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From Fellrnr.com, Running tips
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==GLONASS Accuracy==With GLONASS enabled, the Fenix 5X demonstrates even worse performance than its peers. The values shown below are rather dramatically worse with GLONASS enabled than without. My anecdotal observation is that sometimes the Fenix 5X does a little worse with GLONASS than normal, possibly in line with other Garmin devices, and sometimes it seems to just get lost and produce dramatically worse results. {| class="wikitable sortable"!Device!Accuracy!Trueness!Precision!Repeatability|-|Fenix 5X 4.30|style="background-color: #FEC97E;"|5.6|style="background-color: #EDE683;"|6.9|style="background-color: #FCA377;"|4.5|style="background-color: #F3E783;"|6.8|-|Fenix 5X 4.30 GLONASS|style="background-color: #F97B6F;"|3.5|style="background-color: #FFE082;"|6.2|style="background-color: #F8696B;"|1.6|style="background-color: #F97B6F;"|3.5|}Below you can see a visual representation of the problems. Many of the tracks are a little worse than normal, but you generally follow the path. A small subset of the tracks are dramatically worse, either showing and offset from the actual path, or sometimes it looks like the sampling frequency has dropped, suggesting that the watch is only periodically able to get a location fix. This latter phenomenon is rather surprising to me and goes against my hypothesis of why GLONASS has worse accuracy. I would expect there would be more satellites available with GLONASS enabled, which would result in the watch selecting the subset with the strongest signal that are more likely to have a narrow angle of separation, which would result in increased Dilution of Precision (DoP). These tracks suggest that the Fenix 5X is unable to get any location fix for brief periods. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"|- valign="top"|[[File:BridgeFenix 5X 4.30 GLONASS.jpg|center|thumb|x300px| The GPS tracks from the Fenix 5X with GLONASS enabled. This diagram has tracks color coded with green indicating good accuracy through to red indicating poor accuracy, and the lap markers as blue dots.]]|- valign="top"|[[File:BridgeFenix 5X 4.30.jpg|none|thumb|x300px|Here's the tracks from testing with GLONASS disabled for comparison.]]|}
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Within each model, there are choices of wristband type, with a metal wristband adding $150 and sapphire glass adding $100. There are also a number of color choices, though the 5X is only available in gray. So, which to buy? I'd recommend getting the 935 as it's cheapest. The only reasons to buy the Fenix version are if you need maps (5X), you want the Fenix look, or you want the smaller size of the 5S. Of course, with the release of the Fenix 5, you can get the [[Garmin Fenix 3]] at a lower cost. The Garmin Epix is also worth considering as it has the maps of the 5X, but Garmin has stopped releasing firmware updates and it's nowhere near as attractive.
'''<br/>There are credible reports that the smaller Fenix 5 and 5S models are having issues with sensor support. The reports suggest that Ant+ connectivity is poor, as well as some suggestions that the Bluetooth range is more limited than it should be. '''
=The Big Questions=
For a [[Best Running Watch| simple evaluation of a GPS watch]], I look at how well it can answer some basic questions. There are many things a runner might look for in a running watch, but I feel these four questions are critical.
|[[File:BridgeFenix 5X 4.30.jpg|none|thumb|x300px| For comparison, here's the tracks from the normal GPS mode.]]
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=Optical Heart Rate Monitoring=
I've not found any [[Optical Heart Rate Monitoring ]] (OHRM) implementation that's good enough to be useful, and the Fenix 5X is no exception. I believe that you're better off having no heart rate data than bad heart rate data. While a chest strap based heart rate monitor can have accuracy issues, these are generally dramatic and obvious, whereas OHRM can be quite misleading. (The issues with a chest strap heart rate monitor are also usually fairly easy to remediate, either with some electrode gel or a new battery.) The accuracy of [[Optical Heart Rate Monitoring]] (OHRM) will depend on a number of factors:
* The watch needs to fit just right. Because of the sensor is measuring the expansion of the capillaries with each heartbeat, too much pressure will prevent this expansion. However, to lose and the watch won't get a good reading due to lack of contact. Getting this tension just right can be tricky, especially if you're wrist expands or contracts over time.
* Movement seems to confuse OHRM systems, possibly because it changes the papillary filling. Some users have noted that their OHRM systems seem to lock on to their Cadence rather than their heart rate.
