In the older beam systems, the accuracy of the equisignal area was a function of the pattern of the two directional signals, which demanded that they be relatively narrow. The ILS pattern can be much wider. ILS systems are normally required to be usable within 10 degrees on either side of the runway centerline at 25 nautical miles (46 km; 29 mi), and 35 degrees on either side at 17 nautical miles (31 km; 20 mi).