L-Band Radiometers Inter Calibration over Natural Targets
[11-Apr-2016] Cabot, F., and Anterrieu, E.
Presented at the 14th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment
Since the launch of the SMOS mission in 2009, two other satellites carrying L-band radiometers joined it on orbit. Aquarius was launched in June 2011 and SMAP in January 2015. Unfortunately, Aquarius ceased operation later that year. All 3 instruments have been operating simultaneously between April and June 2015. Although this golden age of L-band on orbit radiometry was short lived, it allowed for sound comparison of the performances of these 3 radiometers. Moreover, its untimely termination emphasizes the need for reliable inter calibration to build long term consistent archives of brightness temperature and higher level products. Still, since all these instruments do not share the same technology and even principle of acquisitions, direct comparison and synergistic use of their measurements is not straightforward. The objective of this paper is to demonstrate a set of methods to make them inter-comparable, down to a common reference. Instead of a ground reference, we use here SMOS as a transfer radiometer. This method can be applied over different types of surfaces: i) making use of a stable target to assess the consistency and stability of both data sets. This is done over the area surrounding Dome Concordia in Antarctica. After careful selection and filtering, statistics of the comparison are retrieved along with long term trends in both data sets. ii) Once every so often, satellites overpass the same area within a very short time period. Due to different orbit inclinations these alignments occur essentially along the equator, but over different surfaces, giving access to wide dynamic range in brightness temperature. iii) At last, all radiometers are aimed at the deep sky for calibration purposes. But despite this use of a common reference, it can be shown that the retrieved brightness temperature exhibit some differences, traceable to the differences in calibration strategies and acquisition principles. This presentation will briefly describe the already established methods, along with a full record of results over the lifetimes of SMOS, SMAP and Aquarius. A more general conclusion on common use of these data sets will also be given.