Experiments and improving GPSLib4J

Garmin Foretrex 101 Started to work with GPSLib4J. First got some NPEs when I added a general packet listener, but no time or position listener. Found the lib makes too many assumptions... Had to modify GPSLib4J in minor ways: the Garmin packet listener now sends specific packets instead of the raw version; improved text output of PVT packets (the Garmin "all info in one" packet); improved text output of position; improved documentation (which, frankly, is a bit sparse to say the least. Added info from Garmin protocol spec to the comments).

Found that position, altitude, speed etc. varies all the time, which is good. Why? It allows me to set up my own "differential GPS". How that? Knowing my laptop does not move I know that all skipping around is due to artificially introduced and other errors. I know precisely where I am, too, if I look up my position on a map. Then I select one data set as a reference on the laptop and submit the delta to to ship every second. The ship then corrects its own data. Hm, this only works when a) both GPS receivers get cheated the same way (for a point in time t the errors on both devices are the same) and b) receive the data in sync (or is the time stamp exact enough for matching packets?). Anyways, it's an idea, which I have to try out.