Recent studies [1][2][3] indicated that co-located microwave and millimeter wavelength radars can estimate liquid water content and drop size parameters of precipitation. Given its ability to aide in mapping regions of supercooled liquid water, multi-frequency radar is now being investigated for in-flight detection of icing potential ahead of aircraft. Although the retreival algorithms slightly vary, the approaches are similar: moments of the drop size distribution are estimated from profiles of reflectivity measured at a weakly attenuating microwave frequency, typically X-band, and at a more attenuating millimeter wavelength, typically Ka-band. The microwave channel is highly sensitive to drop size, as the reflected power is proportional to the sixth moment (Rayleigh scatterers) of the drop size distribution, while the differential attenuation, between the microwave and millimeter wave channels, can be related to liquid water content.

After successful ground-based tests, the Icing branch of the NASA Glenn Research Center began collaborating with ProSensing Inc. to develop an airborne, multi-frequency radar to test the technique in realistic in-flight icing conditions.

The radar system, now under development, was designed with two millimeter wavelength channels (1.0 and 0.3 cm) to extend the liquid water range of the sensor and to maintain accuracy in non-Rayleigh precipitation. The radar system consists of pulsed X (9.4 GHz) and Ka-band (35.5 GHz) radars, both employing 25 kW magnteron transmitters, and a dual-antenna FMCW W-band (95.0 GHz) radar. With the RF sections housed in a single forward looking pod, the three radars will sample similar sample volumes with 12”, 5.5” and 3” diameter antennas at 9.4, 35.5 and 95.0 GHz respectively. Engineering test flights of the multi-frequency radar system are planned for the NASA Twin Otter research aircraft in early 2004.

This paper will introduce the measurement technique, show examples of multi-frequency radar retrieved liquid water and drop size parameters from past ground-based experiments, and describe the airborne radar system.

[1] Vivekanandan, J., G. Zhang, M. K. Politovich, 2001: An Assessment of Droplet Size and Liquid Water Content Derived from Dual-Wavelength Radar Measurements to the Application of Aircraft Icing Detection. Journal of Atmospheric and Oceanic Technology: Vol. 18, No. 11, pp. 1787–1798.

[2] Pazmany, A. L. 2001 :Millimeter-Wave Radar Field Measurements and inversion of cloud parameters for the 1999 Mt. Washington icing sensors project, NASA Tech. Briefs CR-2001-211103, August.

[3] Pazmany, A. L. and J. B. Mead, 2000: Estimation of LWC and Drop Size from Multi-Frequency Radar Reflectivity Profiles with an Artificial Neural Network, 2000 In-Flight Icing Remote Sensing Workshop, Nov. 16-17.