Regarding the built-in data field for the GPS signal strength

You can always add the data field and start up the activity indoor and see how it works. Try to get it to show 1-3 bars.

I have seen that before, while on an airplane. But never while just recording normal outdoor activities.

1-3 bars effectively means it sees a couple satellites, but not determine a valid location. Whenever I've had 1-3 bars it was not able to calculate my position.

AndersB said:

"GPS

The strength of the GPS satellite signal."

I think "The strength of the GPS satellite signal." means the same as RSSI in IEEE 802.11 .

https://en.wikipedia.org/wiki/Received_signal_strength_indication

There is a relationship between GPS signal strength and GPS quality. And if you compare the colors of the GPS quality symbol (arc) with the GPS signal strength symbol (bar), you will see this relationship. But the two things are not the same.

AndersB said:

Also indoors in concrete house with 3 layered windows. The GPS signal strength cannot be the best possible (5 of 5) in these places.

Just as you can listen to the radio or watch TV in your house, the GPS signal is also present with more or less strength, unless you shield it with e.g. aluminum foil

Garmin staff, any information or news regarding this? On ALL my runs the GPS signal strength data field shows full (5 of 5 blue bars) and there must be some kind of trouble with the data field and the API data behind it. I have started to log currentLocationAccuray in the FIT file and it always are 4 (maximum value) even though the device are under heavy tree cover etc. It shows too good accuracy and makes it hard to use the data when making own data fields etc. 

I use a 3rd party widget that writes the history into the activity but also I've only seen full strength or nothing, during a 4h hike through the woods. When we went through a tunnel it was no signal otherwise full power not even a gradual change on entering or exiting, it might really not give out more details than full power or nothing. 

Same when I'm on the water, under bridges should diminish reception, but its always logged as full strength.

Unfortunately this is the way it has been since Fenix3 and ConnectIQ API v1.0 and I doubt they will change it. I know I did the same tests on my F3 with the same results as you.

Question is what Garmin consider marginal vs. good-to-excellent. From my point of view it would be better to give us access to the detailed information (number of sats, HDOP and VDOP).

Yes, I have seen the API docs but the watch never gives QUALITY_POOR or QUALITY_USABLE in my activities. After the initial green GPS fix it never goes down to 2 or 3 in the values. Its always 4. And that must be wrong. 

Yes, it would be nice if Garmin could give us developers more GPS info through the API. It shouldn't be so hard for them to give us that.

AndersB said:

Its always 4. And that must be wrong. 

I'm not so sure it is wrong. If 4 is a HDOP of 5 or better it is a big span.

As a test you can download a GPS Test app to your phone that displays various DOP values (I use GPSTest for Android). You will notice that in most cases you get a very low HDOP, even indoors. 

https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)

Did some indoor tests at home. Started the walk activity, waited for the green GPS and let it soak a little, pressed start, walked around a bit indoors, the data field showed 5 of 5 bars all the time. Then walked in a room with extremely poor GPS signal.

The GPSTest app (don't know if its exactly the same as yours though) showed H/V DOP 99,0 and the watch data field still showed 5 of 5 blue bars. Stayed in the room and after a relative long time (maybe a minute or so) it started to fluctuate between 5/4/0 bars. It never showed 1/2/3 bars in that room.

So, it also seems like there are relative long polling delays for the GPS location accuracy and maybe that's the reason i never see it dip under normal activities while running in under tree cover etc.

AndersB said:

showed H/V DOP 99,0

Wow! If I walk around inside (both upstairs and downstairs) I seldom get values above 1:

If I switch to the Accuracy page in GPSTest I can see how I move around inside the house with quite good precision. I still wouldn't say that the Garmin Status is wrong it is rather the phone that has terrible accuracy in your case.