Some thoughts on Ultratrac mode

I bought the Fenix 5x a few years ago, partly based on the expected battery life of 25hrs plus more if I used Ultratrac.

Well, at the time I bought it, that turned out to be a crock of lies.  (Version 7 or so of the firmware)

In Normal GPS (Navstar only) mode, I was lucky to get 11 or 12 hours.  (Event that was a great improvement over my then current watch)

Ultratrac turned out to be useless, at least for race events, so I never really got an idea of what it was capable of in terms of extended battery life. That first fuzzy hedgehog of a recorded track and inaccurate distance made me so disgusted I just turned back to normal GPS.

Then however (After version 12.something when they introduced some changes that affected the GPS) I tested Normal GPS and found it could work for longer, especially if you turned off heart monitor and paid attention to limiting display updates and such. Maybe as much as about 25 hrs, so Garmin made good on that eventually.  I still haven't measured how Ultratrac might go for less accuracy demanding activities, say hiking where I just need to know I haven't wandered too far off track.

Anyone tested Accutrac whilst on the move from full to empty to see how long it would last?

(One reason I am interested in this thread is that I am toying with an ultra-low power gps device idea that uses e-paper to hold the last position measured and only wakes up every 10 mins or so to get a new fix. No maps, just map coords. Use a real paper map for ultra-ultra-low power display.   Would like it to work for weeks at a time on one battery.)

With version 13.30 I did some simple tests whilst at work, just letting Ultratrac try to track me at the office. I got some interesting results over a couple of hours in Ultratrac and then in Normal GPS. The drop in battery % was about the same for both and looking at the track recorded, Ultratrac wasn't much different than the Normal GPS recording. I'm beginning to wonder... Anyway, extrapolating out the losses over 100% battery use I get 21 hrs for Ultra and 20 hrs for Normal GPS.  Something funny going on. Maybe because the GPS reception was pretty poor in the office. So no conclusion. Did the experiment again with Utratrac on the way to work in the car. Seemed accurate enough. Dropped 5% in 1 hour, so again, about 20 hrs over 100%. Doesn't seem to be any point to using Ultratrac at that rate. I really need to get out in the wild and try it again. (Oh, I had wrist heart rate monitor still on. Maybe should turn it off. Was putting it back to watchface when not looking at it, so no map updates on the screen most of the time)

"Your mileage may vary" is apparently true!

I got the same experience as you. The UltraTrac mode is a completely useless feature to me, as the accuracy is far too low to be accurate. I experienced much higher offset than your +50%, more like +100%.

Here is an activity, the first half (more or less) was recorded with UltraTrac, than I switched to GPS+GLONASS:

https://connect.garmin.com/modern/activity/3709130139?share_unique_id=4

The activity gives 20km, while in reality it was only 10km. The difference would be probably even higher if I kept UltraTrac throughout the full activity. 

i wanted to use it for hiking. thought that was just right for that. but i got allways some crazy line, so i don't use it again. as i found out. gps runs 20 seconds and the is 40 seconds off. in between this 40 seconds, it runs the direction and speed from the last 20 sec, then corrects the if you took a turn. 

i wish it would just set a point every 30 or 60 seconds. distance would be off too, but the the track would be usable for "x"day hike.

Yeah, I want that mode too. Even more, ability to set the 'sleep' time from say 30s to 10 mins (or even more). (I don't walk that fast that I get very far in 10 mins)  This would approximate how I use my handheld GPS when hiking. I turn it completely off and then every hour or so I turn it on, wait for a fix, discover I haven't wandered off the edge of the world (or not) then turn it off again. Batteries last me about 1 week using it like this.

I can use the Fenix 5x similarly, leaving it in watch mode most of the time rather than a hiking activity, I suppose, and only turn on a tracking activity every now and then.

Exactly. This is want I expected this mode to be like. 

That's one messed up recording, but looks like a lovely place to be out in! It's funny but my recent tests with Ultratrac have been pretty good, but I'll test it out some more in the wilderness this weekend and see how it goes.

So I did test it on a 10k run and it did surprisingly well. Here's a link to the trace. The only obvious perturbation is at the far turnaround  (top of red track) where I paused for a few minutes and then resumed. It added in a 2 hundred m 'glitch' at that point making it look like I ran halfway around the sports oval and back, when I definitely didn't. For my purposes whilst hiking, that's something I can live with.  It looks a lot cleaner than my first attempts with Ultratrac a few years ago. Note that this is from Strava, but the map on my watch looks much the same so I don't think Strava modified it.

Same here as the other replies. Because next week I will be in the mountains for a week, without constant access to a power supply, so last week I did a test hike to see how far I can go, when I disable everything and set the GPS to Ultratrac mode.
The result is this:
https://connect.garmin.com/modern/activity/3776116015

As some other people said the distance gap is more like +100%. I originally got 9.9km for this track, when it's actually ~4km. In the activity I already corrected the distance, based on a manual correction with OpenstreetMaps.

Side note: I disabled smart notifications, etc. and then started hiking. The watch then consumed ~4% in ~1.5h. Which brings me to ~38h (I think 42h are advertised). So I assume I can roughly get to the 40-42h. But with this tracking precision this mode is really useless. Otherwise I would have to flatten all tracks at home afterwards with a Mapping tool. Which I don't want to do. So I will probably bring a PowerBank with me next week to track everything.

Thanks for sharing your track (nice area ! Love the Belledonne massif).
I have the exact same experience with my multiple tryouts. Connected dots, which are completely unaccurate, and report funny distance values. Makes GPS tracking completely pointless.

I think Ultratrac has its merits as a last ditch effort to get some track points with a really low battery consumption. It would be really useful, if Garmin could give us a battery threshold below which the F5+ switches from normal GPS mode to Ultratrac mode. I am a bit worried that my F5+'s battery time isn't really as advertised in a real world scenario. I probably need between 10:30 and 12h max to be on the safe side during next year's IM and not totally sure that the F5+ will make it.