Fenix 6X Pro - why do my GPS tracks have straight line sections?

Do you have smart recording turned on instead of every-second recording?  (page 96) https://www8.garmin.com/manuals/webhelp/fenix66s6xpro/EN-US/fenix_6_6S_6X_Pro_OM_EN-US.pdf

Smart recording will ignore "boring" parts of your run where you aren't turning or winding alot and replace those sections with straight lines when saving the file (it doesn't affect how often the watch actually aquires location from GPS/Galileo/etc just how often a point is written to the file)

No. I'm using 'Every second'. It was the unusually long straight sections using smart recording that made me change the data recording to every second. What appears to be a fault, persists.

do you have autopause enabled?

Auto-pause is off.

Straight lines would indicate that your Garmin lost its GPS signal, therefore creating a line between one point and the next point where the signal was picked up again. We could take a look at it if you zip then post the fit file from the \GARMIN\ACTIVITY folder on your watch.

Also, you might try - Expired EPO on a Garmin Outdoor Watch | Garmin Support

Thanks for that offer. Here's the link to the FIT file. You can download it there. Your help is much appreciated.
https://www.dropbox.com/s/2lu8q0twpjsi0zf/2022-01-11-10-23-09.fit?dl=0
I was always under a pretty open sky accessing two GNSS constellations. My phone tracked it OK and I'm certain that my eTrex 20x would have had no bother either.
You can download my phone track for comparison here: www.dropbox.com/.../2022-01-11_10-21-28 - LM4 track rec.gpx

The FIT file was recorded using 'every second' and two constellations. One straight section occurs between 1241 and 1246. This is definitely wrong. My phone was tracking this route and accurately recorded this bit no problem. Another supiciously straight section occurs between 1327 and 1339.

This is a rough mountain route so normally there will be little wiggles in the track.

I'm curious to know how you examine the FIT file as they're not text files?  I convert mine to GPX files or upload 'em to a planner etc.

geoffmozz said:

I'm curious to know how you examine the FIT file as they're not text files?

I usually go to Fit File Repair Tool. (uses Microsoft Access). Other options are FIT File Viewer (free web-based viewer) or a hex editor, such as Hex Editor Neo

However, these tools are mostly for repairing corrupted fit files. There are some visualizer tools that will probably do better in this casel. I used Google Earth to compare your tracks side-by-side.

A section of the activity where there was the most discrepancy. Red = Garmin, Green = Phone - Compare.kmz

Yep...that's the section. I've examined both tracks myself on the map. The green phone track looks right and the red Fenix 6X Pro track looks like it's just getting a fix every now and then. I am climbing a hill so part of the sky is obscured but I would expect a device that's accessing two constellations to do better than the watch. My eTrex 20x only accesses two constellations but it's really solid and has tracked me accurately in deep roclky gullys. 

I was expecting good reliable tracking for hillwalking duties from the Fenix.  I don't think it's an EPO issue.  Thanks for chiming in :-)

geoffmozz said:

I don't think it's an EPO issue.

Maybe not but wouldn't hurt to go through the deletion process to create a new epo and sync with Garmin Express. Then if still no joy, contact Garmin support, in which case they'll probably want you to do a factory reset.

The EPO data should come from the satellites anyway. It just takes a few minutes if an old EPO file needs replacing. Those straight line sections were well over 2 hours into the walk,

Thanks for your help. I'm going to Garmin...