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GLONASS ON/OFF battery use and position accuracy

I just left my 935 to record an 'activity' outside in a fixed position for a couple of hours to look at the difference in battery use between GPS and GPS+GLONASS. I have done this during an actual run before but I got strange results which I was unable to explain, possibly due to changing exposure to the sky and other variables. By leaving the watch in a fixed position with good view of the sky all round and at a fairly stable temperature I hoped to be able to see just the effect on battery use caused by GLONASS.

During this test everything else was switched off, so no OHR, no phone connections, all ANT sensors disabled (switched off in the watch) and recording set to 1 sec.

The resulting 'activity' is here:

https://connect.garmin.com/modern/activity/1862203704

Battery level and calculated discharge rate were written to the FIT file by my battery monitoring data field, which itself uses an unknown amount of power.

It is clear to see the point around 1H40 at which I enabled GLONASS. Before the discharge rate was around 4%/hour, after that the discharge rate went up by more than 1%/hour

Switching on GLONASS had a very noticeable effect on position stability though. In the first 1H40 the recorded distance (ie drift) was 200 meters, in the next 1H30 with GLONASS on only 70 meters. The data points were also much more tightly grouped with GLONASS on.

So based on this not overly scientific test I would say leave GLONASS enabled provided you can life with the additional 1%/hour battery drain.

Next test is going to be 1s recording vs smart recording.

0 FlipStone over 7 years ago

What are these "drift" figures you refer to?
Visual examination of these 4 suggests the ones with Glonass have a much greater "circle" of locations than the ones with GPS only.
So to me GLONASS here makes things worse not better?

As the watch was just lying still in the window all movement I see is what I consider drift. Inaccuracy in GPS location leading to virtual movement.

Wrt the circle, not sure if we are seeing the same thing then?

Also with test 3 and 4, the scale is different between them. 10m for test3 vs 20m for test4. If you zoom in 1 step with test 4 you'll see the difference. Also the variation in test 4 is present all the time, it jumps around a lot. With test 3 it's pretty localized, then jumps a bit and then is localized there. Less variation so to say.

With test 1 and 2, the horizontal spread is about the same but the vertical spread seems smaller in test2. Also again, in test1 the movement is all around the lines all the time while in test 2 it's more localized so to say.

But yeah, it's not as if this is all saying but there IS a difference between gps and gps+globass as 720m vs 500m is quite a percentage. Lets see how it does in my new tests, although the weather is a bit tricky today so not sure how that will mess things up.

0 timgrose over 7 years ago

I meant on the GLONASS one https://connect.garmin.com/modern/activity/1863863315 you have positions stretching across both carriageways but with the GPS only one https://connect.garmin.com/modern/activity/1863861545 they do not seem to stretch as far.

Thus the maximum difference between the lon/lat being reported by the device and the true value is seemingly less with GLONASS off.

Then again what we are seeing here with the positions jumping around a bit is no surprise and actually neither "drift" is overly large.

So interesting about the battery drain but not sure can really say whether or not GLONASS will actually help in "real" runs

0 FlipStone over 7 years ago

I see what you mean, the actual location was on the right of both carriageways though. Anyway, the signal seems a bit more stable in between points with gps+glonass, but must say I'm not convinced about the use either. I usually have my runs set to GPS only and smart recording nowadays and that is perfectly fine, used to do 1sec and gps+glonass but seems like a bit of overkill. Especially for my regular runs, can imagine with races I would enable both.

Anyway, did another few tests... This time with OHR enabled as well and for a 1:30 hour timeframe...

GPS Smart: https://connect.garmin.com/modern/activity/1865363608
GPS+Glonass Smart: https://connect.garmin.com/modern/activity/1865363655
GPS+Glonass 1sec: https://connect.garmin.com/modern/activity/1865663235
GPS 1sec: https://connect.garmin.com/modern/activity/1865663278

Summary:
GPS Smart: 3.8% per hr and 1100m drift (5.7 battery total)
GPS+Glonass Smart: 5.4% per hr and 920m drift (8% battery total)
GPS+Glonass 1sec: 5.5% per hr and 730m drift (5.7% battery over 1:15? Meaning about 7% over 1:30. Taking the last hr: ~5.5% per hr)
GPS 1sec: 3.8% hr and 1110m drift (5.6% battery total)

Something went wrong with test3 though as the battery actually increased in the beginning. So kinda hard to say anything about that... Took the last hour to determine battery usage per hr.

Anyway, GLONASS seems to add some more stableness but at the expense of battery (~1% to 1.5% an hour). 1sec doesn't seem to matter, can ignore the battery use compared to all other factors. Say it takes up another 0.1% per hr max...

Today it is more cloudy compared to yesterday... Perhaps that explains the increased glonass battery use a bit?

0 FlipStone over 7 years ago

For some reason I can't seem to edit, hangs on "working" all the time... sigh.

But wanted to add that today it was/is (way) more cloudy compared to yesterday... Perhaps that explains the increased glonass battery use a bit?

0 anttinn over 7 years ago

Battery level and calculated discharge rate were written to the FIT file by my battery monitoring data field, which itself uses an unknown amount of power.

Would it be possible to add a realtime display for battery drain, in percentage per hour?

This could be configurable option, and would be really handy to experiment effect of various settings on battery drain.

0 CloggyAbroad over 7 years ago

Any effect of changing settings takes time to show up in battery drain. At the moment battery discharge (or charge) rate is written to the FIT file. As you can see in the 'activity' I posted at the start of this thread there is a clear increase in battery drain after switching on GLONASS but it takes time to stabilise. Only the graph in Garmin Connect gives a clear indication what this change is. An instantaneous readout of battery drain on the watch itself is of very limited value which is why I have not included it as a display option.

0 FlipStone over 7 years ago

I just made a quick datafield displaying discharge rate but will test it a bit tomorrow before I make it available... Just a quick and dirty one because of the question asked above.

Seems to be working ok, let me know if you have any issues:

https://apps.garmin.com/en-US/apps/80d67ebb-78e9-4e07-9b82-f52e05ebfe89