Monday, March 12, 2018

The Difference Between Good and Bad

No, this is not a philosophy post. It's about GPS reception. Surprised?

When I looked at GPS test results from yesterday and today, one thing puzzled me. Look at the data and take a guess:
Why was the phone GPS so inaccurate on 3/11, with large errors in every GPSTC discipline, but quite accurate on 3/12? I'll give you some clues, so you can see how early you can guess the answer.

Clue #1: A cold front pulled in yesterday afternoon. Temperatures dropped from the 80s into high 60s yesterday evening, and to 50s today.

Clue #2: That made me wear a 4 mm wetsuit today, while I was sailing in a lycra top yesterday.

The ardent reader of my blog may have a suspicion on what was going on by now (ha!).

Clue #3: The arm band I use for my phone tends to slip on the lycra, but much less so on the wetsuit. Yesterday, I stopped several times just to re-orient the phone so that it was pointing to the sky; it often was on the side of my arm.

Clue #4: Compare the track points from one run where the phone reported about 1 knot less speed than the GW-60 shown here:
to a run where both devices gave almost the same speed:
Clue #5: Check the "Sats" column for the phone data (the numbers on the right side) for the picture above.

So: yesterday, the armband with the phone slipped often, so that the phone was on the side of my arm, instead of pointing up. The runs that deviate most from the GW-60 data had reception from fewer GPS satellites, and changes in the number of satellites during the run. It seems that having the phone facing sideways on the arm during speed runs screws things up. That's not really very surprising - perhaps the more surprising thing is that the USB dongle gave accurate results yesterday (it was in the same bag, lying on the screen of the phone).

Can we verify that turning the phone sideways and blocking one side screws up GPS reception? Sure! I used GPSLogit, which has a graph that shows satellite reception. Here's a screen shot showing the reception with the phone facing up:
The phone used GPS 13 satellites, and had a decent horizontal accuracy of 4.6 meters. Just turning the phone onto its side did not change much, but then putting a hand close to the top of the phone on the back side did:
Now, the phone had good signal (green bars and dots) from only 7 satellites, and the estimated positional accuracy was 19.7 meters - four times worse!

If you want to reproduce this, but don't have GPSLogit, there are plenty of free GPS test apps on the Google Play Store available that could be used instead. The phone's GPS antenna is typically located near one of the top corners of the phone. You'll have to wait a few seconds to see changes in the satellite reception (probably because that information is not updated as often as position and speed info).

So, if you want to get accurate speeds from your phone with GPSLogit or Windsport Tracker, make sure the phone is facing to the sky! Of course, that's the same for the GW-60.

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