Garmin Instinct adds 20km in first 5 seconds of activity

Have a look at the article What Can Cause GPS Accuracy Issues on My Fitness Device? | Garmin Customer Support and pay especially attention to the section speaking about GPS Soaking. 

Thanks, will try 2-5mins of GPS soaking.

That said, the fact that this happens consistently and is always the same crazy 19km distance in the same 3-10 seconds sounds more like some other glitch than a soaking issue? Will wait and see.

I have no doubt that GPS soaking can improve the initial GPS accuracy in some cases, but I have never worried about it at all.  I always just wait until the GPS Ready alert then start the activity.  I have never had any initial discernible inaccuracy in initial GPS readings, at least not enough to make me look at it afterwards and think something looks odd.  And this applies to activities started indoors, under dense bush/forest cover, mountainous areas, high-rise city areas etc.  And "even hundreds of meters aside" is significantly less than 19km.  So I am inclined to agree that it sounds like a defective device, or some other very specific external factor at play.

If the altitude auto-calibration is giving rise to almost 20km (20000), it might be related to the comments from gehcoss in the 19.01 Public Release Candidate thread.  E.g.

gehcoss said:

I use: INSTINCT - Standard

Installed: 19.01 Pblic Release Candidate
Used before: 18.54 release

Both versions "do not!" store the HEIGHT given after GPS-Signals Height Kalibration.

Both versions go to 20000 within miliseconds. Wich you can see on the GPX protocol.

ALSO: Barometer does not work proberly. Elevation floors are not stored.

regards: [email protected]

As trux has said, it may be worth checking the physical condition of the sensor opening etc, but honestly it is hard to believe that would result in a consistent 20km re-calibration on every use.  I've cleaned decent amounts of dirt etc out of my sensor port in the past but never seen any significant discrepancy before or after.  Seems more likely a physical or software fault causing the watch to reset to/from some fixed reference baseline due to a lack of data.  

Out of interest, does the altitude look correct before you start an activity - just from the altimeter widget? It is possible to disable the auto-calibration on activity start, so that might be another workaround.

Clock is set to hight by GPS! The clock always askes for calibration. So i calibrate with GPS.  the anounced hights looks good.
but when I start the activity the watch immediatly goes to 20000. The immediate change of elevetion is stored and is shown on the track.

If the clock is set to hights by baro, it sticks to the same hights even when I cycle, walk up a hill (20 hight meter).
The track does not show any change.

For me it looks like software not as hardware problem. Cause it immediatly changes after calibration .
It i am asked for calibration on and on.

All this also aplies to 18.54. which I have reinstalled ..

gehcoss said:

but when I start the activity the watch immediatly goes to 20000. The immediate change of elevetion is stored and is shown on the track.

Yes, the watch stores the elevations of your last locations and uses it once you start an activity. Reset the watch to factory defaults with deleting all data - that should clear up the cache of the locations. You could also try backing up and deleting the files in the folder //GARMIN/Location on the watch, but I do not know for sure it is really that folder that keeps the cached elevation data. Another option that could help, and that I already mentioned in my previous post, is disabling the altimeter auto-calibration. It should then stop recalibrating automatically at each activity start.

gehcoss said:

If the clock is set to hights by baro, it sticks to the same hights even when I cycle, walk up a hill (20 hight meter).
The track does not show any change.

Yes, that's right, but if you do not care about the elevation profile (i.e. flat runs or flat rides), it will at least help to avoid the added 20km distance that the OP is asking about.

I did a quick search and the "jump" to 20000m after a couple of seconds is not unique to the Instinct.  It has been seen on the Garmin Edge 520, the Fenix 5, and also on the Forerunner 305 (14 or 15 years ago!).  So I would assume it is a standard Garmin "feature" which is triggered by some set of conditions.  Surely there is somebody at Garmin support that could shed light on this?  Have you tried calling up Garmin support and asking them directly?  They have always been helpful when I've spoken to them in the past.

KMB said:

I did a quick search and the "jump" to 20000m after a couple of seconds is not unique to the Instinct.

That's not surprising. The altitude of 20,000 m corresponds to an unpowered pressure sensor (the pressure at 20 km is just ~50 millibars, or ~5% of the SL pressure). So if you get 20,000 meters immediately after calibrating the altimeter, it means the sensor does not deliver any voltage. In other words, it is either physically dead, or there is a serious corruption of the Sensor Hub Firmware (but in that case you would probably experience problems with other sensors as well).

So on my mind, you have the following options:

1. Have the watch replaced, either under the warranty, when still possible (2 years warranty in EU), or against a payment for a refurbished discounted device
2. Disable the altimeter by switching to the mode "barometer only", and use the topographic elevation profiles instead (use the feature "Elevation Corrections" in GC Web). You can also customize the activity screens to show you the GPS elevation live, instead of the non-functional barometric elevation.
3. Get a new watch

You could also try reinstalling the Sensor Hub firmware from an older beta installation package, but I would put any high bets on it.

trux said:

The altitude of 20,000 m corresponds to an unpowered pressure sensor (the pressure at 20 km is just ~50 millibars, or ~5% of the SL pressure). So if you get 20,000 meters immediately after calibrating the altimeter, it means the sensor does not deliver any voltage. In other words, it is either physically dead,

I was assuming the same thing, but I couldn't find any definitive confirmation that 20000m = dead sensor.  I've since found similar threads about dead sensors and 20000m altitude on the Forerunner 935 forum too.

7800448 I realise you have already concluded the sensor is dead but I'll post this here for future reference in case someone else with a similar issue ends up here via a search:

The Instinct (like most Garmin watches) has a service mode that you can enter by turning the watch on while you hold down the Down button.  You can use this to confirm if the sensor is dead.  Scroll though the screens (press Back  button) you will come to a Baro Test screen.  If it's working you should see "Valid" in the inset dial, together with a Local Pressure, Local Altitude and Temp.  The Local Alt. should be something sensible based on where you are, and should vary slightly over time.

Here's a screenshot of the Baro Test from my watch for comparison:

THANKS. My Baro shows "VALID" and the alti shows 20000. Smile.
This means SOFTWARE (S/W) not hardware-problem to me.

This means after START and CALIBRATION the GPS-data are not correctly stored.(S/W)