Old US GMR systems not very efficient but FAA rolling out ASDE-X which has automated controller alerts and which Sensis and Honeywell can modify to create a cockpit alert on aircraft TCAS. The multilateration (MLAT) elements of ASDE-X have also been sold to European airports, India and Hong Kong. It's latest version known as A-SMGCS is being installed in BNE, SYD and MEL, Australia. Watch for the MLAT sensor being used instead of radar and long before ADS-B becomes widespread. Already in use in places like Innsbruck and being installed in the North Sea to track helicopter movements.