Slice what I am talking about is what is the noise floor of the SDR unit itself. Not the SNR. If the unit itself has a -40dbm noise floor then any signal lower than that will be buried in the SDR's own internal noise and even signals that may be at -30dbm may be highly degraded. A fairly low end Rigol DSA815 SSA may have a noise floor of -100dbm which is good enough for most hobbyist testing. BTW the noise floor change on an SSA depending on the RBW you are using. As I stated above I am not sure what you are trying to achieve. If your just trying to see the different transponders and their center Freq and bandwidth then an SDR will do the job just fine. If your trying to decode some digital Data then that is a whole different ball game. BTW with the AIM meter the strongest signals I am seeing are -38db and the SNR is 17.6db The SQ is a solid 100. I am very interested in helping you if I knew more about what you are trying to achieve. It's kind of sad that I have the test gear to do a lot of this stuff but I have never really seen a purpose in looking further into it. Maybe you have an idea that might peak my interest. At one point I was thinking about looking for where the guide data is and I did locate the transponder but never went much further.