code: public static boolean withinThreshold (double timeDifference, double distance, boolean surface) {
  		double x = abs(timeDifference);
  		double d = abs(distance);
  
  //		if (d/x >= (surface?51.44:514.4)*2.5)
  //			System.err.format("%.2f/%.2f=%.2f\n", d, x, d/x);
  
  		// may be due to Internet jitter; distance is realistic
  		if (x < 0.7 && d < 2000) return true;
  		else return d/x < (surface?51.44:514.4)*2.5; // 1000 knots for airborne, 100 for surface
  	}
docs: This function is used to induce some tolerance if messages
  were received shortly after (due to Internet jitter in timestmaps).
  This function is derived empirically
  @param timeDifference the time between the reception of both messages
  @param distance covered
  @param surface is the airplane e.g. taxiing?
  @return whether distance is realistic