up to above ground level (AGL). Frequently, the weather radar can be directed downwards to give a reading from a longer range, up to above ground level (AGL). , all airliners are equipped with at least two and possibly more radar altimeters, as they are essential to autoland capabilities. (, determining height through other methods such as GPS is not permitted by regulations.) Older airliners from the 1960s (such as the British Aircraft Corporation BAC 1-11) and smaller airliners in the sub-50 seat class (such as the ATR 42 and BAe Jetstream series) are equipped with them. Radar altimeters are an essential part in ground proximity warning systems (GPWS), warning the pilot if the aircraft is flying too low or descending too quickly. However, radar altimeters cannot see terrain directly ahead of the aircraft, only that below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar. Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft to the ground (specifically to the nearest large reflecting object). This is because range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20°. Radio altimeters used in civil aviation operate in the IEEE C-band between 4.2 and 4.4 GHz. In early 2022, potential interference from 5G cell phone towers caused some flight delays and a few flight cancellations in the United States. In military aviation Radar altimeters are also used in military aircraft to fly quite low over the land and the sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles. A related use of radar altimeter technology is terrain-following radar, which allows fighter bombers to fly at very low altitudes. The F-111s of the Royal Australian Air Force and the U.S. Air Force have a forward-looking, terrain-following radar (TFR) system connected via digital computer to their automatic pilots. Beneath the nose radome are two separate TFR antennae, each providing individual information to the dual-channel TFR system. In case of a failure in that system, the F-111 has a back-up radar altimeter system, also connected to the