The Soil Moisture Active Passive Experiments (SMAPEx): Towards Soil Moisture Retrieval from the SMAP Mission

21 NASA's Soil Moisture Active Passive (SMAP) mission will carry the first combined spaceborne L-22 band radiometer and Synthetic Aperture Radar (SAR) system with the objective of mapping near-23 surface soil moisture and freeze/thaw state globally every 2-3 days. SMAP will provide three soil 24 moisture products; (i) high-resolution from radar (~3km), (ii) low-resolution from radiometer 25 (~36km), and (iii) intermediate-resolution from the fusion of radar and radiometer (~9km). The Soil 26 Moisture Active Passive Experiments (SMAPEx) are a series of three airborne field experiments 27 designed to provide prototype SMAP data for the development and validation of soil moisture 28 retrieval algorithms applicable to the SMAP system. This paper describes the SMAPEx sampling 29 strategy and presents an overview of the data collected during the three experiments: SMAPEx-1 30 SMAPEx experiments were conducted in a semi-arid agricultural and grazing area located in 32 southeastern Australia, timed so as to acquire data over a seasonal cycle at various stages of the crop 33 growth. Airborne L-band brightness temperature (~1km) and radar backscatter (~10m) observations 34 were collected over an area the size of a single SMAP footprint (38 km × 36 km at 35° latitude) 35 with a 2-3 days revisit time, providing SMAP-like data for testing of radiometer-only, radar-only 36 and combined radiometer-radar soil moisture retrieval and downscaling algorithms. Airborne 37 observations were supported by continuous monitoring of near-surface (0-5cm) soil moisture along 38 with intensive ground monitoring of soil moisture, soil temperature, vegetation biomass and 39 structure, and surface roughness. 40