[FOSS-GPS] Accurate timestamp in rtkpost

David Kelley DavidKelley at iTSware.net
Sat Dec 5 16:48:51 PST 2015

> I'm recording
> video at 30 fps with an iPhone and simultaneously recording GPS data using
> the Topcon receiver while driving.

This seems to be to the classic time problem of to two clock, each with 
it own offset and drift rate , and no common observable to obtains their 
offsets and hence sync.  I am sure the Topcon has an aligned PPS output 
signal and it very likely has some sort of data event logger that would 
capture GPS time when a pin is toggled. Not clear to me how you could 
use these to create your sync event. but that is a common way when you 
can control the hardware.

Please look closely  at your video stream in an editor, it may be a true 
30Hz but is may also be 29.7x Hz in which case you will need to account 
for that as well.  Is there any SMPTE time code here you can use here?  
The Samsung note tablet I use does not sync for time, I have an app for 
that. and it drift a minute each week (most of these devices will be 
about 10^5).  My S4 phone does sync, so it more of less remains 
disciplined.  But here another minor annoyance to keep in mind, some 
phone networks (Verizon in US) align their network time not to UTC but 
to GPS, so of course it looks to be 17 seconds ahead.  Perhaps that is 
why you saw the image grossly off from the GPS track.

 From the prior post
> So is it possible that rtkpost is just reporting the receiver
> timestamp, and not attempting to produce accurate solution for time from the
> GPS data.
No your data seems to be working.  What you posted was the 'trace' file, 
look right next to it for the 'pos' file and you will see the number of 
digits in the file vary with what you set it.  And as I said in another 
post, with 10 digits you will see that was estimates moving about at the 
nSec level.  Still, this value is the time of RTKpost using a least 
squared filter on the data, and it will vary from the time of you iPhone 
reports based on an initial unknown offset and an initial draft rate 
between the two.  Your challenge is to determine these two values, and 
then hope that they are stationary (they do not change) during the data 

I have heard people do with sort of thing GoPro, perhaps someone on the 
list can chime in how that does the time stamp sync.  We cold sure use 
such a thing for our own mobile vehicle testing.
DC Kelley

On 12/5/2015 3:53 PM, RickHwang wrote:
> It's good to double check accuracy of your iPhone clock without making any
> assumption. I'm using a Samsung Galaxy Note 3, configured to use network
> provided time to have automatic date and time. Puzzled by your finding of
> displaced location and video, when calling 117 phone service to check exact
> time of day, the clock on my phone is slow by 7 seconds. The time on a
> laptop seems to have correct time to the second, though. It's rather
> disappointing that Galaxy Note 3 was once a flagship phone back in its days,
> yet time is off by 7 seconds!
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