[sokoloff]

If the expense and lead time for the redesign now that more reliable and precise position (and first derivative thereof) are available from GPS via ADS-B are to be considered, it's fair to ask "well, what are the downsides as compared to using altitude for separation as today?" and one of them is "a higher percentage of aircraft are transmitting mode-C than will be transmitting ADS-B out".

Given that the clear priority is now to follow a TCAS RA (resolution advisory) over an ATC instruction, I think that the current TCAS approach is good enough to cause aircraft to miss each other and that a GPS-based redesign is unlikely.