[time-nuts] Synchronization

[Tom Van Baak]

Hi Anton,

 > My question is what good synchronization of a gps clock in Nano seconds?

That's not much to go on; there are so many variables. To start with, 
almost any cheap eBay GPS/1PPS receiver these days will give you time to 
within a couple 100 ns with no special effort on your part.

If you have a fixed location, a good antenna, a clear view of sky, a 
modern GPS receiver with 1PPS output, and have the ability to apply 
sawtooth correction in h/w or s/w, then you can probably get within 10 
ns. Many commercial and DIY GPSDO are based on this assumption.

Note that this "10 ns" is relative timing. To obtain 10 ns absolute UTC 
is much harder because you have to calibrate and compensate for antenna 
delay, amplifier delay, cable and connector delay, receiver delay, 1PPS 
buffer amplifier, output cable, and edge detection delay, etc. So almost 
nobody can do absolute timing on the cheap.

Fortunately for many applications (e.g., GPSDO) it's not necessary 
because most of those fixed phase corrections cancel.

Then there's the question if your application is based on a surveyed 
fixed location -- if static, or ground mobile, or airborne. Do you have 
any size, mass, or power constraints? Do you need a local oscillator / 
time base or is this just raw, live 1PPS ticks from the receiver? Do you 
need good results now in real-time or can you wait a day or a week to 
get better results after some post-processing?

So the rough answer is that these days 100 ns is easy for under $50; 10 
ns is possible for under $500; and 1 ns absolute is near impossible 
unless you have a lot of development time and money, not to mention 
atomic clocks and test equipment to validate that extreme level of 
performance. Plus the expense of trip(s) to your national NMI for UTC 
calibration at the ns level.

Does that help? If not, can you summarized your technical requirements 
in more detail for the group? There are a number of people on the 
mailing list who have done recent measurements using the ublox F9-series 
receivers and those results should be helpful in your quest.

Precise timing and 3D imaging sounds like an interesting application. 
You mention clouds though; do they move fast enough that milliseconds or 
nanoseconds matter? Can we see your math? I'm curious but confused. For 
example, nanoseconds matter for triangulation of high energy atmospheric 
cosmic rays, but I've not heard where nanoseconds matter for photogrammetry.

/tvb



On 12/2/2019 12:01 AM, Anton Strydom wrote:
> Good day All
>
> I am new here.
>
> I have been busy with GPS systems for the last couple of years and have
> also developed a number of low cost high accuracy L1 units.
>
> I also play around with photography and especially in the field of
> photogrammetry and 3D point cloud situations.
>
> Time being the one thing that influences everything to do with accuracy.
>
> My question is what good synchronization of a gps clock in Nano seconds?
>
> Obviously the closer to 0 the better I would guess.
>
> Thank you in advance
>
> Yours sincerely
>
> Anton Strydom
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