Yesterday at Budgy I tried the GPS Logit app for the first time. I used the earphones that came with the $59 Telstra 4GX Buzz. I attached the phone to the top of my helmet to give it the best view of the sky (and to look as ridiculous as possible)
I could hear the sound quiet clearly, it gave a really useful feedback of current speed.
At the end of each run I compared the 2sec peak to the 2 sec peak displayed on the GT31, which I was wearing on my arm. Generally they were only 0.1 kt apart, but some times up 0.3 or even 0.4.
As my confidence grew I stopped looking at the GT31 as I was getting a reasonable guide of my speed from Logit.
At home I examined the track logs in RealSpeed. I was not surprised to see the fairly poor results from Logit.
See pics below.
What did surprise me was the HDOP and Sats Figures
The GT31 had HDOP of 1 with 8 sats
The Logit had HDOP of 0.2 with 12 Sats
I have very limited understanding of this stuff, but I thought that a lower HDOP was a reflection of overall better Satellite geometry and therefore better results.
I then thought that the Logit app did not calculate the speeds using Doppler, but I read on the net that it does.
So if GPS Logit is using Doppler speeds, has a better HDOP, more satellites tracked then why can it not not produce the smooth logs that the GT31 can? Is it simply the poor quality GPS in the phone?
Can I link my blue tooth GT31 to the Logit app? I don't see in the app how to do that?
I think Daffy would know all about this. He was running his old BT G-Ray 10 Hz gps on Logit.
see
www.seabreeze.com.au/forums/Windsurfing/Gps/GPS-Logit-with-10hz-external-BT-receiver/
Should be no problem
The Buzz tracks look like my old Xperia which had 0.9km/hr speed steps. Pixi's and other phones have 0.09km/hr steps and give smoother graphs.
I agree with yoyo here, some phones have approx 0.5kt resolution and others 0.05kt resolution. So changing to a better phone will give smoother tracks.
The other problem with phones is their gps antennas, there's not a lot of room in a phone to put in a decent sized one.
I've though about using a BGT31 with logit, as yoyo says, daffy has successfully linked blue tooth loggers to logit, but I have a feeling he's using different software from Manfred, (it may be available from the play store, but I'm not sure).
I think this is the future for us, an accurate logger on the head, with speed talk in the ears, and all our divisions on your arm.
I don't think it is a resolution problem. When sorting the data by speed in RealSpeed I counted 180 different speeds between 20 and 21 kts
In theory, you should be able to get the BGT-31 to talk to a BT version of GPS-Logit, but Logit would need to be tweaked. The Beta BT version is specifically tuned to Ublox 6, 7 and 8 series chipsets and 1, 2, 5, 10 and 18Hz settings. It is still in development by Manfred, but he probably would not see value in putting the effort needed into adding the Sirf sbn protocol as there would be so few potential users. Edit: I just realised that the BT output from the BGT-31 is probably NMEA only. That can be read now, but has no error data in it.
It must be stressed that the lack of accuracy or consistency you see in this comparison is NOT the fault of GPS-Logit. It can only work with the data that is fed to it. The GPS's built into phones are heavily compromised for accuracy by their tiny antenna which means very low gain. I recently saw a comparison between survey grade antenna (huge), 25mm patch antenna (like GT-31etc) and tiny, chip phone antenna. The difference between the Survey grade antenna and the patch antenna was something like a loss of a couple of db. The difference between the Patch antenna and the phone chip antenna was over 10db! That is a HUGE loss of signal strength! Phone app developers know this well and are using WiFi and Cell tower location to supplement/correct to get the accuracy they think is needed.
There are experiments being run now in Universities to find ways to process the weak phone signals to allow for RTK processing and cm positional accuracy, but even the most optimistic researchers seem to think practical application of it is at least 5 or 6 years away.
In contrast, When they can crack cm accuracy in smartphones, it will open up a huge new range of apps and uses that most of us don't think we need yet. 
RTK with cm accuracy is possible and being done now with 25mm patch antenna and relatively low priced GPS modules feeding the data to smartphones and tablets using BT. It's still a bit too expensive for us though, but not by much.
Select to expand quotetbwonder said..
I don't think it is a resolution problem. When sorting the data by speed in RealSpeed I counted 180 different speeds between 20 and 21 kts
Well you certainly wouldn't have done that if it was .5kt, and that's closer to .005 kts. guess it comes down to low gain antenna.
I had a bit more of a think about your HDOP values. The only explanation I can think of is that the HDOP is based purely on satellite geometry, and does not take into account the very weak signal and possible muti-path errors that such a small antenna gives. The HDOP would of course be theoretically better with more satellites as you observed (Multi GNSS?).
I have to say that I have never found HDOP a very reliable indicator of Doppler speed errors.
