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

[Bob kb8tq]

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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