Why is the system considered a real-time ES? Gate assignment, the responsibility of gate controllers and their assistants, is a complex and demanding task at any airport. At O’Hare Airport in Chicago, for example, two gate controllers typically plan berthing for about 500 flights a day at about 50 gates. Flights arrive in clusters for the convenience of customers who must transfer to connecting flights, and so controllers must sometimes accommodate a cluster of 30 or 40 planes in 20 to 30 minutes. To complicate the matter, each flight is scheduled to remain at its gate a different length of time, depending on the schedules of connecting flights and the amount of servicing needed.

Mix these problems with the need to juggle gates constantly because of flight delays caused by weather and other factors, and you get some idea of the challenges. The problem is even more complex because of its interrelationship with remote parking and constraints related to ground equipment availability and customer requirements.