[time-nuts] Synchronization

[Bob kb8tq]

Hi

Ok, for survey work there *is* an alternative. It takes care of the location 
of the “rover” as well as the time. uBlox has a ZED-F9P module for around
$100 that will do the trick. It is L1/L2, but the cost penalty isn’t all that great
in this case. I don’t know if that would be a practical thing in this case or
not ( = if the entire budget is $20 …. it’s not). 

Bob

> On Dec 3, 2019, at 3:05 AM, Anton Strydom <agstrydom at gmail.com> wrote:
> 
> Hi Tom
> 
> Thank you for all your input thus far it is appreciated
> 
> The "CLOUD" I am talking about is a Point Cloud and I am attaching an
> example for your perusal.
> 
> I am also attaching a screengrab of a real time stereo video where you can
> see the misalignment of the images
> 
> Presently the system is purely experimental and has to be real time.
> 
> Post processing is done to forecast possible movement and once a "trend"
> has been established it can be accelerated over time using the point cloud
> 3D model and the mesh it is built on
> 
> The points monitored (targets) are surveyed in points as are the camera
> placements
> 
> Using a combination of OpenCV and Tensor Flow a number of observations and
> precise measurements are possible thus allowing modeling of the structure
> over accelerated time using the movement data collected from the structure
> etc etc.
> 
> Yours sincerely
> 
> Anton
> 
> On Tue, Dec 3, 2019 at 9:00 AM Tom Van Baak <tvb at leapsecond.com> wrote:
> 
>> 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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