Analyzing satellite and airborne Ka-band passive microwave observations over land for temperature and vegetation monitoring

Richard De Jeu^1*Yoann Malbeteau²Ruxandra Maria Zotta³Wouter Dorigo³Xiaoling Wu⁴Jeff Walker⁴Diego G. Miralles⁵

• ¹Transmissivity B.V., Alphen aan den Rijn, Netherlands
• ²Planet Labs Inc, San Francisco, California, United States
• ³Vienna University of Technology, Vienna, Vienna, Austria
• ⁴Monash University, Melbourne, Victoria, Australia
• ⁵Ghent University, Ghent, East Flanders, Belgium

Passive microwave observations at frequency have been widely available for decades, but their full potential for land applications has been hardly exploited. In this study these past observations are analyzed to better understand their sensitivity to temperature and vegetation dynamics. Between September and October 2019, a series of airborne flights carrying L-and Kaband instruments were conducted at the Yanco study area in southeastern Australia. In addition to the airborne data, satellite passive microwave observations, including the Ka-band observations from the Advanced Microwave Scanning Radiometer 2 (AMSR2), were collected for the same region. The vertically polarized brightness temperature of the Ka-band frequency, as observed by the satellite, exhibited a very strong correlation with Land Surface Temperature (LST) from the Moderate Resolution Imaging Spectroradiometer (MODIS) based on a period of 12 years (R 2 =0.98). The airborne data also revealed a strong spatial correlation (R 2 =0.70) with LST images from Landsat 8, but only for fields with a substantial vegetation cover (Normalized Difference Vegetation Index (NDVI) ≥0.6). At lower NDVI values, the observations became more sensitive to soil surface characteristics, particularly soil wetness (soil moisture > 0.3 m 3 m -3 ), causing up to 20 K drops in brightness temperature. To isolate the vegetation signal, the Ka-band Vegetation Optical Depth (VOD) was derived using a radiative transfer model for both the satellite and airborne data. The satellite-derived Ka-band VOD closely matched published VOD products from other frequencies, and the aircraft-based VOD provided realistic spatial patterns over different landscapes. At the satellite scale, a clear relationship between VOD and NDVI was observed. The aircraft-based VOD signal was noisier and had a weak spatial correlation with NDVI, although it demonstrated similar trends as at the satellite scale. Overall, our findings highlight the potential of Ka-band for land applications, and its varying sensitivity across scales, with local variability being more pronounced at higher spatial resolutions.