I doubt that it is atmospheric as I find some transponders are affected while others are very stable. I find that TP 01, 05, 09, 13, 17, 21, 25, 29 and TP 04, 08, 12, 16, 20, 24, 28 and 32 all don't vary much (Note the groupings of 4) whereas all the others can change by around 6-8 points on my...