I'm putting in GRT's Horizon I EFIS that has an altitude encoder output. Is there any reason to install a seperate altitude encoder? Has anyone had any issues with their transponder check if they don't have a TSO's encoder? Do the regs say anything about a TSO's encoder for IFR operation? An encoder is fairly cheap but it takes up space and just adds one more thing. If I do put one in, what is the opnion on the best choice? There is an obvious advantage to using the output from the GRT as your transponder would agree with your instruments.