A microwave-based system has been developed as a means of detecting ice on aircraft surfaces, with enough sensitivity to provide a warning before the ice accretes to a dangerous thickness. The system can measure the thickness of ice from a few mils (1 mil = 0.0254 mm) to about 1/4 in. (≈6 mm) and can distinguish among (1) ice, (2) water (or deicing fluid), and (3) a mixture of ice and water (or deicing fluid). Sensors have been ruggedized to withstand the rain erosion environment.

Figure 1. The Sensor Is a Section of CPW or SL Transmission Line flush with an aircraft surface. The magnitude and phase of the microwave signal arriving at the I/Q detector is affected by the amount of ice and/or water coating the sensory section of the transmission line.
Figure 2. This Normalized Response in the Magnitude/Phase Plane was obtained in tests of a slot-line sensor at three temperatures. Three distinct regions of the magnitude-vs.-phase plane make it possible to distinguish among coats of ice, water, and mixtures thereof. In the important case of pure ice, the amplitude shift is negligible and the thickness of ice is indicated by the amount of phase shift.

The system (see Figure 1) includes a microwave module that contains a continuous-wave microwave signal source, the output of which is split onto a sensor path and a reference path. The sensor path consists of a microwave transmission line that includes a sensory portion of either the ground-plane coplanar-waveguide (CPW) type or the slot-line (SL) type. Whichever type is used, the sensory portion of the transmission line is mounted flush with the aircraft surface at the desired ice-detection location. With the exception of the sensory portion of the transmission line, the aforementioned circuitry is enclosed within an electrically conductive box. The sensor- and reference-path outputs are processed through an in-phase/quadrature (I/Q) detector, then through an analog-to-digital (A/D) converter. The output of the A/D converter is sent to data-processing and control circuitry in a cockpit display unit.

The data-processing subsystem computes the magnitude and phase of the sensor signal relative to those of the reference signal, and uses the sensor signal obtained when the sensor is bare to normalize the response of the system when water and/or ice are present. The normalized magnitude and phase response of the system serves as an indication of the thickness of ice and or water (see Figure 2).

This work was done by Philip J. Joseph, Dennis P. Glynn III, and John C. Joseph of Dedicated Electronics, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research
Center, Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4–8,
21000 Brookpark Road,
Cleveland, Ohio 44135.

Refer to LEW-17135.