I haven't researched it since I rent but I believe there is a new Mode S. I
would check into it and see if it made any sense.

http://www.rockwellcollins.com/ecat/...html?smenu=111

talks about it. It may be way out of reach but if it isn't too expensive it
may have some value because of the TCAS features,

KEY OWNER/USER BENEFITS
a.. Replies to ATC radar on Modes A, C and S with altitude and ident
b.. Antenna diversity as required for TCAS II (TDR-94D)
c.. Data link capability for TCAS II crosslink and ATC data communications
d.. ICAO Level 3A for receiving extended length messages
e.. Capable of GPS/FMS position reports ADS-B
f.. Provisions for ICAO FANS
g.. High reliability with crystal controlled solid-state transmitter
h.. Flexible air data interfaces for compatibility with all aircraft
i.. Diagnostics for easy fault isolation and shop maintenance
j.. Provides new Mode S Elementary and Enhanced Surveillance functionality