Well, the easiest way would be Mode S (ADS-B out) transponders on the microlights and an groundstation at the base. That will easily cover the 50NM, however costs quite a bit of money to equip the aircraft.
FLARM is actually not that bad, but only covers around 10NM max with one groundstation. And yes, those exist, have seen a very nice installation at a local glider club that has a radar like situation screen on the ground where it can follow all FLARM equipped aircraft, which includes their whole fleet.