There are a lot of complaints about Ultratrac, mostly that the maps are horrible and the distance calculated is crazily inaccurate. The Fenix manual page isn't that great, but the one from 920XT has more information and perhaps the implementation is the same on Fenix 5.* The key thing I learned there is that the GPS is not used for speed and distance.
- Distance: Not sure if everyone has read the manual pages on it, but the way it works is that the distance is calculated based on the accelerometer in the watch, based on autocalibrations from the GPS. My opinion here is that the calibration might be roughly ok when the conditions are the same, but I would think that mostly this was calibrated during daily runs, but when people use Ultratrac it would be when they need long term battery and thus are power hiking or at least ultra running pace. Using ultratrac on a daily run course I have done before and about the same pace, I found it was 0.7 miles off on a 6 miler run. But in walking, especially with trekking poles, it was almost like a random number off. I'm actually not sure how to calibrate this, I went with a footpod and the calibration of it is defined. To force the calibration for your target event, I imagine that you can clear the current calibration, turn on autocalibrate, go for equivalent trail run/walk with decent GPS sky availability and well within the battery range. The to keep that calibration, it turn off autocalibrate.
- Maps: Ultratrac track maps are often described as figures drawn by small children. Since the gps isn't used for speed or distance, it's use here is just to have some idea of where you went, and of course so there is some kind of map for the Strava activity. I've looked through the track files from an Ultratrac activity, and what it looks like it does is turn on the GPS, and while the watch is still acquiring satellites it starts writing the GPS location into the track. If any of you have ever turned on your watch and started before getting satellites, you know that it is much harder to acquire satellites while moving than while stopped, I've seen it take several minutes on suburban roads to acquire satellites, and on trails I've been with people that their watch took dozens of minutes on the move to get satellite lock. In ultratrac these lousy locations are written to the track, and it may not even get a 3D fix. What Garmin could do here is change the firmware so that low quality GPS data is not written to the track, I think folks would prefer known bad data not be added, even if it meant that good points were only written when available. Otherwise, it seems like the best thing would be turn off the GPS entirely after starting the activity, and then maybe manually turn it on from time to time when you are at rest stops, with the side benefit being that the battery would last even longer and you'll still get a location map.
*920XT Ultratrac manual page: https://www8.garmin.com/manuals/webhelp/forerunner920XT/EN-US/GUID-01F41A53-9B56-433B-846E-C6F5A6D4660F.html