[time-nuts] Synchronization

Wed Dec 4 12:56:34 UTC 2019

Hi

The advantage of the F9P parts is that they generate and process the 
corrections in the module. You supply a relatively low bandwidth com link
and they do all the rest. There are demos of them plotting modules spinning
around on turntables.  The corrections are mainly targeted at real time location. 
To the degree they help with time, that’s a side benefit.

If you go with the LEA 6T’s you will need to pull out the sawtooth correction 
information and apply that to the measured offset of time pulse <-> picture.

Bob

> On Dec 4, 2019, at 1:01 AM, Anton Strydom <agstrydom at gmail.com> wrote:
> 
> Hi Bob
> 
> " If you set up several units as base stations and then stream corrections
> from then,
> you can indeed get down to the CM level real time with a number of L1/L2
> systems.
> The key is that you need to stream the corrections"
> 
> I have a number of LEA 6T modules so therefor instead of running a NTP
> server I am going to configure a LEA 6T unit as a Rover on each of the
> RPi's and setup a separate Base on another RPi and have the base broadcast
> RTCM using the WiFi this should theoretically be sub decimeter accurate and
> that should equate to at least microsecond synchronization I think
> 
> 
> 
> 
> On Tue, Dec 3, 2019 at 10:01 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> It looks like “your thread” just ran into “my thread” :)
>> 
>> I’m doing some digging on a low power WiFi based NTP setup. RPi’s will be
>> part of (but not all of) the mix there. So far the conclusion is that
>> milisecond
>> timing is what you are going to get from a NTP on a WiFi based RPi.
>> 
>> ====
>> 
>> If you set up several units as base stations and then stream corrections
>> from then,
>> you can indeed get down to the CM level real time with a number of L1/L2
>> systems.
>> The key is that you need to stream the corrections.
>> 
>> Bob
>> 
>>> On Dec 3, 2019, at 11:20 AM, Anton Strydom <agstrydom at gmail.com> wrote:
>>> 
>>> Thank you everyone for your input I am very grateful
>>> 
>>> The cameras are pairs sitting on a multiplexer so the problem is not the
>>> camera pair synchronization but the RPi synchronization
>>> 
>>> The ZED-F9P modules work well but still does not give continuous
>> centimeter
>>> accuracy as does the Hexacon (Leica) or the Hemisphere equipment that I
>>> have here
>>> 
>>> Yes basically Lidar but with a twist in that I use at present normal 8MP
>>> RPi V2 roller shutter cameras that adds to the problem in a different
>> way.
>>> I am switching to Global Shutter Cameras that will alleviate the shutter
>>> issue.
>>> 
>>> Each of the units that I use is equiped with 1 to 3 pairs of cameras.
>>> Therefore multiple streams into a single platform.
>>> 
>>> GPS stability I do not think is my biggest problem and I wish I could do
>> a
>>> screen grab of the image the moment the RPI's synchronize to a NTP server
>>> at the same time.
>>> 
>>> Reading through the replies and has prompted me to do the following
>>> experiment:
>>> 
>>> I have a number of different gps units here, I am going to test the
>>> different ones as follows L1 recievers on each of 4 RPI units deployed
>>> around a bridge at the university, sync each unit to it's gps time and
>> then
>>> repeat the experiment with precision timing L1 units such as the UBlox
>> LEA
>>> 6T units both as autonomous and Base Rover combinations doing RTK, L1,
>>> L1/L2  autonomous, L1, L1/L2 RTK and the Dual frequency autonomous and
>> also
>>> RTK
>>> 
>>> That should give me an idea of what I am looking for and I will report
>> back
>>> with the results
>>> 
>>> Thank you all for your input once again
>>> 
>>> Yours sincerely
>>> 
>>> Anton
>>> 
>>> On Tue, Dec 3, 2019 at 4:22 PM David C. Partridge <
>>> david.partridge at perdrix.co.uk> wrote:
>>> 
>>>> So you're processing something like a lidar image - from one lidar
>> device
>>>> on a single platform or multiple many on multiple platforms?  Is your
>>>> concern how stable ONE GPS receiver is, or do you need to have multiple
>>>> GPSs synchronised within a certain number of nS?
>>>> 
>>>> If the latter how close do you need then to be synchronised ?
>>>> 
>>>> David
>>>> 
>>>> 
>>>> -----Original Message-----
>>>> From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of
>>>> Anton Strydom
>>>> Sent: 03 December 2019 08:06
>>>> To: Discussion of precise time and frequency measurement
>>>> Subject: Re: [time-nuts] Synchronization
>>>> 
>>>> 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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>>>>> 
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>>>> 
>>>> 
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