suggest starting with David Allan’s paper (referenced in the link above) as a 
pretty good starting point.

Bob

> 
> Regarding Q3, yes I'm aware that *some* GPS receivers do the estimation of
> ionospheric delay.  What I was asking was:  Do any of the relatively
> inexpensive
> receivers to which we time-nuts have access do this?  Here I'm speaking of
> those being sold for no more than a few hundred USD.
> 
> Dana
> 
> 
> 
> On Sat, Feb 27, 2021 at 9:08 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> 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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