I have seen some reports of some Skylab GPS boards that had some rather sketchy GPS code. A friend had one that was off around 0.5 degrees in longitude... but only if your location was between +/- 90 degrees longitude. He was on the east coast and had the problem. He sent it to me (96 dgrees west) and I didn't see it. Same for a tester on the west coast. He spend weeks going over his code and could not find what the problem was. He sent me a raw data dump and sure enough, I got the same (wrong) location that he did. I seem to remember the NMEA messages were correct, but the binary message (which was in Ublox format) had the issue. ---------------------------------- For comparison with your data, attached is the ADEV+MDEV plot [1] for the SkyLab / MG1613S GPS board [2], the one with the 350 ms offset and 11 ms stdev.