code: public void gc() {
  		List<Integer> toRemove = new ArrayList<Integer>();
  		for (Integer transponder : decoderData.keySet())
  			if (decoderData.get(transponder).posDec.getLastUsedTime()<latestTimestamp-3600000)
  				toRemove.add(transponder);
  
  		for (Integer transponder : toRemove)
  			decoderData.remove(transponder);
  	}
docs: Clean state by removing decoders not used for more than an hour. This happens automatically
  every 1 Mio messages if more than 50000 aircraft are tracked.