Creation of a Lambertian Microwave Surface for Retrieving the Downwelling Contribution in Ground-Based Radiometric Measurements

Downwelling radiative fluxes from inhomogeneous environments are not easy to quantify from the total measured signal contribution. We propose the use of a Lambertian reflector to assess this issue. Using ray tracing to extrapolate the bidirectional reflectance distribution function of a surface with high reflectivity in the microwave spectrum, we characterized a surface that acts as a near-Lambertian reflector at 37 GHz. After implementing such a plate using molded aluminum and getting its emissivity, we validated its Lambertian reflectance properties based on microwave surface-based radiometer measurements. This analysis also gives guidelines on how to make a Lambertian surface for different wavelengths in the microwave spectrum. Field experiments show the usefulness of such a Lambertian plate to estimate the downwelling contribution in ground-based radiometric measurements.