Aircraft Ice Detection using Time Domain Reflectometry with Coplanar Sensors

Ice accretion on the wings and various parts of an aircraft remains a serious problem in aviation. Ice on the external tank of a space shuttle can also be a source of debris hazard at liftoff. Recent efforts to combat this problem have employed time domain reflectometry (TDR) technique with limited success. This is because the structure of conventional TDR sensors is not suitable for accurate measurements. This work proposes the use of more compatible coplanar transmission lines as TDR sensors for ice detection. The coplanar lines, being one-sided structures, allow the test material to be placed conveniently on the sensor for non-destructive measurements. The fringing fields of the coplanar line enhance a strong coupling between the test material and the sensor, while the dielectric substrate provides base support. This paper examines the feasibility of using coplanar transmission lines as TDR sensors for thickness measurements. Numerical analysis is carried out using quasi-static design equations obtained by conformal mapping techniques. The results show that the propagation characteristics of electromagnetic waves in a loaded coplanar line depend on the permittivity and thickness of the test material. By measuring these characteristics (the travel time, phase velocity, effective dielectric constant, etc.) when the sensor is loaded with known dielectric materials, their presence and thickness can be detected. Since ice and water do not show dispersive behavior at low microwave frequencies, accurate measurements can be made using time-domain techniques. This work, which is motivated by the recent call by the National Transportation Safety Board (NTSB) for more icing research, will enhance the development of effective ice detectors.