[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?

[Carsten Andrich]

In late 2019 we successfully employed 4 SRS FS740 (with Rb option; GNSS 
receiver: Trimble RES SMT 360) to synchronize spatially distributed SDR 
receivers over distances of 1~2.5 km for the purpose of locating RF 
emitters via time difference of arrival. As a side product of that 
measurement campaign we managed to assess the accuracy of the time 
synchronization of the FS740, which was about 5 ns over the course of 4 
hours with a little post-processing relying on log data from the FS740. 
We circumvented the issues mentioned by Bob via a reference measurement 
with known emitter location and line of sight to all receivers.

If you're interested in details, I'd refer you to a paper we published 
on the measurement [1].

Best regards,
Carsten

[1] https://ieeexplore.ieee.org/document/9128562 (behind IEEE paywall; I 
can send you a personal copy of the paper upon request)

On 01.05.22 02:50, Bob kb8tq wrote:
> Hi
>
> If you are looking at time ( = the absolute offset from GPS’s version of UTC) then
> there are a number of issues.
>
> The antenna you use will have a delay and it may well vary more than a bit. The
> cable to the antenna is in the same category. If both your modules run off a power
> splitter then those will not show up in an A-B comparison.
>
> The modules themselves likely have SAW filters in them. These have group delay
> just like any other bandpass filter. The tolerance on this is likely in the 10’s of ns
> module to module. There are other bit and pieces that can contribute at the “nanoseconds”
> level.
>
> A typical set of modules from a good supplier should come in to a +/- 20 ns sort of
> window for 2/3 of the parts ( = 1 sigma).
>
> Geometry errors are fairly simple. A meter is 3 ns in free space. Each meter you
> are off from “correct” will add 3 ns of “wobble” in the results. Just how this shows
> up is very dependent on the direction of the error an what sort of satellite view you
> happen to have.
>
> Ionosphere can ( in high sunspot years) contribute 50 ns or more to timing errors.
> Tropospheric issues can also get into the mix at a bit lower level. You might think
> these would wash out on co-located units. Unfortunately they are not going to
> start /stop using this or that sat at exactly the same time.
>
> Lots of fun stuff to look for ….
>
> Bob
>
>> On Apr 30, 2022, at 6:41 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
>>
>> Some more info
>> The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the
>> absolute time difference of their PPS pulses  was, after a cold start,
>> stable within +/- 20ns but  the average value could be up to 100ns and
>> differed after every cold start.
>> The two GPS antenna cables had a length difference of 1 meter, but that
>> should cater for only 5 ns (?) One module is connected to the antenna with
>> only a C, the other has a 1 GHz CLC high pass filter between antenna and
>> module
>> Erik
>>
>>
>>
>> Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek <erik at kaashoek.com>:
>>
>>> The PPS jitter of a cheap Chinese GPS module was measured at about +/-
>>> 10 ns.
>>> But the phase of the PPS compared to a Rb varied substantial more.
>>> To verify if this was possibly due to ionospheric or atmospheric
>>> conditions the time difference between the PPS of two identical modules
>>> using two identical rooftop antenna was measured. Both only used the GPS
>>> constellation.
>>> This showed difference of up to 100 ns. Switching to GPS+GLN did not
>>> make a visible difference.
>>> It was tried to set both GPS modules into fixed position mode but the
>>> reported position still kept moving a bit (within 3 m) and the fixed
>>> mode did not have a visible impact on the time difference variations.
>>> Is a time difference of up to 100 ns to be expected when using two GPS
>>> receivers or is this difference possibly due to bad application or
>>> performance of the cheap Chinese GPS modules
>>> Erik.
>>>
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-- 
M.Sc. Carsten Andrich

Technische Universität Ilmenau
Fachgebiet Elektronische Messtechnik und Signalverarbeitung (EMS)
Helmholtzplatz 2
98693 Ilmenau
T +49 3677 69-4269