GPS NMEA sentence named $GNACCURACY

Hello guys, I know this question is a little bit off topic but may be useful also to others using something more accurated location sensor then the Location Sensor component in AI2.
As probaby you already know, I am updating an app that uses the GPS; during some tests I look at the sentences receved by my smartphone and I see this sentence I did not remember to see before:


I see one of it every second or every couple of seconds. I search the web to find its description but I was not able to find any helpful documentation. If someone has a datasheet, product manual or every else describing the format, purpose and anything else would be very happy to read it.
P.S. I posted the same/similar question in stackoverlow also.
Thanks, Massimo