Monday, November 7, 2011

GPS Accuracy: GT-31 vs. Android Phone

I finally managed to get some recorded GPS tracks from my Android phone for comparison with the GT-31 data from the same run. Just listening to the GPS Speed Talker app, I knew there were some spikes in the GPS data from the phone. Here is an example:
According to the GPS data from the Android phone, I hit 30 knots here. I wish! Within one second, the speed jumped from 24 to 29 knots, and stayed around 30 knots for 3 seconds. That's using the Doppler data - according to the GPS position data from the phone, I actually hit 40 knots!

The tracks show that the GPS points from the phone are not accurate here. I was going straight - I did not suddenly go upwind and accelerate 5 knots within one second. The jibe in the lower right corner also shows that the GT-31 is much more accurate: the blue curve from the GT-31 shows a nice curve; in contrast, the red curve from the phone GPS wiggles all over the place.

I have seen spikes in GT-31 data, too, but they tend to be rare (as long as you look at the Doppler results), and are usually very easy to identify. Most of the time, it's just a single value that's wrong, which gives a very sharp peak. Also, GT-31 spikes tend to be linked to crashes (although that's not always the case). Either way, they are easy to see and remove.

The 4-second, 5-knot spike shown above it the worst in the 2-hour recording, but there are a number of other spikes, too. Many of the other spikes affect only one point, which seems to be off by 1-2 knots (an example is in the track before the jibe). More than 98% of the time, the speed values from the GT-31 and the phone are very close.

So using an Android phone with the GPS Speed Talker app can be helpful, although one has to take the announced speeds with a bit of skepticism. Perhaps other phones are more accurate, although the only thing I have seen was a description that some other phones have less accurate GPS units. Posting results from Android phones to sites like or the GPS Team Challenge is, at best, questionable. The standard filters in GPS action replay did NOT remove the spike above, and my 2-second speed for the day would have been 4 knots to high. There are obviously good reasons why these sites suggest to use GT-31 units.

If you want to hear how fast you are going while sailing with the highest possible accuracy, you can always use a bluetooth-enabled GT-31 (a BGT-31), and have the GPS Speed Talker app use the BGT-31 data for the announcements.

No comments:

Post a Comment