The Third Soil Moisture Active Passive Experiment

Abstract

The National Aeronautics and Space Administration's (NASA) Soil Moisture Active Passive (SMAP) mission is scheduled for launch in 2014. This soil moisture dedicated mission will carry a combined L-band radar and radiometer system with the objective of mapping near surface soil moisture globally at an unprecedented spatial resolution. The scientific rationale for SMAP is an improved accuracy and spatial resolution of the soil moisture estimates through the unique combination of high resolution (3km) but noisy radar derived soil moisture information and the more accurate yet lower resolution (40km) radiometer derived soil moisture information, yielding a 10km active/passive soil moisture product. In order to achieve these objectives, algorithms need to be developed and tested using field data that simulate the future radar and radiometer SMAP data. The Soil Moisture Active Passive Experiment (SMAPEx) contributes to the development and validation of such algorithms, by providing prototype SMAP observations collected with a unique active and passive airborne facility over a heavily monitored study area. SMAPEx comprised three campaigns across an approximately one year timeframe. The field work required by this project was undertaken in the Yanco intensive study area, located in the Murrumbidgee catchment in south-eastern Australia. The SMAP configuration is replicated by an airborne SMAP simulator using the Polarimetric L-band Multibeam Radiometer (PLMR; 1.41GHz) and the Polarimetric L-band Imaging Synthetic aperture radar (PLIS; 1.26GHz). Both instruments were mounted under the fuselage of the same aircraft to acquire concurrent observations of the same ground area. The Soil Moisture Active Passive Experiment (SMAPEx) comprises three campaigns across an approximately one year timeframe. This paper outlines the airborne and ground sampling rationale of the recent third experiment (SMAPEx-3) and summarises the data collected. This campaign was conducted in the austral spring from 5 to 23 September, 2011. While the two previous 1-week experiments focused on the early stage of crops growth and on mature crops, respectively, the 3-weeks long SMAPEx-3 aimed to capture the crop growth process. These data will complete the seasonal SMAPEx data set and will be used for the validation of temporal change detection algorithms, since a wide range of soil moisture and crop conditions were observed during the campaign. Ground and airborne observations were acquired throughout crop growing season every 2-3 days, an interval similar to SMAP revisit time. A total of nine PLMR/PLIS flights over an area the size of the SMAP pixel were conducted during SMAPEx-3, underpinned by extensive ground soil moisture, vegetation, and soil roughness data. Radar only soil moisture retrieval over crops will also be attempted as an add-on of the third experiment. With this purpose, two airborne flights with LIDAR and hyperspectral sensors were conducted for validation of radar retrieval of vegetation parameters. Detailed vegetation structure ground samples will be used to forward modelling of L-band radar backscatter using a discrete scatterer approach.