[time-nuts] GNSS Disciplined Clock

[Mike Cook]

> Le 27 mai 2017 à 21:26, Ebrahim Roghanizad <e.roghanizad at gmail.com> a écrit :
> 
> Dear Chris
> 
> As far as I know, atmospheric effect can not be compensated by looking at
> satellites from all over the sky and averaging, since it does not have a
> random nature, rather it introduces bias to the solution. For example, if
> atmospheric effect is not removed, one can not get a relative position
> accuracy of sub-meter in long distances even by employing the method of
> RTK. All I said here is about position. Now, I would like to know about the
> output of time in this condition. What is the utmost reachable accuracy for
> a timing output from a GNSS receiver? I do not mean the precision that
> reflects the noise behavior. I think that the best result is obtained when
> the receiver supports dual frequency in order be able to deal with
> ionospheric delay. Am I right? In that case, is there any GNSS receiver
> with this ability?
> 

 I don’t know of any and over time have been looking for one . I guess there is no market for a pure GNSS solution. Current L1 only timing receivers can offer down to +/-6ns accuracy with quantization error data allowing correction of their PPS output down to the stability of the GPS signal. Using that data and available cheap delay line chips the 1PPS accuracy deliverable can be reduced to around +/-2-3ns .
It appears to be cheaper to use just the L1 derived time to lock better oscillators for better precision than that. 


> Thanks a lot
> 
> On Thu, May 25, 2017 at 10:01 PM, Chris Albertson <albertson.chris at gmail.com
>> wrote:
> 
>> The long term stability of GPS is very good.  Some one here will point
>> out exactly how one measures it.   But roughly when speaking of
>> accuracy you always need to specify a time interval.     For example
>> if the 1PPS is "off" by 15ns that is not bad and yes there are much
>> better systems if you need to measure time intervals on the order of
>> one second.   But if the signal is "off" by 15 ns over 100,000 seconds
>> that is well, 100,000 time better.
>> 
>> This is a basic reference and for some specialized end use case you
>> might couple it with other equipment.  Many of the concerns you had,
>> such as effects of the atmosphere get averaged out because the unit is
>> looking at satellites from all over the sky.  (averaging over space)
>> And you do git better results with better antenna locations that are
>> away from multi-path and have a 360 degree view of the horizon.  But
>> notice the unit has an temperature stabilized crystal oscillator that
>> is stable over many seconds. an is much more stable in the short term
>> then is a GPS receiver.  Trimble uses this crystal to average over
>> time
>> 
>> You also have to ask where is the tine data going to be used.  Are you
>> synchronizing a computer's internal clock or trying to measure the
>> frequency of a microwave transmitter
>> 
>> SO it falls back to the old thing about there being no "better" only
>> better for a specific use case.
>> 
>> Some of use were lucky enough to buy Trimble Thunderbolts, a previous
>> version of this unit when they were on eBay for $100 each.   For those
>> without 5 digits budget they ar pretty much the Gold Standard.  I have
>> mine installed with a good filtered DC power supply and an outdoor
>> antenna on mast well above the roofs of surrounding buildings.     I
>> get long term stability of about one part in 10E13.   Yes 13 digits
>> over long periods.   (I think?)  It is really hard to know because my
>> measurement system is a little circular referenced
>> 
>> On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
>> <e.roghanizad at gmail.com> wrote:
>>> Dear members
>>> 
>>> I am a new amateur member in your group. Maybe my question has been
>> asked.
>>> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a
>> good
>>> GNSS disciplined time reference. I would like to know if there exists a
>>> more accurate one, since it does not employ dual frequencies to
>> compensate
>>> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
>>> anyone guide me about the presented accuracy in the datasheet? There, it
>> is
>>> stated that "When operating in Over Determined Timing Mode, the accuracy
>> of
>>> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it
>> mean
>>> that it includes both bias and the noise? In other words, is it true to
>> say
>>> that "The time-synchronization error between two of them with a long
>>> distance is less than 2*15 ns"?
>>> 
>>> Best Regards
>>> 
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>> 
>> 
>> --
>> 
>> Chris Albertson
>> Redondo Beach, California
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