I agree completely endecopt. I did have to also shift IOS time by ~ 1sec. I did not reported that in my original post because I too could rationalize such an offset. But, 0.6% is 6000ppm! That kind of accuracy is the kind that you find on really poor oscillator. Unless I have made some kind of mistake in my analysis, I conclude that the time associated with the position/location can NOT be a UTC time.

Thanks so much for the help!

I am now able to write my files to iCloud. My big break thru was realizing the a file's URL (returned from the Swift code) needs to be modified (replace, for example "%20" with " ") before using them as a file name in the fopen() call. Now, I need to figure out why these files are not visible in the Files app. The files are visible on my iPhone via Settings->Accounts->icloud->Manage Storage->. And, I don't yet see them being uploaded to my desktops (either Windows or Mac). Still, pretty good progress for a couple days effort!

WRT enabling CloudKit: I think the answer is yes as I checked both "iCloud Documents" and "CloudKit" under iCloud on my target's "Signing & Capabilities" page. WRT directory creation: I did try to write my files into: /private/var/mobile/Library/Mobile%20Documents/iCloud~TrackInstructor/ thus bypassing the need (I believe) for the Documents/ directory. How would I manually create the Documents/ directory? I don't know what a coordinated write block is, so I assume the answer is "No." I assume the lack of a coordinate write block would affect the fopen() command as well as fwrite(). Correct? BTW, I omitted from the original posting that I have updated the Info.plist file with 3 key/value pairs which I am told are required to write to the iCloud. It there a particular naming convention for the container name? I may have incorrectly taken some liberties with the container name using something other than the project name. I don't know why I did that...

Yes, good questions. I anticipate that I will initially log a fair amount of data while the app is running, logging the data in a "batch mode." That is, I will log data for a number of minutes (10 - 20) and then retrieve that data and analyze it. I suspect that the quantity of data I log will decrease with time as I gain confidence in the app and fix errors. When analyzing the data I expect that some of the tabular, numeric data might be plotted, possibly against other "truth" data using engineering tools such as Matlab. (For example, I might plot the filtered acceleration data recorded by the app vs the raw acceleration data sent to the iPhone app.) There might also be debug data consisting of time stamped text strings with an associated severity eg, info, warning, error, fatal. (For example, I might generate a log entry with the text version of verbal feedback produced by the app.) So, yes, I will be processing the collected data, largely for development but then a smaller subset for ongoing verification of the app. Thanks.

Somewhat off topic: do Apples guidelines/recommendations/rules prohibit use of IPv4 on a WiFi connection to an Access Point which is NOT connected to the internet? Thannks