hah...well TOPO was a failure. The controllers called us "HOBO", "TOEHO", "BOBO" and the best was "TADPOLE". Of course, with a three letter designator of "TOE", most controllers thought that it was a VFR callsign of Toe.
So...we changed had to change it. We tried to get FOCUS and OPTICS, but they were taken. ICAO selected our last choice of "FOOT PRINT". The good news is that is an aerial survey term, but it does sound alittle funny.
oh well....