I do not understand why requiring 2 different tones lessens QRM. If the transponder is set to turn off after 10 minutes, why can't the regular 67 hz tone be the trigger to turn on? Is it that bootleggers aren't smart enough to send two different tones?
73, Bill NZ5N
I hope no one changes the tone requirement on SO-50. If you listen to AO-51 passes over the USA and compare it to an SO-50 pass you will know why - lack of QRM (as well as lack of users - but that's the same on most birds anyway).
SO-50 is similar to working simplex terrestrially, except that you hear your downlink. There is no constant carrier as in the other FM birds as you mentioned. That does make it more difficult to tune, however (and again, not knowing the history of this bird), I'm sure it conserves battery power and transmitter life. I will take a guess and say that the 'turn on' code actually turns on the primary receiver, which again would conserve it's life.