[time-nuts] Quality of timing mode GPS vs survey accuracy

[jimlux]

On 5/8/20 2:44 PM, Bob kb8tq wrote:
> Hi
> 
>> On May 8, 2020, at 5:12 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>
>> Is there any published data on what happens to the quality of time if the
>> survey is off by xxx meters?
> 
> Speed of light plus distance off “correct” plus constellation geometry are what
> are usually tossed into examples. Turning that into an exact error number for
> this receiver design at time = x …. never seen it taken that far. You could do it
> yourself for a hypothetical processing approach.
> 
>>
>> Do all the GPS receivers use the same coordinate system?
> 
> GPS uses a coordinate system, receivers translate this into another system to display
> the results. Many can be set to a variety of coordinate systems.  Unless there are
> bug in the firmware, when set to the same system they should be talking the same
> way. Mr Google will give you a long list of hits going into this for each GNSS system.
> 
>>   If I take a survey
>> with several GPS receivers will they all get the same answer?
> 
> Nope.
> 
> If you take a survey with the *same* receiver, there will be some level of “this pass
> does not agree with that pass”.
> 
> How big the errors are depends a lot  on:
> 
> 1) what sort of receiver you have (single band / multi band / design era / make / model)
> 2) what sort of antenna you have (fancy geodetic / simple pole mount / mag mount …)
> 3) how well you have things set up (sky view ….)
> 4) how bad your local situation is ( multi-path, jammers, RFI / EMI ….)
> 5) how long the survey is ( minutes / hours / days / weeks / month ….)
> 6) what the conditions are (space weather / local weather / GNSS system issues …)
> 7 )what sort of post processing you do ( antenna corrections …. IGS data sets used ….)
> 
> (and on and on and on ….)
> 
> For timing, you may not care about errors dimensioned in mm, but there are people
> who do. Yes, this is a basic issue with wide open questions :)
> 


To give you an idea of magnitude of uncertanties

Getting a GPS position (and time) to 10 meters (30ns) is "easy"
To "meters" (10ns, 3m) is "pretty much every receiver does it, with some 
provisos"
To "less than a meter, less than 1 ns" is "lots of factors, among which 
Bob's list is some" (solid earth tides, ionospheric variations, etc. are 
all in that "sub-meter"

To "less than a mm" is hard.

I'm attaching a screen shot of 4 receivers, taken at the same time 
(within seconds,not time-nuts same time) All sitting on the table together.
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