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Time format and absolute time when exporting data
#11
(08-31-2020, 11:07 AM)Jens Noritzsch Wrote: At least three Satellites are required, so there is not one time stamp… the OS –somehow– decides the time stamp that is given with a location.

Well, I think your comment contains the root cause of this issue, and I could prove this locally now with an indoor measurement of the acceleration and the location at the same time:

While the acceleration data was recorded from the start of the experiment, the data from the GPS signal was started to be recorded only 10 seconds later, because the GPS satellite connection was so bad indoors. This resulted in a measurement delay of the location data compared to the acceleration data of 10 seconds as well. In the end the acceleration data covered 40 seconds, but the location data only 30 seconds.

When automatically aligning both data files in the end, this is what happens:

Acceleration data             | GPS Location data
------------------------------+---------------------------
0.000s (acc_datapoint_0)      | 0.000s (gps_datapoint_0)
0.001s ([font=Courier New]acc_datapoint_1)[/font]             |
0.002s ([font=Courier New]acc_datapoint_2[/font])      |
...                           | ...
9.999s ([font=Courier New]acc_datapoint_999[/font]     |
10.000s ([font=Courier New]acc_datapoint_10000) | 10.000s (gps_datapoint_[/font]10)
10.001s ([font=Courier New]acc_datapoint_10001)[/font] |


But actually, this is what should happen because of the delay in the retrieval of GPS location data:


[font=Courier New]Acceleration data             | GPS Location data[/font]
[font=Courier New]------------------------------+---------------------------
0.000s ([font=Courier New]acc_datapoint_0[/font])      | (no data collected yet)
0.001s ([font=Courier New]acc_datapoint_1)[/font]             |
0.002s ([font=Courier New]acc_datapoint_2[/font])      |
...                           |
9.999s ([font=Courier New]acc_datapoint_9999)[/font]   |
10.000s ([font=Courier New]acc_datapoint_10000) | [font=Courier New]10.000s (gps_datapoint_0)[/font][/font]
[font=Courier New]10.001s ([font=Courier New]acc_datapoint_10001)[/font] |[/font]
[font=Courier New]...                           |[/font]
[font=Courier New]20.001s (acc_datapoint_20000) | 20.000s (gps_datapoint_10)
[/font]
In a nutshell the issue is that both sensors are readout independently and that the time stamps of each data set is not the same from the beginning (when the play button is pressed).

Can this be fixed? I think this is a problem which could be solved on the application source code. When the first GPS data is retrieved, the time which passed already in the experiment should be added to the time of each consecutive data point.[/font]
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#12
Could you please specify your OS and phyphox version. I cannot reproduce it on current phyphox/iOS*: Location data for the full time, constant 1Hz rate after about 7 seconds. Velocity data (understandably) sets in then, too.

* + new simple experiment with accelerometer and location
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#13
(08-31-2020, 02:08 PM)Jens Noritzsch Wrote: Could you please specify your OS and phyphox version. I cannot reproduce it on current phyphox/iOS*: Location data for the full time, constant 1Hz rate after about 7 seconds. Velocity data (understandably) sets in then, too.

* + new simple experiment with accelerometer and location

It's an Android 8.0.0 device (Sony Xperia X Compact), phyphox is version 1.1.6, build 1010603, file format 1.10.

Same experiment like you have. Below are two screenshots from a quick experiment I took right now: While the accelerometer measured roughly 15 seconds, the GPS sensor recorded only 2 seconds of data, but the later dataset should actually start at somewhat >10.00s instead of at 0.00s


Attached Files Thumbnail(s)
       
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#14
Thanks. It is mostly an Android specific problem as on iDevices there is no way to enforce the use of GPS. That's why I have got a location right from the beginning whereas your smartphone has to wait for a first fix, leading to the gap. BTW, this is just a problem of the location service.

This is less straightforwardly fixable: the various “time standards” run at different accuracies, i.e. perhaps no exact sync possible, and only absolute time carries leap seconds that should be taken care of.

For the moment, there is apparently no way around to establish your location first, for instance by a test run, and then start the actual measurement.
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#15
I gave it a try using a "GPS keeper" or "GPS locker" app in the background, which establishes a GPS connection without having the associated app open.

Running such an app in the background before actually starting the phyphox measurement helps reducing the misplacement significantly as the GPS connection is available quite fast. At least I can tell this from a first quick test this afternoon.
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