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Multi-Band GNSS in Open Water (Failure)

jpbergin
over 2 years ago

On Sunday, June 12th I completed the Great Chesapeake Bay Swim.  It's a 4.4mi swim if you track perfectly.  For me it registered on my Epix 2 as 4.95mi which makes sense given how I zig-zagged across the course (strong current and lots of chop).  The final time on my Epix matched the final chip time.  However, as you can see from the image, the GNSS/GPS stopped recording about halfway through.  I believe I was using the default setting for open water and I don't believe that includes the activation of Multi-Band.  Even without Multi-Band turned on, the only obstructions were the very high bridges I was swimming between. 

This poses the following questions:

1) Why would it have lost signal halfway?

2) Why would it not have re-aquired a signal?

3) What did it use to calculate my distance after it lost signal? (I was also wearing an HRM Pro chest strap under my wetsuit)

Thanks,

Joe

  • 0 over 2 years ago

    I can try to answer a couple of your questions.

    1. While the watch (hand/wrist) is in the water there is no GPS signal. If your swim style makes the BAR in that your hand is in the water under xyz time and above the water (recovery) for abc time, the watch may not lose GPS signal. So there are variations across users and their particular swim styles. I for one have a similar issue, and again, never had it until the F6, ie no issues with prior watch generations.

    2. When GPS signal is unavailable the watch uses the "cadence" or swim strokes per minute to determine speed/distance. That's how you get additional distance recorded even in the absence of GPS signal.

    See my post from earlier today regarding some of the issues.

    forums.garmin.com/.../my-ows-distance-issue-analysis-report-with-potential-improvements-garmin-please-read-this

  • 0 over 2 years ago in reply to avikoren1

    Thanks for the reply. 

    On the first topic: I am aware of the challenges with GPS underwater, however, it didn't make sense that it worked for the first half but not the second half.  I never broke stroke or changed stroke.  Freestyle the entire time.  

    On the second topic: I assumed it used some kind of cadence but I'm wondering if it was using the cadence from the watch or from the HRM Pro.  My guess is that it's the watch but I wasn't positive.

  • 0 over 2 years ago in reply to jpbergin

    With regard to your first question, did you change your breathing side or every how many strokes you turn to breathe? In my case I am so marginal that I have noticed after many trials that I do a little better if I wear the watch on the opposite of my breathing side. So if I breathe on my left side, I get better results if I wear the watch on my right hand.

    With regard to the second question, I do not know for sure but I think cadence or swim stroke can only be counted by the hand, therefore watch. I don't know if there are some minor improvements or additional information that can be obtained from the HRM but I do know that most of my swims I do not use the HRM in OWS and still get the "cadence" to be used when GPS signal is lost.  

  • 0 over 2 years ago in reply to avikoren1

    I breathe on both sides, alternating every third stroke.

  • 0 over 2 years ago in reply to jpbergin

    okay, that should provide an even balance. Not sure, maybe you got a little tired :) towards the second half and glided longer with the watch in the water, not sure. If you use a FIT decoder you should be able to see what happened.  This is the one I have been using which has been an eye opener to what is really going on.

    https://www.fitfileviewer.com/