Therefor it is undesirable that GA is equipped with uncertified ADS-B transponders.
I am confused by that! In Europe we are using Mode S transponders with extended squitter to emit ADS-B out. I think all the transponders available are certified?
Sorry to cause confusion. You are correct that the transponder is certified. The certification usually covers Mode S, as described in (E)TSO-C112.
However in general it does not cover the ADS-B specifications (E)TSO-C166b.
Some Mode S certified transponders radiate extended squitters, but are not ADS-B certified. Potentially they could send out corrupt ADS-B data, and in a few cases they do so.
In addition to corruption of data by the tranponder, the attached systems can cause problems. For example the time between the validity of the GPS position and the moment that it is send out by the transponder is too long in some systems.
At the moment all those issues are not really a problem since ADS-B is not used operationally within Europe. But to start operational use of ADS-B these problems need to be addressed.
ATCast