I have used my GPS app twice now since I added the command to turn on the use of SBAS satellites and I think AT6558 may be actually using them. This is my impression after looking the results of two sessions with the changed app. I also tried logging NMEA messages and checking for the presence of the SBAS satellites but I can't find any sign of them. I use GPS and Beidou satellites and those are present in the NMEA messages.
The NMEA message logs shows that the AT6558 software version is SW=URANUS5,V184.108.40.206. A CASIC spec that I have says:
Satellite system control commands, you can configure whether to receive any satellite in the system
Subsequent versions of V5200 support this command
An example of this command from the spec:
$PCAS15,4,FFFF*31, turn on satellites 1-16 of SBAS, that is, PRN=120-135
So, it would appear that the AT6558 in the Bangle.js 2 supports SBAS corrections. Since WAAS (SBAS) can be used for aircraft landing systems, the typical position error of 2m horizontally would be quite an improvement. But, as I could not find any evidence that the Bangle can actually use SBAS, I decided to try it out and see what happens.
Unfortunately, I did not do this test in a controlled manner, but for me the results show much fewer and smaller position errors than what I had experienced before. So far, I have recorded a short 2km walk going up a hill on the left hand side of the road, and then turning around at the 1km mark and returning to the start point on the opposite side of the road (always on the left facing traffic). The recorded tracks show no crossovers, although there is one small section where there was convergence. I then mapped this walk online on the gmap-pedometer site. The distances compare favourably.
Since I added the PCAS15 command to the app, I discovered that there are more than 16 SBAS satellites in service. I have changed the PCAS15 command to:
so 19 satellites (up to PRN 138) can be used. With this new version, I ran just over 10km this morning in hilly terrain with varied tree cover. The horizons were variable so all in all, not great GPS environment. Viewing the track up close on a computer screen shows no gross errors. This is definitely a noticeable change from previous runs in this area. I am very impressed with the Bangle.js 2 GPS now.
Once again, this was not a controlled test, but there is a definite improvement over my previous uses of this app. It may be coincidental with my software changes, but I just want to let people know that this Bangle.js 2 watch is capable of good positioning. Also, maybe others will be encouraged to try this PCAS15 command. It would be good to know if it actually works.